diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index b17038f8b..78c9a6674 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -782,6 +782,133 @@ jobs:
           CI_JOB_TIMEOUT_MINUTES: '45'
         run: node scripts/ci-check-job-near-cap.cjs
 
+  # #4591 (epic #4589 Phase 2): runs ONLY the platform-conformance-tier file
+  # list (scripts/lib/platform-conformance-tier.generated.cjs) on real
+  # Windows/macOS — the OS-agnostic bulk of the suite already ran once on
+  # ubuntu-latest in the `test` job above. Gated the same way `test-full` is
+  # (product code changed AND full_matrix). Windows is sharded 3 ways
+  # (matching test-full's own precedent) after the first real CI run measured
+  # it: unsharded, windows-latest hit its 45-minute timeout and was CANCELLED
+  # (started 03:41:19Z, cancelled 04:26:25Z, run 34434252144) while
+  # macos-latest finished the identical file set in 26m58s — this conformance
+  # tier is dominated by subprocess-spawning tests (343/565 files), and
+  # Windows process-spawn overhead is the documented reason test-full's own
+  # windows lane needed the same fix (#3057). macos-latest stays unsharded; it
+  # has real headroom (27m against the 45m cap).
+  # `test-full` (above) keeps running the WHOLE suite on the
+  # same OSes as a non-gating safety net for one release cycle (see
+  # `required-tests` below) — this job is the new GATING signal for
+  # windows/macos coverage.
+  test-conformance:
+    name: conformance test (${{ matrix.os }}, ${{ matrix.node-version }}${{ matrix.shard && format(', shard {0}', matrix.shard) || '' }})
+    needs: [changes, preflight]
+    if: needs.changes.outputs.code_changed == 'true' && needs.changes.outputs.full_matrix == 'true'
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        shell: ${{ matrix.shell }}
+    # No LANE_COSTS entry exists yet for this brand-new job — see
+    # tests/ci-test-job-timeout-budget.test.cjs's own header: "no unit test
+    # can prove a lane fits its budget — only a real CI run measures that."
+    # Generous and unchanged from test-full's own budget until a real
+    # measurement exists; the smaller file count should comfortably undercut
+    # this ceiling.
+    timeout-minutes: 45
+    env:
+      GSD_PLUGIN_ROOT: .ci-gsd-plugin-root-disabled
+      GSD_STRICT_LIVE_CONFIG_GUARD: ${{ matrix.os != 'windows-latest' && '1' || '' }}
+      GSD_EMITTED_BASE: ${{ github.event.pull_request.base.sha }}
+      AUDIT_BASELINE_REF: ${{ github.event_name == 'pull_request' && github.event.pull_request.base.sha || (github.event_name == 'push' && github.event.before) || (github.event_name == 'merge_group' && github.event.merge_group.base_sha) || '' }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: windows-latest
+            node-version: 24
+            shell: pwsh
+            shard: 1/3
+          - os: windows-latest
+            node-version: 24
+            shell: pwsh
+            shard: 2/3
+          - os: windows-latest
+            node-version: 24
+            shell: pwsh
+            shard: 3/3
+          - os: macos-latest
+            node-version: 24
+            shell: 'zsh {0}'
+
+    steps:
+      - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd  # v5.0.1 (Windows)
+        if: runner.os == 'Windows'
+        with:
+          fetch-depth: 0
+          persist-credentials: true
+          token: ${{ github.token }}
+
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2 (Linux/macOS)
+        if: runner.os != 'Windows'
+        with:
+          fetch-depth: 0
+          persist-credentials: true
+          token: ${{ github.token }}
+
+      - name: Record job start time (near-cap check)
+        run: node -e 'require("fs").appendFileSync(process.env.GITHUB_ENV, "CI_JOB_START_EPOCH_MS=" + Date.now() + "\n")'
+
+      - name: Guard — require GitHub-hosted runner
+        run: node scripts/ci-guard-runner.cjs
+
+      - name: Rebase check — merge PR base branch into PR head
+        if: github.event_name == 'pull_request'
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+          CI_REBASE_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        run: node scripts/ci-rebase-check.cjs
+
+      - name: Set up Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f  # v6.3.0
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: 'npm'
+
+      - name: Environment check
+        run: npm run check:env
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Dependency integrity gate
+        run: node scripts/check-npm-integrity.cjs
+
+      - name: Restore emitted-baseline cache
+        if: github.event_name == 'pull_request'
+        uses: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830  # v4.3.0
+        with:
+          path: .gsd-cache/emitted-baseline.json
+          key: emitted-baseline-${{ github.event.pull_request.base.sha }}
+
+      # #4591: the generated conformance-tier list is a .cjs module (so
+      # scripts/gen-platform-conformance-tier.cjs's own tests and other
+      # scripts can `require()` it directly) — run-tests.cjs --files-from
+      # expects a plain newline-delimited text file, so this step bridges
+      # the two, one path per line, matching the existing scoped-lane
+      # convention (.ci-selected-tests.txt).
+      - name: Prepare conformance-tier test list
+        run: node -e "require('./scripts/lib/platform-conformance-tier.generated.cjs').CONFORMANCE_TIER_FILES.forEach(f => console.log(f))" > .ci-conformance-tests.txt
+
+      - name: Run platform-conformance-tier tests
+        run: node scripts/run-tests.cjs --files-from .ci-conformance-tests.txt${{ matrix.shard && format(' --shard {0}', matrix.shard) || '' }}
+
+      - name: Check job budget (near-cap advisory)
+        if: always()
+        continue-on-error: true
+        env:
+          CI_JOB_LABEL: "conformance test (${{ matrix.os }}, ${{ matrix.node-version }})"
+          CI_JOB_TIMEOUT_MINUTES: '45'
+        run: node scripts/ci-check-job-near-cap.cjs
+
   # #2952: the coverage gate that the `test` lane used to run inline. Sharding
   # the unit suite means no single runner sees the whole picture, so the gate
   # moves here, downloads every shard's raw V8 dumps into one coverage/tmp, and
@@ -929,6 +1056,7 @@ jobs:
       - test
       - test-inert
       - test-full
+      - test-conformance
       - coverage-gate
       - qa-loop-walk
     if: always()
@@ -944,6 +1072,7 @@ jobs:
           CHANGES_RESULT: ${{ needs.changes.result }}
           LINT_RESULT: ${{ needs.lint-tests.result }}
           TEST_RESULT: ${{ needs.test.result }}
+          TEST_CONFORMANCE_RESULT: ${{ needs.test-conformance.result }}
           INERT_RESULT: ${{ needs.test-inert.result }}
           FULL_TEST_RESULT: ${{ needs.test-full.result }}
           COVERAGE_GATE_RESULT: ${{ needs.coverage-gate.result }}
@@ -957,6 +1086,7 @@ jobs:
           echo "changes=$CHANGES_RESULT"
           echo "lint-tests=$LINT_RESULT"
           echo "test=$TEST_RESULT"
+          echo "test-conformance=$TEST_CONFORMANCE_RESULT"
           echo "test-inert=$INERT_RESULT"
           echo "test-full=$FULL_TEST_RESULT"
           echo "coverage-gate=$COVERAGE_GATE_RESULT"
@@ -1002,13 +1132,27 @@ jobs:
               echo "::error::test matrix did not pass"
               exit 1
             fi
+            # #4591 (epic #4589 Phase 2): test-conformance is the new gating
+            # signal for real Windows/macOS coverage — the conformance-tier
+            # file list, not the whole suite.
+            if [ "$TEST_CONFORMANCE_RESULT" != "success" ] && [ "$TEST_CONFORMANCE_RESULT" != "skipped" ]; then
+              echo "::error::platform-conformance-tier matrix did not pass"
+              exit 1
+            fi
             # #2952: the coverage gates no longer run inside the `test` matrix —
             # sharding moved them to the `coverage-gate` job, which merges every
             # shard's dumps. TEST_RESULT therefore does NOT cover them any more;
             # COVERAGE_GATE_RESULT below is what gates coverage.
+            # #4591 (epic #4589 Phase 2): downgraded from a hard gate to a
+            # non-blocking safety net for one release cycle, per the epic's
+            # phased-rollout design — test-conformance (above) is now the real
+            # gate for windows/macos coverage. A red full-suite run here is
+            # still surfaced (it means the conformance-tier classifier missed
+            # something) but does not block merge; retire this job entirely
+            # in a follow-up once the classifier has proven itself over one
+            # release cycle with no such miss.
             if [ "$FULL_TEST_RESULT" != "success" ] && [ "$FULL_TEST_RESULT" != "skipped" ]; then
-              echo "::error::full parity matrix did not pass"
-              exit 1
+              echo "::warning::full legacy parity matrix did not pass (non-gating safety net — see #4591)"
             fi
 
             # #2952: the coverage gate is skipped when product code did not change
diff --git a/bin/install.js b/bin/install.js
index 00ebb795d..fe07435bf 100755
--- a/bin/install.js
+++ b/bin/install.js
@@ -436,7 +436,7 @@ const GSD_CHANGESET_FILES = [
   'github-release-notes.cjs', 'lint.cjs', 'new.cjs',
   'README.md', // documentation only — not user-authored
 ];
-const GSD_SCRIPTS_LIB_FILES = ['cli-exit.cjs', 'allowlist-ratchet.cjs', 'drift-scan.cjs', 'alias-drift-families.cjs', 'exit-code-registry.cjs', 'ndjson-reporter.cjs', 'ci-job-timing.cjs', 'shellcheck-fetch.cjs', 'npm-version-check-diagnosis.cjs'];
+const GSD_SCRIPTS_LIB_FILES = ['cli-exit.cjs', 'allowlist-ratchet.cjs', 'drift-scan.cjs', 'alias-drift-families.cjs', 'exit-code-registry.cjs', 'ndjson-reporter.cjs', 'ci-job-timing.cjs', 'shellcheck-fetch.cjs', 'npm-version-check-diagnosis.cjs', 'platform-conformance-tier.generated.cjs', 'suite-detection.cjs'];
 
 /**
  * Resolve a runtime's shared-hooks directory name from its descriptor.
diff --git a/docs/TESTING-SUITES.md b/docs/TESTING-SUITES.md
index e07c584ad..3e75d0fea 100644
--- a/docs/TESTING-SUITES.md
+++ b/docs/TESTING-SUITES.md
@@ -404,7 +404,7 @@ Two properties are load-bearing and easy to break:
   failure message only.
 
 Gated: `test.yml` (`lint-tests`, `test`, `test-inert`, `test-full`,
-`coverage-gate`, `qa-loop-walk`, `required-tests`), `install-smoke.yml`,
+`test-conformance`, `coverage-gate`, `qa-loop-walk`, `required-tests`), `install-smoke.yml`,
 `mutation.yml`, `security-scan.yml`, `docs-required.yml`,
 `changeset-required.yml`, `default-flip-documentation.yml`, `branch-naming.yml`.
 
@@ -547,6 +547,17 @@ cap stays at least the **headroom factor** (1.5x) above a documented,
 hand-measured cost for that job — now all four of the jobs above, not just
 `test`/`test-full`/`coverage-gate`/`test-inert` as before.
 
+`test-conformance` in `test.yml` (#4591, epic #4589 Phase 2) runs the
+`scripts/lib/platform-conformance-tier.generated.cjs` file list on
+`windows-latest` (sharded three ways) and `macos-latest` (unsharded) — the
+gating signal for real-OS coverage, replacing `test-full`'s old gating role
+for one release cycle while `test-full` runs as a non-gating safety net. It
+also declares a `timeout-minutes` cap and runs the same in-job near-cap check
+described below, but it has no `LANE_COSTS` entry in
+`tests/ci-test-job-timeout-budget.test.cjs` yet — no real, completed
+(non-cancelled) per-shard measurement exists — so the headroom-factor gate
+does not cover it until one lands.
+
 Two runtime mechanisms sit on top of that static gate, both new in #4036:
 
 - **In-job near-cap check** (`scripts/ci-check-job-near-cap.cjs`) — the last
@@ -561,7 +572,7 @@ Two runtime mechanisms sit on top of that static gate, both new in #4036:
   `scripts/ci-timeout-report.cjs`) — runs daily and on `workflow_dispatch`. It
   polls GitHub's Actions REST API for recently completed jobs across
   `test.yml`, `mutation.yml`, and `install-smoke.yml`, resolves each job's
-  declared cap (a literal `timeout-minutes` for `test`/`test-full`/`smoke`, or
+  declared cap (a literal `timeout-minutes` for `test`/`test-full`/`test-conformance`/`smoke`, or
   `scripts/mutation-matrix.cjs`'s `COVERED[<module>].timeoutMinutes` for
   `mutate`'s per-module shards), and appends any new `(runId, jobName)`
   records to `tests/ci-timeout-budget-history.jsonl`. Unlike the in-job check,
diff --git a/docs/how-to/read-ci-timeout-signals.md b/docs/how-to/read-ci-timeout-signals.md
index 81285ad47..64dc56790 100644
--- a/docs/how-to/read-ci-timeout-signals.md
+++ b/docs/how-to/read-ci-timeout-signals.md
@@ -1,9 +1,12 @@
 # How to read CI timeout budget signals
 
-Every matrixed CI job (`test`, `test-full` in `.github/workflows/test.yml`; `mutate` in
-`mutation.yml`; `smoke` in `install-smoke.yml`) now reports how close it ran to its
+Every matrixed CI job (`test`, `test-full`, `test-conformance` in `.github/workflows/test.yml`;
+`mutate` in `mutation.yml`; `smoke` in `install-smoke.yml`) now reports how close it ran to its
 `timeout-minutes` cap. This page is for a maintainer trying to answer: *is a lane drifting
-toward its cap, and where do I look?*
+toward its cap, and where do I look?* (`test-conformance` runs the platform-conformance-tier file
+list — `scripts/lib/platform-conformance-tier.generated.cjs` — on `windows-latest`, sharded three
+ways, and `macos-latest`, unsharded; it is the gating signal for real-OS coverage, replacing
+`test-full`'s old gating role for one release cycle.)
 
 ## 1. A single run crossed 90% of its budget
 
diff --git a/package.json b/package.json
index da75abfa6..f20c28ab8 100644
--- a/package.json
+++ b/package.json
@@ -110,7 +110,7 @@
     "gen:registry": "node scripts/gen-registry.cjs --write",
     "gen:install-tree": "node scripts/gen-install-tree-fixtures.cjs",
     "gen:section-manifest": "node scripts/gen-section-manifest.cjs --write",
-    "regen:derived": "npm run build && npm run gen:registry && node scripts/gen-adr-index.cjs --write && node scripts/gen-features.cjs --write && node scripts/gen-capability-matrix.cjs --write && node scripts/gen-inventory-manifest.cjs --write && node scripts/gen-context-index.cjs --write && node scripts/gen-state-md-docs.cjs --write && npm run gen:section-manifest && node scripts/sync-manifest-versions.cjs && npm run gen:install-tree && node scripts/gen-scripts-cli-exit.cjs --write && node scripts/gen-hooks-cli-exit.cjs --write && node scripts/gen-exit-code-registry.cjs --write && node scripts/gen-exit-code-docs.cjs --write",
+    "regen:derived": "npm run build && npm run gen:registry && node scripts/gen-adr-index.cjs --write && node scripts/gen-features.cjs --write && node scripts/gen-capability-matrix.cjs --write && node scripts/gen-inventory-manifest.cjs --write && node scripts/gen-context-index.cjs --write && node scripts/gen-state-md-docs.cjs --write && npm run gen:section-manifest && node scripts/sync-manifest-versions.cjs && npm run gen:install-tree && node scripts/gen-scripts-cli-exit.cjs --write && node scripts/gen-hooks-cli-exit.cjs --write && node scripts/gen-exit-code-registry.cjs --write && node scripts/gen-exit-code-docs.cjs --write && node scripts/gen-platform-conformance-tier.cjs --write",
     "validate:registry": "node scripts/validate-registry.cjs",
     "prepack": "npm run build:lib",
     "prepare": "npm run build:lib",
@@ -132,7 +132,7 @@
     "lint:test-file-count": "node scripts/lint-test-file-count.cjs",
     "lint:pr-checks": "node scripts/lint-pr-check-project-dir.cjs",
     "lint:changeset": "node scripts/changeset/lint.cjs",
-    "lint:generated-sync": "node scripts/gen-capability-registry.cjs --check && node scripts/gen-loop-host-contract.cjs --check && node scripts/gen-capability-matrix.cjs --check && node scripts/sync-manifest-versions.cjs --check && node scripts/gen-inventory-manifest.cjs --check && node scripts/generate-package-identity.cjs --check && node scripts/gen-plugin-skills.cjs --check && node scripts/gen-registry.cjs --check && node scripts/gen-adr-index.cjs --check && node scripts/gen-features.cjs --check && node scripts/check-glossary-refs.cjs --check && node scripts/lint-compiled-artifact-sync.cjs --check && node scripts/gen-context-index.cjs --check && node scripts/gen-section-manifest.cjs --check && node scripts/gen-health-docs.cjs --check && node scripts/gen-state-md-docs.cjs --check && node scripts/gen-scripts-cli-exit.cjs --check && node scripts/gen-hooks-cli-exit.cjs --check && node scripts/gen-exit-code-registry.cjs --check && node scripts/gen-exit-code-docs.cjs --check",
+    "lint:generated-sync": "node scripts/gen-capability-registry.cjs --check && node scripts/gen-loop-host-contract.cjs --check && node scripts/gen-capability-matrix.cjs --check && node scripts/sync-manifest-versions.cjs --check && node scripts/gen-inventory-manifest.cjs --check && node scripts/generate-package-identity.cjs --check && node scripts/gen-plugin-skills.cjs --check && node scripts/gen-registry.cjs --check && node scripts/gen-adr-index.cjs --check && node scripts/gen-features.cjs --check && node scripts/check-glossary-refs.cjs --check && node scripts/lint-compiled-artifact-sync.cjs --check && node scripts/gen-context-index.cjs --check && node scripts/gen-section-manifest.cjs --check && node scripts/gen-health-docs.cjs --check && node scripts/gen-state-md-docs.cjs --check && node scripts/gen-scripts-cli-exit.cjs --check && node scripts/gen-hooks-cli-exit.cjs --check && node scripts/gen-exit-code-registry.cjs --check && node scripts/gen-exit-code-docs.cjs --check && node scripts/gen-platform-conformance-tier.cjs --check",
     "lint:docs": "node scripts/lint-docs-required.cjs",
     "lint:qa-smells": "node scripts/qa-smell-ratchet.cjs",
     "lint:legacy-name": "node scripts/lint-legacy-dir-name.cjs",
diff --git a/scripts/ci-timeout-report.cjs b/scripts/ci-timeout-report.cjs
index d75289cff..ab0ed21ef 100644
--- a/scripts/ci-timeout-report.cjs
+++ b/scripts/ci-timeout-report.cjs
@@ -35,6 +35,7 @@ const JOB_RULES = [
   { workflowFile: 'test.yml', jobKey: 'test', test: (name) => name.startsWith('test (') && name !== 'test (inert CI)' },
   { workflowFile: 'test.yml', jobKey: 'test-full', test: (name) => name.startsWith('full test (') },
   { workflowFile: 'test.yml', jobKey: 'coverage-gate', test: (name) => name === 'Coverage gate (merged shards)' },
+  { workflowFile: 'test.yml', jobKey: 'test-conformance', test: (name) => name.startsWith('conformance test (') },
   { workflowFile: 'install-smoke.yml', jobKey: 'smoke', test: (name) => name.startsWith('smoke (') },
 ];
 
diff --git a/scripts/gen-platform-conformance-tier.cjs b/scripts/gen-platform-conformance-tier.cjs
new file mode 100644
index 000000000..8691c90fa
--- /dev/null
+++ b/scripts/gen-platform-conformance-tier.cjs
@@ -0,0 +1,336 @@
+#!/usr/bin/env node
+'use strict';
+
+/**
+ * Generates scripts/lib/platform-conformance-tier.generated.cjs — the list of
+ * test files under tests/**\/*.test.cjs whose CONTENT signals they exercise
+ * platform-specific behavior (Windows/macOS path quirks, raw child_process
+ * usage, chmod mode bits, etc.) and therefore need REAL-OS coverage rather
+ * than a Linux-only conformance lane (#4591).
+ *
+ * Only `unit`-suite test files (no suite suffix, per `scripts/run-tests.cjs`'s
+ * `suiteOf()`) are considered for the conformance tier. `install`-,
+ * `security`-, `slow`-, `integration`-, and `qa`-suffixed files are excluded
+ * entirely — never merely deprioritized — for three independent reasons: (1)
+ * `install`/`slow` are explicitly PR-excluded suites
+ * (`scripts/affected-tests-lib.cjs`'s `PR_EXCLUDED_SUITES`; "PRs must never
+ * select or run these"), and this generator's output feeds a `pull_request`-
+ * triggered job; (2) `integration`/`security` already run via their own
+ * separate, dedicated, unsharded, shard-1-only steps in the `test`/
+ * `test-full` jobs (.github/workflows/test.yml) — folding any of them into
+ * this job's generic `--files-from` + `--shard` invocation is unproven and,
+ * per the incident below, unsafe. (3) `qa` (loop-walk-suite files) already
+ * runs via its own separate, dedicated `qa-loop-walk` job
+ * (.github/workflows/test.yml) — the same rationale as (2): a purpose-built
+ * home already exists, so folding it into this job's generic invocation
+ * duplicates coverage without benefit. Real incident that surfaced this: a live
+ * CI run's `conformance test (windows-latest, shard 2/3)` job was killed with
+ * 11 tests in flight — including `tests/release-tarball-smoke.install.test.cjs`
+ * — because this generator had (wrongly) placed an `install`-suite file into
+ * the Linux-conformance candidate pool with no suite filtering at all.
+ *
+ * `classifyContent(content)` is the pure, exported classifier: it favors
+ * simple, auditable substring/regex matching over AST parsing, mirroring
+ * eslint-rules/lib/portability-vocab.cjs's own design stance (over-inclusion
+ * is the safe direction — a false positive costs one extra test running on a
+ * real OS; a false negative silently drops real-OS coverage).
+ *
+ * KNOWN LIMIT, disclosed deliberately: this is a STATIC content classifier,
+ * not a real per-file, per-OS behavioral diff. Epic #4589's issue #4591 asked
+ * for the cutover to be validated by "running the existing full matrix one
+ * more time as a parity baseline, diffing pass/fail per file between the
+ * real-OS runs and the Linux run" before moving any file into the Linux-only
+ * bulk. That literal per-file diff was NOT performed — no historical
+ * per-file, per-OS pass/fail dataset exists to diff against (GitHub Actions
+ * publishes coverage/QA artifacts from CI runs, not per-file JUnit results).
+ * What stands in for it: (1) the most recent push-triggered run on `next`
+ * (unconditionally full-matrix) is green on every OS for every file in this
+ * classification, confirmed before this classifier was built; (2) the
+ * existing `test-full` job keeps running the WHOLE suite on real Windows/
+ * macOS as a non-gating safety net for one release cycle (.github/workflows/
+ * test.yml) — a classifier miss surfaces as a visible warning there, not a
+ * silent gap, before the safety net is retired. This is the same
+ * static-analysis-substitutes-for-real-OS-execution stance ADR-1703's whole
+ * rule catalog already takes; it is a real, disclosed limit, not a silent
+ * substitution.
+ *
+ * Usage:
+ *   node scripts/gen-platform-conformance-tier.cjs                      # print summary to stdout
+ *   node scripts/gen-platform-conformance-tier.cjs --write              # write the generated file
+ *   node scripts/gen-platform-conformance-tier.cjs --check              # exit 1 if the committed file is stale
+ *   node scripts/gen-platform-conformance-tier.cjs --tests-dir <path>   # override the tests/ root (tests only)
+ *   node scripts/gen-platform-conformance-tier.cjs --out <path>         # override the generated-file path (tests only)
+ *
+ * `--tests-dir`/`--out` (or the TESTS_DIR/OUT_PATH env vars, flag takes
+ * precedence) exist solely so tests/platform-conformance-tier.test.cjs can
+ * point the CLI at a small temp fixture tree instead of this repo's real,
+ * 900+-file tests/ tree. Production usage (package.json's lint:generated-sync
+ * / regen:derived chains) passes no flags and gets the real repo paths.
+ */
+
+const fs = require('node:fs');
+const path = require('node:path');
+
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+const { suiteOf } = require('./lib/suite-detection.cjs');
+
+const ROOT = path.resolve(__dirname, '..');
+const DEFAULT_TESTS_DIR = path.join(ROOT, 'tests');
+const DEFAULT_OUT_PATH = path.join(ROOT, 'scripts', 'lib', 'platform-conformance-tier.generated.cjs');
+
+const GENERATED_HEADER =
+  '// GENERATED FILE — do not hand-edit. Run `node scripts/gen-platform-conformance-tier.cjs --write` to regenerate.\n';
+
+/**
+ * Detection categories (#4591 design doc). Each entry's `test` receives the
+ * raw file content string and returns true when that category's signal is
+ * present. Order matches the design doc's enumeration; `signals` in
+ * `classifyContent`'s return value preserves this order.
+ */
+const CATEGORIES = [
+  {
+    name: 'process-platform',
+    test: (content) => /process\.platform/.test(content),
+  },
+  {
+    name: 'os-platform',
+    test: (content) => /os\.platform\(\)/.test(content),
+  },
+  {
+    name: 'win32-darwin-literal',
+    test: (content) => /\bwin32\b/.test(content) || /\bdarwin\b/.test(content),
+  },
+  {
+    name: 'chmod-mode-bit',
+    test: (content) => /chmodSync|chmod\(/.test(content) || /0o[0-7]{3,4}\b/.test(content),
+  },
+  {
+    name: 'windows-shell-token',
+    test: (content) => /cmd\.exe|powershell|pwsh|ComSpec/i.test(content),
+  },
+  {
+    name: 'windows-env-var',
+    test: (content) => /\bPATHEXT\b|\bUSERPROFILE\b|\bHOMEDRIVE\b|\bHOMEPATH\b/.test(content),
+  },
+  {
+    name: 'process-seam-subprocess',
+    test: (content) => /\brunNode\(|\brunGit\(|\brunHook\(|\brunGsdTools\(|\bgitOrThrow\(/.test(content),
+  },
+  {
+    name: 'raw-child-process',
+    // Requires BOTH the child_process import/reference token AND one of the
+    // three call names in the same file — this is what keeps a same-named
+    // local identifier (e.g. a variable called `spawnResult`) from false-
+    // positiving: `spawnResult` never forms the substring `spawnSync(`.
+    test: (content) => {
+      if (/require\((['"])(?:node:)?child_process\1\)/.test(content)) return true;
+      if (!content.includes('child_process')) return false;
+      return /\bspawnSync\(|\bexecSync\(|\bexecFileSync\(/.test(content);
+    },
+  },
+  {
+    name: 'symlink-keyword',
+    // A leading `\b` with no trailing one, case-insensitive: this is
+    // deliberately NOT `/\bsymlink\b|\bSymlink\b/` (that literal pair would
+    // never match the dominant real-world call shape `symlinkSync(` /
+    // `readlinkSync(` — no word boundary exists between "symlink" and the
+    // immediately-following "Sync", both \w characters). The leading `\b`
+    // alone still excludes a mid-word embedding like "presymlink".
+    test: (content) => /\bsymlink/i.test(content),
+  },
+  {
+    name: 'hardcoded-path-vs-path-call',
+    // Intentionally coarse (design doc: over-inclusion is the safe
+    // direction): a path.* call ANYWHERE in the file plus a quoted
+    // forward-slash-leading string literal ANYWHERE in the file, with no
+    // attempt at proximity/scoping.
+    test: (content) =>
+      /path\.(join|resolve|dirname|basename|normalize|relative)\(/.test(content) &&
+      /['"`]\/[\w.\-/]*['"`]/.test(content),
+  },
+];
+
+/**
+ * Pure classifier: given a test file's raw string content, returns which
+ * platform-conformance categories matched and whether the file needs real-OS
+ * coverage (true iff at least one category matched).
+ *
+ * @param {string} content
+ * @returns {{needsRealOs: boolean, signals: string[]}}
+ */
+function classifyContent(content) {
+  const text = typeof content === 'string' ? content : '';
+  const signals = [];
+  for (const category of CATEGORIES) {
+    if (category.test(text)) signals.push(category.name);
+  }
+  return { needsRealOs: signals.length > 0, signals };
+}
+
+/**
+ * Recursively collect every `*.test.cjs` file beneath `dir`.
+ * @param {string} dir
+ * @returns {string[]} absolute paths
+ */
+function walkTestFiles(dir) {
+  const out = [];
+  let entries;
+  try {
+    entries = fs.readdirSync(dir, { withFileTypes: true });
+  } catch {
+    return out;
+  }
+  for (const entry of entries) {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      out.push(...walkTestFiles(full));
+    } else if (entry.isFile() && entry.name.endsWith('.test.cjs')) {
+      out.push(full);
+    }
+  }
+  return out;
+}
+
+/**
+ * Classify every test file under `testsDir`, returning `{ total, files }`
+ * where `files` is the SORTED array of `tests/<...>.test.cjs`-relative paths
+ * (POSIX-normalized, per RULESET.CONTENT-PATH-NORMALIZATION) whose content
+ * needs real-OS coverage.
+ *
+ * @param {string} testsDir
+ * @returns {{ total: number, files: string[] }}
+ */
+function classifyTree(testsDir) {
+  const absoluteFiles = walkTestFiles(testsDir);
+  // Only unit-suite files (no suite suffix) are eligible for the conformance
+  // tier — see the header doc-comment for why suite-tagged files are excluded
+  // entirely rather than merely deprioritized.
+  const unitFiles = absoluteFiles.filter((absPath) => suiteOf(absPath) === null);
+  const flagged = [];
+  for (const absPath of unitFiles) {
+    const content = fs.readFileSync(absPath, 'utf8');
+    const { needsRealOs } = classifyContent(content);
+    if (needsRealOs) {
+      const rel = path.relative(testsDir, absPath).replace(/\\/g, '/');
+      flagged.push('tests/' + rel);
+    }
+  }
+  flagged.sort();
+  return { total: absoluteFiles.length, files: flagged };
+}
+
+/**
+ * Render the generated `.cjs` module body — one array entry per line for a
+ * readable diff, matching scripts/lib/portability-vocab.cjs's array-literal
+ * style.
+ *
+ * @param {string[]} files - already sorted.
+ * @returns {string}
+ */
+function renderGeneratedFile(files) {
+  const lines = files.map((f) => `  ${JSON.stringify(f)},`).join('\n');
+  return (
+    GENERATED_HEADER +
+    "'use strict';\n\n" +
+    'module.exports = {\n' +
+    '  CONFORMANCE_TIER_FILES: [\n' +
+    (lines.length > 0 ? lines + '\n' : '') +
+    '  ],\n' +
+    '};\n'
+  );
+}
+
+/** Resolve the effective tests-dir / out-path from argv/env, flag beats env. */
+function resolveOverrides(argv) {
+  let testsDir = process.env.TESTS_DIR || DEFAULT_TESTS_DIR;
+  let outPath = process.env.OUT_PATH || DEFAULT_OUT_PATH;
+
+  for (let i = 0; i < argv.length; i++) {
+    if (argv[i] === '--tests-dir') {
+      const value = argv[i + 1];
+      if (!value || value.startsWith('--')) {
+        throw new ExitError(1, 'gen-platform-conformance-tier: --tests-dir requires a path value');
+      }
+      testsDir = path.resolve(value);
+      i++;
+    } else if (argv[i] === '--out') {
+      const value = argv[i + 1];
+      if (!value || value.startsWith('--')) {
+        throw new ExitError(1, 'gen-platform-conformance-tier: --out requires a path value');
+      }
+      outPath = path.resolve(value);
+      i++;
+    }
+  }
+
+  return { testsDir: path.resolve(testsDir), outPath: path.resolve(outPath) };
+}
+
+function main() {
+  const argv = process.argv.slice(2);
+  const { testsDir, outPath } = resolveOverrides(argv);
+  const mode = argv.includes('--check') ? 'check' : argv.includes('--write') ? 'write' : 'print';
+
+  const { total, files } = classifyTree(testsDir);
+
+  if (mode === 'write') {
+    fs.mkdirSync(path.dirname(outPath), { recursive: true });
+    fs.writeFileSync(outPath, renderGeneratedFile(files));
+    process.stdout.write(`Wrote ${outPath} (${files.length} conformance-tier file(s))\n`);
+    return;
+  }
+
+  if (mode === 'check') {
+    // Never trust a stale require cache — the committed file may have been
+    // rewritten (by --write, or by hand) since this process started.
+    let committed;
+    try {
+      const resolved = require.resolve(outPath);
+      delete require.cache[resolved];
+      committed = require(resolved);
+    } catch (err) {
+      throw new ExitError(
+        1,
+        `gen-platform-conformance-tier: could not load ${outPath} — run ` +
+          '`node scripts/gen-platform-conformance-tier.cjs --write` first ' +
+          `(${err && err.message ? err.message : err})`,
+      );
+    }
+    const committedFiles = Array.isArray(committed.CONFORMANCE_TIER_FILES)
+      ? committed.CONFORMANCE_TIER_FILES
+      : [];
+    const committedSet = new Set(committedFiles);
+    const liveSet = new Set(files);
+
+    const added = files.filter((f) => !committedSet.has(f));
+    const removed = committedFiles.filter((f) => !liveSet.has(f));
+
+    if (added.length > 0 || removed.length > 0) {
+      process.stderr.write(
+        `${path.relative(ROOT, outPath).replace(/\\/g, '/')} is stale. Run:\n` +
+          '  node scripts/gen-platform-conformance-tier.cjs --write\n\n',
+      );
+      for (const f of added) process.stderr.write('  + ' + f + '\n');
+      for (const f of removed) process.stderr.write('  - ' + f + '\n');
+      throw new ExitError(1);
+    }
+
+    process.stdout.write(
+      `ok gen-platform-conformance-tier: ${files.length} conformance-tier files, list matches\n`,
+    );
+    return;
+  }
+
+  // No flag: print a classification summary, write nothing.
+  process.stdout.write(
+    `gen-platform-conformance-tier: ${total} file(s) scanned, ` +
+      `${files.length} need real OS, ${total - files.length} excluded (Linux-only conformance tier eligible)\n`,
+  );
+}
+
+/* c8 ignore next 3 -- CLI entry guard; this repo measures coverage with c8, which does not honor istanbul pragmas */
+if (require.main === module) {
+  runMain(main);
+}
+
+module.exports = { classifyContent, CATEGORIES, walkTestFiles, classifyTree, renderGeneratedFile };
diff --git a/scripts/lib/platform-conformance-tier.generated.cjs b/scripts/lib/platform-conformance-tier.generated.cjs
new file mode 100644
index 000000000..29c23413a
--- /dev/null
+++ b/scripts/lib/platform-conformance-tier.generated.cjs
@@ -0,0 +1,553 @@
+// GENERATED FILE — do not hand-edit. Run `node scripts/gen-platform-conformance-tier.cjs --write` to regenerate.
+'use strict';
+
+module.exports = {
+  CONFORMANCE_TIER_FILES: [
+  "tests/active-workstream-store.test.cjs",
+  "tests/active-workstream-store.unit.test.cjs",
+  "tests/adr-15-progress-converge.test.cjs",
+  "tests/adr-22-plan-drift-guard.test.cjs",
+  "tests/adr-612-bracket-coherence.test.cjs",
+  "tests/adr-612-bracket-phase-counting.test.cjs",
+  "tests/adr-612-bracket-read-tolerance.test.cjs",
+  "tests/adr-index-gate.test.cjs",
+  "tests/adr857-core-without-capabilities.test.cjs",
+  "tests/advance-plan-ambiguous-phase.test.cjs",
+  "tests/agent-frontmatter.test.cjs",
+  "tests/agent-hint-routing-1689.test.cjs",
+  "tests/agent-install-check.test.cjs",
+  "tests/agent-install-validation.test.cjs",
+  "tests/agent-skills.test.cjs",
+  "tests/ai-evals.test.cjs",
+  "tests/antigravity-upgrades.test.cjs",
+  "tests/api-coverage-gate-e2e.test.cjs",
+  "tests/api-coverage.test.cjs",
+  "tests/assumption-delta-checkpoint-e2e.test.cjs",
+  "tests/assumption-delta.test.cjs",
+  "tests/audit-command-cutover.test.cjs",
+  "tests/audit-open-remainder-count.test.cjs",
+  "tests/audit-uat-acknowledged.test.cjs",
+  "tests/audit-uat-summary-segmentation.test.cjs",
+  "tests/audit-workstream-layouts.test.cjs",
+  "tests/augment-upgrades.test.cjs",
+  "tests/autonomous-converge.test.cjs",
+  "tests/autonomous-interactive.test.cjs",
+  "tests/benchmark-compact-content.test.cjs",
+  "tests/branch-no-track-guard.test.cjs",
+  "tests/broken-windows-description.test.cjs",
+  "tests/broken-windows.test.cjs",
+  "tests/bugs-1656-1657.test.cjs",
+  "tests/canary-version-leak-lint.test.cjs",
+  "tests/capability-cli.test.cjs",
+  "tests/capability-command-dispatch.test.cjs",
+  "tests/capability-consent.test.cjs",
+  "tests/capability-ledger.test.cjs",
+  "tests/capability-lifecycle.test.cjs",
+  "tests/capability-loader.test.cjs",
+  "tests/capability-lock-mkdir-failure-3987.test.cjs",
+  "tests/capability-precedence-parity.test.cjs",
+  "tests/capability-probe-fallback.test.cjs",
+  "tests/capability-registry.test.cjs",
+  "tests/capability-source.test.cjs",
+  "tests/capability-state.test.cjs",
+  "tests/capability-trust.test.cjs",
+  "tests/capability-writer.test.cjs",
+  "tests/changeset-cli.test.cjs",
+  "tests/changeset-github-release-notes.test.cjs",
+  "tests/changeset-lint.test.cjs",
+  "tests/changeset-new.test.cjs",
+  "tests/check-contract-drift.test.cjs",
+  "tests/check-env.test.cjs",
+  "tests/check-gap-analysis-plan-post-e2e.test.cjs",
+  "tests/check-glossary-refs.test.cjs",
+  "tests/check-tdd-review-checkpoint-e2e.test.cjs",
+  "tests/check-ui-plan-gate.test.cjs",
+  "tests/check-ui-safety-gate.test.cjs",
+  "tests/check-update-config-dir.test.cjs",
+  "tests/chunked-planning-parallel.test.cjs",
+  "tests/ci-docs-guard-registry.test.cjs",
+  "tests/ci-next-health.test.cjs",
+  "tests/ci-pr-mergeability.test.cjs",
+  "tests/ci-rebase-check.test.cjs",
+  "tests/ci-test-scope.test.cjs",
+  "tests/cjs-command-router-adapter.test.cjs",
+  "tests/claude-md-path.test.cjs",
+  "tests/claude-md.test.cjs",
+  "tests/claude-orchestration-command-router.test.cjs",
+  "tests/claude-orchestration.test.cjs",
+  "tests/claude-skills-migration.test.cjs",
+  "tests/cli-exit.test.cjs",
+  "tests/cline-install.test.cjs",
+  "tests/clock-seam.test.cjs",
+  "tests/clock.test.cjs",
+  "tests/close-phase-todos-padded-resolves.test.cjs",
+  "tests/code-review-command.test.cjs",
+  "tests/code-review-depth.test.cjs",
+  "tests/code-review-fix-pipeline-regression.test.cjs",
+  "tests/code-review-pipeline-regression.test.cjs",
+  "tests/code-review-tier3-files-override-scoping.test.cjs",
+  "tests/code-review.test.cjs",
+  "tests/codebuddy-install.test.cjs",
+  "tests/codebuddy-upgrades.test.cjs",
+  "tests/codex-config-agents.test.cjs",
+  "tests/codex-config-hooks.test.cjs",
+  "tests/codex-config-install.test.cjs",
+  "tests/codex-config.test.cjs",
+  "tests/codex-declarative-reference.test.cjs",
+  "tests/codex-inherit-smoke.test.cjs",
+  "tests/command-contract.test.cjs",
+  "tests/command-routing-hub.test.cjs",
+  "tests/commands.test.cjs",
+  "tests/commit-docs-bypass.test.cjs",
+  "tests/commit-files-deletion.test.cjs",
+  "tests/commit-files-pathspec.test.cjs",
+  "tests/commonjs-marker.test.cjs",
+  "tests/compact-content-4139.test.cjs",
+  "tests/compact-content-partition-guard.test.cjs",
+  "tests/completion-predicate-drift-guard.test.cjs",
+  "tests/completion-ratio-scope-withholding.test.cjs",
+  "tests/completion-ratio-single-owner.test.cjs",
+  "tests/concurrency-safety.test.cjs",
+  "tests/config-defaults-runtime-exclusion.test.cjs",
+  "tests/config-field-docs.test.cjs",
+  "tests/config-get-default.test.cjs",
+  "tests/config-loader.test.cjs",
+  "tests/config-schema.property.test.cjs",
+  "tests/config.test.cjs",
+  "tests/configuration-migrate-config.test.cjs",
+  "tests/context-drift.test.cjs",
+  "tests/context-predicates-query.test.cjs",
+  "tests/copilot-install.test.cjs",
+  "tests/copilot-upgrades.test.cjs",
+  "tests/core-utils.test.cjs",
+  "tests/coverage-metadata-parser.test.cjs",
+  "tests/coverage-uat-routing.test.cjs",
+  "tests/cursor-hook-workspace-roots.test.cjs",
+  "tests/cursor-hooks.test.cjs",
+  "tests/cursor-reviewer.test.cjs",
+  "tests/cursor-subagent-isolation.test.cjs",
+  "tests/debug-session-manager-commit.test.cjs",
+  "tests/decisions.test.cjs",
+  "tests/declarative-reference-antigravity.test.cjs",
+  "tests/declarative-reference-augment.test.cjs",
+  "tests/declarative-reference-codebuddy.test.cjs",
+  "tests/declarative-reference-copilot.test.cjs",
+  "tests/declarative-reference-windsurf.test.cjs",
+  "tests/declarative-reference-zcode.test.cjs",
+  "tests/default-flip-documentation-lint.test.cjs",
+  "tests/dispatcher.test.cjs",
+  "tests/docs-hooks-table-parity.test.cjs",
+  "tests/docs-parity-live-registry.test.cjs",
+  "tests/docs-update.test.cjs",
+  "tests/drift-detection.test.cjs",
+  "tests/edge-probe-spec-phase-contract.test.cjs",
+  "tests/edge-probe.test.cjs",
+  "tests/edit-phase-milestone-scope-guard.test.cjs",
+  "tests/edit-phase.test.cjs",
+  "tests/effort-surface-axis.test.cjs",
+  "tests/effort-sync-installed-runtime.test.cjs",
+  "tests/emitted-ack-trailer.test.cjs",
+  "tests/emitted-attribution.test.cjs",
+  "tests/emitted-caps-gate.test.cjs",
+  "tests/emitted-provenance.test.cjs",
+  "tests/emitted-sizes.test.cjs",
+  "tests/ensure-runtime-build.test.cjs",
+  "tests/enumeration-single-owner.test.cjs",
+  "tests/eslint-glob-coverage.test.cjs",
+  "tests/eslint-rules.test.cjs",
+  "tests/estimate-calibrate.test.cjs",
+  "tests/estimate-loop-convergence.test.cjs",
+  "tests/execute-mvp-tdd-gate.test.cjs",
+  "tests/execute-phase-wave.test.cjs",
+  "tests/execute-phase-worktree-guard.test.cjs",
+  "tests/execute-plan-update-codebase-map-diff-base.test.cjs",
+  "tests/execute-wave-post-gate-pipeline-e2e.test.cjs",
+  "tests/executed-plan.test.cjs",
+  "tests/executor-mvp-tdd-section.test.cjs",
+  "tests/exit-code-registry.test.cjs",
+  "tests/explore-command.test.cjs",
+  "tests/external-descriptor-confinement.test.cjs",
+  "tests/failing-direction.test.cjs",
+  "tests/fallow-runner.test.cjs",
+  "tests/faulty-deps.test.cjs",
+  "tests/feat-2296-provider-escalation.test.cjs",
+  "tests/feat-2483-review-claude-mds-guard.test.cjs",
+  "tests/feat-2646-deferred-items-audit-scanner.test.cjs",
+  "tests/feat-3881-yaml-parser-consequences.test.cjs",
+  "tests/features-index-gate.test.cjs",
+  "tests/fixture-builder.test.cjs",
+  "tests/frontmatter-cli.test.cjs",
+  "tests/frontmatter.test.cjs",
+  "tests/gap-checker.property.test.cjs",
+  "tests/gate-predicate-evaluator-missing.test.cjs",
+  "tests/gemini-runtime-removed.test.cjs",
+  "tests/gen-context-index.test.cjs",
+  "tests/gen-health-docs.test.cjs",
+  "tests/gen-registry.test.cjs",
+  "tests/gen-section-manifest.test.cjs",
+  "tests/gen-state-md-docs.test.cjs",
+  "tests/git-base-branch.test.cjs",
+  "tests/git-fixture.test.cjs",
+  "tests/golden-install-tree.test.cjs",
+  "tests/graphify-command-cutover.test.cjs",
+  "tests/graphify-graph-path.test.cjs",
+  "tests/graphify-visualization.test.cjs",
+  "tests/graphify.test.cjs",
+  "tests/gsd-agent-isolation-guard.test.cjs",
+  "tests/gsd-check-update-worker-atomic-cache.test.cjs",
+  "tests/gsd-check-update-worker-platform-gate.test.cjs",
+  "tests/gsd-mcp-server-bin.test.cjs",
+  "tests/gsd-quick-batch-merge-integration.test.cjs",
+  "tests/gsd-quick-batch-workflow.test.cjs",
+  "tests/gsd-secret-read-guard.test.cjs",
+  "tests/gsd-settings-advanced.test.cjs",
+  "tests/gsd-statusline.test.cjs",
+  "tests/gsd-tools-path-refs.test.cjs",
+  "tests/gsd-validate-commit-crash-policy.test.cjs",
+  "tests/gsd-write-guard.property.test.cjs",
+  "tests/gsd-write-guard.test.cjs",
+  "tests/gsd2-import.test.cjs",
+  "tests/hardcoded-paths.test.cjs",
+  "tests/health-diagnostic-rules/agent-install.test.cjs",
+  "tests/health-diagnostic-rules/config-validation.test.cjs",
+  "tests/health-diagnostic-rules/consistency.test.cjs",
+  "tests/health-diagnostic-rules/phase-structure.test.cjs",
+  "tests/health-diagnostic-rules/root-existence.test.cjs",
+  "tests/health-diagnostic-rules/state-consistency.test.cjs",
+  "tests/health-diagnostic-rules/worktree-health.test.cjs",
+  "tests/health-diagnostic.test.cjs",
+  "tests/health-validation.test.cjs",
+  "tests/helpers-cleanup.test.cjs",
+  "tests/helpers-process-isolation.test.cjs",
+  "tests/hermes-skills-migration.test.cjs",
+  "tests/hooks-commonjs-marker.test.cjs",
+  "tests/hooks-crash-policy.test.cjs",
+  "tests/hooks-opt-in.test.cjs",
+  "tests/host-integration.test.cjs",
+  "tests/host-runtime-detection.test.cjs",
+  "tests/ingest-docs.test.cjs",
+  "tests/init-debug.test.cjs",
+  "tests/init-manager.test.cjs",
+  "tests/init.test.cjs",
+  "tests/injection-blocking-config.test.cjs",
+  "tests/inline-plan-threshold.test.cjs",
+  "tests/install-fs-adapter-seam.test.cjs",
+  "tests/install-minimal-hooks.test.cjs",
+  "tests/install-nested-layout.test.cjs",
+  "tests/install-path-detection.test.cjs",
+  "tests/install-regressions.test.cjs",
+  "tests/install-runtime-artifacts.test.cjs",
+  "tests/install-scope.test.cjs",
+  "tests/install-write-confinement.test.cjs",
+  "tests/install.test.cjs",
+  "tests/installed-surface-resolver.test.cjs",
+  "tests/installer-migration-antigravity-retire-confighome-artifacts.test.cjs",
+  "tests/installer-migration-authoring.test.cjs",
+  "tests/installer-migration-config-root-marker.test.cjs",
+  "tests/installer-migration-pi-retire-hooks-dir.test.cjs",
+  "tests/installer-migration-prune-stale-pristine.test.cjs",
+  "tests/installer-migration-rename-gsd-core.test.cjs",
+  "tests/installer-migration-report.test.cjs",
+  "tests/installer-migrations-manifest-schema.test.cjs",
+  "tests/installer-migrations.test.cjs",
+  "tests/intel-command-cutover.test.cjs",
+  "tests/intel.test.cjs",
+  "tests/inventory-manifest-sync.test.cjs",
+  "tests/inventory-nested-families.test.cjs",
+  "tests/io.test.cjs",
+  "tests/isolation-sentinel.test.cjs",
+  "tests/kilo-upgrades.test.cjs",
+  "tests/kimi-agent-converter.test.cjs",
+  "tests/kimi-normalize-payload.property.test.cjs",
+  "tests/kimi-upgrades.test.cjs",
+  "tests/kimi-variant-disambiguation.test.cjs",
+  "tests/learnings.test.cjs",
+  "tests/legacy-cleanup-config-dir.test.cjs",
+  "tests/lint-allow-test-rule-refs.test.cjs",
+  "tests/lint-compiled-artifact-sync.test.cjs",
+  "tests/lint-docs-command-form.test.cjs",
+  "tests/lint-frontmatter-scalar-broad-grep.test.cjs",
+  "tests/lint-hooks-runtime-build-seam.test.cjs",
+  "tests/lint-legacy-dir-name.test.cjs",
+  "tests/lint-planning-artifact-writer-drift.test.cjs",
+  "tests/lint-pr-check-project-dir.test.cjs",
+  "tests/lint-regression-test-names.test.cjs",
+  "tests/lint-seam-enforcement.test.cjs",
+  "tests/lint-skill-deps.test.cjs",
+  "tests/lint-test-file-count.test.cjs",
+  "tests/lint-workflow-shellcheck-fetch.test.cjs",
+  "tests/list-seeds.test.cjs",
+  "tests/live-config-guard.test.cjs",
+  "tests/live-dom-uat.test.cjs",
+  "tests/lockfile-cve-audit.test.cjs",
+  "tests/locking-bugs-1909-1916-1925-1927.test.cjs",
+  "tests/loop-hooks-empty-points-e2e.test.cjs",
+  "tests/loop-hooks-ship-pre-e2e.test.cjs",
+  "tests/loop-hooks-verify-post-e2e.test.cjs",
+  "tests/loop-render-hooks.test.cjs",
+  "tests/managed-hooks.test.cjs",
+  "tests/manifest-version-sync.test.cjs",
+  "tests/markdown-table.test.cjs",
+  "tests/mcp-catalog.property.test.cjs",
+  "tests/mcp-catalog.test.cjs",
+  "tests/milestone-archive.test.cjs",
+  "tests/milestone-lock.test.cjs",
+  "tests/milestone-prefixed-convention.test.cjs",
+  "tests/milestone-window-drift-guard.test.cjs",
+  "tests/milestone-window-single-owner.test.cjs",
+  "tests/milestone.test.cjs",
+  "tests/model-omit-when-inherit-guard.test.cjs",
+  "tests/model-profiles.test.cjs",
+  "tests/model-resolver.test.cjs",
+  "tests/mutation-matrix-ratchet.test.cjs",
+  "tests/mutation-score-ratchet.test.cjs",
+  "tests/mutation-workflow-base-ref.test.cjs",
+  "tests/mvp-phase-integration.test.cjs",
+  "tests/new-milestone-clear-phases.test.cjs",
+  "tests/next-decimal-roadmap-scan.test.cjs",
+  "tests/next-up-clear-order.test.cjs",
+  "tests/no-bare-gsd-tools-command-position.test.cjs",
+  "tests/no-bare-npm-exec.rule.test.cjs",
+  "tests/no-dead-sdk-refs.test.cjs",
+  "tests/no-exact-case-env-access.rule.test.cjs",
+  "tests/no-hardcoded-home-gsd-tools.test.cjs",
+  "tests/no-hardcoded-tmp.rule.test.cjs",
+  "tests/no-path-literal-in-assert.rule.test.cjs",
+  "tests/no-pending-3212-markers.test.cjs",
+  "tests/no-phantom-issue-refs.test.cjs",
+  "tests/no-posix-mode-bit-assert.rule.test.cjs",
+  "tests/no-private-binary-resolution.rule.test.cjs",
+  "tests/no-rendered-text-length-assert.rule.test.cjs",
+  "tests/no-swallowed-precondition.rule.test.cjs",
+  "tests/no-unbounded-dirname-walk.rule.test.cjs",
+  "tests/no-unbounded-spawn-allowlist.test.cjs",
+  "tests/no-unbounded-spawn.test.cjs",
+  "tests/no-unguarded-nonportable-exec.rule.test.cjs",
+  "tests/normalize-path-in-content.rule.test.cjs",
+  "tests/normalize-test-command.test.cjs",
+  "tests/npm-audit-baseline.test.cjs",
+  "tests/npm-integrity-gate.test.cjs",
+  "tests/onboard-command.test.cjs",
+  "tests/opencode-command-dir-plural.test.cjs",
+  "tests/opencode-permissions.test.cjs",
+  "tests/opencode-plugin-adapter.test.cjs",
+  "tests/orphan-worktree-detection.test.cjs",
+  "tests/orphaned-hooks.test.cjs",
+  "tests/overlay-repo-helpers.test.cjs",
+  "tests/package-name-single-source.test.cjs",
+  "tests/packaging-shipped-scripts-require-only-shipped.test.cjs",
+  "tests/parallel-dependent-plans.test.cjs",
+  "tests/path-replacement.test.cjs",
+  "tests/pattern-mapper.test.cjs",
+  "tests/pattern.test.cjs",
+  "tests/pause-work-context-detection.test.cjs",
+  "tests/perf-317-context-monitor-fs.test.cjs",
+  "tests/phase-command-router.test.cjs",
+  "tests/phase-completion-single-owner.test.cjs",
+  "tests/phase-estimation.test.cjs",
+  "tests/phase-id-drift-guard.test.cjs",
+  "tests/phase-locator.test.cjs",
+  "tests/phase-resolution-parity.test.cjs",
+  "tests/phase-tdd-applicable.test.cjs",
+  "tests/phase.test.cjs",
+  "tests/phase6-capstone-conformance.test.cjs",
+  "tests/phases-command-router.test.cjs",
+  "tests/pi-config-dir-env-override.test.cjs",
+  "tests/pi-extension-reachability.test.cjs",
+  "tests/pick-flag.test.cjs",
+  "tests/plan-bounce.test.cjs",
+  "tests/plan-count-single-owner.test.cjs",
+  "tests/plan-phase-drift-guard.test.cjs",
+  "tests/plan-phase-mvp-flag.test.cjs",
+  "tests/plan-phase-stall-detection.test.cjs",
+  "tests/plan-pre-hook-e2e.test.cjs",
+  "tests/plan-review-convergence.test.cjs",
+  "tests/planning-inspect.test.cjs",
+  "tests/planning-inspect.unit.test.cjs",
+  "tests/planning-lock-mkdir-failure-1884.test.cjs",
+  "tests/planning-prompt-drift.test.cjs",
+  "tests/planning-snapshot-bypass-drift.test.cjs",
+  "tests/planning-snapshot.test.cjs",
+  "tests/planning-workspace.test.cjs",
+  "tests/platform-conformance-tier.test.cjs",
+  "tests/platform-guard.unit.test.cjs",
+  "tests/playwright-ui-verify.test.cjs",
+  "tests/plugin-manifest.test.cjs",
+  "tests/policy-160-route0-resume.test.cjs",
+  "tests/policy-shell-pinning.test.cjs",
+  "tests/portability-rule-disable-ban.test.cjs",
+  "tests/portability-vocab-drift.test.cjs",
+  "tests/post-planning-gaps-2493.test.cjs",
+  "tests/pr-branch-planning-filter.test.cjs",
+  "tests/precommit-alias-drift-hook.test.cjs",
+  "tests/precondition-element.test.cjs",
+  "tests/prepush-enterprise-email-hook.test.cjs",
+  "tests/probe-core.test.cjs",
+  "tests/process-seam.test.cjs",
+  "tests/profile-output.test.cjs",
+  "tests/profile-pipeline.test.cjs",
+  "tests/prohibition-enforcement.test.cjs",
+  "tests/prohibition-probe.verify-tier.test.cjs",
+  "tests/project-instruction-file-parity.test.cjs",
+  "tests/project-root.test.cjs",
+  "tests/prompt-budget-cli.test.cjs",
+  "tests/prune-orphaned-worktrees.test.cjs",
+  "tests/quick-batch-command-router.test.cjs",
+  "tests/quick-batch.test.cjs",
+  "tests/quick-branching.test.cjs",
+  "tests/quick-research.test.cjs",
+  "tests/quick-review-scope-tip-bound.test.cjs",
+  "tests/qwen-upgrades.test.cjs",
+  "tests/read-guard.test.cjs",
+  "tests/read-injection-scanner.property.test.cjs",
+  "tests/reapply-patches.test.cjs",
+  "tests/reapply-verify-hunks.test.cjs",
+  "tests/refactor-trigger-cli.test.cjs",
+  "tests/registry-reviewer-parity.test.cjs",
+  "tests/release-hotfix-empty-cherry-pick.test.cjs",
+  "tests/removed-but-needed-lint.test.cjs",
+  "tests/repo-invariants.test.cjs",
+  "tests/repo-layout.test.cjs",
+  "tests/representative-corpus.test.cjs",
+  "tests/require-fs-op-fallback.rule.test.cjs",
+  "tests/require-full-tmpdir-triad.rule.test.cjs",
+  "tests/require-issue-link-policy.test.cjs",
+  "tests/require-userprofile-with-home.rule.test.cjs",
+  "tests/research-cli.test.cjs",
+  "tests/research-store.test.cjs",
+  "tests/resolve-execution-dynamic-routing.test.cjs",
+  "tests/resolver-hoist-guard.test.cjs",
+  "tests/response-language-coverage.test.cjs",
+  "tests/retired-artifact-cleanup.test.cjs",
+  "tests/reversibility-tagging.test.cjs",
+  "tests/review-build-prompt-optional-sections.test.cjs",
+  "tests/review-default-reviewers-config.test.cjs",
+  "tests/review-default-reviewers-workflow.test.cjs",
+  "tests/review-lane-descriptor.test.cjs",
+  "tests/review-lane-invocation.test.cjs",
+  "tests/review-lane-runner.test.cjs",
+  "tests/review-lane-windows-spawn-resolution.test.cjs",
+  "tests/review-model-config.test.cjs",
+  "tests/review-parallel-lanes.test.cjs",
+  "tests/review-plan-coverage-manifest.test.cjs",
+  "tests/review-reviewer-instances-config.test.cjs",
+  "tests/reviewer-config-federation.test.cjs",
+  "tests/reviewer-docs-parity.test.cjs",
+  "tests/reviewer-lane-declarations.test.cjs",
+  "tests/reviewer-manifest-body.test.cjs",
+  "tests/reviewer-step-dispatch.test.cjs",
+  "tests/reviewer-trust-disclosure.test.cjs",
+  "tests/revision-remediation-binding.test.cjs",
+  "tests/roadmap-mode-field.test.cjs",
+  "tests/roadmap-parser.test.cjs",
+  "tests/roadmap-phase-fallback.test.cjs",
+  "tests/roadmap-upgrade.test.cjs",
+  "tests/roadmap.test.cjs",
+  "tests/roadmapper-granularity.test.cjs",
+  "tests/run-tests-harness.test.cjs",
+  "tests/run-tests-temp-root.test.cjs",
+  "tests/run-with-timeout.test.cjs",
+  "tests/runtime-artifact-install-plan.test.cjs",
+  "tests/runtime-artifact-layout-descriptor-drive.test.cjs",
+  "tests/runtime-artifact-layout-install-profiles.test.cjs",
+  "tests/runtime-artifact-layout-surface.test.cjs",
+  "tests/runtime-artifact-layout.test.cjs",
+  "tests/runtime-converters.test.cjs",
+  "tests/runtime-homes-descriptor-drive.test.cjs",
+  "tests/runtime-homes.property.test.cjs",
+  "tests/runtime-identity.test.cjs",
+  "tests/runtime-launcher-parity.test.cjs",
+  "tests/runtime-name-policy.test.cjs",
+  "tests/safe-resume-gate-anchoring.test.cjs",
+  "tests/schema-drift.test.cjs",
+  "tests/sdk-removal-query-family-dispatch.test.cjs",
+  "tests/section-manifest-init-facts.test.cjs",
+  "tests/section-manifest.test.cjs",
+  "tests/security.test.cjs",
+  "tests/settings-integrations.test.cjs",
+  "tests/settings-jsonc.test.cjs",
+  "tests/sh-hook-paths.test.cjs",
+  "tests/shadow-report.test.cjs",
+  "tests/shared-hooks-dir-resolution.test.cjs",
+  "tests/shell-command-projection-dispatch.test.cjs",
+  "tests/shell-command-projection-path-sep.test.cjs",
+  "tests/ship-notes-wedged-pr.test.cjs",
+  "tests/shipped-reference-cites.test.cjs",
+  "tests/skill-frontmatter-contract.test.cjs",
+  "tests/skill-manifest.test.cjs",
+  "tests/slash-command-namespace.test.cjs",
+  "tests/slug-derivation-drift-guard.test.cjs",
+  "tests/smart-entry.unit.test.cjs",
+  "tests/spec-section.test.cjs",
+  "tests/stale-bake-guard.test.cjs",
+  "tests/state-command-cutover.test.cjs",
+  "tests/state-contract.test.cjs",
+  "tests/state-document.test.cjs",
+  "tests/state-field-drift.test.cjs",
+  "tests/state-io.test.cjs",
+  "tests/state-prune.test.cjs",
+  "tests/state-rebuild-cli.test.cjs",
+  "tests/state-todos-render.test.cjs",
+  "tests/state-transition.test.cjs",
+  "tests/state-write-path-drift-guard.test.cjs",
+  "tests/state.test.cjs",
+  "tests/stats-mvp-display.test.cjs",
+  "tests/stats-phase-id-shape.test.cjs",
+  "tests/subagent-timeout.test.cjs",
+  "tests/summary-status-blocked-3345.test.cjs",
+  "tests/surface-md-paths.regression.test.cjs",
+  "tests/sync-skills-cross-runtime-refuse.test.cjs",
+  "tests/table-schema-drift-lint.test.cjs",
+  "tests/tdd-backend-wiring.test.cjs",
+  "tests/tdd-mode.test.cjs",
+  "tests/tdd-red-evidence.test.cjs",
+  "tests/tdd-single-statement.test.cjs",
+  "tests/teams-status.test.cjs",
+  "tests/template.test.cjs",
+  "tests/thinking-partner.test.cjs",
+  "tests/todos-done-rename-guard.test.cjs",
+  "tests/todos-workstream-scope.test.cjs",
+  "tests/tracer-bullet.test.cjs",
+  "tests/trae-imperative-reference.test.cjs",
+  "tests/tsconfig-noemit.test.cjs",
+  "tests/uat-predicate.test.cjs",
+  "tests/uat.test.cjs",
+  "tests/ui-spec-inventory-provenance.test.cjs",
+  "tests/ultraplan-phase.test.cjs",
+  "tests/unreachable-guard-drift.test.cjs",
+  "tests/unreachable-shell-guard.test.cjs",
+  "tests/unusable-input.test.cjs",
+  "tests/update-context.test.cjs",
+  "tests/update-custom-backup.test.cjs",
+  "tests/update-workflow.test.cjs",
+  "tests/user-artifact-staging.test.cjs",
+  "tests/validate-context.test.cjs",
+  "tests/validate-registry.test.cjs",
+  "tests/verification-status.test.cjs",
+  "tests/verify-archive-dirs-live-path.test.cjs",
+  "tests/verify-command-grounding.test.cjs",
+  "tests/verify-health.test.cjs",
+  "tests/verify.test.cjs",
+  "tests/vscode-browser-no-node-api.test.cjs",
+  "tests/vscode-extension-reachability.test.cjs",
+  "tests/windows-robustness.test.cjs",
+  "tests/windsurf-conversion.test.cjs",
+  "tests/windsurf-hooks-bridge.test.cjs",
+  "tests/windsurf-install.test.cjs",
+  "tests/workflow-compat.test.cjs",
+  "tests/workflow-fragments.test.cjs",
+  "tests/workflow-guard.test.cjs",
+  "tests/workflow-maintainer-skip.test.cjs",
+  "tests/workflow-shell-pinning.test.cjs",
+  "tests/workspace.test.cjs",
+  "tests/workstream-inventory.test.cjs",
+  "tests/workstream-scoped-paths.test.cjs",
+  "tests/workstream.test.cjs",
+  "tests/worktree-base-ref.test.cjs",
+  "tests/worktree-baseref-install.test.cjs",
+  "tests/worktree-cleanup.test.cjs",
+  "tests/worktree-safety-reap.test.cjs",
+  "tests/worktree-safety.test.cjs",
+  "tests/worktree.test.cjs",
+  ],
+};
diff --git a/scripts/lib/suite-detection.cjs b/scripts/lib/suite-detection.cjs
new file mode 100644
index 000000000..a2c342dd1
--- /dev/null
+++ b/scripts/lib/suite-detection.cjs
@@ -0,0 +1,32 @@
+'use strict';
+
+/**
+ * Suite classification by filename suffix (#4591 packaging fix — extracted
+ * from scripts/run-tests.cjs so a SHIPPED script, e.g.
+ * scripts/gen-platform-conformance-tier.cjs, can depend on it: run-tests.cjs
+ * itself does not ship (it is a repo-development-only tool), so a shipped
+ * file requiring it directly is MODULE_NOT_FOUND in a published install
+ * (tests/packaging-shipped-scripts-require-only-shipped.test.cjs, #2858).
+ * run-tests.cjs re-exports suiteOf from here unchanged for full backward
+ * compatibility — this is a pure relocation, not a behavior change.
+ *
+ * A file with no suite suffix (plain `foo.test.cjs`) belongs to the `unit`
+ * suite and returns `null` here; a suite-tagged file (`foo.install.test.cjs`)
+ * returns its marker string.
+ */
+
+const { basename } = require('node:path');
+
+const MARKED_SUITES = ['integration', 'install', 'security', 'slow', 'qa'];
+
+function suiteOf(filename) {
+  const name = basename(filename);
+  if (!name.endsWith('.test.cjs')) return null;
+  const base = name.slice(0, -'.test.cjs'.length);
+  const lastDot = base.lastIndexOf('.');
+  if (lastDot === -1) return null;
+  const marker = base.slice(lastDot + 1);
+  return MARKED_SUITES.includes(marker) ? marker : null;
+}
+
+module.exports = { MARKED_SUITES, suiteOf };
diff --git a/scripts/run-tests.cjs b/scripts/run-tests.cjs
index e39b32aa4..f5568c410 100644
--- a/scripts/run-tests.cjs
+++ b/scripts/run-tests.cjs
@@ -41,6 +41,7 @@ const { tmpdir } = require('os');
 const { pathToFileURL } = require('url');
 const { execFileSync } = require('child_process');
 const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+const { suiteOf } = require('./lib/suite-detection.cjs');
 const {
   resolveLiveConfigRoots,
   resolveExtraWatchTargets,
@@ -179,8 +180,6 @@ function ensureBuiltHooks(overrides = {}) {
     runBuild();
   }
 }
-const MARKED_SUITES = ['integration', 'install', 'security', 'slow', 'qa'];
-
 // Recursively collect *.test.cjs files under dir, returning paths relative to dir.
 // Skips node_modules to avoid accidentally picking up decoy files.
 function walkTestFiles(dir, relBase) {
@@ -794,20 +793,8 @@ function parseArgs(argv) {
   return { suite, files, filesFrom, shard };
 }
 
-// Return the marked suite name embedded in a filename, or null if it's unmarked.
-// foo.security.test.cjs -> "security"
-// foo.test.cjs          -> null (unit)
-// Accepts either a bare filename or a relative subdir path; classification is
-// based on the basename only so subdir paths classify identically to root files.
-function suiteOf(filename) {
-  const name = basename(filename);
-  if (!name.endsWith('.test.cjs')) return null;
-  const base = name.slice(0, -'.test.cjs'.length);
-  const lastDot = base.lastIndexOf('.');
-  if (lastDot === -1) return null;
-  const marker = base.slice(lastDot + 1);
-  return MARKED_SUITES.includes(marker) ? marker : null;
-}
+// suiteOf (and its backing MARKED_SUITES) is imported from
+// ./lib/suite-detection.cjs — see that module's header comment for why.
 
 function selectFiles(allFiles, suite) {
   if (suite === null || suite === 'all') {
diff --git a/tests/ci-test-job-timeout-budget.test.cjs b/tests/ci-test-job-timeout-budget.test.cjs
index 29e81d7b0..d09b200b2 100644
--- a/tests/ci-test-job-timeout-budget.test.cjs
+++ b/tests/ci-test-job-timeout-budget.test.cjs
@@ -253,6 +253,7 @@ test('near-cap check CI_JOB_TIMEOUT_MINUTES literals match each job\'s own timeo
   const staticLanes = [
     { workflowFile: 'test.yml', jobKey: 'test', envLiteral: '32' },
     { workflowFile: 'test.yml', jobKey: 'test-full', envLiteral: '45' },
+    { workflowFile: 'test.yml', jobKey: 'test-conformance', envLiteral: '45' },
     { workflowFile: 'install-smoke.yml', jobKey: 'smoke', envLiteral: '12' },
   ];
 
@@ -294,5 +295,10 @@ test('near-cap check CI_JOB_TIMEOUT_MINUTES literals match each job\'s own timeo
     assert.equal(testWorkflow.jobs['coverage-gate'].name, 'Coverage gate (merged shards)',
       'test.yml jobs.coverage-gate.name changed — update JOB_RULES to match');
     assert.equal(coverageGateRule.test('Coverage gate (merged shards)'), true);
+
+    const testConformanceRule = JOB_RULES.find((r) => r.workflowFile === 'test.yml' && r.jobKey === 'test-conformance');
+    assert.ok(testWorkflow.jobs['test-conformance'].name.startsWith('conformance test ('),
+      'test.yml jobs.test-conformance.name no longer starts with "conformance test (" — update JOB_RULES to match');
+    assert.equal(testConformanceRule.test('conformance test (windows-latest, 24, shard 1/3)'), true);
   });
 });
diff --git a/tests/fixtures/install-tree/antigravity.json b/tests/fixtures/install-tree/antigravity.json
index f747670c9..6394abe9c 100644
--- a/tests/fixtures/install-tree/antigravity.json
+++ b/tests/fixtures/install-tree/antigravity.json
@@ -624,7 +624,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/augment.json b/tests/fixtures/install-tree/augment.json
index 99c30cf15..0c93b86c4 100644
--- a/tests/fixtures/install-tree/augment.json
+++ b/tests/fixtures/install-tree/augment.json
@@ -695,7 +695,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/claude-local.json b/tests/fixtures/install-tree/claude-local.json
index d3b12fc4c..f013bdedd 100644
--- a/tests/fixtures/install-tree/claude-local.json
+++ b/tests/fixtures/install-tree/claude-local.json
@@ -559,5 +559,7 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
-  "scripts/lib/shellcheck-fetch.cjs"
+  "scripts/lib/platform-conformance-tier.generated.cjs",
+  "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs"
 ]
diff --git a/tests/fixtures/install-tree/claude.json b/tests/fixtures/install-tree/claude.json
index d46fcb517..611519571 100644
--- a/tests/fixtures/install-tree/claude.json
+++ b/tests/fixtures/install-tree/claude.json
@@ -623,7 +623,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/cline.json b/tests/fixtures/install-tree/cline.json
index ec6bcad8f..64d22d177 100644
--- a/tests/fixtures/install-tree/cline.json
+++ b/tests/fixtures/install-tree/cline.json
@@ -585,7 +585,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/codebuddy.json b/tests/fixtures/install-tree/codebuddy.json
index 926d035c4..7b035ffe7 100644
--- a/tests/fixtures/install-tree/codebuddy.json
+++ b/tests/fixtures/install-tree/codebuddy.json
@@ -695,7 +695,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/codex.json b/tests/fixtures/install-tree/codex.json
index 79fe0174a..a7bb472f8 100644
--- a/tests/fixtures/install-tree/codex.json
+++ b/tests/fixtures/install-tree/codex.json
@@ -623,5 +623,7 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
-  "scripts/lib/shellcheck-fetch.cjs"
+  "scripts/lib/platform-conformance-tier.generated.cjs",
+  "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs"
 ]
diff --git a/tests/fixtures/install-tree/copilot.json b/tests/fixtures/install-tree/copilot.json
index f28a7b9fe..4b60ade55 100644
--- a/tests/fixtures/install-tree/copilot.json
+++ b/tests/fixtures/install-tree/copilot.json
@@ -585,7 +585,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/cursor.json b/tests/fixtures/install-tree/cursor.json
index dd7521351..2d8c8c71f 100644
--- a/tests/fixtures/install-tree/cursor.json
+++ b/tests/fixtures/install-tree/cursor.json
@@ -596,7 +596,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/hermes.json b/tests/fixtures/install-tree/hermes.json
index a6053ff59..ed62ec76a 100644
--- a/tests/fixtures/install-tree/hermes.json
+++ b/tests/fixtures/install-tree/hermes.json
@@ -623,7 +623,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd/DESCRIPTION.md",
   "skills/gsd/gsd-ns-context/SKILL.md",
   "skills/gsd/gsd-ns-context/skills/docs-update/SKILL.md",
diff --git a/tests/fixtures/install-tree/kilo.json b/tests/fixtures/install-tree/kilo.json
index d19a48a7d..52a04da47 100644
--- a/tests/fixtures/install-tree/kilo.json
+++ b/tests/fixtures/install-tree/kilo.json
@@ -698,7 +698,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/kimi-code.json b/tests/fixtures/install-tree/kimi-code.json
index 49924e27d..cec17e031 100644
--- a/tests/fixtures/install-tree/kimi-code.json
+++ b/tests/fixtures/install-tree/kimi-code.json
@@ -624,7 +624,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/kimi.json b/tests/fixtures/install-tree/kimi.json
index 5c05af055..37683b3be 100644
--- a/tests/fixtures/install-tree/kimi.json
+++ b/tests/fixtures/install-tree/kimi.json
@@ -591,7 +591,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/opencode.json b/tests/fixtures/install-tree/opencode.json
index 95dbb2a13..0d0cb6b41 100644
--- a/tests/fixtures/install-tree/opencode.json
+++ b/tests/fixtures/install-tree/opencode.json
@@ -698,7 +698,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/pi.json b/tests/fixtures/install-tree/pi.json
index 65e02bf89..319b699ba 100644
--- a/tests/fixtures/install-tree/pi.json
+++ b/tests/fixtures/install-tree/pi.json
@@ -425,5 +425,7 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
-  "scripts/lib/shellcheck-fetch.cjs"
+  "scripts/lib/platform-conformance-tier.generated.cjs",
+  "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs"
 ]
diff --git a/tests/fixtures/install-tree/qwen.json b/tests/fixtures/install-tree/qwen.json
index 43733896a..906b2e318 100644
--- a/tests/fixtures/install-tree/qwen.json
+++ b/tests/fixtures/install-tree/qwen.json
@@ -623,7 +623,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/trae.json b/tests/fixtures/install-tree/trae.json
index d326780b2..57ca1a0b5 100644
--- a/tests/fixtures/install-tree/trae.json
+++ b/tests/fixtures/install-tree/trae.json
@@ -583,7 +583,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/windsurf.json b/tests/fixtures/install-tree/windsurf.json
index 37a238793..6baca149a 100644
--- a/tests/fixtures/install-tree/windsurf.json
+++ b/tests/fixtures/install-tree/windsurf.json
@@ -518,5 +518,7 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
-  "scripts/lib/shellcheck-fetch.cjs"
+  "scripts/lib/platform-conformance-tier.generated.cjs",
+  "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs"
 ]
diff --git a/tests/fixtures/install-tree/zcode.json b/tests/fixtures/install-tree/zcode.json
index 6d3a1380a..ee09be87a 100644
--- a/tests/fixtures/install-tree/zcode.json
+++ b/tests/fixtures/install-tree/zcode.json
@@ -655,7 +655,9 @@
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
   "scripts/lib/npm-version-check-diagnosis.cjs",
+  "scripts/lib/platform-conformance-tier.generated.cjs",
   "scripts/lib/shellcheck-fetch.cjs",
+  "scripts/lib/suite-detection.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/platform-conformance-tier.test.cjs b/tests/platform-conformance-tier.test.cjs
new file mode 100644
index 000000000..9864b6cc1
--- /dev/null
+++ b/tests/platform-conformance-tier.test.cjs
@@ -0,0 +1,307 @@
+'use strict';
+
+/**
+ * Test matrix: .gsd/phase/chore-4591-conformance-tier-linux-primary/50-test-matrix.md
+ *
+ * Rows 1-15 exercise the pure, exported `classifyContent(content)` classifier
+ * directly on short string fixtures. Rows 16-18 exercise the
+ * `--check`/`--write` CLI behavior against a small temp fixture tree, driven
+ * through the process seam (tests/helpers/process-seam.cjs's `runNode`) per
+ * CONTRIBUTING.md's "spawning a subprocess: use the process seam" rule. Row
+ * 19 is a real-tree regression proving the committed generated file matches a
+ * fresh sweep of this repo's actual tests/ tree. Rows 20-21 prove the
+ * suite-exclusion fix (#4591 CI incident): a suite-tagged file must never
+ * enter the conformance-tier pool even when its content would otherwise
+ * qualify, and the committed generated file must contain zero such files.
+ */
+
+const { describe, test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const { createTempDir, cleanup } = require('./helpers.cjs');
+const { runNode } = require('./helpers/process-seam.cjs');
+const { classifyContent, classifyTree } = require('../scripts/gen-platform-conformance-tier.cjs');
+
+const ROOT = path.resolve(__dirname, '..');
+const SCRIPT = path.join(ROOT, 'scripts', 'gen-platform-conformance-tier.cjs');
+const GENERATED_PATH = path.join(ROOT, 'scripts', 'lib', 'platform-conformance-tier.generated.cjs');
+
+// ─── Rows 1-15: classifyContent, pure fixtures ────────────────────────────────
+
+describe('classifyContent — happy-path signals', () => {
+  test('flags process.platform', () => {
+    const { needsRealOs, signals } = classifyContent("if (process.platform === 'win32') { doThing(); }");
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('process-platform'));
+  });
+
+  test('flags os.platform()', () => {
+    const { needsRealOs, signals } = classifyContent("const os = require('node:os');\nconst p = os.platform();");
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('os-platform'));
+  });
+
+  test('flags win32 literal', () => {
+    const { needsRealOs, signals } = classifyContent("const platforms = ['win32', 'linux'];");
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('win32-darwin-literal'));
+  });
+
+  test('flags darwin literal', () => {
+    const { needsRealOs, signals } = classifyContent("const platforms = ['darwin', 'linux'];");
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('win32-darwin-literal'));
+  });
+
+  test('flags chmod mode-bit octal', () => {
+    const { needsRealOs, signals } = classifyContent('fs.chmodSync(target, 0o755);');
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('chmod-mode-bit'));
+  });
+
+  test('flags Windows-shell tokens', () => {
+    for (const fixture of ["spawn('cmd.exe', args)", "spawn('powershell', args)", 'const e = process.env.ComSpec;']) {
+      const { needsRealOs, signals } = classifyContent(fixture);
+      assert.equal(needsRealOs, true, fixture);
+      assert.ok(signals.includes('windows-shell-token'), fixture);
+    }
+  });
+
+  test('flags Windows env-var names', () => {
+    for (const fixture of [
+      'const home = process.env.USERPROFILE;',
+      'const drive = process.env.HOMEDRIVE;',
+      'const p = process.env.HOMEPATH;',
+    ]) {
+      const { needsRealOs, signals } = classifyContent(fixture);
+      assert.equal(needsRealOs, true, fixture);
+      assert.ok(signals.includes('windows-env-var'), fixture);
+    }
+  });
+
+  test('flags process-seam subprocess helpers', () => {
+    for (const fixture of [
+      "runNode(['--check'])",
+      "runGit(['status'])",
+      "runHook(HOOK_PATH, [])",
+      "runGsdTools(['state', 'show'])",
+    ]) {
+      const { needsRealOs, signals } = classifyContent(fixture);
+      assert.equal(needsRealOs, true, fixture);
+      assert.ok(signals.includes('process-seam-subprocess'), fixture);
+    }
+  });
+
+  test('flags raw child_process usage', () => {
+    const fixture = "const { execFileSync } = require('child_process');\nexecFileSync('ls', []);";
+    const { needsRealOs, signals } = classifyContent(fixture);
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('raw-child-process'));
+  });
+
+  test('flags symlink keyword', () => {
+    const { needsRealOs, signals } = classifyContent('fs.symlinkSync(target, link);');
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('symlink-keyword'));
+  });
+
+  test('flags hardcoded path literal vs path.* call', () => {
+    const fixture = "const p = path.join(root, 'x');\nassert.equal(rendered, '/etc/passwd');";
+    const { needsRealOs, signals } = classifyContent(fixture);
+    assert.equal(needsRealOs, true);
+    assert.ok(signals.includes('hardcoded-path-vs-path-call'));
+  });
+});
+
+describe('classifyContent — negative / hostile inputs', () => {
+  test('does not flag a clean in-process unit test', () => {
+    const fixture = "const assert = require('node:assert/strict');\ntest('adds numbers', () => { assert.equal(1 + 1, 2); });";
+    const { needsRealOs, signals } = classifyContent(fixture);
+    assert.equal(needsRealOs, false);
+    assert.deepEqual(signals, []);
+  });
+
+  test('does not flag incidental use of the word "path"', () => {
+    const fixture = '// This test verifies the correct path through the state machine.\nassert.ok(true);';
+    const { needsRealOs } = classifyContent(fixture);
+    assert.equal(needsRealOs, false);
+  });
+
+  test('does not crash on empty content', () => {
+    assert.doesNotThrow(() => classifyContent(''));
+    const { needsRealOs, signals } = classifyContent('');
+    assert.equal(needsRealOs, false);
+    assert.deepEqual(signals, []);
+  });
+
+  test('does not flag an unrelated identifier containing "spawn"', () => {
+    const fixture = 'const spawnResult = computeSomething();\nassert.ok(spawnResult);';
+    const { needsRealOs, signals } = classifyContent(fixture);
+    assert.equal(needsRealOs, false);
+    assert.deepEqual(signals, []);
+  });
+});
+
+// ─── Rows 16-18: CLI --check/--write against a temp fixture tree ──────────────
+
+/** Spawn the real generator CLI via the process seam. */
+function runGen(args) {
+  return runNode([SCRIPT, ...args]);
+}
+
+describe('gen-platform-conformance-tier.cjs CLI (temp fixture tree)', () => {
+  test('--check passes when the generated file is fresh', () => {
+    const tmpDir = createTempDir('gen-platform-conformance-tier-');
+    try {
+      const testsDir = path.join(tmpDir, 'tests');
+      fs.mkdirSync(testsDir, { recursive: true });
+      fs.writeFileSync(path.join(testsDir, 'flagged.test.cjs'), "if (process.platform === 'win32') {}\n");
+      fs.writeFileSync(path.join(testsDir, 'clean.test.cjs'), "assert.equal(1 + 1, 2);\n");
+      const outPath = path.join(tmpDir, 'platform-conformance-tier.generated.cjs');
+
+      const write = runGen(['--write', '--tests-dir', testsDir, '--out', outPath]);
+      assert.equal(write.exitCode, 0, write.stderr);
+      assert.ok(fs.existsSync(outPath));
+
+      const check = runGen(['--check', '--tests-dir', testsDir, '--out', outPath]);
+      assert.equal(check.exitCode, 0, check.stderr);
+      assert.match(check.stdout, /ok gen-platform-conformance-tier/);
+    } finally {
+      cleanup(tmpDir);
+    }
+  });
+
+  test('--check fails and names the drift when the list is stale', () => {
+    const tmpDir = createTempDir('gen-platform-conformance-tier-');
+    try {
+      const testsDir = path.join(tmpDir, 'tests');
+      fs.mkdirSync(testsDir, { recursive: true });
+      fs.writeFileSync(path.join(testsDir, 'flagged.test.cjs'), "if (process.platform === 'win32') {}\n");
+      fs.writeFileSync(path.join(testsDir, 'clean.test.cjs'), "assert.equal(1 + 1, 2);\n");
+      const outPath = path.join(tmpDir, 'platform-conformance-tier.generated.cjs');
+
+      const write = runGen(['--write', '--tests-dir', testsDir, '--out', outPath]);
+      assert.equal(write.exitCode, 0, write.stderr);
+
+      // Introduce drift: a brand-new signal-bearing file the committed list
+      // has never seen.
+      fs.writeFileSync(path.join(testsDir, 'new-signal.test.cjs'), 'fs.symlinkSync(target, link);\n');
+
+      const check = runGen(['--check', '--tests-dir', testsDir, '--out', outPath]);
+      assert.equal(check.exitCode, 1);
+      const combined = check.stdout + check.stderr;
+      assert.match(combined, /tests\/new-signal\.test\.cjs/, 'the drift report must name the new file');
+      assert.match(combined, /\+/, 'a newly-added file is reported with a + marker');
+    } finally {
+      cleanup(tmpDir);
+    }
+  });
+
+  test('--write is deterministic across repeated runs', () => {
+    const tmpDir = createTempDir('gen-platform-conformance-tier-');
+    try {
+      const testsDir = path.join(tmpDir, 'tests');
+      fs.mkdirSync(testsDir, { recursive: true });
+      fs.writeFileSync(path.join(testsDir, 'a.test.cjs'), "process.env.PATHEXT;\n");
+      fs.writeFileSync(path.join(testsDir, 'b.test.cjs'), 'fs.symlinkSync(target, link);\n');
+      const out1 = path.join(tmpDir, 'out1.generated.cjs');
+      const out2 = path.join(tmpDir, 'out2.generated.cjs');
+
+      assert.equal(runGen(['--write', '--tests-dir', testsDir, '--out', out1]).exitCode, 0);
+      assert.equal(runGen(['--write', '--tests-dir', testsDir, '--out', out2]).exitCode, 0);
+
+      const content1 = fs.readFileSync(out1, 'utf8');
+      const content2 = fs.readFileSync(out2, 'utf8');
+      assert.equal(content1, content2, '--write must be byte-identical across independent runs over the same input');
+    } finally {
+      cleanup(tmpDir);
+    }
+  });
+});
+
+// ─── Row 20: suite-tagged files are excluded even when their content qualifies
+
+describe('gen-platform-conformance-tier.cjs CLI (temp fixture tree) — suite exclusion', () => {
+  test('a suite-suffixed file is excluded even with a qualifying content signal', () => {
+    const tmpDir = createTempDir('gen-platform-conformance-tier-suite-');
+    try {
+      const testsDir = path.join(tmpDir, 'tests');
+      fs.mkdirSync(testsDir, { recursive: true });
+      const signal = "if (process.platform === 'win32') {}\n";
+      fs.writeFileSync(path.join(testsDir, 'foo.test.cjs'), signal);
+      fs.writeFileSync(path.join(testsDir, 'foo.install.test.cjs'), signal);
+      const outPath = path.join(tmpDir, 'platform-conformance-tier.generated.cjs');
+
+      const write = runGen(['--write', '--tests-dir', testsDir, '--out', outPath]);
+      assert.equal(write.exitCode, 0, write.stderr);
+
+      delete require.cache[require.resolve(outPath)];
+      const generated = require(outPath);
+
+      assert.deepEqual(
+        generated.CONFORMANCE_TIER_FILES,
+        ['tests/foo.test.cjs'],
+        'the install-suite file must be excluded even though its content would otherwise qualify',
+      );
+    } finally {
+      cleanup(tmpDir);
+    }
+  });
+});
+
+// ─── Row 19: real tests/ tree, regression against the committed artifact ─────
+
+describe('gen-platform-conformance-tier.cjs — real repo tree (regression)', () => {
+  test('real tests/ tree classification matches the committed list', () => {
+    const realTestsDir = path.join(ROOT, 'tests');
+
+    let fresh;
+    assert.doesNotThrow(() => {
+      fresh = classifyTree(realTestsDir);
+    }, 'a full sweep of the real tests/ tree must complete without throwing');
+
+    // Measured 546/952 at authoring time (#4591, post suite-exclusion fix) —
+    // the range below is a sanity ballpark with headroom for organic
+    // test-suite growth in either direction, not a brittle exact-match on
+    // that literal.
+    assert.ok(
+      fresh.files.length >= 450 && fresh.files.length <= 700,
+      `expected a real, current, sanity-checked count in [450, 700], got ${fresh.files.length}`,
+    );
+
+    delete require.cache[require.resolve(GENERATED_PATH)];
+    const committed = require(GENERATED_PATH);
+
+    assert.equal(
+      committed.CONFORMANCE_TIER_FILES.length,
+      fresh.files.length,
+      'the committed generated file must be fresh — run `node scripts/gen-platform-conformance-tier.cjs --write`',
+    );
+    assert.deepEqual(
+      committed.CONFORMANCE_TIER_FILES.slice().sort(),
+      fresh.files.slice().sort(),
+      'the committed list must match a fresh sweep exactly, not just in length',
+    );
+  });
+
+  // ─── Row 21: no suite-tagged file ever reaches the committed conformance
+  // tier — the exact assertion that would have caught the CI incident before
+  // it ever shipped.
+  test('the committed conformance-tier list contains zero suite-tagged files', () => {
+    delete require.cache[require.resolve(GENERATED_PATH)];
+    const { CONFORMANCE_TIER_FILES } = require(GENERATED_PATH);
+
+    const suiteTaggedPattern = /\.(install|security|slow|integration|qa)\.test\.cjs$/;
+    const offenders = CONFORMANCE_TIER_FILES.filter((f) => suiteTaggedPattern.test(f));
+
+    assert.deepEqual(
+      offenders,
+      [],
+      'suite-tagged files (install/security/slow/integration/qa) must never appear in the ' +
+        'conformance-tier list — install/slow are PR-excluded suites and integration/security ' +
+        'already run via their own dedicated steps',
+    );
+  });
+});