diff --git a/.changeset/proud-koalas-jump.md b/.changeset/proud-koalas-jump.md
new file mode 100644
index 000000000..c476225a2
--- /dev/null
+++ b/.changeset/proud-koalas-jump.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 4476
+---
+**Sequential phase execution stays on the orchestrator's checkout** — non-isolated executors now receive the orchestrator's validated root as a literal prompt pin and halt loudly before any write or commit when their actual root differs, instead of silently committing onto whatever checkout their spawn cwd resolved to. (#4254)
diff --git a/docs/INVENTORY-MANIFEST.json b/docs/INVENTORY-MANIFEST.json
index 5e3b0415a..7adf3fe53 100644
--- a/docs/INVENTORY-MANIFEST.json
+++ b/docs/INVENTORY-MANIFEST.json
@@ -627,6 +627,7 @@
       "execute-phase/steps/protected-branch.md",
       "execute-phase/steps/regression-gate-run.md",
       "execute-phase/steps/regression-gate.md",
+      "execute-phase/steps/sequential-root-pin.md",
       "execute-phase/steps/tdd-applicability-resolution.md",
       "execute-phase/steps/wave-post-gate-hooks.md",
       "execute-phase/steps/worktree-recovery-policy.md",
diff --git a/docs/INVENTORY.md b/docs/INVENTORY.md
index f1e418d65..1aa6e65ee 100644
--- a/docs/INVENTORY.md
+++ b/docs/INVENTORY.md
@@ -367,7 +367,7 @@ Full roster at `gsd-core/references/*.md`. References are shared knowledge docum
 | `worktree-branch-check.md` | Canonical spawn-time worktree HEAD/base guard (worktree_branch_check): verify-only and fail-closed — per-agent-branch assertion, protected-ref refusal (#2924), and an exact-base assertion that halts with `exit 42` on mismatch so the orchestrator (worktree lifecycle owner) performs recovery (#48). Embedded into worktree sub-agent prompts at dispatch. |
 | `runtime-aware-dispatch.md` | Runtime-aware subagent dispatch protocol (#2508 Phase 4 Option A): before any `Agent(subagent_type="gsd-*")` call, resolve the type via `gsd_run query resolve-dispatch-type --requested <name> --raw`. On named-dispatch runtimes (Claude/OpenCode/…) the name is returned unchanged; on built-in-only runtimes (kimi-code) it maps to `coder`/`explore`/`plan` by role-suffix. The persona rides `${AGENT_SKILLS_<ROLE>}` (Phase 3) regardless. Documents why a PreToolUse-remap hook (the epic's original Option B) is infeasible — Kimi Code's hook API supports only allow/deny, not tool_input rewriting. |
 | `dispatch-isolation-gate.md` | Canonical gate deciding whether a dispatch site may run an agent isolated (#2584/#2652): resolves `ISOLATION` from the negotiated `dispatch.isolation` capability — never from a runtime id — fails closed to `none`, resolves the host's declared `harnessFlag` instead of hardcoding Claude Code's `isolation="worktree"` literal, and degrades single-agent sites to sequential on `orchestrator-worktree` hosts. Read by `quick.md`, `diagnose-issues.md`, and `execute-plan.md`. |
-| `worktree-path-safety.md` | Worktree guard suite: HEAD assertion, cwd-drift sentinel (step 0a, #3097), and absolute-path guard (step 0b, #3099) — loaded into executor spawn prompts via `<execution_context>`. |
+| `worktree-path-safety.md` | Executor path guards: supplied-root pin (step 0p, #4254 — every mode; execute-phase.md binds the orchestrator-validated root into sequential dispatches as `<project_root_pin>`), cwd-drift sentinel (step 0a, #3097), and absolute-path guard (step 0b, #3099) — loaded into executor spawn prompts via `<execution_context>`. |
 | `untrusted-input-boundary.md` | Shared prompt-injection boundary (#1577) `@`-included by the 10 research/doc-ingest agents (`gsd-project-researcher`, `gsd-phase-researcher`, `gsd-ui-researcher`, `gsd-assumptions-analyzer`, `gsd-advisor-researcher`, `gsd-doc-classifier`, `gsd-doc-synthesizer`, `gsd-research-synthesizer`, `gsd-ai-researcher`, `gsd-domain-researcher`): treat fetched/read text as data-not-instructions, self-scan before use (PromptArmor 2507.15219), task-anchor (2504.20472), and fence quoted text with a fresh random delimiter per wrap (PPA 2506.05739). Prompt-level defense-in-depth (2503.00061); the hook scanner is a separate pattern pre-filter. |
 | `artifact-types.md` | Planning artifact type definitions. |
 | `phase-argument-parsing.md` | Phase argument parsing conventions. |
diff --git a/docs/ja-JP/INVENTORY.md b/docs/ja-JP/INVENTORY.md
index b68d3e8cf..98c72fa1f 100644
--- a/docs/ja-JP/INVENTORY.md
+++ b/docs/ja-JP/INVENTORY.md
@@ -302,7 +302,7 @@
 | `scout-codebase.md` | discuss-phase スカウトステップ向けのフェーズタイプ→コードベースマップ選択テーブル（discuss-phase/modes プログレッシブディスクロージャー分割により抽出、#717）。 |
 | `revision-loop.md` | プラン修正の反復パターン。 |
 | `universal-anti-patterns.md` | 検出して避けるべきユニバーサルアンチパターン。 |
-| `worktree-path-safety.md` | ワークツリーガードスイート: HEAD アサーション、cwd ドリフトセンチネル（ステップ 0a、#3097）、絶対パスガード（ステップ 0b、#3099）— `<execution_context>` 経由でエグゼキュータースポーンプロンプトに読み込まれる。 |
+| `worktree-path-safety.md` | エグゼキューターパスガード: 供給ルート pin（ステップ 0p、#4254 — 全モード。execute-phase.md がオーケストレーター検証済みルートをシーケンシャルディスパッチに `<project_root_pin>` として束縛する）、cwd ドリフトセンチネル（ステップ 0a、#3097）、絶対パスガード（ステップ 0b、#3099）— `<execution_context>` 経由でエグゼキュータースポーンプロンプトに読み込まれる。 |
 | `artifact-types.md` | 計画アーティファクトタイプの定義。 |
 | `phase-argument-parsing.md` | フェーズ引数の解析規約。 |
 | `decimal-phase-calculation.md` | 小数サブフェーズの番号付けルール。 |
diff --git a/docs/ko-KR/INVENTORY.md b/docs/ko-KR/INVENTORY.md
index b015cc182..04faad000 100644
--- a/docs/ko-KR/INVENTORY.md
+++ b/docs/ko-KR/INVENTORY.md
@@ -302,7 +302,7 @@
 | `scout-codebase.md` | discuss-phase 스카우트 단계를 위한 단계 유형→코드베이스 맵 선택 테이블(discuss-phase/modes 프로그레시브 디스클로저 분할을 통해 추출, #717). |
 | `revision-loop.md` | 계획 수정 반복 패턴. |
 | `universal-anti-patterns.md` | 감지하고 피해야 할 보편적인 안티패턴. |
-| `worktree-path-safety.md` | 워크트리 가드 스위트: HEAD 어설션, cwd-드리프트 센티널(0a단계, #3097), 절대 경로 가드(0b단계, #3099) — `<execution_context>`를 통해 executor 스폰 프롬프트에 로드됨. |
+| `worktree-path-safety.md` | 실행기 경로 가드: 공급 루트 pin(0p단계, #4254 — 모든 모드. execute-phase.md가 오케스트레이터 검증 루트를 시퀀셜 디스패치에 `<project_root_pin>`으로 바인딩), cwd-드리프트 센티널(0a단계, #3097), 절대 경로 가드(0b단계, #3099) — `<execution_context>`를 통해 executor 스폰 프롬프트에 로드됨. |
 | `artifact-types.md` | 계획 아티팩트 유형 정의. |
 | `phase-argument-parsing.md` | 단계 인수 파싱 관례. |
 | `decimal-phase-calculation.md` | 소수점 하위 단계 번호 규칙. |
diff --git a/docs/pt-BR/INVENTORY.md b/docs/pt-BR/INVENTORY.md
index 532556e0b..2baeb1b3d 100644
--- a/docs/pt-BR/INVENTORY.md
+++ b/docs/pt-BR/INVENTORY.md
@@ -302,7 +302,7 @@ Registro completo em `gsd-core/references/*.md`. Referências são documentos de
 | `scout-codebase.md` | Tabela de seleção de tipo de fase → mapa de base de código para a etapa de scout da discuss-phase (extraída via a divisão progressiva discuss-phase/modes, #717). |
 | `revision-loop.md` | Padrões de iteração de revisão de plano. |
 | `universal-anti-patterns.md` | Antipadrões universais a detectar e evitar. |
-| `worktree-path-safety.md` | Suite de guarda do worktree: asserção de HEAD, sentinela de drift de cwd (etapa 0a, #3097) e guarda de caminho absoluto (etapa 0b, #3099) — carregados nos prompts de spawn do executor via `<execution_context>`. |
+| `worktree-path-safety.md` | Guardas de caminho do executor: pin de raiz fornecida (etapa 0p, #4254 — todo modo; o execute-phase.md binda a raiz validada pelo orquestrador em dispatches sequenciais como `<project_root_pin>`), sentinela de drift de cwd (etapa 0a, #3097) e guarda de caminho absoluto (etapa 0b, #3099) — carregados nos prompts de spawn do executor via `<execution_context>`. |
 | `artifact-types.md` | Definições de tipos de artefato de planejamento. |
 | `phase-argument-parsing.md` | Convenções de análise de argumentos de fase. |
 | `decimal-phase-calculation.md` | Regras de numeração de subfases decimais. |
diff --git a/docs/zh-CN/INVENTORY.md b/docs/zh-CN/INVENTORY.md
index bc5e92625..5edb65594 100644
--- a/docs/zh-CN/INVENTORY.md
+++ b/docs/zh-CN/INVENTORY.md
@@ -302,7 +302,7 @@
 | `scout-codebase.md` | discuss-phase 侦察步骤的阶段类型→代码库映射选择表（通过 discuss-phase/modes 渐进式披露分割提取，#717）。 |
 | `revision-loop.md` | 计划修订迭代模式。 |
 | `universal-anti-patterns.md` | 需要检测和避免的通用反模式。 |
-| `worktree-path-safety.md` | Worktree 守卫套件：HEAD 断言、cwd 漂移哨兵（步骤 0a，#3097）和绝对路径守卫（步骤 0b，#3099）— 通过 `<execution_context>` 加载到执行器生成提示中。 |
+| `worktree-path-safety.md` | 执行器路径守卫：供给根 pin（步骤 0p，#4254 — 所有模式；execute-phase.md 将编排器已验证的根绑定为顺序派发中的 `<project_root_pin>`）、cwd 漂移哨兵（步骤 0a，#3097）和绝对路径守卫（步骤 0b，#3099）— 通过 `<execution_context>` 加载到执行器生成提示中。 |
 | `artifact-types.md` | 规划产物类型定义。 |
 | `phase-argument-parsing.md` | 阶段参数解析约定。 |
 | `decimal-phase-calculation.md` | 十进制子阶段编号规则。 |
diff --git a/gsd-core/references/worktree-path-safety.md b/gsd-core/references/worktree-path-safety.md
index dac806918..bf7b9bb41 100644
--- a/gsd-core/references/worktree-path-safety.md
+++ b/gsd-core/references/worktree-path-safety.md
@@ -1,7 +1,117 @@
 # Worktree Path Safety
 
-Guards for executor agents running inside Claude Code worktrees. Three checks
-must run before any staging, Edit, or Write operation in worktree mode.
+Guards for executor agents running inside Claude Code worktrees. The
+supplied-root pin (step 0p) runs in EVERY mode; the remaining checks run before
+any staging, Edit, or Write operation in worktree mode.
+
+---
+
+## Supplied-root pin — step 0p (#4254, EVERY mode)
+
+Sequential-mode dispatch (no `isolation="worktree"`) gives the executor no
+spawn-time cwd guarantee, and the worktree-only guards below do not apply — so
+a sequential executor whose process cwd resolved to a different checkout of
+the same repo would self-derive that checkout as its root and commit there,
+silently. Step 0p closes that hole by comparing the executor's actual root
+against a root the ORCHESTRATOR already validated — never against anything the
+executor derives itself.
+
+**Runtime contract (executor):** if your prompt contains a `<project_root_pin>`
+block, run its guard script verbatim before your first Edit/Write and again
+before every commit, in the same cwd as that write or commit. On FATAL, halt
+and report — recovery (moving commits between checkouts) is an
+orchestrator/human decision, never agent self-repair. If your prompt contains
+NO `<project_root_pin>` block (worktree/isolated dispatch, or a legacy
+orchestrator), emit one warning line and continue with steps 0a/0b below — do
+not fail closed on dispatches that never carried a pin. **Never bind
+`{PINNED_ROOT}` yourself**: if this template reaches you unbound it is
+reference prose, not your pin — only the orchestrator's build-time
+substitution produces a valid guard.
+
+**Composition contract (orchestrator — build time, NOT a sub-agent runtime
+step):** copy the guard below into the dispatched prompt inside a
+`<project_root_pin>` block, substituting `{PINNED_ROOT}` with the literal value
+of `$ORCHESTRATOR_WT` captured at execute_waves entry, shell-single-quoted:
+wrap the path in `'…'` and escape any embedded `'` as `'\''`. A path that
+cannot be quoted this way must halt the phase (surface a blocker) rather than
+ship a pin that could mis-parse. The comparison is git-vs-git on BOTH sides —
+`git -C` resolves the pinned path to its repo's canonical toplevel in git's
+own path representation, so symlink aliases, trailing slashes, `/var` vs
+`/private/var` spellings, and Windows drive-letter forms — forward- or
+backslash-separated, `RUNNER~1`-style short names included — compare equal by
+construction (shell `pwd -P` normalization does NOT match git's emission on
+Windows — do not re-introduce it).
+
+Two portability rules baked into the guard below, learned from the #4254 CI
+Windows legs: (1) a backslash comparator must be GENERATED at runtime
+(`printf '\134'`), because a backslash written twice in the script text does
+not survive the Windows command-line round-trip into bash — the doubled form
+arrives halved, which silently rewrites any escape pattern that relies on it;
+(2) every FATAL names its `Guard stage` and, where a git capture failed,
+git's own stderr in a `Diagnostic` line, so a platform failure self-describes
+instead of surfacing as a bare `Actual root: <none>`.
+
+```bash
+# gsd:guard=supplied-root-pin (#4254) — run before the first Edit/Write and before every commit.
+PINNED_ROOT='{PINNED_ROOT}'  # orchestrator build-time substitution — the only valid source of this value
+PIN_STAGE=''
+PIN_DIAG=''
+gsd_pin_fail() {
+  echo "FATAL: executor root does not match the orchestrator-supplied PROJECT_ROOT pin (#4254)." >&2
+  echo "  Pinned root: ${PINNED_ROOT:-<empty or unexpanded>}" >&2
+  echo "  Actual root: ${ACTUAL_ROOT:-<none>}" >&2
+  echo "  Guard stage: ${PIN_STAGE:-<unset>}" >&2
+  if [ -n "$PIN_DIAG" ]; then echo "  Diagnostic: $PIN_DIAG" >&2; fi
+  echo "  No writes or commits are permitted from this checkout. HALT and report; recovery is an" >&2
+  echo "  orchestrator/human decision. Only the IMMEDIATE submodule of the pinned checkout is a" >&2
+  echo "  legitimate other cwd — nested submodules must surface as a blocker, not self-route." >&2
+  exit 1
+}
+# Backslash comparator, generated at runtime: a backslash written twice in this
+# script does not survive the Windows spawn path into bash (the command-line
+# round-trip halves the doubled form), which rejected every C:\ pin at the form
+# gate on the #4254 CI Windows legs. printf's octal escape is a lone backslash,
+# which does survive; the quoted expansion below is literal in a case pattern.
+BS=$(printf '\134')
+# Fail closed if the comparator could not be generated: an empty BS would widen
+# the drive-form arm below to drive-RELATIVE pins (C:foo) — the one fail-open
+# seam in this construction, closed loudly rather than trusted to the shell.
+if [ -z "$BS" ]; then
+  PIN_STAGE=form-gate
+  PIN_DIAG='backslash comparator generation failed (printf octal escape returned empty)'
+  gsd_pin_fail
+fi
+case "$PINNED_ROOT" in
+  ''|'{PINNED_ROOT}') PIN_STAGE=pin-unbound; gsd_pin_fail ;;  # empty or unexpanded pin — fail closed, never warn-and-proceed
+  /*) ;;                                                     # absolute POSIX form
+  [A-Za-z]:/*|[A-Za-z]:"$BS"*) ;;                            # Windows drive form, forward- or backslash-separated
+  *) PIN_STAGE=form-gate; gsd_pin_fail ;;                    # relative pin — never trustworthy across cwds
+esac
+ACTUAL_ROOT=$(git rev-parse --show-toplevel 2>/dev/null)
+if [ -z "$ACTUAL_ROOT" ]; then
+  PIN_STAGE=actual-capture
+  PIN_DIAG="git rev-parse --show-toplevel from the cwd failed: $(git rev-parse --show-toplevel 2>&1 1>/dev/null)"
+  gsd_pin_fail
+fi
+PINNED_TL=$(git -C "$PINNED_ROOT" rev-parse --show-toplevel 2>/dev/null)
+if [ -z "$PINNED_TL" ]; then
+  PIN_STAGE=pinned-capture
+  PIN_DIAG="git -C <pinned root> rev-parse --show-toplevel failed: $(git -C "$PINNED_ROOT" rev-parse --show-toplevel 2>&1 1>/dev/null)"
+  gsd_pin_fail
+fi
+if [ "$ACTUAL_ROOT" != "$PINNED_TL" ]; then
+  # Registered-submodule allowance: sub_repos plans legitimately commit inside an
+  # immediate submodule of the pinned checkout. The superproject working tree is
+  # git-emitted in the same representation as PINNED_TL, so the equality is
+  # representation-safe on every platform.
+  SUPER_TL=$(git rev-parse --show-superproject-working-tree 2>/dev/null)
+  if [ "$SUPER_TL" != "$PINNED_TL" ]; then
+    PIN_STAGE=root-mismatch
+    PIN_DIAG="actual=${ACTUAL_ROOT} pinned=${PINNED_TL} superproject=${SUPER_TL:-<none>}"
+    gsd_pin_fail
+  fi
+fi
+```
 
 ---
 
diff --git a/gsd-core/workflows/execute-phase.md b/gsd-core/workflows/execute-phase.md
index a5ef4ac59..32e7bbf9c 100644
--- a/gsd-core/workflows/execute-phase.md
+++ b/gsd-core/workflows/execute-phase.md
@@ -815,14 +815,23 @@ increases monotonically across waves. `{status}` is `complete` (success),
 
    **Sequential mode** (`USE_WORKTREES_FOR_PLAN` is `false` — either project-level `USE_WORKTREES=false`, or per-plan submodule intersection forced it false in step 2.5):
 
-   Omit `isolation="worktree"` from the Agent call. Replace the `<parallel_execution>` block with:
+   Omit `isolation="worktree"` from the Agent call. Before composing the prompt, read and execute
+   `execute-phase/steps/sequential-root-pin.md` (#4254) — it owns the sequential root-pin build-time
+   embed and the wave serialization rules.
+
+   Replace the `<parallel_execution>` block with:
 
    ```
        <sequential_execution>
        You are running as a SEQUENTIAL executor agent on the main working tree.
        Use normal git commits (with hooks). Do NOT use --no-verify.
+       Run the `<project_root_pin>` guard before your first Edit/Write and before every commit (#4254).
        REQUIRED ORDER: Write SUMMARY.md → commit → only then any narration. No text between Write and commit (truncation risk; #2070 rescue is not primary defense).
        </sequential_execution>
+
+       <project_root_pin>
+       {ORCHESTRATOR build-time embed: bound step-0p guard per sequential-root-pin.md — never this note}
+       </project_root_pin>
    ```
 
    The sequential mode Agent prompt uses the same structure as worktree mode but with these differences in success_criteria — since there is only one agent writing at a time, there are no shared-file conflicts:
@@ -837,8 +846,6 @@ increases monotonically across waves. `{status}` is `complete` (success),
        </success_criteria>
    ```
 
-   When worktrees are disabled for a plan (per-plan or project-level), that plan's executor runs on the main working tree. If **any** plan in the current wave dropped to sequential mode, execute the affected plan(s) **one at a time** to avoid concurrent writes to the main working tree — plans in the same wave that retained worktree isolation can still run in parallel alongside the sequential ones, but two non-worktree plans in the same wave must serialize. When the project-level `USE_WORKTREES=false`, all plans in the wave serialize regardless of the `PARALLELIZATION` setting.
-
 4. **Wait for all agents in wave to complete.**
 
    **Plan-complete heartbeat (#2410):** as each executor returns (or is verified
diff --git a/gsd-core/workflows/execute-phase/steps/sequential-root-pin.md b/gsd-core/workflows/execute-phase/steps/sequential-root-pin.md
new file mode 100644
index 000000000..a9a87d613
--- /dev/null
+++ b/gsd-core/workflows/execute-phase/steps/sequential-root-pin.md
@@ -0,0 +1,35 @@
+# Sequential root pin (#4254)
+
+Apply response_language to all user-facing prose — narration between tool calls, status
+updates, progress notes, and findings included; preserve code, paths, and identifiers.
+
+Read and execute this fragment from `execute-phase.md`'s **Sequential mode** branch before
+composing any sequential dispatch prompt. It owns the sequential root-pin build-time embed,
+the `<required_reading>` root substitution, and the wave serialization rules.
+
+## Root pin — ORCHESTRATOR build-time embed (#4254; NOT a sub-agent runtime step)
+
+Before this dispatch, read `gsd-core/references/worktree-path-safety.md` step 0p
+("Supplied-root pin") and copy its guard template into this prompt inside a
+`<project_root_pin>` block, substituting `{PINNED_ROOT}` with the literal value of
+`$ORCHESTRATOR_WT` resolved at execute_waves entry, shell-single-quoted per that section's
+composition contract — the dispatched prompt must carry the bound, runnable guard verbatim;
+do not pass this instruction through in its place.
+
+In this dispatch's `<required_reading>`, also replace the self-derivation line
+`PROJECT_ROOT=$(git rev-parse --show-toplevel 2>/dev/null)` with
+`PROJECT_ROOT='<the same literal $ORCHESTRATOR_WT>'` — a sequential executor must never
+re-derive its root from its own (possibly drifted) cwd. This substitution is sequential-mode
+ONLY: worktree-mode dispatches keep the self-derived line — an isolated executor's own
+toplevel IS its correct (and intentionally different) worktree, and substituting the
+orchestrator's root there would break every worktree-mode dispatch.
+
+## Wave serialization (moved verbatim from the host step — ADR-857 Phase 6 ceiling, #1168)
+
+When worktrees are disabled for a plan (per-plan or project-level), that plan's executor runs
+on the main working tree. If **any** plan in the current wave dropped to sequential mode,
+execute the affected plan(s) **one at a time** to avoid concurrent writes to the main working
+tree — plans in the same wave that retained worktree isolation can still run in parallel
+alongside the sequential ones, but two non-worktree plans in the same wave must serialize.
+When the project-level `USE_WORKTREES=false`, all plans in the wave serialize regardless of
+the `PARALLELIZATION` setting.
diff --git a/tests/executor-mvp-tdd-section.test.cjs b/tests/executor-mvp-tdd-section.test.cjs
index c3c69338c..2e42932e0 100644
--- a/tests/executor-mvp-tdd-section.test.cjs
+++ b/tests/executor-mvp-tdd-section.test.cjs
@@ -238,3 +238,309 @@ describe('bug #3099: absolute-path safety guidance in gsd-executor.md', () => {
 });
   });
 }
+
+// ────────────────────────────────────────────────────────────────────────
+// #4254 — sequential (non-isolated) executor dispatch had no hard pin to the
+// orchestrator's own worktree root: the dispatched prompt told the executor to
+// self-derive PROJECT_ROOT via `git rev-parse --show-toplevel`, and every
+// existing guard (steps 0a/0b worktree-only, step 0 branch-scoped) is
+// self-referential — so an executor spawned with a drifted cwd committed onto
+// the wrong checkout silently. The fix ships a mode-agnostic "supplied-root
+// pin" guard (step 0p) in worktree-path-safety.md, bound at dispatch time by
+// execute-phase.md's SEQUENTIAL branch only (worktree mode keeps its own,
+// intentionally different, self-derived root). These tests EXECUTE the shipped
+// guard against real git fixtures — no string-only vacuity (#4296 review
+// precedent, Blocker 2).
+// ────────────────────────────────────────────────────────────────────────
+describe('bug #4254: sequential executor supplied-root pin', () => {
+  // allow-test-rule: source-text-is-the-product (see #3097) — the reference
+  // and the workflow markdown ARE the product under test. ROOT and
+  // executePhaseSrc are re-declared here: the folded #3097/#3099 block above
+  // declares its own copies inside a closure, out of this describe's scope.
+  const ROOT = path.join(__dirname, '..');
+  const executePhaseSrc = fs.readFileSync(
+    path.join(ROOT, 'gsd-core', 'workflows', 'execute-phase.md'), 'utf8',
+  );
+  const safetyRefPath = path.join(ROOT, 'gsd-core', 'references', 'worktree-path-safety.md');
+  const pinStepPath = path.join(ROOT, 'gsd-core', 'workflows', 'execute-phase', 'steps', 'sequential-root-pin.md');
+  const safetySrc = fs.readFileSync(safetyRefPath, 'utf8');
+  const pinStepSrc = fs.readFileSync(pinStepPath, 'utf8');
+  const { createTempGitProject, cleanup } = require('./helpers.cjs');
+  const { runHook } = require('./helpers/process-seam.cjs');
+  const { gitOrThrow } = require('./helpers/git-fixture.cjs');
+  const { HOOK_FANOUT_TIMEOUT_MS } = require('./helpers/timeouts.cjs');
+
+  const PIN_MARKER = '# gsd:guard=supplied-root-pin';
+
+  // Extract the shipped guard — the exact ```bash block that carries the
+  // marker — so the tests can never pass against a stale or hand-copied body.
+  function extractPinGuard() {
+    const body = safetySrc.split('```bash\n').find((b) => b.startsWith(PIN_MARKER));
+    assert.ok(body, 'worktree-path-safety.md must ship the #4254 supplied-root-pin guard');
+    return body.split('```')[0].trim();
+  }
+
+  // The composition contract the orchestrator follows at dispatch: a
+  // shell-single-quoted literal with `'\''` escaping for embedded quotes.
+  const shellQuote = (v) => `'${String(v).replace(/'/g, `'\\''`)}'`;
+
+  function composeGuard(pinQuotedLiteral) {
+    return extractPinGuard().replace("PINNED_ROOT='{PINNED_ROOT}'", `PINNED_ROOT=${pinQuotedLiteral}`);
+  }
+
+  // Run the composed guard in `cwd`; `probe` (optional bash line) runs only if
+  // the guard passes — proving the write barrier, not just the exit code.
+  function runPinGuard(cwd, pin, probe) {
+    return runHook('-c', [composeGuard(shellQuote(pin)) + (probe ? `\n${probe}` : '')], {
+      interpreter: 'bash',
+      cwd,
+      timeoutMs: HOOK_FANOUT_TIMEOUT_MS,
+      env: { ...process.env, GIT_TERMINAL_PROMPT: '0' },
+    });
+  }
+
+  // Fixture: a primary checkout plus a linked worktree (the orchestrator's
+  // lane). Sibling path — a worktree inside the primary's tree would show up
+  // as an untracked directory and muddy the fixtures.
+  function makeOrchestratorLane(prefix) {
+    const primary = createTempGitProject(prefix);
+    const lane = `${primary}-orchestrator-wt`;
+    gitOrThrow(['worktree', 'add', '-q', '-b', 'phase/2-1', lane], { cwd: primary });
+    return { primary, lane };
+  }
+
+  test('#4254: reference ships the supplied-root pin section with composition + runtime contracts', () => {
+    assert.match(safetySrc, /## Supplied-root pin — step 0p \(#4254, EVERY mode\)/);
+    // Runtime contract: run before first Edit/Write and every commit; HALT on
+    // FATAL; warn-and-proceed ONLY when the prompt carries no pin block.
+    assert.match(safetySrc, /before your first Edit\/Write and again\s+before every commit/);
+    assert.match(safetySrc, /NO `<project_root_pin>` block[\s\S]*?warning line and continue/);
+    // The executor must never bind the placeholder itself — that is exactly
+    // the #4254 failure mode re-armored as a "fix".
+    assert.match(safetySrc, /Never bind\s+`\{PINNED_ROOT\}` yourself/);
+    // Composition contract: build-time literal substitution, single-quoted,
+    // git-vs-git comparison rationale (the #4296 Windows lesson).
+    assert.match(safetySrc, /substituting[\s\S]*`\{PINNED_ROOT\}`[\s\S]*shell-single-quoted/);
+    assert.match(safetySrc, /git-vs-git on BOTH sides/);
+  });
+
+  test('#4254: RED regression — drifted-cwd executor halts before the wrong-checkout write', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-drift-');
+    try {
+      // The orchestrator pinned its own lane; the executor's process cwd
+      // resolved to the PRIMARY checkout (the issue's exact shape).
+      const marker = path.join(primary, 'write-marker');
+      const res = runPinGuard(primary, lane, `printf 'W' > ${shellQuote(marker)}`);
+      assert.equal(res.exitCode, 1, `mismatched root must halt, got:\n${res.stdout}\n${res.stderr}`);
+      assert.equal(fs.existsSync(marker), false, 'no write may run after a root mismatch');
+      assert.match(res.stderr, /FATAL[^\n]*#4254/);
+      // Loud detection: the FATAL names BOTH roots — the pinned lane and the
+      // actual (wrong) primary checkout. The primary's basename is unique to
+      // it (the lane appends '-orchestrator-wt'), so matching it inside the
+      // Actual-root line proves the right checkout is being named.
+      assert.match(res.stderr, /Pinned root:[^\n]*orchestrator-wt/, 'FATAL must name the pinned root');
+      const { escapeRegex } = require('../gsd-core/bin/lib/pattern.cjs');
+      const primaryName = escapeRegex(path.basename(primary));
+      assert.match(res.stderr, new RegExp(`Actual root:[^\\n]*${primaryName}`), 'FATAL must name the actual (wrong) root');
+      assert.doesNotMatch(res.stderr, new RegExp(`Actual root:[^\\n]*orchestrator-wt`), 'the actual root must NOT be the pinned lane');
+      // The FATAL self-describes its stage (and, on capture failures, git's own
+      // stderr) — the discriminator whose absence let this class of failure read
+      // as "capture returns empty" when the form gate was what fired on Windows.
+      assert.match(res.stderr, /Guard stage: root-mismatch/, 'drifted cwd is a root mismatch, not a capture or form failure');
+      assert.match(res.stderr, /Diagnostic: actual=.*pinned=.*superproject=<none>/, 'the mismatch diagnostic names both roots and the absent superproject');
+    } finally {
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: correct-cwd control — matching root permits the write', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-match-');
+    try {
+      const marker = path.join(lane, 'write-marker');
+      const res = runPinGuard(lane, lane, `printf 'W' > ${shellQuote(marker)}`);
+      assert.equal(res.exitCode, 0, `matching root must permit the write, got:\n${res.stderr}`);
+      assert.equal(fs.readFileSync(marker, 'utf8'), 'W');
+    } finally {
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: unexpanded or empty pin fails closed (never warn-and-proceed inside a pin block)', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-unbound-');
+    try {
+      const marker = path.join(lane, 'write-marker');
+      // Unexpanded: the orchestrator never performed the build-time substitution.
+      const unexpanded = runHook('-c', [extractPinGuard() + `\nprintf 'W' > ${shellQuote(marker)}`], {
+        interpreter: 'bash', cwd: lane, timeoutMs: HOOK_FANOUT_TIMEOUT_MS,
+      });
+      assert.equal(unexpanded.exitCode, 1, 'an unexpanded {PINNED_ROOT} must halt');
+      assert.match(unexpanded.stderr, /Guard stage: pin-unbound/, 'an unexpanded pin halts at the pin-unbound stage');
+      // Empty: substituted to nothing — indistinguishable from a forgotten transplant.
+      const empty = runHook('-c', [composeGuard("''") + `\nprintf 'W' > ${shellQuote(marker)}`], {
+        interpreter: 'bash', cwd: lane, timeoutMs: HOOK_FANOUT_TIMEOUT_MS,
+      });
+      assert.equal(empty.exitCode, 1, 'an empty pin must halt');
+      assert.match(empty.stderr, /Guard stage: pin-unbound/, 'an empty pin halts at the pin-unbound stage');
+      assert.equal(fs.existsSync(marker), false);
+    } finally {
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: normalization — trailing slash, symlink alias, subdirectory cwd, and unresolved temp-root spelling all match', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-norm-');
+    try {
+      const alias = `${primary}-alias`;
+      fs.symlinkSync(lane, alias, 'junction');
+      const subdir = path.join(lane, 'nested');
+      fs.mkdirSync(subdir);
+      // The unresolved spelling of the temp root (macOS: /var/... vs git's
+      // resolved /private/var/...): both sides are git-emitted, so the
+      // comparison is representation-safe by construction. On platforms
+      // without the /var symlink this degenerates to the realpath form and
+      // still asserts the matching property.
+      const unresolvedLane = fs.realpathSync(lane).replace('/private/var/', '/var/');
+      for (const pin of [lane, `${lane}/`, alias, unresolvedLane]) {
+        const marker = path.join(lane, 'write-marker');
+        const res = runPinGuard(subdir, pin, `printf 'W' > ${shellQuote(marker)}`);
+        assert.equal(res.exitCode, 0, `pin form ${pin} must normalize to the same checkout:\n${res.stderr}`);
+        fs.unlinkSync(marker);
+      }
+    } finally {
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: boundary — registered submodule of the pinned checkout commits; unregistered and sibling checkouts halt', () => {
+    const primary = createTempGitProject('gsd-4254-super-');
+    const subSource = createTempGitProject('gsd-4254-subsource-');
+    try {
+      gitOrThrow(['-c', 'protocol.file.allow=always', 'submodule', 'add', '-q', subSource, 'module'], { cwd: primary });
+      gitOrThrow(['commit', '-qm', 'chore: register submodule'], { cwd: primary });
+      const moduleRoot = path.join(primary, 'module');
+      // Registered immediate submodule: legitimate sub_repos work, permitted.
+      const inModule = path.join(moduleRoot, 'write-marker');
+      assert.equal(runPinGuard(moduleRoot, primary, `printf 'W' > ${shellQuote(inModule)}`).exitCode, 0);
+      fs.unlinkSync(inModule);
+      // Unregistered nested clone inside the pinned checkout: halts.
+      const rogue = path.join(primary, 'rogue-clone');
+      gitOrThrow(['clone', '-q', subSource, rogue], { cwd: primary });
+      const rogueMarker = path.join(rogue, 'write-marker');
+      assert.equal(runPinGuard(rogue, primary, `printf 'W' > ${shellQuote(rogueMarker)}`).exitCode, 1);
+      assert.equal(fs.existsSync(rogueMarker), false);
+      // Sibling linked worktree of the SAME repo (right repo, wrong checkout —
+      // the incident's exact shape): halts.
+      const sibling = `${primary}-sibling-wt`;
+      gitOrThrow(['worktree', 'add', '-q', '-b', 'phase/9-9', sibling], { cwd: primary });
+      const siblingMarker = path.join(sibling, 'write-marker');
+      assert.equal(runPinGuard(sibling, primary, `printf 'W' > ${shellQuote(siblingMarker)}`).exitCode, 1);
+      assert.equal(fs.existsSync(siblingMarker), false);
+    } finally {
+      cleanup(primary);
+      cleanup(subSource);
+    }
+  });
+
+  test('#4254: pin outside any git repo, and cwd outside any git repo, both fail closed', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-norepo-');
+    const outside = fs.mkdtempSync(path.join(require('node:os').tmpdir(), 'gsd-4254-outside-'));
+    try {
+      assert.equal(runPinGuard(lane, outside).exitCode, 1, 'pin outside a repo must halt');
+      assert.equal(runPinGuard(outside, lane).exitCode, 1, 'cwd outside a repo must halt');
+    } finally {
+      cleanup(outside);
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: shell-metacharacter pin is safely quoted — no command substitution executes', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-meta-');
+    try {
+      const tricky = path.join(primary, `q' $HOME $(touch PWNED) x`);
+      fs.symlinkSync(lane, tricky, 'junction');
+      const marker = path.join(lane, 'write-marker');
+      const res = runPinGuard(lane, tricky, `printf 'W' > ${shellQuote(marker)}`);
+      assert.equal(res.exitCode, 0, `quoted metacharacter pin must match its checkout:\n${res.stderr}`);
+      assert.equal(fs.existsSync(path.join(lane, 'PWNED')), false, 'command substitution in the pin must not execute');
+      assert.equal(fs.existsSync(marker), true);
+    } finally {
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: relative pin is rejected; Windows drive-letter forms pass the form gate and fail only at the git lookup', () => {
+    const { primary, lane } = makeOrchestratorLane('gsd-4254-forms-');
+    try {
+      // Relative pins are never trustworthy across cwds — rejected outright,
+      // and the FATAL says so at the form-gate stage.
+      const rel = runPinGuard(lane, 'relative/path');
+      assert.equal(rel.exitCode, 1);
+      assert.match(rel.stderr, /Guard stage: form-gate/, 'a relative pin must halt at the form gate');
+      // The drive-letter forms Windows produces must be ACCEPTED by the shipped
+      // guard's absolute-form gate and then fail only on the git lookup — never
+      // on the form rejection: the forward-slash form git EMITS (C:/…) and the
+      // backslash form Node's path.join and cmd.exe PRODUCE (C:\…, including
+      // RUNNER~1-style short names). The Guard stage line is the discriminator:
+      // pinned-capture means the form passed and `git -C` spoke (its stderr rides
+      // the Diagnostic line), form-gate means the gate ate the pin — the exact
+      // CI defect where every backslash pin died before the actual root was ever
+      // computed. Driving the SHIPPED guard also removes the hand-rolled
+      // duplicate `case` this row used to carry (the #4296 Minor 1 duplication
+      // smell, and itself a transit-fragile copy of the broken pattern).
+      for (const driveForm of ['C:/definitely/not/a/repo', 'C:\\definitely\\not\\a\\repo']) {
+        const res = runPinGuard(lane, driveForm);
+        assert.equal(res.exitCode, 1, `nonexistent drive-letter pin must fail closed (${driveForm})`);
+        assert.match(
+          res.stderr, /Guard stage: pinned-capture/,
+          `drive form ${driveForm} must pass the form gate and halt at the pinned-capture stage, got:\n${res.stderr}`,
+        );
+        assert.match(res.stderr, /Diagnostic: git -C <pinned root> rev-parse --show-toplevel failed:/, 'the capture failure carries git stderr');
+      }
+      // Transit hardening, pinned on the shipped text itself: the guard must
+      // generate its backslash comparator at RUNTIME (printf octal) and must not
+      // contain a doubled backslash anywhere — a backslash written twice does
+      // not survive the Windows command-line round-trip into bash (it arrives
+      // halved, rewriting any escape pattern that relies on it). Two earlier
+      // pattern spellings failed the Windows CI leg on exactly this.
+      const guardBody = extractPinGuard();
+      assert.doesNotMatch(guardBody, /\\\\/, 'the shipped guard must contain no doubled backslash (Windows transit halves it)');
+      assert.match(guardBody, /printf '\\134'/, 'the drive-form gate must generate its backslash comparator at runtime');
+    } finally {
+      cleanup(primary);
+    }
+  });
+
+  test('#4254: execute-phase.md sequential branch pins the orchestrator root at build time', () => {
+    const sequential = executePhaseSrc.split('**Sequential mode**')[1].split('4. **Wait for all agents')[0];
+    assert.ok(sequential.length > 0, 'sequential-mode section not found');
+    // The host step delegates the composition detail to the fragment (ADR-857
+    // Phase 6 frozen ceiling — execute-phase.md cannot grow) and the prompt
+    // gains the pin block plus the per-write/commit instruction.
+    assert.match(sequential, /read and execute\s+`execute-phase\/steps\/sequential-root-pin\.md`/);
+    assert.match(sequential, /<project_root_pin>/);
+    assert.match(sequential, /Run the `<project_root_pin>` guard before your first Edit\/Write and before every commit/);
+    // The fragment carries the full build-time embed contract.
+    assert.match(pinStepSrc, /ORCHESTRATOR build-time embed/);
+    assert.match(pinStepSrc, /\{PINNED_ROOT\}/);
+    assert.match(pinStepSrc, /ORCHESTRATOR_WT/);
+    assert.match(pinStepSrc, /do not pass this instruction through/i);
+    // The self-derivation is replaced with the literal, preserving the
+    // `PROJECT_ROOT=` binding the rest of <required_reading> depends on.
+    assert.match(pinStepSrc, /replace the self-derivation line/);
+    assert.match(pinStepSrc, /PROJECT_ROOT='<the same literal/);
+    // Scoping: the fragment must say worktree mode keeps the self-derived form.
+    assert.match(pinStepSrc, /sequential-mode\s+ONLY/i);
+    // The wave serialization rules moved with it, verbatim in substance.
+    assert.match(pinStepSrc, /two non-worktree plans in the same wave must serialize/);
+  });
+
+  test('#4254: worktree-mode dispatch is untouched — keeps self-derivation, gains no pin', () => {
+    const isolated = executePhaseSrc.slice(
+      executePhaseSrc.indexOf('<parallel_execution>'),
+      executePhaseSrc.indexOf('**Sequential mode**'),
+    );
+    assert.ok(isolated.length > 0, 'worktree-mode dispatch slice not found');
+    assert.match(isolated, /PROJECT_ROOT=\$\(git rev-parse --show-toplevel/, 'isolated executor keeps its own (correct) root derivation');
+    assert.doesNotMatch(isolated, /<project_root_pin>/, 'isolated prompt must not inherit the orchestrator root');
+  });
+});
diff --git a/tests/fixtures/install-tree/antigravity.json b/tests/fixtures/install-tree/antigravity.json
index 8943408be..b0f94cefa 100644
--- a/tests/fixtures/install-tree/antigravity.json
+++ b/tests/fixtures/install-tree/antigravity.json
@@ -394,6 +394,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/augment.json b/tests/fixtures/install-tree/augment.json
index 588734b0b..00f8f292e 100644
--- a/tests/fixtures/install-tree/augment.json
+++ b/tests/fixtures/install-tree/augment.json
@@ -466,6 +466,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/claude-local.json b/tests/fixtures/install-tree/claude-local.json
index 3cf79d7da..dbdefbd0f 100644
--- a/tests/fixtures/install-tree/claude-local.json
+++ b/tests/fixtures/install-tree/claude-local.json
@@ -359,6 +359,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/claude.json b/tests/fixtures/install-tree/claude.json
index 95195ce26..cad6eb778 100644
--- a/tests/fixtures/install-tree/claude.json
+++ b/tests/fixtures/install-tree/claude.json
@@ -394,6 +394,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/cline.json b/tests/fixtures/install-tree/cline.json
index 6149f0fd9..edce069ac 100644
--- a/tests/fixtures/install-tree/cline.json
+++ b/tests/fixtures/install-tree/cline.json
@@ -396,6 +396,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/codebuddy.json b/tests/fixtures/install-tree/codebuddy.json
index 18f097b05..14f1133bc 100644
--- a/tests/fixtures/install-tree/codebuddy.json
+++ b/tests/fixtures/install-tree/codebuddy.json
@@ -466,6 +466,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/codex.json b/tests/fixtures/install-tree/codex.json
index c0b65addc..cdb5f8445 100644
--- a/tests/fixtures/install-tree/codex.json
+++ b/tests/fixtures/install-tree/codex.json
@@ -430,6 +430,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/copilot.json b/tests/fixtures/install-tree/copilot.json
index 92efd5ee9..8108a27ea 100644
--- a/tests/fixtures/install-tree/copilot.json
+++ b/tests/fixtures/install-tree/copilot.json
@@ -395,6 +395,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/cursor.json b/tests/fixtures/install-tree/cursor.json
index eb4503859..868a8cf07 100644
--- a/tests/fixtures/install-tree/cursor.json
+++ b/tests/fixtures/install-tree/cursor.json
@@ -394,6 +394,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/hermes.json b/tests/fixtures/install-tree/hermes.json
index d0a1b9ac0..dbcf40faf 100644
--- a/tests/fixtures/install-tree/hermes.json
+++ b/tests/fixtures/install-tree/hermes.json
@@ -394,6 +394,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/kilo.json b/tests/fixtures/install-tree/kilo.json
index d43d47aa4..7f9faf98e 100644
--- a/tests/fixtures/install-tree/kilo.json
+++ b/tests/fixtures/install-tree/kilo.json
@@ -466,6 +466,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/kimi-code.json b/tests/fixtures/install-tree/kimi-code.json
index b0970bd63..46870aaea 100644
--- a/tests/fixtures/install-tree/kimi-code.json
+++ b/tests/fixtures/install-tree/kimi-code.json
@@ -395,6 +395,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/kimi.json b/tests/fixtures/install-tree/kimi.json
index dcc0bac5c..f551fbeea 100644
--- a/tests/fixtures/install-tree/kimi.json
+++ b/tests/fixtures/install-tree/kimi.json
@@ -431,6 +431,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/opencode.json b/tests/fixtures/install-tree/opencode.json
index 366cf8ec7..6faa1ca62 100644
--- a/tests/fixtures/install-tree/opencode.json
+++ b/tests/fixtures/install-tree/opencode.json
@@ -466,6 +466,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/pi.json b/tests/fixtures/install-tree/pi.json
index d4fb2bc2d..8b8b68466 100644
--- a/tests/fixtures/install-tree/pi.json
+++ b/tests/fixtures/install-tree/pi.json
@@ -254,6 +254,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/qwen.json b/tests/fixtures/install-tree/qwen.json
index 99ac81efa..edc7463cc 100644
--- a/tests/fixtures/install-tree/qwen.json
+++ b/tests/fixtures/install-tree/qwen.json
@@ -394,6 +394,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/trae.json b/tests/fixtures/install-tree/trae.json
index b1e4423f6..8050cd141 100644
--- a/tests/fixtures/install-tree/trae.json
+++ b/tests/fixtures/install-tree/trae.json
@@ -394,6 +394,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/windsurf.json b/tests/fixtures/install-tree/windsurf.json
index d067cde15..38cebc17d 100644
--- a/tests/fixtures/install-tree/windsurf.json
+++ b/tests/fixtures/install-tree/windsurf.json
@@ -322,6 +322,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/fixtures/install-tree/zcode.json b/tests/fixtures/install-tree/zcode.json
index 9b4bc9ae2..510831b62 100644
--- a/tests/fixtures/install-tree/zcode.json
+++ b/tests/fixtures/install-tree/zcode.json
@@ -466,6 +466,7 @@
   "gsd-core/workflows/execute-phase/steps/protected-branch.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md",
+  "gsd-core/workflows/execute-phase/steps/sequential-root-pin.md",
   "gsd-core/workflows/execute-phase/steps/tdd-applicability-resolution.md",
   "gsd-core/workflows/execute-phase/steps/wave-post-gate-hooks.md",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md",
diff --git a/tests/worktree-safety.test.cjs b/tests/worktree-safety.test.cjs
index 02cd6f9c6..ecd24c2ee 100644
--- a/tests/worktree-safety.test.cjs
+++ b/tests/worktree-safety.test.cjs
@@ -6502,11 +6502,30 @@ describe('execute-phase.md dispatch wires USE_WORKTREES_FOR_PLAN (#2772)', () =>
   });
 
   test('"Worktrees disabled" sequential rule is documented per-plan, not project-level', () => {
+    // #4254 moved the wave-serialization rules into the sequential-root-pin step
+    // fragment (ADR-857 Phase 6 frozen ceiling — the host step cannot grow), so the
+    // rule is asserted where it now lives AND that the host step still wires the
+    // fragment in at the Sequential mode branch.
+    const pinFragmentPath = path.join(
+      __dirname,
+      '..',
+      'gsd-core',
+      'workflows',
+      'execute-phase',
+      'steps',
+      'sequential-root-pin.md'
+    );
+    const frag = fs.readFileSync(pinFragmentPath, 'utf-8');
+    assert.match(
+      frag,
+      /worktrees are disabled for a plan/i,
+      'sequential-execution rule must be expressed per-plan (in the sequential-root-pin fragment)'
+    );
     const md = fs.readFileSync(workflowPath, 'utf-8');
     assert.match(
       md,
-      /worktrees are disabled for a plan/i,
-      'sequential-execution rule must be expressed per-plan'
+      /execute-phase\/steps\/sequential-root-pin\.md/,
+      'execute-phase.md must wire the sequential-root-pin fragment at the Sequential mode branch'
     );
   });