diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index ca1bf511d..a7555357c 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -90,9 +90,11 @@ jobs:
           cache: 'npm'
 
       - name: Install dependencies
+        shell: bash
         run: npm ci
 
       - name: Build SDK dist (required by installer)
+        shell: bash
         run: npm run build:sdk
 
       # Seam contract gate: keep manifest -> generated aliases -> registry/CJS adapters aligned.
@@ -214,8 +216,10 @@ jobs:
           node-version: 24
           cache: 'npm'
       - name: Install dependencies
+        shell: bash
         run: npm ci
       - name: Build SDK dist
+        shell: bash
         run: npm run build:sdk
       - name: Unit coverage
         shell: bash
diff --git a/tests/workflow-shell-pinning.test.cjs b/tests/workflow-shell-pinning.test.cjs
new file mode 100644
index 000000000..1d0721440
--- /dev/null
+++ b/tests/workflow-shell-pinning.test.cjs
@@ -0,0 +1,395 @@
+'use strict';
+
+process.env.GSD_TEST_MODE = '1';
+
+/**
+ * Asserts that every `run:` step whose command begins with `npm ` in any
+ * .github/workflows/*.yml file has an effective `shell:` directive.
+ *
+ * "Effective shell" is resolved as:
+ *   step.shell ?? job.defaults.run.shell ?? workflow.defaults.run.shell
+ *
+ * Without an effective shell, GitHub Actions defaults to pwsh on
+ * windows-latest.  The npm.cmd → node.exe → npm-cli.js child-process chain
+ * under pwsh can swallow stderr, making `npm ci` / `npm run` failures
+ * invisible in CI logs.
+ *
+ * Scope: only workflow files that reference windows-latest (directly as a
+ * literal `runs-on:` value, or as a member of a `strategy.matrix.os` list).
+ * Steps in jobs that cannot run on Windows cannot trigger the class of failure
+ * described above, but the file must still be scanned once any job in it
+ * includes a windows target (to catch unshelled npm steps in sibling jobs that
+ * could be copy-pasted to a windows context).
+ *
+ * Acceptable shell values: bash, pwsh, sh, cmd.
+ */
+
+const { test, describe } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const REPO_ROOT = path.join(__dirname, '..');
+const WORKFLOWS_DIR = path.join(REPO_ROOT, '.github', 'workflows');
+
+/**
+ * Collect all .yml / .yaml files under the workflows directory.
+ *
+ * Only files that reference "windows-latest" (as a literal runs-on value or
+ * inside a matrix.os list) are returned — the pwsh-stderr-swallow failure
+ * class can only manifest in workflows that target Windows runners.
+ */
+function listWorkflowFiles() {
+  const entries = fs.readdirSync(WORKFLOWS_DIR);
+  const all = entries
+    .filter((e) => /\.ya?ml$/.test(e))
+    .map((e) => path.join(WORKFLOWS_DIR, e));
+
+  // Filter to files that have at least one windows-latest reference.
+  // allow-test-rule: file-scope prefilter, not a test assertion — we need to
+  // detect whether a workflow file targets Windows runners at all. The pwsh
+  // stderr-swallow class is windows-only, so files that never mention
+  // windows-latest are out of scope. Exposing a typed IR from production
+  // code is not appropriate here because the source-of-truth is the YAML
+  // itself; the actual test assertions below ARE structural (parse runs-on,
+  // strategy.matrix.os, defaults.run.shell, etc.).
+  return all.filter((f) => {
+    const raw = fs.readFileSync(f, 'utf8');
+    return raw.indexOf('windows-latest') !== -1;
+  });
+}
+
+/**
+ * Return the number of leading spaces in a line.
+ */
+function indentOf(line) {
+  const m = line.match(/^(\s*)/);
+  return m ? m[1].length : 0;
+}
+
+/**
+ * Parse a workflow YAML file with a line-based scanner.
+ *
+ * Returns an array of violation objects:
+ *   { file, job, stepIndex, stepName, runLine }
+ *
+ * A violation is a `run:` step whose command starts with `npm ` and that
+ * does NOT have an effective `shell:` directive.  Effective shell is:
+ *   step.shell ?? job.defaults.run.shell ?? workflow.defaults.run.shell
+ *
+ * Strategy:
+ * 1. Walk lines top-to-bottom tracking workflow-level defaults.run.shell.
+ * 2. Track job keys (jobs.<key>) and their defaults.run.shell.
+ * 3. Detect step boundaries: a line matching /^\s+-\s+(name:|uses:|run:)/ at
+ *    "step list" indentation (8 spaces for most workflows, detected
+ *    dynamically) opens a new step context.
+ * 4. Within a step context, collect all keys (name, run, shell, uses, …).
+ * 5. At the END of each step context (next step boundary or end-of-job),
+ *    emit a violation if `run` starts with `npm ` and effective shell is null.
+ */
+function findViolations(filePath) {
+  const relFile = path.relative(REPO_ROOT, filePath);
+  const lines = fs.readFileSync(filePath, 'utf8').split(/\r?\n/);
+  const violations = [];
+
+  // Workflow-level defaults.run.shell
+  let workflowDefaultShell = null;
+
+  // ── State machine ─────────────────────────────────────────────────────────
+  let inJobs = false;
+  let currentJob = null;
+  let jobDefaultShell = null;   // job-level defaults.run.shell
+
+  // Section tracking for defaults blocks
+  // We need to detect:
+  //   defaults:         (at col 0 = workflow level, or col 2 = job level)
+  //     run:
+  //       shell: bash
+  let inDefaultsBlock = false;    // currently inside a `defaults:` mapping
+  let inDefaultsRunBlock = false; // currently inside `defaults: run:`
+  let defaultsBlockOwner = null;  // 'workflow' or 'job'
+  let defaultsBlockCol = null;    // column of the `defaults:` key
+
+  // Strategy/matrix tracking
+  let inStrategyBlock = false;
+  let inMatrixBlock = false;
+  let inMatrixOsBlock = false;
+  let strategyCol = null;
+  let matrixCol = null;
+
+  // Step tracking
+  let stepIndent = null;      // indent level of the `- name:/run:/uses:` items
+  let inStep = false;
+  let stepIndex = -1;
+  let stepProps = null;       // { name, run, shell }
+
+  /**
+   * Flush the current step: emit a violation if it qualifies.
+   * Effective shell = step.shell ?? jobDefaultShell ?? workflowDefaultShell
+   */
+  function flushStep() {
+    if (!inStep || stepProps === null) return;
+    const { name, run, shell } = stepProps;
+    const effectiveShell = shell !== null ? shell
+      : jobDefaultShell !== null ? jobDefaultShell
+      : workflowDefaultShell;
+    if (run !== null && /^\s*(?:npm|npx)(\s|$)/.test(run) && effectiveShell === null) {
+      violations.push({
+        file: relFile,
+        job: currentJob,
+        stepIndex,
+        stepName: name || '(unnamed)',
+      });
+    }
+    inStep = false;
+    stepProps = null;
+  }
+
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
+    const trimmed = line.trimStart();
+    const col = indentOf(line);
+
+    // Skip blank lines and comments
+    if (trimmed === '' || trimmed.startsWith('#')) continue;
+
+    // ── Workflow-level `defaults:` block (col 0) ──────────────────────────
+    // Detect `defaults:` at the root level (before `jobs:`)
+    if (!inJobs && /^defaults\s*:/.test(line)) {
+      inDefaultsBlock = true;
+      inDefaultsRunBlock = false;
+      defaultsBlockOwner = 'workflow';
+      defaultsBlockCol = 0;
+      continue;
+    }
+
+    if (inDefaultsBlock && defaultsBlockOwner === 'workflow') {
+      // A key at col 0 that isn't blank/comment ends the defaults block
+      if (col === 0 && !/^\s/.test(line)) {
+        inDefaultsBlock = false;
+        inDefaultsRunBlock = false;
+      } else if (/^\s+run\s*:/.test(line) && col === 2) {
+        inDefaultsRunBlock = true;
+      } else if (inDefaultsRunBlock && /^\s+shell\s*:\s*(\S+)/.test(line)) {
+        const m = line.match(/^\s+shell\s*:\s*(\S+)/);
+        if (m) workflowDefaultShell = m[1];
+      }
+    }
+
+    // ── `jobs:` section ───────────────────────────────────────────────────
+    if (/^jobs\s*:/.test(line)) {
+      inJobs = true;
+      inDefaultsBlock = false;
+      inDefaultsRunBlock = false;
+      continue;
+    }
+
+    if (!inJobs) continue;
+
+    // ── Job-level keys at indent 2 ────────────────────────────────────────
+    if (col === 2 && /^[a-zA-Z0-9_-]+\s*:/.test(trimmed)) {
+      flushStep();
+      currentJob = trimmed.replace(/\s*:.*/, '');
+      stepIndent = null;
+      inStep = false;
+      stepIndex = -1;
+      jobDefaultShell = null;
+      // Reset sub-section tracking
+      inDefaultsBlock = false;
+      inDefaultsRunBlock = false;
+      inStrategyBlock = false;
+      inMatrixBlock = false;
+      inMatrixOsBlock = false;
+      continue;
+    }
+
+    if (currentJob === null) continue;
+
+    // ── Job-level `defaults:` block (col 4) ──────────────────────────────
+    if (col === 4 && /^defaults\s*:/.test(trimmed)) {
+      inDefaultsBlock = true;
+      inDefaultsRunBlock = false;
+      defaultsBlockOwner = 'job';
+      defaultsBlockCol = 4;
+      continue;
+    }
+
+    if (inDefaultsBlock && defaultsBlockOwner === 'job') {
+      if (col <= 4 && !/^\s{5}/.test(line)) {
+        // Back to job level or above — end defaults block
+        inDefaultsBlock = false;
+        inDefaultsRunBlock = false;
+      } else if (col === 6 && /^run\s*:/.test(trimmed)) {
+        inDefaultsRunBlock = true;
+      } else if (inDefaultsRunBlock && col === 8 && /^shell\s*:\s*(\S+)/.test(trimmed)) {
+        const m = trimmed.match(/^shell\s*:\s*(\S+)/);
+        if (m) jobDefaultShell = m[1];
+      }
+    }
+
+    // ── Step list detection ───────────────────────────────────────────────
+    const stepStartMatch = line.match(
+      /^(\s+)-\s+(name|run|uses|shell|if|id|env|with|continue-on-error|timeout-minutes|working-directory)\s*[:\|]/,
+    );
+    if (stepStartMatch) {
+      const thisIndent = stepStartMatch[1].length;
+
+      if (stepIndent === null) {
+        stepIndent = thisIndent;
+      }
+
+      if (thisIndent === stepIndent) {
+        // New step boundary
+        flushStep();
+        stepIndex += 1;
+        inStep = true;
+        stepProps = { name: null, run: null, shell: null };
+        // Reset defaults sub-tracking when we enter the steps section
+        inDefaultsBlock = false;
+        inDefaultsRunBlock = false;
+
+        // Parse the key on this same line
+        const keyMatch = line.match(/^\s+-\s+(name|run|shell|uses)\s*:\s*(.*)/);
+        if (keyMatch) {
+          const key = keyMatch[1];
+          const val = keyMatch[2].trim();
+          if (key === 'name') stepProps.name = val || null;
+          else if (key === 'run') stepProps.run = val || null;
+          else if (key === 'shell') stepProps.shell = val || null;
+        }
+        continue;
+      }
+    }
+
+    // ── Inside a step: parse continuation key-value pairs ─────────────────
+    if (inStep && stepIndent !== null && col > stepIndent) {
+      const kvMatch = line.match(/^\s+(name|run|shell|uses)\s*:\s*(.*)/);
+      if (kvMatch) {
+        const key = kvMatch[1];
+        const val = kvMatch[2].trim();
+        if (key === 'run') {
+          if (val && val !== '|') {
+            stepProps.run = val;
+          } else {
+            // Multi-line run block — find first non-empty continuation line
+            let j = i + 1;
+            while (j < lines.length) {
+              const contLine = lines[j];
+              const contTrimmed = contLine.trimStart();
+              if (contTrimmed === '' || contTrimmed.startsWith('#')) { j++; continue; }
+              if (indentOf(contLine) <= col) break;
+              stepProps.run = contTrimmed;
+              break;
+            }
+          }
+        } else if (key === 'shell') {
+          stepProps.shell = val || null;
+        } else if (key === 'name') {
+          stepProps.name = val || null;
+        }
+      }
+    }
+  }
+
+  // Flush the last step
+  flushStep();
+
+  return violations;
+}
+
+// ── Unit helper: scanner exercised against a synthetic YAML string ──────────
+
+/**
+ * Parse violation list from a raw YAML string (written to a temp file).
+ * Used by the defaults.run.shell unit test below.
+ */
+function findViolationsInString(yamlContent) {
+  const tmpPath = path.join(require('os').tmpdir(), `gsd-shell-test-${process.pid}.yml`);
+  fs.writeFileSync(tmpPath, yamlContent, 'utf8');
+  try {
+    return findViolations(tmpPath);
+  } finally {
+    fs.unlinkSync(tmpPath);
+  }
+}
+
+// ── Test suite ──────────────────────────────────────────────────────────────
+
+describe('GitHub Actions workflow shell pinning', () => {
+  test('npm ci/run steps in workflow files must pin shell', () => {
+    const workflowFiles = listWorkflowFiles();
+    assert.ok(workflowFiles.length > 0, 'No windows-targeting workflow files found — check WORKFLOWS_DIR path');
+
+    const allViolations = [];
+    for (const wf of workflowFiles) {
+      const v = findViolations(wf);
+      allViolations.push(...v);
+    }
+
+    if (allViolations.length > 0) {
+      const details = allViolations.map(
+        (v) => `  jobs.${v.job}.steps[${v.stepIndex}].name = ${v.stepName}  (${v.file})`,
+      ).join('\n');
+      assert.fail(
+        `${allViolations.length} npm run/ci step(s) are missing an explicit shell: directive.\n` +
+        `On windows-latest, steps without shell: default to pwsh, which can swallow npm stderr.\n` +
+        `Add  shell: bash  (or another explicit shell) to each listed step:\n\n` +
+        details,
+      );
+    }
+  });
+
+  test('workflow-level defaults.run.shell satisfies shell requirement', () => {
+    // A workflow with defaults.run.shell: bash at the root level should NOT
+    // produce violations for npm steps that lack their own shell: directive.
+    const yaml = `
+name: Test
+on: push
+defaults:
+  run:
+    shell: bash
+jobs:
+  build:
+    runs-on: windows-latest
+    steps:
+      - name: Install
+        run: npm ci
+      - name: Build
+        run: npm run build
+`.trimStart();
+    const violations = findViolationsInString(yaml);
+    assert.strictEqual(
+      violations.length,
+      0,
+      `Expected 0 violations when workflow-level defaults.run.shell is set, got:\n` +
+        violations.map((v) => `  steps[${v.stepIndex}] ${v.stepName}`).join('\n'),
+    );
+  });
+
+  test('job-level defaults.run.shell satisfies shell requirement', () => {
+    // A job with defaults.run.shell: bash should NOT produce violations for
+    // npm steps in that job that lack their own shell: directive.
+    const yaml = `
+name: Test
+on: push
+jobs:
+  build:
+    runs-on: windows-latest
+    defaults:
+      run:
+        shell: bash
+    steps:
+      - name: Install
+        run: npm ci
+      - name: Build
+        run: npm run build
+`.trimStart();
+    const violations = findViolationsInString(yaml);
+    assert.strictEqual(
+      violations.length,
+      0,
+      `Expected 0 violations when job-level defaults.run.shell is set, got:\n` +
+        violations.map((v) => `  steps[${v.stepIndex}] ${v.stepName}`).join('\n'),
+    );
+  });
+});