diff --git a/.changeset/3170-graphify-commit-staleness.md b/.changeset/3170-graphify-commit-staleness.md
new file mode 100644
index 000000000..3329c2b67
--- /dev/null
+++ b/.changeset/3170-graphify-commit-staleness.md
@@ -0,0 +1,4 @@
+---
+type: Enhancement
+---
+**`/gsd-graphify status` surfaces graphify v0.7+ commit-based staleness (#3170)** — `graphifyStatus()` now reads `built_at_commit` from `graph.json` (written by graphify v0.7+ at build time), compares it against `git HEAD`, and returns four new fields: `built_at_commit`, `current_commit`, `commits_behind`, and `commit_stale`. The `commit_stale` flag is tri-state — `true` / `false` / `null`, where `null` means the signal is unavailable (pre-v0.7 graph, non-git checkout, or unreachable commit) and callers should fall back to the existing mtime-based `stale` flag. The skill renders `Source commit: <hash> (N commits behind HEAD | current | freshness unknown)` when the signal is present, and omits the line entirely for pre-v0.7 graphs. The `built_at_commit` value is validated as 4–40 hex chars before reaching `git`, so a hostile `graph.json` cannot smuggle dashed options (e.g. `--upload-pack=…`) into the argv. Also documents `graphify hook install` in `docs/CONFIGURATION.md` for multi-dev teams who would otherwise hit `graph.json` merge conflicts on parallel rebuilds.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3af7e8a1d..6754c6610 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,6 +55,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ### Enhancement
 
+- **`/gsd-graphify status` surfaces commit-based staleness from graphify v0.7+** — `graphifyStatus()` now reads `built_at_commit` from `graph.json` (graphify v0.7+ embeds it at build time), compares against `git HEAD`, and returns four new fields: `built_at_commit`, `current_commit`, `commits_behind`, and `commit_stale`. The `commit_stale` flag is tri-state (`true`/`false`/`null`) — `null` means the signal is unavailable (pre-v0.7 graph, non-git checkout, or unreachable commit) and callers should fall back to the existing mtime-based `stale` flag. The skill renders `Source commit: <hash> (N commits behind HEAD | current | freshness unknown)` when the signal is present, and omits the line entirely for pre-v0.7 graphs. The `built_at_commit` value is validated as 4–40 hex chars before reaching `git`, so a hostile `graph.json` cannot smuggle dashed options into the argv. Also documents `graphify hook install` in `docs/CONFIGURATION.md` for multi-dev teams who would otherwise hit `graph.json` merge conflicts on parallel rebuilds. Regression covered by `tests/enh-3170-graphify-commit-staleness.test.cjs` (8 assertions across git-aware, non-git, and back-compat groups). (#3170)
 - **Test suite for `config-schema.cjs` is now mutation-resistant** — Stryker measured a 4.62% mutation score on `get-shit-done/bin/lib/config-schema.cjs` (6 killed, 124 survived out of 130). Surviving mutants flagged that existing tests were exercising paths but not verifying outputs: a polarity flip (`return true` → `return false`), a predicate swap (`.some` → `.every`), or a guard removal (`if (VALID_CONFIG_KEYS.has(...)) return true;` → unguarded fallthrough) all passed every test. New `tests/bug-2986-config-schema-mutation-killers.test.cjs` adds 95 tests across four suites that target each surviving mutant class: (1) parameterized `isValidConfigKey('${key}') === true` for every member of `VALID_CONFIG_KEYS` (kills the static-key-fast-path mutation), (2) representative dynamic-pattern keys that match exactly one pattern (kills the `.some` → `.every` mutation, with an inline mutual-exclusivity invariant check), (3) `strictEqual` against the literal boolean `true`/`false` instead of `assert.ok` truthy checks (kills polarity-flip mutations), (4) anchor-tightening cases that differ from valid keys by one character beyond the documented shape (kills regex-loosening mutations on `^`, `$`, and character-class boundaries). Tests use the lib's public surface (typed boolean assertions on `isValidConfigKey` return values), no source-grep. (#2986)
 - **Hotfix release flow now auto-incorporates fixes from `main` and bundles the SDK** — `hotfix.yml create` auto-cherry-picks every `fix:`/`chore:` commit on `origin/main` not yet shipped (oldest-first; patch-equivalents skipped via `git cherry`; `feat:`/`refactor:` excluded; conflicts halt with the offending SHA; run summary lists every included SHA). `hotfix.yml finalize` adds the `install-smoke` cross-platform gate, bundles `sdk-bundle/gsd-sdk.tgz` inside the CC tarball (parity with `release-sdk.yml`), tightens the `next` dist-tag re-point, and marks the GitHub Release `--latest`. `release-sdk.yml` gains `action: publish | hotfix` plus an `auto_cherry_pick` toggle, with a new `prepare` job that branches `hotfix/X.YY.Z` from the highest existing `vX.YY.*` tag and runs the same cherry-pick logic — idempotent if the branch was pre-prepared via `hotfix.yml`. Hotfix `vX.YY.Z` is now defined as everything in `vX.YY.{Z-1}` plus every `fix:`/`chore:` since that base, so each tag is the cumulative-fix anchor for the next. (#2955)
 - **Planning workspace seam extracted from `core.cjs` into `planning-workspace.cjs`** — path/workstream/lock behavior now lives in a dedicated module (`planningDir`, `planningPaths`, `planningRoot`, active-workstream routing, `withPlanningLock`). `core.cjs` keeps compatibility re-exports while call-sites migrate to direct imports, improving locality and reducing coupling. (#2900)
diff --git a/commands/gsd/graphify.md b/commands/gsd/graphify.md
index b441c2727..b549ca55f 100644
--- a/commands/gsd/graphify.md
+++ b/commands/gsd/graphify.md
@@ -100,6 +100,16 @@ node $HOME/.claude/get-shit-done/bin/gsd-tools.cjs graphify status
 Parse the JSON output and display:
 - If `exists: false`, display the message field
 - Otherwise show last build time, node/edge/hyperedge counts, and STALE or FRESH indicator
+- If `built_at_commit` is non-null, also display a `Source commit:` line:
+  - `commit_stale === false` (rebuilt at HEAD): `Source commit: <built_at_commit> (current)`
+  - `commit_stale === true` (graph behind HEAD): `Source commit: <built_at_commit> (<commits_behind> commits behind HEAD)`
+  - `commit_stale === null` (unreachable commit / no git): `Source commit: <built_at_commit> (freshness unknown)`
+- If `built_at_commit` is null (pre-graphify-v0.7 graph), omit the source-commit line entirely — do not render "Source commit: unknown"
+
+The mtime-based STALE/FRESH flag and the commit-based `commit_stale` measure
+different things and can disagree (e.g., a CI-built graph rebuilt minutes ago
+against an old checkout reads as FRESH on mtime but `commit_stale: true`).
+Surface both so the agent can choose.
 
 **STOP** after displaying status. Do not spawn an agent.
 
diff --git a/docs/CONFIGURATION.md b/docs/CONFIGURATION.md
index 9e3bf53f6..cd502f6e3 100644
--- a/docs/CONFIGURATION.md
+++ b/docs/CONFIGURATION.md
@@ -362,6 +362,38 @@ Toggle optional capabilities via the `features.*` config namespace. Feature flag
 | `graphify.enabled` | boolean | `false` | Enable the project knowledge graph. When `true`, `/gsd-graphify` builds and queries a graph in `.planning/graphs/`. Added in v1.36 |
 | `graphify.build_timeout` | number (seconds) | `300` | Maximum seconds allowed for a `/gsd-graphify build` run before it aborts. Added in v1.36 |
 
+#### Multi-developer setup
+
+If multiple developers will rebuild the graph in the same repo, run once per
+clone after enabling graphify:
+
+```bash
+graphify hook install
+```
+
+This installs a git merge driver that union-merges concurrent `graph.json`
+writes (no conflict markers in the knowledge graph), plus the post-commit
+rebuild hook. It writes `.gitattributes` and registers `graphify
+merge-driver` in `.git/config`. Solo projects can skip this step; running it
+anyway is harmless. Introduced upstream in graphify v0.7.0 alongside the
+`built_at_commit` freshness signal that `/gsd-graphify status` surfaces.
+
+#### Commit-based staleness
+
+`/gsd-graphify status` reports two orthogonal staleness signals:
+
+- **`stale`** (mtime-based, 24-hour window) — when the graph file was last
+  written. Useful when graphify isn't run automatically.
+- **`commit_stale`** (commit-based, requires graphify v0.7+) — whether the
+  graph was built against the current `git HEAD`. Trustworthy when present.
+  Tri-state: `true` / `false` / `null`. `null` means the signal is
+  unavailable (pre-v0.7 graph, no git, or unreachable commit) — fall back
+  to the mtime flag.
+
+A CI-built graph rebuilt minutes ago against an old checkout will read as
+fresh on mtime but `commit_stale: true`. Surface both when answering
+architecture questions.
+
 ### Usage
 
 ```bash
diff --git a/get-shit-done/bin/lib/graphify.cjs b/get-shit-done/bin/lib/graphify.cjs
index 0c375a944..721166eb7 100644
--- a/get-shit-done/bin/lib/graphify.cjs
+++ b/get-shit-done/bin/lib/graphify.cjs
@@ -3,7 +3,7 @@
 const fs = require('fs');
 const path = require('path');
 const childProcess = require('child_process');
-const { atomicWriteFileSync } = require('./core.cjs');
+const { atomicWriteFileSync, execGit } = require('./core.cjs');
 
 // ─── Config Gate ─────────────────────────────────────────────────────────────
 
@@ -358,9 +358,44 @@ function graphifyQuery(cwd, term, options = {}) {
   };
 }
 
+/**
+ * Strict 4-40 hex fence for graph.built_at_commit values (#3170). Anything
+ * else (dashed, prose, empty) is treated as absent so a hostile graph.json
+ * cannot smuggle a `--upload-pack=…` option into a `git` argv.
+ */
+const COMMIT_HASH_RE = /^[0-9a-f]{4,40}$/i;
+
+/**
+ * Read git HEAD for the project at `cwd`. Returns the full commit hash on
+ * success, or null when cwd is not a git repo / `git` is not on PATH.
+ */
+function readGitHead(cwd) {
+  const r = execGit(cwd, ['rev-parse', 'HEAD']);
+  if (r.exitCode !== 0) return null;
+  return r.stdout.trim() || null;
+}
+
+/**
+ * Count commits between `from` and `to` (exclusive..inclusive, like
+ * `git rev-list --count A..B`). Returns null when either ref is unreachable
+ * or the cwd is not a git repo.
+ */
+function countCommitsBetween(cwd, from, to) {
+  const r = execGit(cwd, ['rev-list', '--count', `${from}..${to}`]);
+  if (r.exitCode !== 0) return null;
+  const n = parseInt(r.stdout.trim(), 10);
+  return Number.isFinite(n) ? n : null;
+}
+
 /**
  * Return status information about the knowledge graph (STAT-01, STAT-02).
  *
+ * Surfaces the graphify v0.7+ commit-staleness signal as four optional
+ * fields when graph.built_at_commit is present and validly formatted
+ * (#3170). Tri-state on commit_stale: null means "we don't know" (pre-v0.7
+ * graph, no git, or unreachable commit), distinct from false ("known
+ * fresh").
+ *
  * @param {string} cwd - Working directory
  * @returns {object}
  */
@@ -382,6 +417,17 @@ function graphifyStatus(cwd) {
   const STALE_MS = 24 * 60 * 60 * 1000; // 24 hours
   const age = Date.now() - stat.mtimeMs;
 
+  // Commit-staleness signal (#3170). Validate before passing to git.
+  const rawBuilt = (graph.built_at_commit || '').toString().trim();
+  const builtAt = COMMIT_HASH_RE.test(rawBuilt) ? rawBuilt : null;
+  const head = readGitHead(cwd);
+  let commitsBehind = null;
+  let commitStale = null;
+  if (builtAt && head) {
+    commitsBehind = countCommitsBetween(cwd, builtAt, head);
+    if (commitsBehind !== null) commitStale = commitsBehind > 0;
+  }
+
   return {
     exists: true,
     last_build: stat.mtime.toISOString(),
@@ -390,6 +436,10 @@ function graphifyStatus(cwd) {
     hyperedge_count: (graph.hyperedges || []).length,
     stale: age > STALE_MS,
     age_hours: Math.round(age / (60 * 60 * 1000)),
+    built_at_commit: builtAt ? builtAt.slice(0, 7) : null,
+    current_commit: head ? head.slice(0, 7) : null,
+    commits_behind: commitsBehind,
+    commit_stale: commitStale,
   };
 }
 
diff --git a/tests/enh-3170-graphify-commit-staleness.test.cjs b/tests/enh-3170-graphify-commit-staleness.test.cjs
new file mode 100644
index 000000000..1013df880
--- /dev/null
+++ b/tests/enh-3170-graphify-commit-staleness.test.cjs
@@ -0,0 +1,224 @@
+'use strict';
+
+/**
+ * Contract for the #3170 commit-staleness signal on graphifyStatus().
+ *
+ * graphify v0.7+ embeds `built_at_commit` (full git HEAD) into graph.json at
+ * write time. GSD's status used to be mtime-only, a poor proxy for "does
+ * this graph reflect the current code." This suite fences the four new
+ * fields surfaced by graphifyStatus():
+ *
+ *   built_at_commit  short hash from graph.built_at_commit, or null
+ *   current_commit   short hash of HEAD, or null if cwd is not a git repo
+ *   commits_behind   git rev-list --count <built>..HEAD, or null
+ *   commit_stale     boolean, true if commits_behind > 0; null when unknown
+ *
+ * Tri-state on commit_stale is load-bearing: null means "we don't know"
+ * (pre-v0.7 graph or no git), which is semantically distinct from false
+ * ("known fresh"). Agents reading null should fall back to mtime; reading
+ * false can confidently skip a rebuild.
+ */
+
+const { describe, test, beforeEach, afterEach } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const path = require('path');
+const { execFileSync } = require('child_process');
+const { createTempProject, createTempGitProject, cleanup } = require('./helpers.cjs');
+const { graphifyStatus } = require('../get-shit-done/bin/lib/graphify.cjs');
+
+function enableGraphify(planningDir) {
+  const cfgPath = path.join(planningDir, 'config.json');
+  const cfg = fs.existsSync(cfgPath) ? JSON.parse(fs.readFileSync(cfgPath, 'utf8')) : {};
+  cfg.graphify = { enabled: true };
+  fs.writeFileSync(cfgPath, JSON.stringify(cfg, null, 2));
+}
+
+function writeGraph(planningDir, data) {
+  const graphsDir = path.join(planningDir, 'graphs');
+  fs.mkdirSync(graphsDir, { recursive: true });
+  fs.writeFileSync(path.join(graphsDir, 'graph.json'), JSON.stringify(data, null, 2));
+}
+
+function gitHead(cwd) {
+  return execFileSync('git', ['rev-parse', 'HEAD'], { cwd, encoding: 'utf-8' }).trim();
+}
+
+function commitEmpty(cwd, message) {
+  execFileSync('git', ['commit', '--allow-empty', '-m', message], { cwd, stdio: 'pipe' });
+}
+
+const SAMPLE_NODES = [
+  { id: 'n1', label: 'A', description: '', type: 'service' },
+  { id: 'n2', label: 'B', description: '', type: 'model' },
+];
+
+describe('enh-3170: graphifyStatus surfaces built_at_commit staleness', () => {
+  let tmpDir;
+  let planningDir;
+
+  // ──────────────────────────────────────────────────────────────────
+  // Group 1 — git-aware cases (real git repo via createTempGitProject)
+  // ──────────────────────────────────────────────────────────────────
+
+  describe('git-aware', () => {
+    beforeEach(() => {
+      tmpDir = createTempGitProject();
+      planningDir = path.join(tmpDir, '.planning');
+      enableGraphify(planningDir);
+    });
+
+    afterEach(() => cleanup(tmpDir));
+
+    test('graph rebuilt at HEAD: commits_behind=0, commit_stale=false', () => {
+      const head = gitHead(tmpDir);
+      writeGraph(planningDir, { nodes: SAMPLE_NODES, edges: [], built_at_commit: head });
+
+      const result = graphifyStatus(tmpDir);
+
+      assert.equal(result.built_at_commit, head.slice(0, 7),
+        'short hash from graph.built_at_commit');
+      assert.equal(result.current_commit, head.slice(0, 7),
+        'short hash of git HEAD');
+      assert.equal(result.commits_behind, 0,
+        'zero commits between HEAD and itself');
+      assert.equal(result.commit_stale, false,
+        'commit_stale is explicitly false when commits_behind === 0');
+    });
+
+    test('graph 5 commits behind HEAD: commits_behind=5, commit_stale=true', () => {
+      const built = gitHead(tmpDir);
+      for (let i = 0; i < 5; i += 1) commitEmpty(tmpDir, `c${i}`);
+      writeGraph(planningDir, { nodes: SAMPLE_NODES, edges: [], built_at_commit: built });
+
+      const result = graphifyStatus(tmpDir);
+
+      assert.equal(result.commits_behind, 5);
+      assert.equal(result.commit_stale, true);
+      assert.equal(result.built_at_commit, built.slice(0, 7));
+      assert.notEqual(result.current_commit, built.slice(0, 7),
+        'current_commit reflects HEAD, not graph build commit');
+    });
+
+    test('built_at_commit absent (pre-v0.7 graph): all four new fields null', () => {
+      // No built_at_commit on the graph -- GSD must not fabricate one.
+      writeGraph(planningDir, { nodes: SAMPLE_NODES, edges: [] });
+
+      const result = graphifyStatus(tmpDir);
+
+      assert.equal(result.built_at_commit, null);
+      assert.equal(result.commits_behind, null);
+      assert.equal(result.commit_stale, null,
+        'tri-state: null means "we do not know", not "fresh"');
+      // current_commit may still be non-null since we are in a git repo,
+      // but without a baseline it cannot drive staleness.
+      assert.notEqual(result.current_commit, undefined,
+        'current_commit field is always present even when null');
+    });
+
+    test('rebased-away built_at_commit: commits_behind=null, commit_stale=null', () => {
+      // built_at_commit references a commit that never existed in this repo.
+      const ghostHash = '0000000000000000000000000000000000000001';
+      writeGraph(planningDir, { nodes: SAMPLE_NODES, edges: [], built_at_commit: ghostHash });
+
+      const result = graphifyStatus(tmpDir);
+
+      assert.equal(result.built_at_commit, ghostHash.slice(0, 7),
+        'echoes the field even if unreachable -- caller can decide what to do');
+      assert.equal(result.commits_behind, null,
+        'cannot count commits to an unreachable commit');
+      assert.equal(result.commit_stale, null,
+        'unknown distance means unknown staleness');
+    });
+
+    test('malformed built_at_commit (dashed argv): rejected before git invocation', () => {
+      // Argument-injection fence: a graph.json with a hostile built_at_commit
+      // must never reach `git` as an argv element. The implementation should
+      // validate /^[0-9a-f]{4,40}$/i and treat anything else as absent.
+      const malicious = '--upload-pack=evil';
+      writeGraph(planningDir, { nodes: SAMPLE_NODES, edges: [], built_at_commit: malicious });
+
+      const result = graphifyStatus(tmpDir);
+
+      assert.equal(result.built_at_commit, null,
+        'malformed value is rejected, not echoed');
+      assert.equal(result.commits_behind, null);
+      assert.equal(result.commit_stale, null);
+    });
+  });
+
+  // ──────────────────────────────────────────────────────────────────
+  // Group 2 — non-git cases (createTempProject, no .git/)
+  // ──────────────────────────────────────────────────────────────────
+
+  describe('non-git cwd', () => {
+    beforeEach(() => {
+      tmpDir = createTempProject();
+      planningDir = path.join(tmpDir, '.planning');
+      enableGraphify(planningDir);
+    });
+
+    afterEach(() => cleanup(tmpDir));
+
+    test('cwd has no .git: current_commit=null, derived fields=null', () => {
+      const built = 'abcdef1234567890abcdef1234567890abcdef12';
+      writeGraph(planningDir, { nodes: SAMPLE_NODES, edges: [], built_at_commit: built });
+
+      const result = graphifyStatus(tmpDir);
+
+      assert.equal(result.built_at_commit, built.slice(0, 7),
+        'graph field is echoed even without a local repo');
+      assert.equal(result.current_commit, null,
+        'no HEAD without git');
+      assert.equal(result.commits_behind, null);
+      assert.equal(result.commit_stale, null);
+    });
+  });
+
+  // ──────────────────────────────────────────────────────────────────
+  // Group 3 — back-compat fences for existing fields
+  // ──────────────────────────────────────────────────────────────────
+
+  describe('back-compat', () => {
+    beforeEach(() => {
+      tmpDir = createTempGitProject();
+      planningDir = path.join(tmpDir, '.planning');
+      enableGraphify(planningDir);
+      writeGraph(planningDir, {
+        nodes: SAMPLE_NODES,
+        edges: [{ source: 'n1', target: 'n2', label: 'x', confidence: 'EXTRACTED' }],
+        hyperedges: [],
+        built_at_commit: gitHead(tmpDir),
+      });
+    });
+
+    afterEach(() => cleanup(tmpDir));
+
+    test('existing fields are unchanged when commit-staleness fields are added', () => {
+      const result = graphifyStatus(tmpDir);
+
+      // Existing contract — must not regress.
+      assert.equal(result.exists, true);
+      assert.equal(result.node_count, 2);
+      assert.equal(result.edge_count, 1);
+      assert.equal(result.hyperedge_count, 0);
+      assert.equal(typeof result.last_build, 'string');
+      assert.equal(typeof result.stale, 'boolean',
+        'mtime-based stale flag stays as-is for back-compat');
+      assert.equal(typeof result.age_hours, 'number');
+    });
+
+    test('disabled response is unchanged (commit-staleness fields not added)', () => {
+      const tmp2 = createTempProject();
+      try {
+        const result = graphifyStatus(tmp2);
+        assert.equal(result.disabled, true,
+          'disabled path returns the existing shape, no commit fields');
+        assert.equal(result.built_at_commit, undefined,
+          'commit-staleness fields are only added on the success path');
+      } finally {
+        cleanup(tmp2);
+      }
+    });
+  });
+});