diff --git a/src/resources/extensions/gsd/git-service.ts b/src/resources/extensions/gsd/git-service.ts
index 88c07f50a..53c8d85a2 100644
--- a/src/resources/extensions/gsd/git-service.ts
+++ b/src/resources/extensions/gsd/git-service.ts
@@ -9,7 +9,7 @@
  */
 
 import { execFileSync, execSync } from "node:child_process";
-import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { existsSync, mkdirSync, readFileSync, readdirSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { gsdRoot } from "./paths.js";
 import { GIT_NO_PROMPT_ENV } from "./git-constants.js";
@@ -488,6 +488,29 @@ export class GitServiceImpl {
     // If .gsd/ IS in .gitignore (the default for external state projects),
     // git add -A already skips it and the exclusions are harmless no-ops.
     const allExclusions = [...RUNTIME_EXCLUSION_PATHS, ...extraExclusions];
+
+    // ── Parallel worker milestone scope (#1991) ──
+    // When GSD_MILESTONE_LOCK is set, this process is a parallel worker that
+    // must only commit files belonging to its own milestone. Exclude all other
+    // milestone directories from staging to prevent cross-milestone pollution
+    // (e.g., an M033 worker fabricating M032 artifacts in the same commit).
+    const milestoneLock = process.env.GSD_MILESTONE_LOCK;
+    if (milestoneLock) {
+      const msDir = join(gsdRoot(this.basePath), "milestones");
+      if (existsSync(msDir)) {
+        try {
+          const entries = readdirSync(msDir, { withFileTypes: true });
+          for (const entry of entries) {
+            if (entry.isDirectory() && entry.name !== milestoneLock) {
+              allExclusions.push(`.gsd/milestones/${entry.name}/`);
+            }
+          }
+        } catch {
+          // Best-effort — if we can't read the milestones dir, proceed without scoping
+        }
+      }
+    }
+
     nativeAddAllWithExclusions(this.basePath, allExclusions);
   }
 
diff --git a/src/resources/extensions/gsd/tests/parallel-commit-scope.test.ts b/src/resources/extensions/gsd/tests/parallel-commit-scope.test.ts
new file mode 100644
index 000000000..f7de95667
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/parallel-commit-scope.test.ts
@@ -0,0 +1,159 @@
+/**
+ * parallel-commit-scope.test.ts — Regression test for #1991.
+ *
+ * Parallel workers must only commit files belonging to their locked milestone.
+ * When GSD_MILESTONE_LOCK is set, smartStage() must exclude .gsd/milestones/<M>/
+ * directories for milestones other than the locked one.
+ *
+ * Without the fix, a worker for M033 can stage and commit fabricated artifacts
+ * under .gsd/milestones/M032/, causing cross-milestone pollution.
+ */
+
+import { describe, test, beforeEach, afterEach } from "node:test";
+import assert from "node:assert/strict";
+import {
+  mkdtempSync,
+  mkdirSync,
+  writeFileSync,
+  rmSync,
+} from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { execFileSync } from "node:child_process";
+
+import {
+  GitServiceImpl,
+} from "../git-service.ts";
+
+function run(command: string, cwd: string): string {
+  const [cmd, ...args] = command.split(" ");
+  return execFileSync(cmd, args, { cwd, stdio: ["ignore", "pipe", "pipe"], encoding: "utf-8" }).trim();
+}
+
+function gitRun(args: string[], cwd: string): string {
+  return execFileSync("git", args, { cwd, stdio: ["ignore", "pipe", "pipe"], encoding: "utf-8" }).trim();
+}
+
+function createFile(base: string, relPath: string, content: string): void {
+  const full = join(base, relPath);
+  mkdirSync(join(full, ".."), { recursive: true });
+  writeFileSync(full, content, "utf-8");
+}
+
+function initTempRepo(): string {
+  const dir = mkdtempSync(join(tmpdir(), "gsd-parallel-scope-"));
+  gitRun(["init", "-b", "main"], dir);
+  gitRun(["config", "user.name", "Test"], dir);
+  gitRun(["config", "user.email", "test@test.com"], dir);
+  createFile(dir, ".gitkeep", "");
+  gitRun(["add", "-A"], dir);
+  gitRun(["commit", "-m", "init"], dir);
+  return dir;
+}
+
+describe("parallel commit scope (#1991)", () => {
+  const savedEnv: Record<string, string | undefined> = {};
+
+  beforeEach(() => {
+    savedEnv.GSD_MILESTONE_LOCK = process.env.GSD_MILESTONE_LOCK;
+    savedEnv.GSD_PARALLEL_WORKER = process.env.GSD_PARALLEL_WORKER;
+  });
+
+  afterEach(() => {
+    for (const key of ["GSD_MILESTONE_LOCK", "GSD_PARALLEL_WORKER"] as const) {
+      if (savedEnv[key] === undefined) {
+        delete process.env[key];
+      } else {
+        process.env[key] = savedEnv[key];
+      }
+    }
+  });
+
+  test("autoCommit excludes other milestone directories when GSD_MILESTONE_LOCK is set", () => {
+    const repo = initTempRepo();
+
+    // Set up parallel worker environment for M033
+    process.env.GSD_MILESTONE_LOCK = "M033";
+    process.env.GSD_PARALLEL_WORKER = "1";
+
+    // Create dirty files in BOTH milestones (simulates cross-milestone pollution)
+    createFile(repo, ".gsd/milestones/M032/M032-SUMMARY.md", "# M032 Summary\nFabricated by M033 worker");
+    createFile(repo, ".gsd/milestones/M032/M032-VALIDATION.md", "# M032 Validation\nFabricated");
+    createFile(repo, ".gsd/milestones/M032/slices/S01/S01-SUMMARY.md", "Fabricated S01 summary");
+    createFile(repo, ".gsd/milestones/M033/slices/S01/tasks/T01-SUMMARY.md", "Legit T01 summary");
+    createFile(repo, "src/feature.ts", "export const x = 1;");
+
+    const svc = new GitServiceImpl(repo);
+    const msg = svc.autoCommit("complete-milestone", "M033/complete");
+    assert.ok(msg !== null, "autoCommit should produce a commit");
+
+    const committed = gitRun(["show", "--name-only", "HEAD"], repo);
+
+    // Source files and own milestone files SHOULD be committed
+    assert.ok(committed.includes("src/feature.ts"), "source files are committed");
+    assert.ok(committed.includes(".gsd/milestones/M033/"), "own milestone files are committed");
+
+    // Other milestone files MUST NOT be committed
+    assert.ok(!committed.includes(".gsd/milestones/M032/"),
+      "M032 files must NOT be committed by M033 worker — cross-milestone pollution (#1991)");
+
+    // Verify M032 files are still dirty (unstaged) in the working tree
+    const status = gitRun(["status", "--porcelain"], repo);
+    assert.ok(status.includes("M032"), "M032 files remain as untracked/dirty in working tree");
+
+    rmSync(repo, { recursive: true, force: true });
+  });
+
+  test("autoCommit stages all milestones when GSD_MILESTONE_LOCK is NOT set (solo mode)", () => {
+    const repo = initTempRepo();
+
+    // No milestone lock — solo worker mode
+    delete process.env.GSD_MILESTONE_LOCK;
+    delete process.env.GSD_PARALLEL_WORKER;
+
+    createFile(repo, ".gsd/milestones/M032/M032-SUMMARY.md", "# M032 Summary");
+    createFile(repo, ".gsd/milestones/M033/slices/S01/tasks/T01-SUMMARY.md", "T01 summary");
+    createFile(repo, "src/feature.ts", "export const x = 1;");
+
+    const svc = new GitServiceImpl(repo);
+    const msg = svc.autoCommit("complete-milestone", "M032/complete");
+    assert.ok(msg !== null, "autoCommit should produce a commit");
+
+    const committed = gitRun(["show", "--name-only", "HEAD"], repo);
+
+    // In solo mode, ALL milestone files should be committed
+    assert.ok(committed.includes(".gsd/milestones/M032/"), "M032 files committed in solo mode");
+    assert.ok(committed.includes(".gsd/milestones/M033/"), "M033 files committed in solo mode");
+    assert.ok(committed.includes("src/feature.ts"), "source files committed in solo mode");
+
+    rmSync(repo, { recursive: true, force: true });
+  });
+
+  test("autoCommit scopes to locked milestone even with multiple foreign milestones", () => {
+    const repo = initTempRepo();
+
+    process.env.GSD_MILESTONE_LOCK = "M035";
+    process.env.GSD_PARALLEL_WORKER = "1";
+
+    // Create files across many milestones
+    createFile(repo, ".gsd/milestones/M032/M032-SUMMARY.md", "foreign");
+    createFile(repo, ".gsd/milestones/M033/M033-SUMMARY.md", "foreign");
+    createFile(repo, ".gsd/milestones/M034/M034-SUMMARY.md", "foreign");
+    createFile(repo, ".gsd/milestones/M035/slices/S01/tasks/T01-SUMMARY.md", "own work");
+    createFile(repo, "src/app.ts", "export const app = {};");
+
+    const svc = new GitServiceImpl(repo);
+    const msg = svc.autoCommit("execute-task", "M035/S01/T01");
+    assert.ok(msg !== null, "autoCommit should produce a commit");
+
+    const committed = gitRun(["show", "--name-only", "HEAD"], repo);
+
+    assert.ok(committed.includes(".gsd/milestones/M035/"), "own milestone committed");
+    assert.ok(committed.includes("src/app.ts"), "source files committed");
+    assert.ok(!committed.includes(".gsd/milestones/M032/"), "M032 excluded");
+    assert.ok(!committed.includes(".gsd/milestones/M033/"), "M033 excluded");
+    assert.ok(!committed.includes(".gsd/milestones/M034/"), "M034 excluded");
+
+    rmSync(repo, { recursive: true, force: true });
+  });
+});