diff --git a/src/resources/extensions/gsd/doctor.ts b/src/resources/extensions/gsd/doctor.ts
index cf26589ad..c727c671e 100644
--- a/src/resources/extensions/gsd/doctor.ts
+++ b/src/resources/extensions/gsd/doctor.ts
@@ -11,6 +11,7 @@ import { RUNTIME_EXCLUSION_PATHS } from "./git-service.js";
 import { nativeIsRepo, nativeWorktreeRemove, nativeBranchList, nativeBranchDelete, nativeLsFiles, nativeRmCached } from "./native-git-bridge.js";
 import { readCrashLock, isLockProcessAlive, clearLock } from "./crash-recovery.js";
 import { ensureGitignore } from "./gitignore.js";
+import { readAllSessionStatuses, isSessionStale, removeSessionStatus } from "./session-status-io.js";
 
 export type DoctorSeverity = "info" | "warning" | "error";
 export type DoctorIssueCode =
@@ -37,6 +38,7 @@ export type DoctorIssueCode =
   | "tracked_runtime_files"
   | "legacy_slice_branches"
   | "stale_crash_lock"
+  | "stale_parallel_session"
   | "orphaned_completed_units"
   | "stale_hook_state"
   | "activity_log_bloat"
@@ -711,6 +713,31 @@ async function checkRuntimeHealth(
     // Non-fatal — crash lock check failed
   }
 
+  // ── Stale parallel sessions ────────────────────────────────────────────
+  try {
+    const parallelStatuses = readAllSessionStatuses(basePath);
+    for (const status of parallelStatuses) {
+      if (isSessionStale(status)) {
+        issues.push({
+          severity: "warning",
+          code: "stale_parallel_session",
+          scope: "project",
+          unitId: status.milestoneId,
+          message: `Stale parallel session for ${status.milestoneId} (PID ${status.pid}, started ${new Date(status.startedAt).toISOString()}, last heartbeat ${new Date(status.lastHeartbeat).toISOString()}) — process is no longer running`,
+          file: `.gsd/parallel/${status.milestoneId}.status.json`,
+          fixable: true,
+        });
+
+        if (shouldFix("stale_parallel_session")) {
+          removeSessionStatus(basePath, status.milestoneId);
+          fixesApplied.push(`cleaned up stale parallel session for ${status.milestoneId}`);
+        }
+      }
+    }
+  } catch {
+    // Non-fatal — parallel session check failed
+  }
+
   // ── Orphaned completed-units keys ─────────────────────────────────────
   try {
     const completedKeysFile = join(root, "completed-units.json");
diff --git a/src/resources/extensions/gsd/parallel-merge.ts b/src/resources/extensions/gsd/parallel-merge.ts
new file mode 100644
index 000000000..9df875180
--- /dev/null
+++ b/src/resources/extensions/gsd/parallel-merge.ts
@@ -0,0 +1,156 @@
+/**
+ * GSD Parallel Merge — Worktree reconciliation for parallel milestones.
+ *
+ * Handles merging completed milestone worktrees back to main branch
+ * with safety checks for parallel execution context.
+ */
+
+import { loadFile } from "./files.js";
+import { resolveMilestoneFile } from "./paths.js";
+import { mergeMilestoneToMain } from "./auto-worktree.js";
+import { MergeConflictError } from "./git-service.js";
+import { removeSessionStatus } from "./session-status-io.js";
+import type { WorkerInfo } from "./parallel-orchestrator.js";
+
+// ─── Types ─────────────────────────────────────────────────────────────────
+
+export interface MergeResult {
+  milestoneId: string;
+  success: boolean;
+  commitMessage?: string;
+  pushed?: boolean;
+  error?: string;
+  conflictFiles?: string[];
+}
+
+export type MergeOrder = "sequential" | "by-completion";
+
+// ─── Merge Queue ───────────────────────────────────────────────────────────
+
+/**
+ * Determine safe merge order for completed milestones.
+ * Sequential: merge in milestone ID order (M001 before M002).
+ * By-completion: merge in the order milestones finished.
+ */
+export function determineMergeOrder(
+  workers: WorkerInfo[],
+  order: MergeOrder = "sequential",
+): string[] {
+  const completed = workers.filter(w => w.state === "stopped" && w.completedUnits > 0);
+  if (order === "by-completion") {
+    return completed
+      .sort((a, b) => a.startedAt - b.startedAt) // earliest first
+      .map(w => w.milestoneId);
+  }
+  return completed
+    .sort((a, b) => a.milestoneId.localeCompare(b.milestoneId))
+    .map(w => w.milestoneId);
+}
+
+/**
+ * Attempt to merge a single milestone's worktree back to main.
+ * Wraps mergeMilestoneToMain with error handling for parallel context.
+ */
+export async function mergeCompletedMilestone(
+  basePath: string,
+  milestoneId: string,
+): Promise<MergeResult> {
+  try {
+    // Load the roadmap content (needed by mergeMilestoneToMain)
+    const roadmapPath = resolveMilestoneFile(basePath, milestoneId, "ROADMAP");
+    if (!roadmapPath) {
+      return {
+        milestoneId,
+        success: false,
+        error: `No roadmap found for ${milestoneId}`,
+      };
+    }
+
+    const roadmapContent = await loadFile(roadmapPath);
+    if (!roadmapContent) {
+      return {
+        milestoneId,
+        success: false,
+        error: `Could not read roadmap for ${milestoneId}`,
+      };
+    }
+
+    // Attempt the merge
+    const result = mergeMilestoneToMain(basePath, milestoneId, roadmapContent);
+
+    // Clean up parallel session status
+    removeSessionStatus(basePath, milestoneId);
+
+    return {
+      milestoneId,
+      success: true,
+      commitMessage: result.commitMessage,
+      pushed: result.pushed,
+    };
+  } catch (err) {
+    if (err instanceof MergeConflictError) {
+      return {
+        milestoneId,
+        success: false,
+        error: `Merge conflict: ${err.conflictedFiles.length} conflicting file(s)`,
+        conflictFiles: err.conflictedFiles,
+      };
+    }
+    return {
+      milestoneId,
+      success: false,
+      error: err instanceof Error ? err.message : String(err),
+    };
+  }
+}
+
+/**
+ * Merge all completed milestones in sequence.
+ * Stops on first conflict and returns results so far.
+ */
+export async function mergeAllCompleted(
+  basePath: string,
+  workers: WorkerInfo[],
+  order: MergeOrder = "sequential",
+): Promise<MergeResult[]> {
+  const mergeOrder = determineMergeOrder(workers, order);
+  const results: MergeResult[] = [];
+
+  for (const mid of mergeOrder) {
+    const result = await mergeCompletedMilestone(basePath, mid);
+    results.push(result);
+
+    // Stop on first conflict — later merges may depend on this one
+    if (!result.success && result.conflictFiles) {
+      break;
+    }
+  }
+
+  return results;
+}
+
+/**
+ * Format merge results for display.
+ */
+export function formatMergeResults(results: MergeResult[]): string {
+  if (results.length === 0) return "No completed milestones to merge.";
+
+  const lines: string[] = ["# Merge Results\n"];
+
+  for (const r of results) {
+    if (r.success) {
+      const pushStatus = r.pushed ? " (pushed)" : "";
+      lines.push(`- **${r.milestoneId}** — merged successfully${pushStatus}`);
+    } else if (r.conflictFiles) {
+      lines.push(`- **${r.milestoneId}** — CONFLICT (${r.conflictFiles.length} file(s)):`);
+      for (const f of r.conflictFiles) {
+        lines.push(`  - \`${f}\``);
+      }
+      lines.push(`  Resolve conflicts manually and run \`/gsd parallel merge ${r.milestoneId}\` to retry.`);
+    } else {
+      lines.push(`- **${r.milestoneId}** — failed: ${r.error}`);
+    }
+  }
+
+  return lines.join("\n");
+}
diff --git a/src/resources/extensions/gsd/tests/parallel-orchestration.test.ts b/src/resources/extensions/gsd/tests/parallel-orchestration.test.ts
index ffa2bbf9a..7cf7b80be 100644
--- a/src/resources/extensions/gsd/tests/parallel-orchestration.test.ts
+++ b/src/resources/extensions/gsd/tests/parallel-orchestration.test.ts
@@ -44,6 +44,9 @@ import {
 
 import { validatePreferences, resolveParallelConfig } from "../preferences.js";
 
+import { determineMergeOrder, formatMergeResults, type MergeResult } from "../parallel-merge.js";
+import type { WorkerInfo } from "../parallel-orchestrator.js";
+
 // ─── Test Helpers ────────────────────────────────────────────────────────────
 
 function makeTmpBase(): string {
@@ -453,3 +456,201 @@ describe("preferences: validatePreferences parallel config", () => {
     assert.ok(result.errors.some(e => e.includes("auto_merge")));
   });
 });
+
+// ─── Test Helpers (parallel-merge) ───────────────────────────────────────────
+
+function makeWorker(overrides: Partial<WorkerInfo> = {}): WorkerInfo {
+  return {
+    milestoneId: "M001",
+    title: "Test Milestone",
+    pid: process.pid,
+    process: null,
+    worktreePath: "/tmp/test-worktree",
+    startedAt: Date.now() - 60_000,
+    state: "stopped",
+    completedUnits: 5,
+    cost: 2.50,
+    ...overrides,
+  };
+}
+
+// ─── parallel-merge: determineMergeOrder ─────────────────────────────────────
+
+describe("parallel-merge: determineMergeOrder sequential", () => {
+  it("returns milestone IDs sorted alphabetically by default", () => {
+    const workers = [
+      makeWorker({ milestoneId: "M003", state: "stopped", completedUnits: 1 }),
+      makeWorker({ milestoneId: "M001", state: "stopped", completedUnits: 2 }),
+      makeWorker({ milestoneId: "M002", state: "stopped", completedUnits: 3 }),
+    ];
+    const order = determineMergeOrder(workers, "sequential");
+    assert.deepEqual(order, ["M001", "M002", "M003"]);
+  });
+
+  it("excludes workers that are still running", () => {
+    const workers = [
+      makeWorker({ milestoneId: "M001", state: "stopped", completedUnits: 5 }),
+      makeWorker({ milestoneId: "M002", state: "running", completedUnits: 0 }),
+      makeWorker({ milestoneId: "M003", state: "stopped", completedUnits: 2 }),
+    ];
+    const order = determineMergeOrder(workers, "sequential");
+    assert.deepEqual(order, ["M001", "M003"]);
+  });
+
+  it("excludes workers with zero completedUnits even if stopped", () => {
+    const workers = [
+      makeWorker({ milestoneId: "M001", state: "stopped", completedUnits: 0 }),
+      makeWorker({ milestoneId: "M002", state: "stopped", completedUnits: 3 }),
+    ];
+    const order = determineMergeOrder(workers, "sequential");
+    assert.deepEqual(order, ["M002"]);
+  });
+
+  it("returns empty array when no workers are completed", () => {
+    const workers = [
+      makeWorker({ milestoneId: "M001", state: "running", completedUnits: 0 }),
+      makeWorker({ milestoneId: "M002", state: "paused", completedUnits: 0 }),
+    ];
+    const order = determineMergeOrder(workers);
+    assert.deepEqual(order, []);
+  });
+
+  it("uses sequential order as the default when no order arg provided", () => {
+    const workers = [
+      makeWorker({ milestoneId: "M002", state: "stopped", completedUnits: 1 }),
+      makeWorker({ milestoneId: "M001", state: "stopped", completedUnits: 1 }),
+    ];
+    // Call with no second argument — should default to "sequential"
+    const order = determineMergeOrder(workers);
+    assert.deepEqual(order, ["M001", "M002"]);
+  });
+});
+
+describe("parallel-merge: determineMergeOrder by-completion", () => {
+  it("returns milestones sorted by startedAt (earliest first)", () => {
+    const now = Date.now();
+    const workers = [
+      makeWorker({ milestoneId: "M003", state: "stopped", completedUnits: 1, startedAt: now - 30_000 }),
+      makeWorker({ milestoneId: "M001", state: "stopped", completedUnits: 1, startedAt: now - 90_000 }),
+      makeWorker({ milestoneId: "M002", state: "stopped", completedUnits: 1, startedAt: now - 60_000 }),
+    ];
+    const order = determineMergeOrder(workers, "by-completion");
+    assert.deepEqual(order, ["M001", "M002", "M003"]);
+  });
+
+  it("excludes paused workers from by-completion order", () => {
+    const now = Date.now();
+    const workers = [
+      makeWorker({ milestoneId: "M001", state: "stopped", completedUnits: 2, startedAt: now - 90_000 }),
+      makeWorker({ milestoneId: "M002", state: "paused",  completedUnits: 1, startedAt: now - 60_000 }),
+      makeWorker({ milestoneId: "M003", state: "stopped", completedUnits: 3, startedAt: now - 30_000 }),
+    ];
+    const order = determineMergeOrder(workers, "by-completion");
+    assert.deepEqual(order, ["M001", "M003"]);
+  });
+});
+
+// ─── parallel-merge: formatMergeResults ──────────────────────────────────────
+
+describe("parallel-merge: formatMergeResults", () => {
+  it("returns a no-op message for an empty results array", () => {
+    const output = formatMergeResults([]);
+    assert.equal(output, "No completed milestones to merge.");
+  });
+
+  it("formats a single successful merge without push", () => {
+    const results: MergeResult[] = [
+      { milestoneId: "M001", success: true, commitMessage: "feat: auth system", pushed: false },
+    ];
+    const output = formatMergeResults(results);
+    assert.ok(output.includes("# Merge Results"));
+    assert.ok(output.includes("**M001**"));
+    assert.ok(output.includes("merged successfully"));
+    assert.ok(!output.includes("(pushed)"));
+  });
+
+  it("includes (pushed) suffix when result.pushed is true", () => {
+    const results: MergeResult[] = [
+      { milestoneId: "M002", success: true, commitMessage: "feat: dashboard", pushed: true },
+    ];
+    const output = formatMergeResults(results);
+    assert.ok(output.includes("(pushed)"));
+  });
+
+  it("formats a conflict result with file list and retry instructions", () => {
+    const results: MergeResult[] = [
+      {
+        milestoneId: "M003",
+        success: false,
+        conflictFiles: ["src/types.ts", "src/utils.ts"],
+        error: "Merge conflict: 2 conflicting file(s)",
+      },
+    ];
+    const output = formatMergeResults(results);
+    assert.ok(output.includes("**M003**"));
+    assert.ok(output.includes("CONFLICT (2 file(s))"));
+    assert.ok(output.includes("`src/types.ts`"));
+    assert.ok(output.includes("`src/utils.ts`"));
+    assert.ok(output.includes("/gsd parallel merge M003"));
+  });
+
+  it("formats a generic error (no conflict files) with the error message", () => {
+    const results: MergeResult[] = [
+      { milestoneId: "M004", success: false, error: "No roadmap found for M004" },
+    ];
+    const output = formatMergeResults(results);
+    assert.ok(output.includes("**M004**"));
+    assert.ok(output.includes("failed: No roadmap found for M004"));
+    assert.ok(!output.includes("CONFLICT"));
+  });
+
+  it("formats multiple results in the order provided", () => {
+    const results: MergeResult[] = [
+      { milestoneId: "M001", success: true, pushed: false },
+      { milestoneId: "M002", success: false, error: "branch not found" },
+      { milestoneId: "M003", success: true, pushed: true },
+    ];
+    const output = formatMergeResults(results);
+    const m1Pos = output.indexOf("M001");
+    const m2Pos = output.indexOf("M002");
+    const m3Pos = output.indexOf("M003");
+    assert.ok(m1Pos < m2Pos, "M001 should appear before M002");
+    assert.ok(m2Pos < m3Pos, "M002 should appear before M003");
+  });
+});
+
+// ─── doctor: stale_parallel_session issue code ───────────────────────────────
+
+describe("doctor: stale_parallel_session issue code exists", () => {
+  it("DoctorIssueCode union includes stale_parallel_session", async () => {
+    // Import doctor.ts and verify the type is real by constructing a DoctorIssue
+    // with code "stale_parallel_session" — TypeScript will reject it at compile
+    // time if the code is not in the union; the runtime assertion confirms the
+    // string value round-trips through the typed object correctly.
+    const { } = await import("../doctor.js");
+    // Construct a value that satisfies DoctorIssue using the code under test
+    const issue: import("../doctor.js").DoctorIssue = {
+      severity: "warning",
+      code: "stale_parallel_session",
+      scope: "project",
+      unitId: "M001",
+      message: "Stale parallel session detected",
+      fixable: true,
+    };
+    assert.equal(issue.code, "stale_parallel_session");
+  });
+
+  it("DoctorIssue with stale_parallel_session has warning severity", () => {
+    const issue: import("../doctor.js").DoctorIssue = {
+      severity: "warning",
+      code: "stale_parallel_session",
+      scope: "project",
+      unitId: "M002",
+      message: "Stale parallel session for M002",
+      fixable: true,
+    };
+    assert.equal(issue.severity, "warning");
+    assert.equal(issue.fixable, true);
+    assert.equal(issue.scope, "project");
+  });
+});