diff --git a/src/resources/extensions/gsd/auto-worktree-sync.ts b/src/resources/extensions/gsd/auto-worktree-sync.ts
index 643576098..8fab45fc3 100644
--- a/src/resources/extensions/gsd/auto-worktree-sync.ts
+++ b/src/resources/extensions/gsd/auto-worktree-sync.ts
@@ -93,6 +93,11 @@ export function syncStateToProjectRoot(
     { force: true },
   );
 
+  // 3. metrics.json — session cost/token tracking (#2313).
+  // Without this, metrics accumulated in the worktree are invisible from the
+  // project root and never appear in the dashboard or skill-health reports.
+  safeCopy(join(wtGsd, "metrics.json"), join(prGsd, "metrics.json"), { force: true });
+
   // 4. Runtime records — unit dispatch state used by selfHealRuntimeRecords().
   // Without this, a crash during a unit leaves the runtime record only in the
   // worktree. If the next session resolves basePath before worktree re-entry,
diff --git a/src/resources/extensions/gsd/auto-worktree.ts b/src/resources/extensions/gsd/auto-worktree.ts
index 784d11276..95e1daba3 100644
--- a/src/resources/extensions/gsd/auto-worktree.ts
+++ b/src/resources/extensions/gsd/auto-worktree.ts
@@ -162,6 +162,7 @@ export function syncGsdStateToWorktree(
     "OVERRIDES.md",
     "QUEUE.md",
     "completed-units.json",
+    "metrics.json",
   ];
   for (const f of rootFiles) {
     const src = join(mainGsd, f);
@@ -325,8 +326,9 @@ export function syncWorktreeStateBack(
   // ── 1. Sync root-level .gsd/ files back ──────────────────────────────
   // The worktree is authoritative — complete-milestone updates REQUIREMENTS,
   // PROJECT, etc. These must overwrite main's copies so they survive teardown.
-  // Also includes QUEUE.md and completed-units.json which are written during
-  // milestone closeout and lost on teardown without explicit sync (#1787).
+  // Also includes QUEUE.md, completed-units.json, and metrics.json which are
+  // written during milestone closeout and lost on teardown without explicit sync
+  // (#1787, #2313).
   const rootFiles = [
     "DECISIONS.md",
     "REQUIREMENTS.md",
@@ -335,6 +337,7 @@ export function syncWorktreeStateBack(
     "OVERRIDES.md",
     "QUEUE.md",
     "completed-units.json",
+    "metrics.json",
   ];
   for (const f of rootFiles) {
     const src = join(wtGsd, f);
diff --git a/src/resources/extensions/gsd/auto/phases.ts b/src/resources/extensions/gsd/auto/phases.ts
index 945c4e1a0..0b4e276ad 100644
--- a/src/resources/extensions/gsd/auto/phases.ts
+++ b/src/resources/extensions/gsd/auto/phases.ts
@@ -28,6 +28,7 @@ import { gsdRoot } from "../paths.js";
 import { atomicWriteSync } from "../atomic-write.js";
 import { PROJECT_FILES } from "../detection.js";
 import { join } from "node:path";
+import { existsSync, cpSync } from "node:fs";
 
 // ─── generateMilestoneReport ──────────────────────────────────────────────────
 
@@ -263,9 +264,17 @@ export async function runPreDispatch(
     // Reset completed-units tracking for the new milestone — stale entries
     // from the previous milestone cause the dispatch loop to skip units
     // that haven't actually been completed in the new milestone's context.
+    // Archive the old completed-units.json instead of wiping it (#2313).
     s.completedUnits = [];
     try {
       const completedKeysPath = join(gsdRoot(s.basePath), "completed-units.json");
+      if (existsSync(completedKeysPath) && s.currentMilestoneId) {
+        const archivePath = join(
+          gsdRoot(s.basePath),
+          `completed-units-${s.currentMilestoneId}.json`,
+        );
+        cpSync(completedKeysPath, archivePath);
+      }
       atomicWriteSync(completedKeysPath, JSON.stringify([], null, 2));
     } catch { /* non-fatal */ }
 
diff --git a/src/resources/extensions/gsd/tests/completed-units-metrics-sync.test.ts b/src/resources/extensions/gsd/tests/completed-units-metrics-sync.test.ts
new file mode 100644
index 000000000..e2bfc550f
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/completed-units-metrics-sync.test.ts
@@ -0,0 +1,113 @@
+/**
+ * completed-units-metrics-sync.test.ts — Regression tests for #2313.
+ *
+ * 1. completed-units.json should be archived (not wiped) on milestone transition
+ * 2. metrics.json should be in the worktree → project root sync file list
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync, mkdtempSync, mkdirSync, writeFileSync, existsSync, cpSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+
+// ─── Bug 1: completed-units.json should be archived, not wiped ─────────────
+
+const phasesSrcPath = join(import.meta.dirname, "..", "auto", "phases.ts");
+const phasesSrc = readFileSync(phasesSrcPath, "utf-8");
+
+test("#2313: completed-units.json should not be blindly wiped to [] on milestone transition", () => {
+  // The milestone transition block should NOT write an empty array to completed-units.json
+  // without first archiving the existing data. Look for the archive/rename pattern.
+  const transitionIdx = phasesSrc.indexOf("Milestone transition");
+  assert.ok(transitionIdx !== -1, "Milestone transition section exists");
+
+  // Find the completed-units handling block
+  const completedUnitsIdx = phasesSrc.indexOf("completed-units", transitionIdx);
+  assert.ok(completedUnitsIdx !== -1, "completed-units handling exists in transition");
+
+  // Get a window around the completed-units handling
+  const windowStart = Math.max(0, completedUnitsIdx - 200);
+  const windowEnd = Math.min(phasesSrc.length, completedUnitsIdx + 500);
+  const window = phasesSrc.slice(windowStart, windowEnd);
+
+  // Should archive/rename the old file before resetting
+  const hasArchive = window.includes("archive") ||
+    window.includes("rename") ||
+    window.includes("cpSync") ||
+    window.includes("safeCopy") ||
+    window.includes("completed-units-");
+
+  assert.ok(
+    hasArchive,
+    "completed-units.json should be archived before reset during milestone transition",
+  );
+});
+
+// ─── Bug 2: metrics.json should be in the sync file lists ──────────────────
+
+test("#2313: syncStateToProjectRoot should sync metrics.json", () => {
+  const syncSrcPath = join(import.meta.dirname, "..", "auto-worktree-sync.ts");
+  const syncSrc = readFileSync(syncSrcPath, "utf-8");
+
+  // syncStateToProjectRoot should copy metrics.json from worktree to project root
+  assert.ok(
+    syncSrc.includes("metrics.json"),
+    "auto-worktree-sync.ts should reference metrics.json for sync",
+  );
+});
+
+test("#2313: syncWorktreeStateBack should include metrics.json in root files list", () => {
+  const autoWorktreeSrcPath = join(import.meta.dirname, "..", "auto-worktree.ts");
+  const autoWorktreeSrc = readFileSync(autoWorktreeSrcPath, "utf-8");
+
+  // Find the rootFiles array in syncWorktreeStateBack
+  const syncBackIdx = autoWorktreeSrc.indexOf("syncWorktreeStateBack");
+  assert.ok(syncBackIdx !== -1, "syncWorktreeStateBack exists");
+
+  const rootFilesIdx = autoWorktreeSrc.indexOf("rootFiles", syncBackIdx);
+  assert.ok(rootFilesIdx !== -1, "rootFiles list exists in syncWorktreeStateBack");
+
+  // Get the rootFiles array content
+  const arrayStart = autoWorktreeSrc.indexOf("[", rootFilesIdx);
+  const arrayEnd = autoWorktreeSrc.indexOf("]", arrayStart);
+  const rootFilesBlock = autoWorktreeSrc.slice(arrayStart, arrayEnd);
+
+  assert.ok(
+    rootFilesBlock.includes("metrics.json"),
+    "metrics.json should be in syncWorktreeStateBack rootFiles list",
+  );
+});
+
+// ─── Functional test: completed-units archive ────────────────────────────────
+
+test("#2313: functional — completed-units archive creates milestone-specific file", () => {
+  const tmpBase = mkdtempSync(join(tmpdir(), "gsd-completed-units-"));
+  const gsdDir = join(tmpBase, ".gsd");
+  mkdirSync(gsdDir, { recursive: true });
+
+  // Simulate existing completed-units.json with data
+  const existing = [
+    { type: "task", id: "T01" },
+    { type: "slice", id: "S01" },
+  ];
+  const completedKeysPath = join(gsdDir, "completed-units.json");
+  writeFileSync(completedKeysPath, JSON.stringify(existing, null, 2));
+
+  // Simulate the archive behavior: copy to milestone-specific file
+  const milestoneId = "M001";
+  const archivePath = join(gsdDir, `completed-units-${milestoneId}.json`);
+  cpSync(completedKeysPath, archivePath);
+
+  // Reset the main file
+  writeFileSync(completedKeysPath, JSON.stringify([], null, 2));
+
+  // Verify archive exists with original data
+  assert.ok(existsSync(archivePath), "archive file should exist");
+  const archived = JSON.parse(readFileSync(archivePath, "utf-8"));
+  assert.deepEqual(archived, existing, "archived data should match original");
+
+  // Verify main file is reset
+  const current = JSON.parse(readFileSync(completedKeysPath, "utf-8"));
+  assert.deepEqual(current, [], "current completed-units should be empty after transition");
+});