diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index 4b939a0ca..17cb3102e 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -610,14 +610,48 @@ export async function stopAuto(
     }
 
     // ── Step 4: Auto-worktree exit ──
+    // When the milestone is complete (has a SUMMARY), merge the worktree branch
+    // back to main so code isn't stranded on the worktree branch (#2317).
+    // For incomplete milestones, preserve the branch for later resumption.
     try {
       if (s.currentMilestoneId) {
         const notifyCtx = ctx
           ? { notify: ctx.ui.notify.bind(ctx.ui) }
           : { notify: () => {} };
-        buildResolver().exitMilestone(s.currentMilestoneId, notifyCtx, {
-          preserveBranch: true,
-        });
+        const resolver = buildResolver();
+
+        // Check if the milestone is complete — SUMMARY file is the authoritative signal.
+        let milestoneComplete = false;
+        try {
+          const summaryPath = resolveMilestoneFile(
+            s.originalBasePath || s.basePath,
+            s.currentMilestoneId,
+            "SUMMARY",
+          );
+          if (!summaryPath) {
+            // Also check in the worktree path (SUMMARY may not be synced yet)
+            const wtSummaryPath = resolveMilestoneFile(
+              s.basePath,
+              s.currentMilestoneId,
+              "SUMMARY",
+            );
+            milestoneComplete = wtSummaryPath !== null;
+          } else {
+            milestoneComplete = true;
+          }
+        } catch {
+          // Non-fatal — fall through to preserveBranch path
+        }
+
+        if (milestoneComplete) {
+          // Milestone is complete — merge worktree branch back to main
+          resolver.mergeAndExit(s.currentMilestoneId, notifyCtx);
+        } else {
+          // Milestone still in progress — preserve branch for later resumption
+          resolver.exitMilestone(s.currentMilestoneId, notifyCtx, {
+            preserveBranch: true,
+          });
+        }
       }
     } catch (e) {
       debugLog("stop-cleanup-worktree", { error: e instanceof Error ? e.message : String(e) });
diff --git a/src/resources/extensions/gsd/tests/stop-auto-merge-back.test.ts b/src/resources/extensions/gsd/tests/stop-auto-merge-back.test.ts
new file mode 100644
index 000000000..464c69c33
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/stop-auto-merge-back.test.ts
@@ -0,0 +1,67 @@
+/**
+ * stop-auto-merge-back.test.ts — Regression test for #2317.
+ *
+ * When auto-mode stops after a milestone is complete, stopAuto should trigger
+ * merge-back (mergeAndExit) instead of just exiting the worktree with
+ * preserveBranch: true. Otherwise milestone code stays stranded on the
+ * worktree branch and never reaches main.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+// ─── Source analysis: stopAuto calls mergeAndExit for complete milestones ────
+
+const autoSrcPath = join(import.meta.dirname, "..", "auto.ts");
+const autoSrc = readFileSync(autoSrcPath, "utf-8");
+
+test("#2317: stopAuto should check milestone completion status before choosing exit strategy", () => {
+  // stopAuto Step 4 should NOT unconditionally call exitMilestone(preserveBranch: true).
+  // It should check if the milestone is complete and call mergeAndExit instead.
+
+  // Find the Step 4 section
+  const step4Idx = autoSrc.indexOf("Step 4: Auto-worktree exit");
+  assert.ok(step4Idx !== -1, "Step 4 comment exists in stopAuto");
+
+  // Extract a reasonable window around Step 4 (up to Step 5)
+  const step5Idx = autoSrc.indexOf("Step 5:", step4Idx);
+  const step4Block = autoSrc.slice(step4Idx, step5Idx);
+
+  // The fix: Step 4 should call mergeAndExit when milestone is complete
+  assert.ok(
+    step4Block.includes("mergeAndExit"),
+    "Step 4 should call mergeAndExit for completed milestones",
+  );
+});
+
+test("#2317: stopAuto should detect milestone completion via SUMMARY file or DB", () => {
+  const step4Idx = autoSrc.indexOf("Step 4: Auto-worktree exit");
+  const step5Idx = autoSrc.indexOf("Step 5:", step4Idx);
+  const step4Block = autoSrc.slice(step4Idx, step5Idx);
+
+  // Should check completion status — either via SUMMARY file, DB getMilestone, or phase
+  const checksCompletion =
+    step4Block.includes("SUMMARY") ||
+    step4Block.includes("getMilestone") ||
+    step4Block.includes("complete") ||
+    step4Block.includes("isMilestoneComplete");
+
+  assert.ok(
+    checksCompletion,
+    "Step 4 should check if milestone is complete before deciding exit strategy",
+  );
+});
+
+test("#2317: stopAuto still preserves branch for incomplete milestones", () => {
+  const step4Idx = autoSrc.indexOf("Step 4: Auto-worktree exit");
+  const step5Idx = autoSrc.indexOf("Step 5:", step4Idx);
+  const step4Block = autoSrc.slice(step4Idx, step5Idx);
+
+  // preserveBranch should still be used as fallback for non-complete milestones
+  assert.ok(
+    step4Block.includes("preserveBranch"),
+    "Step 4 should still preserve branch for incomplete milestones (fallback path)",
+  );
+});