diff --git a/src/resources/extensions/gsd/state.ts b/src/resources/extensions/gsd/state.ts
index ab19f5695..ec6e36210 100644
--- a/src/resources/extensions/gsd/state.ts
+++ b/src/resources/extensions/gsd/state.ts
@@ -710,7 +710,7 @@ async function reconcileSliceTasks(
     const summaryPath = resolveTaskFile(basePath, milestoneId, sliceId, t.id, "SUMMARY");
     if (summaryPath && existsSync(summaryPath)) {
       try {
-        updateTaskStatus(milestoneId, sliceId, t.id, "complete");
+        updateTaskStatus(milestoneId, sliceId, t.id, "complete", new Date().toISOString());
         logWarning("reconcile", `task ${milestoneId}/${sliceId}/${t.id} status reconciled from "${t.status}" to "complete" (#2514)`, { mid: milestoneId, sid: sliceId, tid: t.id });
         reconciled = true;
       } catch (e) {
diff --git a/src/resources/extensions/gsd/tests/completed-at-reconcile.test.ts b/src/resources/extensions/gsd/tests/completed-at-reconcile.test.ts
new file mode 100644
index 000000000..fcf6e4a42
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/completed-at-reconcile.test.ts
@@ -0,0 +1,42 @@
+/**
+ * Regression test for #4129: tasks.completed_at stays NULL when status is
+ * reconciled to 'complete' via the file-existence path in state.ts.
+ *
+ * Root cause: reconcileSliceTasks called
+ *   updateTaskStatus(milestoneId, sliceId, t.id, "complete")
+ * without a completedAt timestamp, so the column stays NULL.
+ *
+ * Fix: pass new Date().toISOString() as the 5th argument.
+ */
+
+import { describe, test } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const stateSource = readFileSync(join(__dirname, "..", "state.ts"), "utf-8");
+
+describe("completed-at reconcile (#4129)", () => {
+  test("reconcileSliceTasks passes a completedAt timestamp when setting status to complete", () => {
+    // Before the fix, state.ts had:
+    //   updateTaskStatus(milestoneId, sliceId, t.id, "complete")
+    // which leaves completed_at NULL in the DB.
+    // After the fix, a timestamp must be passed as the 5th argument.
+    assert.doesNotMatch(
+      stateSource,
+      /updateTaskStatus\(\s*milestoneId\s*,\s*sliceId\s*,\s*t\.id\s*,\s*["']complete["']\s*\)/,
+      "updateTaskStatus must not be called without a completedAt timestamp when reconciling tasks to 'complete' (#4129)",
+    );
+  });
+
+  test("reconcileSliceTasks passes new Date().toISOString() as the completedAt argument", () => {
+    // Positive assertion: the fixed call must include a timestamp.
+    assert.match(
+      stateSource,
+      /updateTaskStatus\(\s*milestoneId\s*,\s*sliceId\s*,\s*t\.id\s*,\s*["']complete["']\s*,\s*new Date\(\)\.toISOString\(\)\s*\)/,
+      "reconcileSliceTasks must pass new Date().toISOString() as completedAt when setting task status to 'complete' (#4129)",
+    );
+  });
+});