diff --git a/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.test.ts b/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.test.ts
index 4471be814bf39..c3efaa7ce8af9 100644
--- a/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.test.ts
+++ b/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.test.ts
@@ -8111,6 +8111,84 @@ describe("CodexAdapterV2 post-settle continuation", () => {
       ),
   );
 
+  it.effect("unarchives an archived Codex source thread before forking it", () =>
+    Effect.gen(function* () {
+      const nativeThreadId = "archived-fork-source";
+      const forkThreadId = "archived-fork-target";
+      const forkParams = {
+        threadId: nativeThreadId,
+        lastTurnId: "native-turn-first",
+        config: CodexAdapterV2.CODEX_THREAD_CONFIG,
+      };
+      const exchange = (
+        id: number,
+        method: string,
+        params: unknown,
+        reply: { readonly result: unknown } | { readonly error: unknown },
+      ): Array<CodexReplay.CodexAppServerReplayEntry> => [
+        { type: "expect_outbound", label: method, frame: { id, method, params } },
+        { type: "emit_inbound", label: method, frame: { id, ...reply } },
+      ];
+      const transcript = makeCodexReplayTranscript({
+        scenario: "codex-archived-fork",
+        entries: [
+          ...codexReplayPreamble({
+            nativeThreadId,
+            nativeTurnId: "archived-fork-turn",
+            prompt: "unused",
+          }).slice(0, 5),
+          ...exchange(3, "thread/fork", forkParams, {
+            error: {
+              code: -32600,
+              message: `session ${nativeThreadId} is archived. Run \`codex unarchive ${nativeThreadId}\` to unarchive it first.`,
+            },
+          }),
+          ...exchange(
+            4,
+            "thread/unarchive",
+            { threadId: nativeThreadId },
+            { result: codexReplayThreadResult({ nativeThreadId, forkedFromId: null }) },
+          ),
+          ...exchange(5, "thread/fork", forkParams, {
+            result: codexReplayThreadResult({
+              nativeThreadId: forkThreadId,
+              forkedFromId: nativeThreadId,
+            }),
+          }),
+        ],
+      });
+      const harness = yield* makeCodexReplayHarness(transcript);
+      const now = yield* DateTime.now;
+      const firstTurn = codexReplaySourceTurn({
+        id: "provider-turn-first",
+        ordinal: 1,
+        nativeId: "native-turn-first",
+        providerThreadId: harness.providerThread.id,
+        now,
+      });
+
+      const forkedProviderThread = yield* harness.runtime.forkThread({
+        sourceProviderThread: harness.providerThread,
+        sourceProviderTurns: [firstTurn],
+        providerTurnId: firstTurn.id,
+        targetThreadId: ThreadId.make("thread-archived-fork-target"),
+      });
+
+      assert.equal(forkedProviderThread.nativeThreadRef?.nativeId, forkThreadId);
+      assert.equal(forkedProviderThread.forkedFrom?.providerTurnId, firstTurn.id);
+    }).pipe(
+      Effect.scoped,
+      Effect.provide(
+        Layer.mergeAll(
+          IdAllocator.layer,
+          McpProviderSessions.layer,
+          NodeServices.layer,
+          NodeCrypto.layer,
+        ),
+      ),
+    ),
+  );
+
   describe("native goals", () => {
     const nativeThreadId = "goal-thread";
     const objective = "Ship the feature";
diff --git a/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.ts b/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.ts
index 688de092c03d0..43e33d1d6692c 100644
--- a/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.ts
+++ b/apps/server/src/orchestration-v2/Adapters/CodexAdapterV2.ts
@@ -7909,24 +7909,32 @@ export const makeCodexAdapterV2 = Effect.fn("makeCodexAdapterV2")(function* (
             function* (threadInput) {
               const threadId = yield* getNativeThreadId(threadInput.sourceProviderThread);
               const boundary = yield* resolveCodexForkBoundary(threadInput);
+              const fork = client.request("thread/fork", {
+                threadId,
+                ...(boundary.lastTurnId === undefined ? {} : { lastTurnId: boundary.lastTurnId }),
+                ...codexThreadRuntimeParams({
+                  threadId: threadInput.targetThreadId,
+                  mcpSession: yield* mcpSessions.read(threadInput.targetThreadId),
+                  ...(threadInput.modelSelection === undefined
+                    ? {}
+                    : { modelSelection: threadInput.modelSelection }),
+                  ...(threadInput.runtimePolicy === undefined
+                    ? {}
+                    : { runtimePolicy: threadInput.runtimePolicy }),
+                }),
+              });
               const response = yield* ensureInitialized.pipe(
                 Effect.andThen(
-                  client.request("thread/fork", {
-                    threadId,
-                    ...(boundary.lastTurnId === undefined
-                      ? {}
-                      : { lastTurnId: boundary.lastTurnId }),
-                    ...codexThreadRuntimeParams({
-                      threadId: threadInput.targetThreadId,
-                      mcpSession: yield* mcpSessions.read(threadInput.targetThreadId),
-                      ...(threadInput.modelSelection === undefined
-                        ? {}
-                        : { modelSelection: threadInput.modelSelection }),
-                      ...(threadInput.runtimePolicy === undefined
-                        ? {}
-                        : { runtimePolicy: threadInput.runtimePolicy }),
+                  fork.pipe(
+                    Effect.catchTags({
+                      CodexAppServerRequestError: (cause) =>
+                        /\bsession \S+ is archived\b|\bcodex unarchive\b/i.test(cause.errorMessage)
+                          ? client.raw
+                              .request("thread/unarchive", { threadId })
+                              .pipe(Effect.andThen(fork))
+                          : Effect.fail(cause),
                     }),
-                  }),
+                  ),
                 ),
                 Effect.mapError(
                   (cause) =>