diff --git a/openspec/changes/agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56/notes.md b/openspec/changes/agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56/notes.md
new file mode 100644
index 00000000..06269dd6
--- /dev/null
+++ b/openspec/changes/agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56/notes.md
@@ -0,0 +1,21 @@
+# agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56 (minimal / T1)
+
+Branch: `agent/<your-name>/<branch-slug>`
+
+Describe the change in a sentence or two. Commit message is the spec of record.
+
+## Handoff
+
+- Handoff: change=`agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56`; branch=`agent/<your-name>/<branch-slug>`; scope=`TODO`; action=`continue this sandbox or finish cleanup after a usage-limit/manual takeover`.
+- Copy prompt: Continue `agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56` on branch `agent/<your-name>/<branch-slug>`. Work inside the existing sandbox, review `openspec/changes/agent-codex-sync-codex-auth-into-cue-runtimes-2026-08-10-11-56/notes.md`, continue from the current state instead of creating a new sandbox, and when the work is done run `gx branch finish --branch agent/<your-name>/<branch-slug> --base dev --via-pr --wait-for-merge --cleanup`.
+
+## Cleanup
+
+- [ ] Run: `gx branch finish --branch agent/<your-name>/<branch-slug> --base dev --via-pr --wait-for-merge --cleanup`
+- [ ] Record PR URL + `MERGED` state in the completion handoff.
+- [ ] Confirm sandbox worktree is gone (`git worktree list`, `git branch -a`).
+# Codex auth sync
+
+- Cause: Cue launches Codex with a profile-isolated `CODEX_HOME`, while AuthMux manages `~/.codex/auth.json`.
+- Fix: copy canonical auth into the selected runtime before launch, then copy refreshed runtime auth back after exit.
+- Verification: `bun test src/lib/codex-auth-sync.test.ts`; `bunx tsc --noEmit`.
diff --git a/src/commands/launch.ts b/src/commands/launch.ts
index 58fa422f..9ba0fce6 100644
--- a/src/commands/launch.ts
+++ b/src/commands/launch.ts
@@ -14,7 +14,7 @@
  */
 
 import { spawn } from "node:child_process";
-import { readFile } from "node:fs/promises";
+import { copyFile, readFile } from "node:fs/promises";
 import { existsSync, readFileSync, readdirSync } from "node:fs";
 import { basename, dirname, join, resolve, sep } from "node:path";
 import { homedir } from "node:os";
@@ -205,6 +205,16 @@ function execAgent(bin: string, args: string[], env: NodeJS.ProcessEnv): Promise
   });
 }
 
+/** Keep Cue's isolated CODEX_HOME in sync with Codex/AuthMux's canonical auth. */
+export async function syncCodexAuth(source: string, destination: string): Promise<boolean> {
+  try {
+    await copyFile(source, destination);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 /**
  * Whether the interactive MCP toggle should open this launch. Only when stdin
  * is a TTY, AND either the user forced it (`--cue-pick-mcps`) or there's no
@@ -2712,6 +2722,11 @@ export async function run(args: string[]): Promise<number> {
   // re-login.
   const stopReconciler =
     agentKind === "claude-code" ? startCredentialReconciler(runtimeKey) : undefined;
+  const canonicalCodexAuth = join(homedir(), ".codex", "auth.json");
+  const runtimeCodexAuth = join(runtime.runtimeDir, "auth.json");
+  if (agentKind === "codex") {
+    await syncCodexAuth(canonicalCodexAuth, runtimeCodexAuth);
+  }
   let exitCode: number;
   try {
     exitCode = await execAgent(realBin, [...briefArgs, ...parsed.passthrough], childEnv);
@@ -2721,6 +2736,9 @@ export async function run(args: string[]): Promise<number> {
   // Persist any /login done inside the session to its account dir now —
   // don't leave the only live rotated token stranded in the per-account runtime.
   if (agentKind === "claude-code") await rescueRuntimeCredsToOwner(runtimeKey);
+  if (agentKind === "codex") {
+    await syncCodexAuth(runtimeCodexAuth, canonicalCodexAuth);
+  }
   // Post-session runtime GC: the child has exited, so this costs zero launch
   // latency. Throttled (~once/day) and never touches the runtime we just used.
   try { await maybeAutoGc(runtimeKey); } catch { /* GC is best-effort */ }
diff --git a/src/lib/codex-auth-sync.test.ts b/src/lib/codex-auth-sync.test.ts
new file mode 100644
index 00000000..ada9258b
--- /dev/null
+++ b/src/lib/codex-auth-sync.test.ts
@@ -0,0 +1,31 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+import { syncCodexAuth } from "../commands/launch";
+
+describe("syncCodexAuth", () => {
+  const dirs: string[] = [];
+
+  afterEach(async () => {
+    await Promise.all(dirs.splice(0).map((dir) => rm(dir, { recursive: true, force: true })));
+  });
+
+  test("copies canonical Codex auth into a Cue runtime", async () => {
+    const dir = await mkdtemp(join(tmpdir(), "cue-codex-auth-"));
+    dirs.push(dir);
+    const source = join(dir, "source.json");
+    const destination = join(dir, "runtime.json");
+    await writeFile(source, '{"tokens":{"access_token":"test"}}\n');
+
+    expect(await syncCodexAuth(source, destination)).toBe(true);
+    expect(await readFile(destination, "utf8")).toBe(await readFile(source, "utf8"));
+  });
+
+  test("fails open when no canonical login exists", async () => {
+    const dir = await mkdtemp(join(tmpdir(), "cue-codex-auth-"));
+    dirs.push(dir);
+    expect(await syncCodexAuth(join(dir, "missing.json"), join(dir, "runtime.json"))).toBe(false);
+  });
+});