diff --git a/src/lib/codex-install.ts b/src/lib/codex-install.ts
index 867b98b..2ae09a8 100644
--- a/src/lib/codex-install.ts
+++ b/src/lib/codex-install.ts
@@ -697,8 +697,42 @@ function testCodexCommand(): string[] | null {
   return parsed as string[];
 }
 
+// Memoized probe result. `null` = not yet probed; `boolean` = final answer for
+// this process. Cleared via `resetCodexCliAvailabilityMemoForTests()` between
+// test scenarios that install/uninstall codex under a synthetic PATH.
+let codexCliAvailabilityMemo: boolean | null = null;
+
+/** Test-only: reset the memoized probe so PATH changes take effect. */
+export function resetCodexCliAvailabilityMemoForTests(): void {
+  codexCliAvailabilityMemo = null;
+}
+
 export function codexCliAvailable(): boolean {
-  return testCodexCommand() !== null || hasCommand("codex");
+  // Test escape hatch: an explicit fixture command bypasses PATH resolution
+  // entirely and is always considered available.
+  if (testCodexCommand() !== null) return true;
+  if (codexCliAvailabilityMemo !== null) return codexCliAvailabilityMemo;
+  if (!hasCommand("codex")) {
+    codexCliAvailabilityMemo = false;
+    return false;
+  }
+  // A binary named `codex` is on PATH, but proxy shims (e.g. cmux CLI shims
+  // at $TMPDIR/cmux-cli-shims/.../codex) can pass the existence check while
+  // failing on invocation. Probe with `codex --version` before trusting the
+  // CLI for real plugin/marketplace queries. A 3s cap keeps a hung shim
+  // from stalling setup.
+  try {
+    const probe = Bun.spawnSync({
+      cmd: ["codex", "--version"],
+      stdout: "ignore",
+      stderr: "ignore",
+      timeout: 3000,
+    });
+    codexCliAvailabilityMemo = probe.exitCode === 0;
+  } catch {
+    codexCliAvailabilityMemo = false;
+  }
+  return codexCliAvailabilityMemo;
 }
 
 const REQUIRED_SOURCE_ARTIFACTS = [
diff --git a/tests/codex-install.test.ts b/tests/codex-install.test.ts
index 1908ab2..89fff39 100644
--- a/tests/codex-install.test.ts
+++ b/tests/codex-install.test.ts
@@ -3830,3 +3830,55 @@ exit 0
     240_000,
   );
 });
+
+describe.skipIf(process.platform === "win32")("codexCliAvailable — shim detection", () => {
+  test("PATH entry named 'codex' that exits non-zero is treated as unavailable", async () => {
+    // Simulates the cmux CLI shim case: a binary named `codex` is on PATH,
+    // but invocation fails. Bun.which passes; the version probe must not.
+    const { codexCliAvailable, resetCodexCliAvailabilityMemoForTests } = await import(
+      "../src/lib/codex-install.ts"
+    );
+    const shimDir = await mkdtemp(join(tmpdir(), "cc-codex-shim-"));
+    try {
+      const shim = join(shimDir, "codex");
+      // POSIX shim that mimics the cmux behavior: exits non-zero with a
+      // "codex not found" stderr message on every invocation.
+      await writeFile(shim, "#!/bin/sh\necho 'Error: codex not found in PATH' >&2\nexit 1\n");
+      await chmod(shim, 0o755);
+
+      const originalPath = process.env.PATH;
+      process.env.PATH = prependTestPath(shimDir);
+      resetCodexCliAvailabilityMemoForTests();
+      try {
+        expect(codexCliAvailable()).toBe(false);
+      } finally {
+        if (originalPath === undefined) delete process.env.PATH;
+        else process.env.PATH = originalPath;
+        resetCodexCliAvailabilityMemoForTests();
+      }
+    } finally {
+      await rm(shimDir, { recursive: true, force: true });
+    }
+  });
+
+  test("no codex on PATH → unavailable (fast path, no spawn)", async () => {
+    const { codexCliAvailable, resetCodexCliAvailabilityMemoForTests } = await import(
+      "../src/lib/codex-install.ts"
+    );
+    const emptyDir = await mkdtemp(join(tmpdir(), "cc-codex-empty-"));
+    try {
+      const originalPath = process.env.PATH;
+      process.env.PATH = emptyDir;
+      resetCodexCliAvailabilityMemoForTests();
+      try {
+        expect(codexCliAvailable()).toBe(false);
+      } finally {
+        if (originalPath === undefined) delete process.env.PATH;
+        else process.env.PATH = originalPath;
+        resetCodexCliAvailabilityMemoForTests();
+      }
+    } finally {
+      await rm(emptyDir, { recursive: true, force: true });
+    }
+  });
+});