diff --git a/.changeset/attach-runtime-hint.md b/.changeset/attach-runtime-hint.md
new file mode 100644
index 00000000..46b165c1
--- /dev/null
+++ b/.changeset/attach-runtime-hint.md
@@ -0,0 +1,8 @@
+---
+"@cotal-ai/manager": patch
+---
+
+`cotal attach` now gives runtime-correct guidance. The manager previously assumed any non-`pty`
+runtime was tmux, so cmux users were told to run a `tmux attach` command that doesn't apply.
+`opAttach` now delegates to the agent handle's own `attach()`, so each runtime speaks for itself:
+tmux → `tmux attach -t cotal-<space>:<name>`, cmux → switch to the `cotal-<name>` tab.
diff --git a/docs/architecture.md b/docs/architecture.md
index f7978171..b9343c98 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -263,12 +263,14 @@ dependency on them). Selectable backends:
   or types in via `cotal attach <name>` (stream the PTY), and the manager keeps full OS-signal
   control (group-kill, restart). No external software to install.
 - **`tmux` / `iTerm2` (opt-in)** — for users already living in a multiplexer who want native
-  panes / persistence; auto-detect (if already inside tmux, use it).
+  panes / persistence; auto-detect (if already inside tmux, use it). You watch it natively, so
+  `cotal attach` points you at `tmux attach -t cotal-<space>:<name>` rather than streaming.
 - **`cmux` (integration)** — each agent gets its own [cmux](https://github.com/) tab. This is a
   true plug-in: the `cmux` runtime lives in **`@cotal-ai/cmux`** and self-registers a `RuntimeProvider`
   on import, so the manager spawns into tabs without depending on the package — a composition root
   opts in with one `import "@cotal-ai/cmux"` (the `cotal` binary does). Like tmux you watch it
-  natively, so `attach` points you at the tab rather than streaming. Teardown is real: the runtime
+  natively, so `cotal attach` points you at the `cotal-<name>` tab rather than streaming (it is
+  *not* tmux — cmux is its own CLI/app). Teardown is real: the runtime
   keeps the tab's workspace + surface ids, so `stop` types `/exit` for a clean leave then closes the
   tab (graceful) or closes it outright (hard). The manager must run inside a live cmux surface (cmux
   only authorizes its control socket from a real pane). Drives
diff --git a/implementations/manager/attach.smoke.ts b/implementations/manager/attach.smoke.ts
new file mode 100644
index 00000000..0c84f57f
--- /dev/null
+++ b/implementations/manager/attach.smoke.ts
@@ -0,0 +1,100 @@
+/**
+ * Runtime attach smoke (no NATS, no test runner) — run with: pnpm smoke:attach
+ *
+ * Guards that `cotal attach` gives runtime-correct guidance: only `pty` streams over the WS
+ * attach endpoint; `tmux`/`cmux` are watched natively, so each handle's attach() must throw its
+ * OWN hint (this is what manager.opAttach surfaces). Regression target: before, opAttach assumed
+ * "not pty == tmux" and told cmux users to run a tmux command. Spawns a throwaway `sleep` so it
+ * needs no claude/mesh; tmux/cmux are skipped (logged) when not present on the machine.
+ */
+import { execFileSync } from "node:child_process";
+import { createRuntime } from "./src/index.js";
+import "@cotal-ai/cmux"; // registers the `cmux` runtime provider
+import type { LaunchSpec } from "@cotal-ai/core";
+
+let failures = 0;
+function check(label: string, cond: boolean): void {
+  console.log(`${cond ? "✓" : "✗"} ${label}`);
+  if (!cond) failures++;
+}
+function skip(label: string, why: string): void {
+  console.log(`• ${label} skipped (${why})`);
+}
+function attachError(fn: () => unknown): string {
+  try {
+    fn();
+    return "";
+  } catch (e) {
+    return (e as Error).message;
+  }
+}
+
+const SESSION = "cotal-smoke";
+const spec: LaunchSpec = { command: "sleep", args: ["60"] };
+const cwd = process.cwd();
+
+// pty (default) — the one streamable backend: attach() returns a live session, never throws.
+{
+  const h = createRuntime("pty", SESSION).spawn("smoke-pty", spec, cwd);
+  let sess: { onData?: unknown; cols?: unknown } | undefined;
+  const err = attachError(() => (sess = h.attach()));
+  check(
+    "pty: attach() returns a live session (streams, no throw)",
+    err === "" && typeof sess?.onData === "function" && typeof sess?.cols === "number",
+  );
+  h.stop({ graceful: false });
+}
+
+// tmux — watched natively: attach() points you at `tmux attach -t <session>:<name>`.
+{
+  let rt: ReturnType<typeof createRuntime> | null = null;
+  try {
+    rt = createRuntime("tmux", SESSION);
+  } catch (e) {
+    skip("tmux", (e as Error).message);
+  }
+  if (rt) {
+    const h = rt.spawn("smoke-tmux", spec, cwd);
+    const err = attachError(() => h.attach());
+    check(
+      "tmux: attach() points to `tmux attach -t cotal-smoke:smoke-tmux` (not a stream)",
+      err.includes("tmux attach -t cotal-smoke:smoke-tmux"),
+    );
+    check("tmux: hint is tmux-specific, never a cmux tab", !err.includes("cmux tab"));
+    h.stop({ graceful: false });
+    try {
+      execFileSync("tmux", ["kill-session", "-t", SESSION], { stdio: "ignore" });
+    } catch {
+      /* session already gone */
+    }
+  }
+}
+
+// cmux — watched natively: attach() points you at the `cotal-<name>` tab, NOT tmux.
+{
+  let rt: ReturnType<typeof createRuntime> | null = null;
+  try {
+    rt = createRuntime("cmux", SESSION);
+  } catch (e) {
+    skip("cmux", (e as Error).message);
+  }
+  if (rt) {
+    let h: ReturnType<typeof rt.spawn> | null = null;
+    try {
+      h = rt.spawn("smoke-cmux", spec, cwd);
+    } catch (e) {
+      skip("cmux", `spawn: ${(e as Error).message}`); // e.g. not inside a live cmux surface
+    }
+    if (h) {
+      const err = attachError(() => h!.attach());
+      check(
+        'cmux: attach() points to the "cotal-smoke-cmux" tab (not tmux)',
+        err.includes('switch to the "cotal-smoke-cmux" cmux tab') && !err.includes("tmux attach"),
+      );
+      h.stop({ graceful: false });
+    }
+  }
+}
+
+console.log(failures ? `\n${failures} check(s) failed` : "\nall checks passed");
+process.exit(failures ? 1 : 0);
diff --git a/implementations/manager/src/manager.ts b/implementations/manager/src/manager.ts
index 378513d6..62ee5533 100644
--- a/implementations/manager/src/manager.ts
+++ b/implementations/manager/src/manager.ts
@@ -265,11 +265,15 @@ export class Manager {
     const name = String(args.name ?? "").trim();
     const a = this.agents.get(name);
     if (!a) return { ok: false, error: `no agent "${name}"` };
-    if (this.runtime.kind !== "pty") {
-      return {
-        ok: false,
-        error: `attach needs the pty runtime; under tmux run \`tmux attach -t cotal-${this.space}:${name}\``,
-      };
+    // Only pty streams over the WS attach endpoint. tmux/cmux are watched natively, and
+    // each handle's attach() throws with the right per-runtime guidance (tmux attach … /
+    // switch to the cmux tab) — surface that instead of assuming tmux.
+    if (a.handle.kind !== "pty") {
+      try {
+        a.handle.attach();
+      } catch (e) {
+        return { ok: false, error: (e as Error).message };
+      }
     }
     return { ok: true, data: { ws: this.attach.url(name) } };
   }
diff --git a/package.json b/package.json
index dd71a6d2..f1c141ed 100644
--- a/package.json
+++ b/package.json
@@ -25,7 +25,8 @@
     "smoke:channels:auth": "tsx packages/core/channels-auth.smoke.ts",
     "smoke:attention": "tsx extensions/connector-core/attention.smoke.ts",
     "smoke:attention:auth": "tsx extensions/connector-core/attention-auth.smoke.ts",
-    "smoke:feedback": "tsx extensions/connector-core/feedback.smoke.ts"
+    "smoke:feedback": "tsx extensions/connector-core/feedback.smoke.ts",
+    "smoke:attach": "tsx implementations/manager/attach.smoke.ts"
   },
   "dependencies": {
     "@cotal-ai/cli": "workspace:*",