diff --git a/README.md b/README.md
index 238dce90..c4757aad 100644
--- a/README.md
+++ b/README.md
@@ -64,7 +64,7 @@ Internal build — all rights reserved.
 
 | Need | Version | Required for |
 |---|---|---|
-| **Node.js** | >= 20 (>= 22.5 for a plugin install) | Everything. Node **24** is what the test suites are verified on |
+| **Node.js** | >= 20 (>= 22.5 for a plugin install) | Everything. Both suites are verified green on **24** and **26** |
 | **npm** | 9+ | Ships with Node |
 | **git** | any | Cloning, and every lane operation (worktrees, branch/status reads) |
 | **tmux** | 3.x | The browser run console, `agenthub lanes shell`, typing `/reload-plugins` into a live pane, the Codex nudge |
@@ -73,8 +73,10 @@ Internal build — all rights reserved.
 | **GitHub CLI (`gh`)** | any | Optional — only for `agenthub lanes ci status` and the PR stages of the ship pipelines |
 | **systemd (user)** | — | Optional — only for `agenthub service install` (Linux auto-start on boot) |
 
-Node 25 currently breaks 6 server tests through a better-sqlite3 ABI mismatch
-and 20 client tests through a global `localStorage` change — stay on 20/22/24.
+Node **25** is the one version to avoid: it breaks 6 server tests through a
+better-sqlite3 ABI mismatch and 20 client tests through a global `localStorage`
+change. Both were gone again by 26 (1712 server + 488 client tests green on
+v26.8.1).
 
 ### Installing tmux
 
diff --git a/README.vi.md b/README.vi.md
index bcec0345..96e13fef 100644
--- a/README.vi.md
+++ b/README.vi.md
@@ -64,7 +64,7 @@ Internal build — all rights reserved.
 
 | Cần | Phiên bản | Dùng cho |
 |---|---|---|
-| **Node.js** | >= 20 (>= 22.5 nếu cài qua plugin) | Tất cả. Test suite được verify trên Node **24** |
+| **Node.js** | >= 20 (>= 22.5 nếu cài qua plugin) | Tất cả. Cả hai test suite verify xanh trên Node **24** và **26** |
 | **npm** | 9+ | Đi kèm Node |
 | **git** | bất kỳ | Clone repo, và mọi thao tác lane (worktree, đọc branch/status) |
 | **tmux** | 3.x | Run console trên trình duyệt, `agenthub lanes shell`, gõ `/reload-plugins` vào pane đang sống, nhắc nhở Codex |
@@ -73,8 +73,9 @@ Internal build — all rights reserved.
 | **GitHub CLI (`gh`)** | bất kỳ | Tùy chọn — chỉ cho `agenthub lanes ci status` và các stage PR của ship pipeline |
 | **systemd (user)** | — | Tùy chọn — chỉ cho `agenthub service install` (tự khởi động khi boot, Linux) |
 
-Node 25 hiện đang làm gãy 6 test server do lệch ABI `better-sqlite3` và 20 test
-client do thay đổi global `localStorage` — hãy ở lại 20/22/24.
+Node **25** là bản duy nhất nên tránh: nó làm gãy 6 test server do lệch ABI
+`better-sqlite3` và 20 test client do thay đổi global `localStorage`. Tới 26 thì
+cả hai đã hết (1712 test server + 488 test client xanh trên v26.8.1).
 
 ### Cài tmux
 
diff --git a/docs/API.md b/docs/API.md
index cec63ea2..819c9d82 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -245,6 +245,8 @@ All fields are optional. The tool (CLI invoked) comes from the lane itself, neve
 - **`approval`** — one of `on-request` or `never`, or omitted/empty for no approval prompt. Passed to `codex -a <value>`.
 - **`resumeSessionId`** or **`resumeLast: true`** — resume an earlier run instead of starting new. For Codex lanes specifically, `resumeLast: true` uses `codex resume --last`; `resumeSessionId` uses `codex resume <sessionId>`. For Claude lanes, `resumeSessionId` re-attaches a Claude session by id.
 
+Error `503 ETMUXMISSING` is returned when the host has no `tmux` binary. Every run the dashboard starts is a tmux session, so this is a missing dependency rather than a bad request — the message carries the install command, and the same code comes back from `POST /api/run`. Nothing else in the dashboard is affected.
+
 Error `409 ERUNLIVE` is returned if the lane already has a live run; stop it first with the `stop` action. The same code and status come back when the lane's tmux pane is already running an agent while a `prompt` or `resumeSessionId` was sent — that argv cannot be typed over a live agent, so the request is refused instead of answered 200 with nothing done. A start that asks for neither still adopts the existing pane silently.
 
 #### Preflight a destructive lane action
diff --git a/server/__tests__/pty-run.test.js b/server/__tests__/pty-run.test.js
index 402c6c9c..82742fe7 100644
--- a/server/__tests__/pty-run.test.js
+++ b/server/__tests__/pty-run.test.js
@@ -58,6 +58,29 @@ describe("pty-run", () => {
     assert.ok(newSessionCall.includes("opus"));
   });
 
+  it("spawnRun refuses with ETMUXMISSING, naming the fix, when tmux is not installed", () => {
+    // Before this guard the ENOENT escaped from three calls down as
+    // `spawnSync tmux ENOENT` and the route answered 400 — blaming the request
+    // for a missing host package, with no hint of what to install.
+    const calls = [];
+    tmux.__setExecImpl((args) => {
+      calls.push(args);
+      const e = new Error("spawnSync tmux ENOENT");
+      e.code = "ENOENT";
+      throw e;
+    });
+
+    assert.throws(
+      () => pty.spawnRun({ laneId: 42, cwd: "/tmp/repo" }),
+      (err) => err.code === "ETMUXMISSING" && /install/i.test(err.message),
+      "a missing tmux must be reported as such, with the install command"
+    );
+    assert.ok(
+      !calls.some((c) => c[0] === "new-session"),
+      "must not attempt a session it already knows cannot be created"
+    );
+  });
+
   it("spawnRun is a no-op (adopts) when the existing session's pane runs claude", () => {
     const calls = [];
     tmux.__setExecImpl((args) => {
diff --git a/server/__tests__/run.test.js b/server/__tests__/run.test.js
index 2f0003c4..cedfe114 100644
--- a/server/__tests__/run.test.js
+++ b/server/__tests__/run.test.js
@@ -118,6 +118,23 @@ describe("/api/run", () => {
     assert.equal(body.error.code, "EBADCWD");
   });
 
+  it("POST / answers 503, not 400, when the host has no tmux", async () => {
+    // The request is fine; the dependency is absent. 400 would tell the user to
+    // fix their input, and the body must carry the install line the UI shows.
+    tmux.__setExecImpl(() => {
+      const e = new Error("spawnSync tmux ENOENT");
+      e.code = "ENOENT";
+      throw e;
+    });
+    const { status, body } = await fetchJson("/api/run", {
+      method: "POST",
+      body: { laneId: 9, cwd: TMP },
+    });
+    assert.equal(status, 503);
+    assert.equal(body.error.code, "ETMUXMISSING");
+    assert.match(body.error.message, /install/i);
+  });
+
   it("POST / spawns a tmux session and GET /:id finds it", async () => {
     let hasSessionCalls = 0;
     tmux.__setExecImpl((args) => {
diff --git a/server/lib/pty-run.js b/server/lib/pty-run.js
index 18e396a7..40024b2b 100644
--- a/server/lib/pty-run.js
+++ b/server/lib/pty-run.js
@@ -181,6 +181,21 @@ function spawnRun(args) {
     throw makeErr("EBADSESSION", "resumeSessionId is not a valid session id");
   }
 
+  // A run IS a tmux session, so an absent binary is a precondition failure, not
+  // a spawn failure. Without this the caller gets `spawnSync tmux ENOENT` from
+  // three directories down — a message that names no fix — and the route maps
+  // it to 400, blaming the request for the host's missing package. Checked here
+  // rather than in each route because both spawn paths (POST /api/run and
+  // POST /api/lanes/:id/start) come through this function.
+  if (!tmux.isTmuxAvailable()) {
+    throw makeErr(
+      "ETMUXMISSING",
+      "tmux is not installed, and every run the dashboard starts is a tmux session. " +
+        "Install it (sudo apt install tmux / brew install tmux; on Windows run AgentHub under WSL2), " +
+        "then restart the dashboard. Everything else keeps working without it."
+    );
+  }
+
   const id = runIdForLane(laneId);
   const startedAt = Date.now();
   const argv = buildArgv({
diff --git a/server/routes/lanes.js b/server/routes/lanes.js
index 572cf652..8c6c0cfe 100644
--- a/server/routes/lanes.js
+++ b/server/routes/lanes.js
@@ -1360,6 +1360,12 @@ router.post("/:id/:action", sameOriginGuard, async (req, res) => {
         break;
     }
   } catch (err) {
+    // A host with no tmux cannot start a run, but nothing is broken on this
+    // side — 503 plus the install line, not a 500 the reader can only read as a
+    // dashboard bug.
+    if (err.code === "ETMUXMISSING") {
+      return res.status(503).json({ error: { code: err.code, message: err.message } });
+    }
     return res.status(500).json({ error: { code: err.code, message: err.message } });
   }
 
diff --git a/server/routes/run.js b/server/routes/run.js
index e8f028b5..1be58cb5 100644
--- a/server/routes/run.js
+++ b/server/routes/run.js
@@ -260,6 +260,11 @@ router.post("/", (req, res) => {
     if (err.code === "ERUNLIVE") {
       return res.status(409).json({ error: { code: err.code, message: err.message } });
     }
+    // Nor is a host missing tmux: the request was fine, the dependency is not
+    // installed. 400 would tell the user to fix their input.
+    if (err.code === "ETMUXMISSING") {
+      return res.status(503).json({ error: { code: err.code, message: err.message } });
+    }
     if (err.code && err.code.startsWith("E")) {
       return res.status(400).json({ error: { code: err.code, message: err.message } });
     }