diff --git a/src/create-managers.test.ts b/src/create-managers.test.ts
index 075080606..1380e40b1 100644
--- a/src/create-managers.test.ts
+++ b/src/create-managers.test.ts
@@ -175,6 +175,28 @@ describe("createManagers", () => {
     expect(markServerRunningInProcess).toHaveBeenCalledTimes(1)
   })
 
+  it("#given tmux is enabled but ctx.serverUrl is undefined #when managers are created #then it does NOT mark the server as running (issue #3894)", () => {
+    // Vanilla `opencode` (no `opencode serve` / `opencode web`) leaves
+    // ctx.serverUrl undefined. Marking the server as in-process running
+    // would short-circuit isServerRunning() in createTeamLayout, letting
+    // it spawn tmux panes whose `opencode attach` then fails because no
+    // server is actually listening on the fallback port.
+    const ctx = createContext("/tmp")
+    const ctxWithoutServerUrl = { ...ctx, serverUrl: undefined as unknown as URL }
+    const args = {
+      ctx: ctxWithoutServerUrl,
+      pluginConfig: OhMyOpenCodeConfigSchema.parse({}),
+      tmuxConfig: createTmuxConfig(true),
+      modelCacheState: createModelCacheState(),
+      backgroundNotificationHookEnabled: false,
+      deps: createDeps(),
+    }
+
+    createManagers(args)
+
+    expect(markServerRunningInProcess).not.toHaveBeenCalled()
+  })
+
   it("#given openclaw is enabled #when the background session-created callback runs #then it dispatches openclaw with the tracked pane id", async () => {
     const args = {
       ctx: createContext("/tmp/project"),
diff --git a/src/create-managers.ts b/src/create-managers.ts
index 842f6cfe3..40b752983 100644
--- a/src/create-managers.ts
+++ b/src/create-managers.ts
@@ -57,7 +57,15 @@ export function createManagers(args: {
   const { ctx, pluginConfig, tmuxConfig, modelCacheState, backgroundNotificationHookEnabled } = args
   const deps = { ...defaultCreateManagersDeps, ...args.deps }
 
-  if (tmuxConfig.enabled) {
+  // Only mark the server as in-process when the SDK actually exposes a
+  // serverUrl. `tmuxConfig.enabled` alone is not proof of a running server —
+  // a vanilla `opencode` session (no `opencode serve`/`opencode web`) leaves
+  // `ctx.serverUrl` undefined, and marking it running would make
+  // `isServerRunning` short-circuit to true. That bypasses the guard in
+  // `createTeamLayout` and lets it spawn tmux panes whose `opencode attach`
+  // command then fails because nothing is actually listening on the
+  // fallback port (issue #3894).
+  if (tmuxConfig.enabled && ctx.serverUrl) {
     deps.markServerRunningInProcessFn()
   }
   const tmuxSessionManager = new deps.TmuxSessionManagerClass(ctx, tmuxConfig)