fix(background-agent): fire promptAsync before tmux callback

Awaiting the tmux callback blocked the prompt for up to 10s (waitForSessionReady downstream). During that window the spawned pane ran 'opencode attach' against an empty session and rendered a blank TUI. Users saw 'pane created but attach not working'.

Start promptWithModelSuggestionRetry immediately after session.create, then invoke the tmux callback as fire-and-forget. The session becomes active before the attach client connects.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/spawner.test.ts

@@ -577,3 +577,84 @@ describe("background-agent spawner fallback model promotion", () => {
     expect(promptCalls[0]?.body?.agent).toBe("sisyphus-junior")
   })
 })
+
+describe("background-agent spawner tmux callback ordering", () => {
+  test("fires promptAsync before tmux callback resolves (no blocking)", async () => {
+    //#given
+    const events: string[] = []
+    let resolveTmuxCallback: () => void = () => {}
+    const tmuxCallbackPromise = new Promise<void>((resolve) => {
+      resolveTmuxCallback = resolve
+    })
+
+    const client = {
+      session: {
+        get: async () => ({ data: { directory: "/tmp/test" } }),
+        create: async () => {
+          events.push("session.create")
+          return { data: { id: "ses_blocking_tmux" } }
+        },
+        promptAsync: async () => {
+          events.push("promptAsync")
+          return { data: {} }
+        },
+      },
+    } as any
+
+    const onSubagentSessionCreated = mock(async () => {
+      events.push("tmux.callback.start")
+      await tmuxCallbackPromise
+      events.push("tmux.callback.end")
+    })
+
+    const task = createTask({
+      description: "Blocking tmux test",
+      prompt: "Do work",
+      agent: "general",
+      parentSessionID: "ses_parent",
+      parentMessageID: "msg_parent",
+    })
+
+    const item = {
+      task,
+      input: {
+        description: task.description,
+        prompt: task.prompt,
+        agent: task.agent,
+        parentSessionID: task.parentSessionID,
+        parentMessageID: task.parentMessageID,
+      },
+    }
+
+    const ctx = {
+      client,
+      directory: "/tmp/test",
+      concurrencyManager: { release: () => {} },
+      tmuxEnabled: true,
+      onSubagentSessionCreated,
+      onTaskError: () => {},
+    }
+
+    const originalTmux = process.env.TMUX
+    process.env.TMUX = "/tmp/fake-tmux-socket"
+
+    try {
+      //#when
+      await startTask(item as any, ctx as any)
+      await new Promise((resolve) => setTimeout(resolve, 20))
+
+      //#then
+      expect(events).toContain("session.create")
+      expect(events).toContain("promptAsync")
+      expect(events).toContain("tmux.callback.start")
+      const promptIdx = events.indexOf("promptAsync")
+      const tmuxStartIdx = events.indexOf("tmux.callback.start")
+      expect(promptIdx < tmuxStartIdx).toBe(true)
+      expect(events).not.toContain("tmux.callback.end")
+    } finally {
+      resolveTmuxCallback()
+      if (originalTmux === undefined) delete process.env.TMUX
+      else process.env.TMUX = originalTmux
+    }
+  })
+})

src/features/background-agent/spawner.ts

@@ -1,6 +1,5 @@
 import type { BackgroundTask, LaunchInput, ResumeInput } from "./types"
 import type { OpencodeClient, OnSubagentSessionCreated, QueueItem } from "./constants"
-import { TMUX_CALLBACK_DELAY_MS } from "./constants"
 import { log, getAgentToolRestrictions, promptWithModelSuggestionRetry, createInternalAgentTextPart } from "../../shared"
 import { applySessionPromptParams } from "../../shared/session-prompt-params-helpers"
 import { subagentSessions } from "../claude-code-session-state"
@@ -115,29 +114,6 @@ export async function startTask(
   const sessionID = createResult.data.id
   subagentSessions.add(sessionID)
 
-  log("[background-agent] tmux callback check", {
-    hasCallback: !!onSubagentSessionCreated,
-    tmuxEnabled,
-    isInsideTmux: isInsideTmux(),
-    sessionID,
-    parentID: input.parentSessionID,
-  })
-
-  if (onSubagentSessionCreated && tmuxEnabled && isInsideTmux()) {
-    log("[background-agent] Invoking tmux callback NOW", { sessionID })
-    await onSubagentSessionCreated({
-      sessionID,
-      parentID: input.parentSessionID,
-      title: input.description,
-    }).catch((err) => {
-      log("[background-agent] Failed to spawn tmux pane:", err)
-    })
-    log("[background-agent] tmux callback completed, waiting")
-    await new Promise(r => setTimeout(r, TMUX_CALLBACK_DELAY_MS))
-  } else {
-    log("[background-agent] SKIP tmux callback - conditions not met")
-  }
-
   task.status = "running"
   task.startedAt = new Date()
   task.sessionID = sessionID
@@ -188,7 +164,8 @@ export async function startTask(
     parts: [createInternalAgentTextPart(input.prompt)],
   }
 
-  promptWithModelSuggestionRetry(client, {
+  // Must fire BEFORE tmux callback: attach client needs session activity to render TUI.
+  const promptChain = promptWithModelSuggestionRetry(client, {
     path: { id: sessionID },
     body: promptBody,
   }).catch(async (error) => {
@@ -214,6 +191,29 @@ export async function startTask(
     log("[background-agent] promptAsync error:", error)
     onTaskError(task, error instanceof Error ? error : new Error(String(error)))
   })
+
+  void promptChain
+
+  log("[background-agent] tmux callback check", {
+    hasCallback: !!onSubagentSessionCreated,
+    tmuxEnabled,
+    isInsideTmux: isInsideTmux(),
+    sessionID,
+    parentID: input.parentSessionID,
+  })
+
+  if (onSubagentSessionCreated && tmuxEnabled && isInsideTmux()) {
+    log("[background-agent] Invoking tmux callback (fire-and-forget)", { sessionID })
+    void onSubagentSessionCreated({
+      sessionID,
+      parentID: input.parentSessionID,
+      title: input.description,
+    }).catch((err) => {
+      log("[background-agent] Failed to spawn tmux pane:", err)
+    })
+  } else {
+    log("[background-agent] SKIP tmux callback - conditions not met")
+  }
 }
 
 export async function resumeTask(