diff --git a/src/features/background-agent/spawner.test.ts b/src/features/background-agent/spawner.test.ts
index b1f486c52..eb2cc294b 100644
--- a/src/features/background-agent/spawner.test.ts
+++ b/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
+    }
+  })
+})
diff --git a/src/features/background-agent/spawner.ts b/src/features/background-agent/spawner.ts
index 675aeb5d9..ab6aaa2f1 100644
--- a/src/features/background-agent/spawner.ts
+++ b/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(