diff --git a/src/tools/background-task/create-background-output.metadata.test.ts b/src/tools/background-task/create-background-output.metadata.test.ts
new file mode 100644
index 000000000..5111667bb
--- /dev/null
+++ b/src/tools/background-task/create-background-output.metadata.test.ts
@@ -0,0 +1,68 @@
+/// <reference types="bun-types" />
+
+import type { ToolContext } from "@opencode-ai/plugin/tool"
+import { describe, expect, test } from "bun:test"
+import type { BackgroundTask } from "../../features/background-agent"
+import { clearPendingStore, consumeToolMetadata } from "../../features/tool-metadata-store"
+import type { BackgroundOutputClient, BackgroundOutputManager } from "./clients"
+import { createBackgroundOutput } from "./create-background-output"
+
+const projectDir = "/Users/yeongyu/local-workspaces/oh-my-opencode"
+
+type ToolContextWithCallID = ToolContext & {
+  callID: string
+}
+
+describe("createBackgroundOutput metadata", () => {
+  test("omits sessionId metadata when task session is not yet assigned", async () => {
+    // #given
+    clearPendingStore()
+
+    const task: BackgroundTask = {
+      id: "task-1",
+      sessionID: undefined,
+      parentSessionID: "main-1",
+      parentMessageID: "msg-1",
+      description: "background task",
+      prompt: "do work",
+      agent: "test-agent",
+      status: "running",
+    }
+    const manager: BackgroundOutputManager = {
+      getTask: id => (id === task.id ? task : undefined),
+    }
+    const client: BackgroundOutputClient = {
+      session: {
+        messages: async () => ({ data: [] }),
+      },
+    }
+    const tool = createBackgroundOutput(manager, client)
+    const context = {
+      sessionID: "test-session",
+      messageID: "test-message",
+      agent: "test-agent",
+      directory: projectDir,
+      worktree: projectDir,
+      abort: new AbortController().signal,
+      metadata: () => {},
+      ask: async () => {},
+      callID: "call-1",
+    } as ToolContextWithCallID
+
+    // #when
+    await tool.execute({ task_id: task.id }, context)
+
+    // #then
+    expect(consumeToolMetadata("test-session", "call-1")).toEqual({
+      title: "test-agent - background task",
+      metadata: {
+        agent: "test-agent",
+        category: undefined,
+        description: "background task",
+        task_id: "task-1",
+      },
+    })
+
+    clearPendingStore()
+  })
+})
diff --git a/src/tools/background-task/create-background-output.ts b/src/tools/background-task/create-background-output.ts
index 446c0888e..3508052cf 100644
--- a/src/tools/background-task/create-background-output.ts
+++ b/src/tools/background-task/create-background-output.ts
@@ -75,7 +75,7 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
             agent: task.agent,
             category: task.category,
             description: task.description,
-            sessionId: task.sessionID ?? "pending",
+            ...(task.sessionID ? { sessionId: task.sessionID } : {}),
           } as Record<string, unknown>,
         }
         ctx.metadata?.(meta)
diff --git a/src/tools/background-task/create-background-task.metadata.test.ts b/src/tools/background-task/create-background-task.metadata.test.ts
new file mode 100644
index 000000000..d21e69c09
--- /dev/null
+++ b/src/tools/background-task/create-background-task.metadata.test.ts
@@ -0,0 +1,84 @@
+/// <reference types="bun-types" />
+
+import type { PluginInput } from "@opencode-ai/plugin"
+import type { ToolContext } from "@opencode-ai/plugin/tool"
+import { describe, expect, mock, test } from "bun:test"
+import type { BackgroundManager } from "../../features/background-agent"
+import { clearPendingStore, consumeToolMetadata } from "../../features/tool-metadata-store"
+import { createBackgroundTask } from "./create-background-task"
+
+const projectDir = "/Users/yeongyu/local-workspaces/oh-my-opencode"
+
+type ToolContextWithCallID = ToolContext & {
+  callID: string
+}
+
+describe("createBackgroundTask metadata", () => {
+  test("omits sessionId metadata when session is not yet assigned", async () => {
+    // #given
+    clearPendingStore()
+
+    const manager = {
+      launch: mock(() => Promise.resolve({
+        id: "task-1",
+        sessionID: null,
+        description: "Test task",
+        agent: "test-agent",
+        status: "pending",
+      })),
+      getTask: mock(() => undefined),
+    } as unknown as BackgroundManager
+    const client = {
+      session: {
+        messages: mock(() => Promise.resolve({ data: [] })),
+      },
+    } as unknown as PluginInput["client"]
+
+    let capturedMetadata: { title?: string; metadata?: Record<string, unknown> } | undefined
+    const tool = createBackgroundTask(manager, client)
+    const originalDateNow = Date.now
+    let dateNowCallCount = 0
+    Date.now = () => {
+      dateNowCallCount += 1
+      return dateNowCallCount === 1 ? 0 : 30001
+    }
+
+    try {
+      // #when
+      const context: ToolContextWithCallID = {
+        sessionID: "test-session",
+        messageID: "test-message",
+        agent: "test-agent",
+        directory: projectDir,
+        worktree: projectDir,
+        abort: new AbortController().signal,
+        ask: async () => {},
+        callID: "call-1",
+        metadata: input => {
+          capturedMetadata = input
+        },
+      }
+
+      const output = await tool.execute(
+        {
+          description: "Test background task",
+          prompt: "Test prompt",
+          agent: "test-agent",
+        },
+        context
+      )
+
+      // #then
+      expect(output).toContain("Session ID: (not yet assigned)")
+      expect(output).not.toContain('Session ID: pending')
+      expect(capturedMetadata?.metadata).toEqual({})
+      expect(consumeToolMetadata("test-session", "call-1")).toEqual({
+        title: "Test background task",
+        metadata: {},
+      })
+    } finally {
+      Date.now = originalDateNow
+      clearPendingStore()
+    }
+  })
+})
diff --git a/src/tools/background-task/create-background-task.ts b/src/tools/background-task/create-background-task.ts
index 9da0d5c50..8f57ed763 100644
--- a/src/tools/background-task/create-background-task.ts
+++ b/src/tools/background-task/create-background-task.ts
@@ -94,9 +94,11 @@ export function createBackgroundTask(
 
         const bgMeta = {
           title: args.description,
-          metadata: { sessionId: sessionId ?? "pending" },
+          metadata: {
+            ...(sessionId ? { sessionId } : {}),
+          },
         }
-        await ctx.metadata?.(bgMeta)
+        ctx.metadata?.(bgMeta)
 
         if (ctx.callID) {
           storeToolMetadata(ctx.sessionID, ctx.callID, bgMeta)
@@ -105,7 +107,7 @@ export function createBackgroundTask(
         return `Background task launched successfully.
 
 Task ID: ${task.id}
-Session ID: ${sessionId ?? "pending"}
+Session ID: ${sessionId ?? "(not yet assigned)"}
 Description: ${task.description}
 Agent: ${task.agent}
 Status: ${task.status}