diff --git a/src/tools/delegate-task/background-task.test.ts b/src/tools/delegate-task/background-task.test.ts
new file mode 100644
index 00000000..cc87586f
--- /dev/null
+++ b/src/tools/delegate-task/background-task.test.ts
@@ -0,0 +1,108 @@
+const bunTest = require("bun:test")
+const describeFn = bunTest.describe
+const testFn = bunTest.test
+const expectFn = bunTest.expect
+const beforeEachFn = bunTest.beforeEach
+const afterEachFn = bunTest.afterEach
+
+const { executeBackgroundTask } = require("./background-task")
+const { __setTimingConfig, __resetTimingConfig } = require("./timing")
+
+describeFn("executeBackgroundTask output/session metadata compatibility", () => {
+  beforeEachFn(() => {
+    //#given - reduce waiting to keep tests fast
+    __setTimingConfig({
+      WAIT_FOR_SESSION_INTERVAL_MS: 1,
+      WAIT_FOR_SESSION_TIMEOUT_MS: 2,
+    })
+  })
+
+  afterEachFn(() => {
+    __resetTimingConfig()
+  })
+
+  testFn("does not emit synthetic pending session metadata when session id is unresolved", async () => {
+    //#given - launched task without resolved subagent session id
+    const metadataCalls: any[] = []
+    const manager = {
+      launch: async () => ({
+        id: "bg_unresolved",
+        sessionID: undefined,
+        description: "Unresolved session",
+        agent: "explore",
+        status: "running",
+      }),
+      getTask: () => undefined,
+    }
+
+    const result = await executeBackgroundTask(
+      {
+        description: "Unresolved session",
+        prompt: "check",
+        run_in_background: true,
+        load_skills: [],
+      },
+      {
+        sessionID: "ses_parent",
+        callID: "call_1",
+        metadata: async (value: any) => metadataCalls.push(value),
+        abort: new AbortController().signal,
+      },
+      { manager },
+      { sessionID: "ses_parent", messageID: "msg_1" },
+      "explore",
+      undefined,
+      undefined,
+      undefined,
+    )
+
+    //#then - output and metadata should avoid fake session markers
+    expectFn(result).not.toContain("<task_metadata>")
+    expectFn(result).not.toContain("session_id: undefined")
+    expectFn(result).not.toContain("session_id: pending")
+    expectFn(metadataCalls).toHaveLength(1)
+    expectFn("sessionId" in metadataCalls[0].metadata).toBe(false)
+  })
+
+  testFn("emits task metadata session_id when real session id is available", async () => {
+    //#given - launched task with resolved subagent session id
+    const metadataCalls: any[] = []
+    const manager = {
+      launch: async () => ({
+        id: "bg_resolved",
+        sessionID: "ses_sub_123",
+        description: "Resolved session",
+        agent: "explore",
+        status: "running",
+      }),
+      getTask: () => ({ sessionID: "ses_sub_123" }),
+    }
+
+    const result = await executeBackgroundTask(
+      {
+        description: "Resolved session",
+        prompt: "check",
+        run_in_background: true,
+        load_skills: [],
+      },
+      {
+        sessionID: "ses_parent",
+        callID: "call_2",
+        metadata: async (value: any) => metadataCalls.push(value),
+        abort: new AbortController().signal,
+      },
+      { manager },
+      { sessionID: "ses_parent", messageID: "msg_2" },
+      "explore",
+      undefined,
+      undefined,
+      undefined,
+    )
+
+    //#then - output and metadata should include canonical session linkage
+    expectFn(result).toContain("<task_metadata>")
+    expectFn(result).toContain("session_id: ses_sub_123")
+    expectFn(metadataCalls).toHaveLength(1)
+    expectFn(metadataCalls[0].metadata.sessionId).toBe("ses_sub_123")
+  })
+})
diff --git a/src/tools/delegate-task/background-task.ts b/src/tools/delegate-task/background-task.ts
index c01538a5..6aad94ea 100644
--- a/src/tools/delegate-task/background-task.ts
+++ b/src/tools/delegate-task/background-task.ts
@@ -56,25 +56,31 @@ export async function executeBackgroundTask(
       SessionCategoryRegistry.register(sessionId, args.category)
     }
 
+    const metadata = {
+      prompt: args.prompt,
+      agent: task.agent,
+      category: args.category,
+      load_skills: args.load_skills,
+      description: args.description,
+      run_in_background: args.run_in_background,
+      command: args.command,
+      ...(sessionId ? { sessionId } : {}),
+      ...(categoryModel ? { model: { providerID: categoryModel.providerID, modelID: categoryModel.modelID } } : {}),
+    }
+
     const unstableMeta = {
       title: args.description,
-      metadata: {
-        prompt: args.prompt,
-        agent: task.agent,
-        category: args.category,
-        load_skills: args.load_skills,
-        description: args.description,
-        run_in_background: args.run_in_background,
-        sessionId: sessionId ?? "pending",
-        command: args.command,
-        model: categoryModel ? { providerID: categoryModel.providerID, modelID: categoryModel.modelID } : undefined,
-      },
+      metadata,
     }
     await ctx.metadata?.(unstableMeta)
     if (ctx.callID) {
       storeToolMetadata(ctx.sessionID, ctx.callID, unstableMeta)
     }
 
+    const taskMetadataBlock = sessionId
+      ? `\n\n<task_metadata>\nsession_id: ${sessionId}\n</task_metadata>`
+      : ""
+
     return `Background task launched.
 
 Task ID: ${task.id}
@@ -82,11 +88,7 @@ Description: ${task.description}
 Agent: ${task.agent}${args.category ? ` (category: ${args.category})` : ""}
 Status: ${task.status}
 
-System notifies on completion. Use \`background_output\` with task_id="${task.id}" to check.
-
-<task_metadata>
-session_id: ${sessionId}
-</task_metadata>`
+System notifies on completion. Use \`background_output\` with task_id="${task.id}" to check.${taskMetadataBlock}`
   } catch (error) {
     return formatDetailedError(error, {
       operation: "Launch background task",