diff --git a/src/tools/background-task/create-background-output.ts b/src/tools/background-task/create-background-output.ts
index 50b9c9b4c..0b63f2f50 100644
--- a/src/tools/background-task/create-background-output.ts
+++ b/src/tools/background-task/create-background-output.ts
@@ -78,7 +78,7 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
         }
 
         const isActive = task.status === "pending" || task.status === "running"
-        const fullSession = args.full_session ?? isActive
+        const fullSession = args.full_session ?? false
         const includeThinking = isActive || (args.include_thinking ?? false)
         const includeToolResults = isActive || (args.include_tool_results ?? false)
 
diff --git a/src/tools/background-task/tools.test.ts b/src/tools/background-task/tools.test.ts
index a74b2f196..79b084ce7 100644
--- a/src/tools/background-task/tools.test.ts
+++ b/src/tools/background-task/tools.test.ts
@@ -232,7 +232,7 @@ describe("background_output full_session", () => {
     expect(output).toContain("Has more: true")
   })
 
-  test("defaults to full_session when task is running", async () => {
+  test("defaults to compact status when task is running", async () => {
     // #given
     const task = createTask({ status: "running" })
     const manager = createMockManager(task)
@@ -242,6 +242,21 @@ describe("background_output full_session", () => {
     // #when
     const output = await tool.execute({ task_id: "task-1" }, mockContext)
 
+    // #then
+    expect(output).toContain("# Task Status")
+    expect(output).not.toContain("# Full Session Output")
+  })
+
+  test("returns full session when explicitly requested for running task", async () => {
+    // #given
+    const task = createTask({ status: "running" })
+    const manager = createMockManager(task)
+    const client = createMockClient({})
+    const tool = createBackgroundOutput(manager, client)
+
+    // #when
+    const output = await tool.execute({ task_id: "task-1", full_session: true }, mockContext)
+
     // #then
     expect(output).toContain("# Full Session Output")
   })