diff --git a/src/features/background-agent/task-poller.test.ts b/src/features/background-agent/task-poller.test.ts
index 3132de21b..b08f78c59 100644
--- a/src/features/background-agent/task-poller.test.ts
+++ b/src/features/background-agent/task-poller.test.ts
@@ -177,6 +177,7 @@ describe("checkAndInterruptStaleTasks", () => {
     //#then
     expect(task.status).toBe("cancelled")
     expect(task.error).toContain("no activity")
+    expect(task.error).toContain("messageStalenessTimeoutMs")
   })
 
   it("should await abort before resolving for no-progress stale interruption", async () => {
@@ -403,7 +404,7 @@ describe("checkAndInterruptStaleTasks", () => {
       progress: undefined,
     })
 
-    //#when — session is running
+    //#when - session is running
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -419,7 +420,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should NOT cancel healthy task on first missing status poll", async () => {
-    //#given — one missing poll should not be enough to declare the session gone
+    //#given - one missing poll should not be enough to declare the session gone
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -445,7 +446,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should NOT cancel task when session.get confirms the session still exists", async () => {
-    //#given — repeated missing polls but direct lookup still succeeds
+    //#given - repeated missing polls but direct lookup still succeeds
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -472,7 +473,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should NOT cancel task when session.get returns a transient error response", async () => {
-    //#given — repeated missing polls but lookup failed with a retryable transport error
+    //#given - repeated missing polls but lookup failed with a retryable transport error
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -504,7 +505,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should use session-gone timeout when session is missing from status map (with progress)", async () => {
-    //#given — lastUpdate 2min ago, session completely gone from status
+    //#given - lastUpdate 2min ago, session completely gone from status
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -516,7 +517,7 @@ describe("checkAndInterruptStaleTasks", () => {
 
     mockClient.session.get.mockRejectedValue(new Error("missing"))
 
-    //#when — empty sessionStatuses (session gone), sessionGoneTimeoutMs = 60s
+    //#when - empty sessionStatuses (session gone), sessionGoneTimeoutMs = 60s
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -526,7 +527,7 @@ describe("checkAndInterruptStaleTasks", () => {
       sessionStatuses: {},
     })
 
-    //#then — cancelled because session gone timeout (60s) < timeSinceLastUpdate (120s)
+    //#then - cancelled because session gone timeout (60s) < timeSinceLastUpdate (120s)
     expect(task.status).toBe("cancelled")
     expect(task.error).toContain("session gone from status registry")
   })
@@ -571,7 +572,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should use session-gone timeout when session is missing from status map (no progress)", async () => {
-    //#given — task started 2min ago, no progress, session completely gone
+    //#given - task started 2min ago, no progress, session completely gone
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 120_000),
       progress: undefined,
@@ -580,7 +581,7 @@ describe("checkAndInterruptStaleTasks", () => {
 
     mockClient.session.get.mockRejectedValue(new Error("missing"))
 
-    //#when — session gone, sessionGoneTimeoutMs = 60s
+    //#when - session gone, sessionGoneTimeoutMs = 60s
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -590,13 +591,13 @@ describe("checkAndInterruptStaleTasks", () => {
       sessionStatuses: {},
     })
 
-    //#then — cancelled because session gone timeout (60s) < runtime (120s)
+    //#then - cancelled because session gone timeout (60s) < runtime (120s)
     expect(task.status).toBe("cancelled")
     expect(task.error).toContain("session gone from status registry")
   })
 
   it("should NOT use session-gone timeout when session is idle (present in status map)", async () => {
-    //#given — lastUpdate 2min ago, session is idle (present in status but not active)
+    //#given - lastUpdate 2min ago, session is idle (present in status but not active)
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -608,7 +609,7 @@ describe("checkAndInterruptStaleTasks", () => {
 
     mockClient.session.get.mockRejectedValue(new Error("missing"))
 
-    //#when — session is idle (present in map), staleTimeoutMs = 180s
+    //#when - session is idle (present in map), staleTimeoutMs = 180s
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -618,12 +619,12 @@ describe("checkAndInterruptStaleTasks", () => {
       sessionStatuses: { "ses-1": { type: "idle" } },
     })
 
-    //#then — still running because normal staleTimeout (180s) > timeSinceLastUpdate (120s)
+    //#then - still running because normal staleTimeout (180s) > timeSinceLastUpdate (120s)
     expect(task.status).toBe("running")
   })
 
   it("should use default session-gone timeout when not configured", async () => {
-    //#given — lastUpdate 2min ago, session gone, no sessionGoneTimeoutMs config
+    //#given - lastUpdate 2min ago, session gone, no sessionGoneTimeoutMs config
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -635,7 +636,7 @@ describe("checkAndInterruptStaleTasks", () => {
 
     mockClient.session.get.mockRejectedValue(new Error("missing"))
 
-    //#when — no config (default sessionGoneTimeoutMs = 60_000)
+    //#when - no config (default sessionGoneTimeoutMs = 60_000)
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -645,13 +646,13 @@ describe("checkAndInterruptStaleTasks", () => {
       sessionStatuses: {},
     })
 
-    //#then — cancelled because default session gone timeout (60s) < timeSinceLastUpdate (120s)
+    //#then - cancelled because default session gone timeout (60s) < timeSinceLastUpdate (120s)
     expect(task.status).toBe("cancelled")
     expect(task.error).toContain("session gone from status registry")
   })
 
   it("should interrupt task when busy session exceeds stale timeout", async () => {
-    //#given — lastUpdate is 5min old and session is still "busy"
+    //#given - lastUpdate is 5min old and session is still "busy"
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -660,7 +661,7 @@ describe("checkAndInterruptStaleTasks", () => {
       },
     })
 
-    //#when — session status is "busy" (not "running" — OpenCode uses "busy" for active LLM processing)
+    //#when - session status is "busy" (not "running" - OpenCode uses "busy" for active LLM processing)
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -676,7 +677,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should interrupt task when retry session exceeds stale timeout", async () => {
-    //#given — lastUpdate is 5min old but session is retrying
+    //#given - lastUpdate is 5min old but session is retrying
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -685,7 +686,7 @@ describe("checkAndInterruptStaleTasks", () => {
       },
     })
 
-    //#when — session status is "retry" (OpenCode retries on transient API errors)
+    //#when - session status is "retry" (OpenCode retries on transient API errors)
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -701,13 +702,13 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it("should interrupt busy session with no progress after message staleness timeout", async () => {
-    //#given — no progress at all, session is still "busy"
+    //#given - no progress at all, session is still "busy"
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 15 * 60 * 1000),
       progress: undefined,
     })
 
-    //#when — session is busy
+    //#when - session is busy
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -771,7 +772,7 @@ describe("checkAndInterruptStaleTasks", () => {
   })
 
   it('should NOT protect task when session has terminal non-idle status like "interrupted"', async () => {
-    //#given — lastUpdate is 5min old, session is "interrupted" (terminal, not active)
+    //#given - lastUpdate is 5min old, session is "interrupted" (terminal, not active)
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -780,7 +781,7 @@ describe("checkAndInterruptStaleTasks", () => {
       },
     })
 
-    //#when — session status is "interrupted" (terminal)
+    //#when - session status is "interrupted" (terminal)
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -790,13 +791,13 @@ describe("checkAndInterruptStaleTasks", () => {
       sessionStatuses: { "ses-1": { type: "interrupted" } },
     })
 
-    //#then — terminal statuses should not protect from stale timeout
+    //#then - terminal statuses should not protect from stale timeout
     expect(task.status).toBe("cancelled")
     expect(task.error).toContain("Stale timeout")
   })
 
   it('should NOT protect task when session has unknown status type', async () => {
-    //#given — lastUpdate is 5min old, session has an unknown status
+    //#given - lastUpdate is 5min old, session has an unknown status
     const task = createRunningTask({
       startedAt: new Date(Date.now() - 300_000),
       progress: {
@@ -805,7 +806,7 @@ describe("checkAndInterruptStaleTasks", () => {
       },
     })
 
-    //#when — session has unknown status type
+    //#when - session has unknown status type
     await checkAndInterruptStaleTasks({
       tasks: [task],
       client: mockClient as never,
@@ -815,7 +816,7 @@ describe("checkAndInterruptStaleTasks", () => {
       sessionStatuses: { "ses-1": { type: "some-weird-status" } },
     })
 
-    //#then — unknown statuses should not protect from stale timeout
+    //#then - unknown statuses should not protect from stale timeout
     expect(task.status).toBe("cancelled")
     expect(task.error).toContain("Stale timeout")
   })
diff --git a/src/features/background-agent/task-poller.ts b/src/features/background-agent/task-poller.ts
index 408db3b83..9190e4994 100644
--- a/src/features/background-agent/task-poller.ts
+++ b/src/features/background-agent/task-poller.ts
@@ -172,7 +172,7 @@ export async function checkAndInterruptStaleTasks(args: {
       const staleMinutes = Math.round(runtime / 60000)
       const reason = sessionGone ? "session gone from status registry" : "no activity"
       task.status = "cancelled"
-      task.error = `Stale timeout (${reason} for ${staleMinutes}min since start). This is a FINAL cancellation - do NOT create a replacement task. If the timeout is too short, increase 'background_task.${sessionGone ? "sessionGoneTimeoutMs" : "staleTimeoutMs"}' in .opencode/${CONFIG_BASENAME}.json.`
+      task.error = `Stale timeout (${reason} for ${staleMinutes}min since start). This is a FINAL cancellation - do NOT create a replacement task. If the timeout is too short, increase 'background_task.${sessionGone ? "sessionGoneTimeoutMs" : "messageStalenessTimeoutMs"}' in .opencode/${CONFIG_BASENAME}.json.`
       task.completedAt = new Date()
 
       if (task.concurrencyKey) {