fix(background-agent): fail abort on SDK errors

Treat resolved abort responses with a non-null error payload the same as rejected aborts. This prevents stale-timeout cancellation bookkeeping from reporting success when the child session was not actually aborted.

Plan: .omo/plans/subagent-timeout-active-output.md

src/features/background-agent/abort-with-timeout.test.ts

@@ -37,6 +37,22 @@ describe("abortWithTimeout", () => {
     expect(logMock).not.toHaveBeenCalled()
   })
 
+  test("#given abort resolves with an SDK error response #when abortWithTimeout runs #then it reports cancellation failure", async () => {
+    // given
+    const error = { message: "session not found" }
+    const abort = mock(async () => ({ error }))
+
+    // when
+    const result = await abortWithTimeout(createClient(abort), "session-error-response", 10)
+
+    // then
+    expect(result).toBe(false)
+    expect(logMock).toHaveBeenCalledWith(
+      "[background-agent] Session abort returned an error response:",
+      { sessionID: "session-error-response", error },
+    )
+  })
+
   test("#given abort hangs indefinitely #when abortWithTimeout runs #then it logs warning and continues", async () => {
     // given
     const abort = mock(() => new Promise<never>(() => {}))

src/features/background-agent/abort-with-timeout.ts

@@ -1,6 +1,13 @@
 import { log } from "../../shared"
+import { isRecord } from "../../shared/record-type-guard"
 import type { OpencodeClient } from "./opencode-client"
 
+function getAbortResponseError(response: unknown): unknown | undefined {
+  if (!isRecord(response)) return undefined
+  const error = response.error
+  return error === undefined || error === null ? undefined : error
+}
+
 export async function abortWithTimeout(
   client: OpencodeClient,
   sessionID: string,
@@ -10,7 +17,26 @@ export async function abortWithTimeout(
 
   try {
     const result = await Promise.race([
-      client.session.abort({ path: { id: sessionID } }).then(() => "aborted" as const),
+      client.session.abort({ path: { id: sessionID } }).then(
+        (response) => {
+          const error = getAbortResponseError(response)
+          if (error !== undefined) {
+            log("[background-agent] Session abort returned an error response:", {
+              sessionID,
+              error,
+            })
+            return "failed" as const
+          }
+          return "aborted" as const
+        },
+        (error) => {
+          log("[background-agent] Session abort failed:", {
+            sessionID,
+            error,
+          })
+          return "failed" as const
+        },
+      ),
       new Promise<"timed_out">((resolve) => {
         timeoutHandle = setTimeout(() => {
           resolve("timed_out")
@@ -26,7 +52,7 @@ export async function abortWithTimeout(
       return false
     }
 
-    return true
+    return result === "aborted"
   } finally {
     if (timeoutHandle) {
       clearTimeout(timeoutHandle)