diff --git a/knowledge-fs/apps/api/src/durable-deletion-options.test.ts b/knowledge-fs/apps/api/src/durable-deletion-options.test.ts
index 5f58f86b86b..da420465234 100644
--- a/knowledge-fs/apps/api/src/durable-deletion-options.test.ts
+++ b/knowledge-fs/apps/api/src/durable-deletion-options.test.ts
@@ -10,6 +10,7 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   assertApiDurableDeletionDataReadiness,
   createApiDurableDeletionAssembly,
+  writeDurableDeletionErrorLog,
 } from "./durable-deletion-options";
 
 type DatabaseExecuteInput = Parameters<
@@ -137,6 +138,29 @@ describe("API durable deletion assembly", () => {
     expect(repository.claimJobs).toHaveBeenCalledTimes(1);
     expect(repository.claimOutbox).toHaveBeenCalledTimes(1);
   });
+
+  it("logs durable deletion failures with job correlation and the original error", () => {
+    const write = vi.spyOn(process.stderr, "write").mockImplementation(() => true);
+
+    writeDurableDeletionErrorLog({
+      error: new Error("derived data cleanup failed"),
+      job: {
+        checkpoint: "deleting_derived_data",
+        executionAttempts: 10,
+        id: "deletion-job-1",
+        knowledgeSpaceId: "knowledge-space-1",
+        runState: "failed",
+        targetId: "source-1",
+        targetType: "source",
+      } as never,
+    });
+
+    expect(write).toHaveBeenCalledOnce();
+    expect(write.mock.calls[0]?.[0]).toContain('"event":"knowledge_fs.durable_deletion.failed"');
+    expect(write.mock.calls[0]?.[0]).toContain('"jobId":"deletion-job-1"');
+    expect(write.mock.calls[0]?.[0]).toContain('"checkpoint":"deleting_derived_data"');
+    expect(write.mock.calls[0]?.[0]).toContain('"errorMessage":"derived data cleanup failed"');
+  });
 });
 
 function secretStoreStub(): Pick<SourceSecretStore, "delete"> {
diff --git a/knowledge-fs/apps/api/src/durable-deletion-options.ts b/knowledge-fs/apps/api/src/durable-deletion-options.ts
index fbc2253d312..3d104c798b8 100644
--- a/knowledge-fs/apps/api/src/durable-deletion-options.ts
+++ b/knowledge-fs/apps/api/src/durable-deletion-options.ts
@@ -1,6 +1,7 @@
 import { randomUUID } from "node:crypto";
 
 import {
+  type DurableDeletionJob,
   type DurableDeletionOutboxDispatcher,
   type DurableDeletionRepository,
   type DurableDeletionRuntime,
@@ -94,6 +95,7 @@ export function createApiDurableDeletionAssembly({
     maxBatchSize: 10,
     maxRetryDelayMs: 5 * 60_000,
     maxStepsPerLease: 25,
+    onError: writeDurableDeletionErrorLog,
     processor,
     repository,
     stepTimeoutMs: 5_000,
@@ -124,3 +126,29 @@ export function createApiDurableDeletionAssembly({
     },
   };
 }
+
+export function writeDurableDeletionErrorLog(input: {
+  readonly error: unknown;
+  readonly job?: DurableDeletionJob | undefined;
+}): void {
+  const error = input.error instanceof Error ? input.error : undefined;
+  process.stderr.write(
+    `${JSON.stringify({
+      checkpoint: input.job?.checkpoint,
+      errorMessage: boundedLogValue(error?.message ?? String(input.error), 16_384),
+      errorName: error?.name ?? "UnknownError",
+      errorStack: error?.stack ? boundedLogValue(error.stack, 32_768) : undefined,
+      event: "knowledge_fs.durable_deletion.failed",
+      executionAttempts: input.job?.executionAttempts,
+      jobId: input.job?.id,
+      knowledgeSpaceId: input.job?.knowledgeSpaceId,
+      runState: input.job?.runState,
+      targetId: input.job?.targetId,
+      targetType: input.job?.targetType,
+    })}\n`,
+  );
+}
+
+function boundedLogValue(value: string, limit: number): string {
+  return value.length <= limit ? value : `${value.slice(0, limit)}...[truncated]`;
+}