diff --git a/runtime/worker.go b/runtime/worker.go
index 23a8228..2295573 100644
--- a/runtime/worker.go
+++ b/runtime/worker.go
@@ -1227,6 +1227,10 @@ func (client sessionLogBatchClient) IngestLogBatch(ctx context.Context, batch pr
 		batch.Checksum = checksum
 	}
 	response, err := client.client.IngestLogBatch(ctx, batch)
+	if err != nil && logBatchNotFoundError(err) {
+		log.Printf("RUN phase=durable_uploaders.logs status=spool_quarantine stream=%s firstSeq=%d lastSeq=%d reason=platform_not_found error=%s", safeOptional(batch.LogStreamID), batch.FirstSeq, batch.LastSeq, RedactText(err.Error()))
+		return protocol.LogBatchIngestResponse{}, spool.PermanentLogBatchRejection("platform_not_found", err)
+	}
 	if err != nil && (logBatchSequenceGapError(err) || logBatchAcknowledgedRangeConflict(err)) {
 		reason := "platform_sequence_gap"
 		if logBatchAcknowledgedRangeConflict(err) {
@@ -1246,6 +1250,11 @@ func (client sessionLogBatchClient) IngestLogBatch(ctx context.Context, batch pr
 	return response, err
 }
 
+func logBatchNotFoundError(err error) bool {
+	var httpErr interface{ HTTPStatus() int }
+	return errors.As(err, &httpErr) && httpErr.HTTPStatus() == http.StatusNotFound
+}
+
 func logBatchSequenceGapError(err error) bool {
 	var gapError interface{ LogBatchSequenceGap() bool }
 	return errors.As(err, &gapError) && gapError.LogBatchSequenceGap()
diff --git a/runtime/worker_test.go b/runtime/worker_test.go
index 9e30e05..dafaaa4 100644
--- a/runtime/worker_test.go
+++ b/runtime/worker_test.go
@@ -640,6 +640,16 @@ func TestSessionLogBatchClientQuarantinesSequenceConflict(t *testing.T) {
 	}
 }
 
+func TestSessionLogBatchClientQuarantinesPlatformNotFound(t *testing.T) {
+	recorder := &recordingDurableLogClient{err: api.PlatformRequestError{Status: http.StatusNotFound, Code: "not_found"}}
+	client := sessionLogBatchClient{client: recorder, runEndpointID: "run-current", sessionToken: "token-current"}
+	_, err := client.IngestLogBatch(context.Background(), protocol.LogBatchIngestRequest{LogStreamID: "run.stale-endpoint.stale-server.session.stdout", FirstSeq: 1, LastSeq: 1, Entries: []protocol.LogEntry{{Seq: 1, Timestamp: workerTestTime(), Line: "line"}}})
+	var permanent spool.PermanentLogBatchError
+	if !errors.As(err, &permanent) || permanent.Reason != "platform_not_found" {
+		t.Fatalf("expected permanent platform_not_found rejection, got %#v", err)
+	}
+}
+
 func TestSessionLogBatchClientQuarantinesSessionMetadataMismatch(t *testing.T) {
 	recorder := &recordingDurableLogClient{err: api.PlatformRequestError{Status: http.StatusBadRequest, Code: "validation_failed", Details: []string{"log session metadata must match stream"}}}
 	client := sessionLogBatchClient{client: recorder, runEndpointID: "run-current", sessionToken: "token-current"}