fix(stream): report aborts after HTTP 200 in-band instead of closing silently

A stream that stalled or lost its upstream was closed with no terminal frame
at all, so clients saw "200 OK, a few chunks, then nothing" and could not tell
a truncated reply from a finished one. The Responses passthrough path already
synthesized response.failed; every other client format got nothing.

The watchdog now hands its reason ("stream stall timeout" or "upstream
connection lost") to onAbortTerminal, and buildStreamErrorBytes frames it per
client format: OpenAI-compatible clients get data: {"error":{...}} followed by
data: [DONE], Anthropic clients get `event: error`. The error frame always
precedes [DONE] (openai-python raises APIError on any data payload carrying an
error key), and no synthetic finish_reason is ever emitted — a truncated
stream must not look like a clean stop.

Co-Authored-By: Claude Code <noreply@anthropic.com>

open-sse/utils/streamHandler.js

@@ -95,6 +95,9 @@ export function createStreamController({ onDisconnect, onError, log, provider, m
  * activity), not here — output of the transform stream may be silent
  * for long periods while raw bytes still flow (e.g. Kiro EventStream
  * binary frames buffering, Claude reasoning streams).
+ *
+ * @param {function} [onAbortTerminal] - Receives a human-readable abort
+ * message and returns terminal SSE bytes to emit downstream.
  */
 export function createDisconnectAwareStream(transformStream, streamController, onAbortTerminal = null) {
   const reader = transformStream.readable.getReader();
@@ -194,6 +197,7 @@ export function pipeWithDisconnect(providerResponse, transformStream, streamCont
   let chunkCount = 0;
   let totalBytes = 0;
   let lastChunkAt = Date.now();
+  let abortMessage = "upstream connection lost";
   const t0 = Date.now();
   const tag = "STREAM";
   const clearStall = () => {
@@ -203,6 +207,7 @@ export function pipeWithDisconnect(providerResponse, transformStream, streamCont
     clearStall();
     stallTimer = setTimeout(() => {
       stallTimer = null;
+      abortMessage = "stream stall timeout";
       dbg(tag, `STALL TIMEOUT ${stallTimeoutMs}ms | chunks=${chunkCount} | bytes=${totalBytes} | sinceLast=${Date.now() - lastChunkAt}ms`);
       streamController.handleError?.(new Error("stream stall timeout"));
       streamController.abort?.();
@@ -249,7 +254,7 @@ export function pipeWithDisconnect(providerResponse, transformStream, streamCont
   return createDisconnectAwareStream(
     { readable: transformedBody, writable: { getWriter: () => ({ abort: () => Promise.resolve() }) } },
     wrappedController,
-    onAbortTerminal
+    onAbortTerminal ? () => onAbortTerminal(abortMessage) : null
   );
 }
 

open-sse/utils/streamHelpers.js

@@ -1,4 +1,8 @@
 import { FORMATS } from "../translator/formats.js";
+import { buildErrorBody } from "./error.js";
+import { SSE_DONE } from "./sseConstants.js";
+
+const sharedEncoder = new TextEncoder();
 
 // Parse SSE data line
 export function parseSSELine(line, format = null) {
@@ -120,3 +124,24 @@ export function formatSSE(data, sourceFormat) {
 
   return `data: ${JSON.stringify(data)}\n\n`;
 }
+
+// Terminal frames for a stream that aborted after HTTP 200 was already sent, so
+// the status code can no longer change. OpenAI-compatible clients (openai-python
+// raises APIError on any `data:` payload carrying an `error` key, checked before
+// [DONE]) need the error frame first, then [DONE]; Anthropic clients need
+// `event: error`. Never fabricate a successful finish_reason instead.
+//
+// Returns encoded bytes: onAbortTerminal callbacks are enqueued verbatim, same
+// as buildAbortedResponsesTerminalBytes.
+//
+// NOTE: non-SSE client formats (Ollama NDJSON) get an SSE frame here — dead in
+// practice because detectFormatByEndpoint never resolves to OLLAMA.
+export function buildStreamErrorBytes(statusCode, message, clientFormat) {
+  const { error } = buildErrorBody(statusCode, message);
+
+  const sse = clientFormat === FORMATS.CLAUDE
+    ? formatSSE({ type: "error", error }, FORMATS.CLAUDE)
+    : formatSSE({ error }, clientFormat) + SSE_DONE;
+
+  return sharedEncoder.encode(sse);
+}