diff --git a/config/quality/file-size-baseline.json b/config/quality/file-size-baseline.json
index 9fcd6f0aad..279335935e 100644
--- a/config/quality/file-size-baseline.json
+++ b/config/quality/file-size-baseline.json
@@ -217,7 +217,8 @@
     "_rebaseline_2026_07_22_8211_gemini_malformed_tool_choice": "PR #8211 (hartmark, fix/gemini-malformed-function-call-tool-choice) own growth: open-sse/translator/response/gemini-to-openai.ts 771->821 (+50, entirely this PR's diff — no other commit touched this file between the PR's merge-base and the release tip). Adds MALFORMED_FUNCTION_CALL/UNEXPECTED_TOOL_CALL handling inside geminiToOpenAIResponse(): synthesizes a `malformed_tool_call` tool_calls entry so finish_reason normalizes to the standard \"tool_calls\" instead of an unrecognized raw enum value that OpenAI-compatible clients (e.g. OpenClaw) silently ignore, and always synthesizes (rather than skipping when a real tool call already exists) so a malformed attempt alongside a real one in the same turn is not silently discarded. Irreducible cohesive addition at the existing candidate/finishReason translation chokepoint (mirrors the 9router#2462 raw-finish-reason precedent immediately below it in the same function). Covered by the PR's own tests/unit test additions for both the malformed-only and malformed-plus-real-call cases.",
     "open-sse/translator/response/gemini-to-openai.ts": 821,
     "_rebaseline_2026_07_22_7936_namespace_roundtrip": "#7936 (@RCrushMe, Responses-Chat namespace round-trip identity seam) own growth: open-sse/translator/response/openai-responses.ts 1092->1125 (+33) and open-sse/utils/stream.ts 2814->2869 (+55) — threading the namespace-identity seam through the Responses↔Chat translation + stream paths so tool-call namespaces survive the round-trip. Cohesive translation/stream wiring at existing chokepoints, frozen at new size.",
-    "open-sse/translator/response/openai-responses.ts": 1137,
+    "_rebaseline_2026_07_22_8210_openrouter_midstream_error": "PR #8210 (hartmark, fix/openrouter-midstream-error-surfacing) own growth: open-sse/translator/response/openai-responses.ts 1137->1163 (+26) measured on the merged tip (release 1137 + this PR own growth). Adds a single new branch inside openaiToOpenAIResponsesResponse() that detects an OpenRouter-style mid-stream aggregator error (HTTP 200 SSE chunk with empty choices + a top-level error object) and surfaces it as state.upstreamError instead of silently falling through to the no-op/awaitingTrailingUsage path, which previously masked the failure as a false empty-success completion and skipped combo fallback. Irreducible call-site addition at the existing chunk-dispatch chokepoint (mirrors the Gemini-to-OpenAI translator's #4177 precedent for the same class of upstream error surfacing). Note: this baseline entry does NOT cover the separate pre-existing +11 drift already on the release tip from #8081/#8162 (1125->1136, unrelated reasoning-placeholder-stripping fix merged after this PR branched) — that drift belongs to the maintainer's rebaseline, not this PR.",
+    "open-sse/translator/response/openai-responses.ts": 1163,
     "open-sse/utils/cursorAgentProtobuf.ts": 1521,
     "_rebaseline_2026_07_23_8143_empty_catch_logging": "#8143 (@chirag127) own growth: open-sse/utils/stream.ts 2869->2887 (+18) — replacing empty catch blocks in the SSE stream subsystem with console.debug logging (Rule #6 silent-swallow fix, issues #8138-#8142). Cohesive logging additions at the existing catch chokepoints, not extractable; frozen at new size. Covered by tests/unit/stream-handler-catch-logging-8143.test.ts.",
     "open-sse/utils/stream.ts": 2887,
diff --git a/open-sse/translator/response/openai-responses.ts b/open-sse/translator/response/openai-responses.ts
index 2371db9adc..f649853552 100644
--- a/open-sse/translator/response/openai-responses.ts
+++ b/open-sse/translator/response/openai-responses.ts
@@ -105,6 +105,32 @@ export function openaiToOpenAIResponsesResponse(chunk, state) {
   }
 
   if (!chunk.choices?.length) {
+    // Mid-stream aggregator error: OpenRouter (and similar OpenAI-compatible
+    // aggregators) can send an HTTP 200 SSE stream whose body carries a chunk with
+    // empty `choices` and a top-level `error` object instead of any delta — e.g. the
+    // underlying provider hitting its own capacity limit mid-request. Without this
+    // branch the chunk has no choices, so it falls into the awaitingTrailingUsage/
+    // no-op path below and the stream silently ends with a false "completed, empty
+    // output" response, masking the failure and skipping combo fallback. Surface it
+    // as state.upstreamError so stream.ts errors the stream out (mirrors the
+    // Gemini-to-OpenAI translator's #4177 fix).
+    if (chunk.error && typeof chunk.error === "object") {
+      const rawCode = chunk.error.code;
+      const status =
+        typeof rawCode === "number" && rawCode >= 400 && rawCode <= 599 ? rawCode : 502;
+      state.upstreamError = {
+        status,
+        type: status === 429 ? "rate_limit_error" : "server_error",
+        code:
+          typeof chunk.error.metadata?.error_type === "string"
+            ? chunk.error.metadata.error_type
+            : status === 429
+              ? "rate_limit_exceeded"
+              : "bad_gateway",
+        message: typeof chunk.error.message === "string" ? chunk.error.message : "Upstream failure",
+      };
+      return [];
+    }
     // #6906: a deferred finish_reason (awaitingTrailingUsage, see below) completes here —
     // the trailing usage-only chunk (choices: [], usage: {...}) is what real
     // stream_options.include_usage=true upstreams send after finish_reason (see the
diff --git a/tests/unit/openrouter-midstream-error-chunk.test.ts b/tests/unit/openrouter-midstream-error-chunk.test.ts
new file mode 100644
index 0000000000..98f2bd960c
--- /dev/null
+++ b/tests/unit/openrouter-midstream-error-chunk.test.ts
@@ -0,0 +1,85 @@
+/**
+ * Regression test: OpenRouter mid-stream error chunk silently swallowed.
+ *
+ * OpenRouter (and other OpenAI-compatible aggregators) can send an HTTP 200 SSE
+ * stream whose body carries a `chat.completion.chunk` with an empty `choices`
+ * array and a top-level `error` object instead of any delta — e.g. when the
+ * underlying provider (Nvidia, hosting a free model) hits its own capacity limit
+ * mid-request:
+ *   {"choices":[],"error":{"code":502,"message":"Upstream error from Nvidia: ...
+ *   Worker local total request limit reached (33/32)","metadata":{"error_type":
+ *   "provider_unavailable"}}}
+ *
+ * Before this fix, `openaiToOpenAIResponsesResponse()`'s `!chunk.choices?.length`
+ * branch treated this exactly like a legitimate trailing-usage/no-op chunk and
+ * dropped it, so the stream ended with `response.completed` / `error: null` /
+ * `output: []` — a false "successful but empty" response instead of surfacing
+ * the real 502 upstream failure. Mirrors the Gemini mid-stream error fix (#4177).
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { openaiToOpenAIResponsesResponse } =
+  await import("../../open-sse/translator/response/openai-responses.ts");
+const { initState } = await import("../../open-sse/translator/index.ts");
+const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+test("OpenRouter mid-stream 502 provider_unavailable is surfaced as upstreamError, not dropped", () => {
+  const state = initState(FORMATS.OPENAI_RESPONSES);
+
+  const errorChunk = {
+    id: "gen-1784726796-AJw13XXRnwqcRL2kieNm",
+    object: "chat.completion.chunk",
+    created: 1784726796,
+    model: "nvidia/nemotron-3-ultra-550b-a55b:free",
+    provider: "Nvidia",
+    choices: [],
+    error: {
+      code: 502,
+      message:
+        "Upstream error from Nvidia: ResourceExhausted: Worker local total request limit reached (33/32)",
+      metadata: { error_type: "provider_unavailable" },
+    },
+  };
+
+  const immediateEvents = openaiToOpenAIResponsesResponse(errorChunk, state);
+  assert.deepEqual(immediateEvents, [], "the error chunk itself emits no delta events");
+
+  // End of stream (chunk === null) flushes the deferred completion.
+  const flushEvents = openaiToOpenAIResponsesResponse(null, state);
+  const completedEvent = flushEvents.find((e) => e.event === "response.completed");
+  assert.ok(completedEvent, "should have a response.completed event");
+  assert.equal(
+    completedEvent.data.response.status,
+    "failed",
+    "status must be 'failed', not a false 'completed'"
+  );
+  assert.ok(completedEvent.data.response.error, "error must not be null");
+  assert.match(
+    completedEvent.data.response.error.message,
+    /Worker local total request limit reached/
+  );
+  assert.equal(completedEvent.data.response.output.length, 0);
+});
+
+test("OpenRouter mid-stream error with a rate-limit code maps to a 429 upstreamError", () => {
+  const state = initState(FORMATS.OPENAI_RESPONSES);
+
+  openaiToOpenAIResponsesResponse(
+    {
+      choices: [],
+      error: {
+        code: 429,
+        message: "Rate limit exceeded, please try again later.",
+        metadata: { error_type: "rate_limited" },
+      },
+    },
+    state
+  );
+
+  const flushEvents = openaiToOpenAIResponsesResponse(null, state);
+  const completedEvent = flushEvents.find((e) => e.event === "response.completed");
+  assert.ok(completedEvent);
+  assert.equal(completedEvent.data.response.status, "failed");
+  assert.equal(completedEvent.data.response.error.code, "429");
+});