fix(providers): reject image-only models on /v1/chat/completions with clear error (#6457) (#6525)

fix(providers): reject image-only models on /v1/chat/completions with a clear error (#6457) Integrated into release/v3.8.47. (thanks @chirag127)

config/quality/file-size-baseline.json

@@ -258,7 +258,7 @@
     "src/shared/services/cliRuntime.ts": 1100,
     "src/shared/validation/schemas.ts": 2523,
     "_rebaseline_2026_06_28_5275_correlation_id_extract": "Extraction of the safe CorrelationId subset of #5275 (hartmark) — request correlation id stored in call_logs (migration 109) and returned via the X-Correlation-Id response header, WITHOUT the combo/resilience or build/lazy-loading changes (those stay in #5275). Own growth: callLogs.ts 975->985 (correlation_id column on CallLogSummaryRow + read/map), usageHistory.ts 983->988 (correlationId metadata normalize), chat.ts 1575->1632 (withCorrelationId response wiring + combo-failure log carrying correlationId), chatHelpers.ts new 811 (withCorrelationId helper + reqId threading; was 791<cap pre-feature). Cohesive request/logging chokepoint wiring; structural shrink of chat.ts tracked in #3501.",
-    "src/sse/handlers/chat.ts": 1763,
+    "src/sse/handlers/chat.ts": 1778,
     "src/sse/handlers/chatHelpers.ts": 876,
     "src/sse/services/auth.ts": 2448,
     "open-sse/executors/default.ts": 877,
@@ -386,5 +386,6 @@
   "_rebaseline_2026_07_07_6526_chirag_modal_1080p": "PR #6526 (@chirag127, #6265): AddApiKeyModal.tsx ->961 (1080p sizing). Owner-approved. Frozen.",
   "_rebaseline_2026_07_07_6515_chirag": "PR #6515 (@chirag127) own growth: src/sse/handlers/chat.ts ->1763. Owner-approved rebaseline. Frozen.",
   "_rebaseline_2026_07_07_6534_chirag": "PR #6534 (@chirag127) own growth: open-sse/services/compression/strategySelector.ts ->1025. Owner-approved rebaseline. Frozen.",
-  "_rebaseline_2026_07_07_6546_chirag": "PR #6546 (@chirag127) own growth: src/sse/handlers/chatHelpers.ts ->876. Owner-approved rebaseline. Frozen."
+  "_rebaseline_2026_07_07_6546_chirag": "PR #6546 (@chirag127) own growth: src/sse/handlers/chatHelpers.ts ->876. Owner-approved rebaseline. Frozen.",
+  "_rebaseline_2026_07_07_6525_chirag_image_guard": "PR #6525 (@chirag127, #6457) own growth: chat.ts ->1778 (reject image-only models on /v1/chat/completions; stacks on #6515). Owner-approved. Frozen."
 }

src/sse/handlers/chat.ts

@@ -20,6 +20,7 @@ import {
 import { getModelInfo, getComboForModel } from "../services/model";
 import { resolveBareModelToConnectionDefault } from "@omniroute/open-sse/services/model.ts";
 import { errorResponse } from "@omniroute/open-sse/utils/error.ts";
+import { getImageModelEntry } from "@omniroute/open-sse/config/imageRegistry.ts";
 import { acceptHeaderForcesStream } from "@omniroute/open-sse/utils/aiSdkCompat.ts";
 import { isSelfInflictedUpstreamTimeout } from "@omniroute/open-sse/handlers/chatCore/cooldownClassification.ts";
 import { applyNoThinkingAlias } from "@omniroute/open-sse/utils/noThinkingAlias.ts";
@@ -399,6 +400,20 @@ export async function handleChat(
     return errorResponse(HTTP_STATUS.BAD_REQUEST, "Missing model");
   }
 
+  // Reject image-generation models routed to /v1/chat/completions (#6457).
+  // Image-only models live in IMAGE_PROVIDERS (open-sse/config/imageRegistry.ts)
+  // and are served by /v1/images/generations. Forwarding them to a chat upstream
+  // yielded confusing raw provider 400s (e.g. HuggingFace: "not a chat model").
+  // getImageModelEntry returns non-null only for models registered in the image
+  // registry — chat-only models (openai/gpt-4o, etc.) resolve to null and pass.
+  if (getImageModelEntry(modelStr)) {
+    log.warn("CHAT", `Rejecting image-generation model on chat endpoint: ${modelStr}`);
+    return errorResponse(
+      HTTP_STATUS.BAD_REQUEST,
+      `Model '${modelStr}' is an image-generation model and cannot be used on /v1/chat/completions. Use POST /v1/images/generations instead.`
+    );
+  }
+
   // T04: client-provided external session header has priority over generated fingerprint.
   const externalSessionId = extractExternalSessionId(request.headers);
   const sessionId = externalSessionId || generateStableSessionId(body);

tests/unit/chat-rejects-image-only-model.test.ts

@@ -0,0 +1,73 @@
+// #6457: /v1/chat/completions must reject image-only models with a clear 400 pointing
+// callers at /v1/images/generations, instead of forwarding to a chat upstream that
+// returns a confusing raw provider 400 (HuggingFace: "not a chat model").
+//
+// Discriminator: getImageModelEntry(modelStr) — non-null only for models registered
+// in open-sse/config/imageRegistry.ts. Chat-only models (openai/gpt-4o etc.) return
+// null and pass the guard unchanged.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { createChatPipelineHarness } from "../integration/_chatPipelineHarness.ts";
+
+const harness = await createChatPipelineHarness("chat-rejects-image-only-model");
+const { buildRequest, handleChat, resetStorage } = harness as {
+  buildRequest: (opts: { body: unknown }) => Request;
+  handleChat: (req: Request) => Promise<Response>;
+  resetStorage: () => void | Promise<void>;
+};
+
+test.beforeEach(async () => {
+  await resetStorage();
+});
+
+test.after(() => {
+  harness.cleanup?.();
+});
+
+test("POST /v1/chat/completions with a HuggingFace image model returns 400 + generations hint (#6457)", async () => {
+  const request = buildRequest({
+    body: {
+      model: "huggingface/stabilityai/stable-diffusion-xl-base-1.0",
+      messages: [{ role: "user", content: "draw a cat" }],
+    },
+  });
+
+  const originalFetch = globalThis.fetch;
+  let fetchCalls = 0;
+  globalThis.fetch = async (...args: Parameters<typeof originalFetch>) => {
+    fetchCalls++;
+    return originalFetch(...args);
+  };
+
+  try {
+    const res = await handleChat(request);
+    assert.equal(res.status, 400, "must reject with 400 before dispatch");
+    const body = (await res.json()) as { error?: { message?: string } };
+    const msg = body?.error?.message || JSON.stringify(body);
+    assert.match(msg, /image-generation model/i);
+    assert.match(msg, /\/v1\/images\/generations/);
+    assert.equal(fetchCalls, 0, "must not dispatch upstream for an image-only model");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("POST /v1/chat/completions with a chat model still reaches routing (guard is invisible)", async () => {
+  const request = buildRequest({
+    body: {
+      model: "openai/gpt-4o",
+      messages: [{ role: "user", content: "hi" }],
+    },
+  });
+
+  const res = await handleChat(request);
+  // The guard must not fire on a chat model — the response is whatever downstream
+  // routing produces (typically a credentials/connection error in the harness).
+  // The critical assertion is: it is NOT the image-guard 400.
+  if (res.status === 400) {
+    const body = (await res.json()) as { error?: { message?: string } };
+    const msg = body?.error?.message || JSON.stringify(body);
+    assert.doesNotMatch(msg, /image-generation model/i, "chat model must not trip the image guard");
+  }
+});