fix(providers): fail fast on empty auto-combo pool instead of 15s timeout (#6458)

config/quality/file-size-baseline.json

@@ -259,7 +259,7 @@
     "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/chatHelpers.ts": 866,
+    "src/sse/handlers/chatHelpers.ts": 876,
     "src/sse/services/auth.ts": 2448,
     "open-sse/executors/default.ts": 877,
     "open-sse/translator/request/openai-responses.ts": 902,
@@ -385,5 +385,6 @@
   "_rebaseline_2026_07_07_6523_chirag_cooldown_body": "PR #6523 (@chirag127, #6460) own growth: chatHelpers.ts 860->866 (+6 = retryAfterAt/credentialsCoolingCount fields on modelCooldownResponse) and auth.ts 2447->2448 (+1 = connectionsCount threaded through no-credentials fallback). Owner-approved rebaseline (file-size cap for contributor PR). Frozen (cannot grow further); release captain's rebaseline-at-release supersedes.",
   "_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_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."
 }

src/sse/handlers/chatHelpers.ts

@@ -204,10 +204,20 @@ export async function resolveModelOrError(
 
     try {
       const virtualCombo = await createBuiltinAutoCombo(modelStr, suffix);
+      const poolSize = virtualCombo.candidatePool?.length || 0;
       log.info(
         "AUTO",
-        `"auto" provider → built-in virtual combo "${modelStr}" (${virtualCombo.candidatePool?.length || 0} candidates)`
+        `"auto" provider → built-in virtual combo "${modelStr}" (${poolSize} candidates)`
       );
+      // #6458: fail fast instead of leaking a silent 15s upstream timeout when
+      // the category/tier filter (e.g. auto/coding:pro, auto/reasoning) matches
+      // zero connected candidates. An empty virtual combo has no targets to
+      // dispatch to, so downstream combo routing stalls on an empty set.
+      if (poolSize === 0) {
+        const msg = `No connected providers match '${modelStr}'. Connect a provider whose models satisfy this category/tier, or use a different auto combo.`;
+        log.warn("AUTO", msg, { model: modelStr });
+        return { error: errorResponse(HTTP_STATUS.SERVICE_UNAVAILABLE, msg) };
+      }
       return { combo: virtualCombo, provider: "auto", model: suffix };
     } catch (err) {
       log.warn("CHAT", `Failed to create built-in auto combo "${modelStr}"`, { err });

tests/unit/auto-empty-pool-fastfail-6458.test.ts

@@ -0,0 +1,31 @@
+/**
+ * #6458 — an `auto/*` category/tier combo that matches ZERO connected
+ * candidates must fail fast with a clear 503, instead of building an empty
+ * virtual combo whose downstream routing stalls on a silent ~15s upstream
+ * timeout. Regression guard (Rule #18).
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-auto-empty-pool-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const core = await import("../../src/lib/db/core.ts");
+const { resolveModelOrError } = await import("../../src/sse/handlers/chatHelpers.ts");
+
+test.beforeEach(() => core.resetDbInstance());
+test.after(() => {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+test("#6458 empty auto-combo pool returns a 503 instead of an empty combo", async () => {
+  // No provider connections seeded → any auto category resolves to an empty pool.
+  const result = await resolveModelOrError("auto/coding:pro", { messages: [{ role: "user", content: "hi" }] });
+  assert.ok(result.error, "expected an error result, not a combo");
+  assert.equal(result.error.status, 503, "empty auto pool must fail fast with 503");
+  assert.equal(result.combo, undefined, "must not return a combo for an empty pool");
+});