test(combo): guard auto/best-free never leaks the combo name as a model (#7754)

changelog.d/fixes/7754-best-free-fallback.md

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+- test(combo): guard auto/best-free never leaks the combo name as a model (#7754)

tests/unit/repro-7754.test.ts

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+import test from "node:test";
+import assert from "node:assert/strict";
+import { createBuiltinAutoCombo } from "@omniroute/open-sse/services/autoCombo/builtinCatalog.ts";
+
+// #7754: `auto/best-free` combo name must never leak downstream as the model id.
+// When the free-tier candidate pool resolves non-empty, every model in the combo
+// must carry a concrete `<provider>/<model>` id — never the literal combo name.
+
+test("#7754 auto/best-free never leaks the combo name as a model", async () => {
+  const combo = await createBuiltinAutoCombo("auto/best-free", "best-free");
+  const models = combo.models || [];
+
+  // The combo id is the modelStr by design (routing resolves it back), but the
+  // models array must never contain it as a target model.
+  const leak = models.filter(
+    (m: any) =>
+      (m.id || "") === "auto/best-free" ||
+      (m.model || "") === "auto/best-free" ||
+      (m.modelStr || "") === "auto/best-free"
+  );
+  assert.equal(
+    leak.length,
+    0,
+    `combo name leaked as a target model: ${JSON.stringify(leak)}`
+  );
+});
+
+test("#7754 every auto/best-free model carries a concrete provider/model", async () => {
+  const combo = await createBuiltinAutoCombo("auto/best-free", "best-free");
+  const models = combo.models || [];
+  for (const m of models as any[]) {
+    assert.ok(
+      m.model && m.model !== "auto/best-free",
+      `model missing concrete id: ${JSON.stringify(m)}`
+    );
+    assert.ok(
+      m.providerId && m.providerId !== "auto",
+      `model missing concrete provider: ${JSON.stringify(m)}`
+    );
+  }
+});
+
+test("#7754 empty free-tier pool degrades with a clear 503, not a name leak", async () => {
+  // When NO free-tier candidate exists, createBuiltinAutoCombo must either
+  // return an empty models[] (which the #6458 route check converts to a clear
+  // 503) or throw — never synthesize a target whose model is the combo name.
+  const combo = await createBuiltinAutoCombo("auto/best-free", "best-free");
+  const models = combo.models || [];
+  if (models.length === 0) {
+    // Empty pool is fine — the route layer (#6458) converts it to a clear 503.
+    assert.equal(combo.candidatePool?.length || 0, 0);
+  } else {
+    // Non-empty pool must not leak.
+    const leak = models.filter((m: any) => (m.model || "") === "auto/best-free");
+    assert.equal(leak.length, 0);
+  }
+});