diff --git a/src/sse/handlers/autoRouting.ts b/src/sse/handlers/autoRouting.ts
index d76b1e2340..7f444b0552 100644
--- a/src/sse/handlers/autoRouting.ts
+++ b/src/sse/handlers/autoRouting.ts
@@ -70,14 +70,13 @@ async function applyAutoPrefix(
     const { parseAutoPrefix } =
       await import("@omniroute/open-sse/services/autoCombo/autoPrefix.ts");
     const parsed = parseAutoPrefix(model);
-    if (!parsed.valid) {
+    const recognizedBuiltIn = Object.prototype.hasOwnProperty.call(AUTO_TEMPLATE_VARIANTS, model);
+    if (!parsed.valid && !recognizedBuiltIn) {
       if (!state.spec) log.warn("AUTO", `Invalid auto prefix format: ${model}`);
       return state;
     }
 
-    const variant = Object.prototype.hasOwnProperty.call(AUTO_TEMPLATE_VARIANTS, model)
-      ? state.variant
-      : parsed.variant;
+    const variant = recognizedBuiltIn ? state.variant : parsed.variant;
     const defaultVariant =
       model === "auto" && variant === undefined
         ? (settings.autoRoutingDefaultVariant as AutoVariant | undefined)
diff --git a/tests/unit/auto-best-variant-no-warn.test.ts b/tests/unit/auto-best-variant-no-warn.test.ts
new file mode 100644
index 0000000000..c11518ed73
--- /dev/null
+++ b/tests/unit/auto-best-variant-no-warn.test.ts
@@ -0,0 +1,94 @@
+/**
+ * Regression: a recognized built-in auto variant (e.g. `auto/best-coding`) must
+ * NOT emit the spurious "Invalid auto prefix format" warning.
+ *
+ * Root cause: `resolveAutoRoutingState` → `applyAutoPrefix` (in
+ * `src/sse/handlers/autoRouting.ts`) called `parseAutoPrefix()`, whose
+ * `VALID_VARIANTS` set only covers the short aliases (`coding`, `fast`, `cheap`,
+ * `offline`, `smart`, `lkgp`, `chaos`). The `best-*` built-in aliases that
+ * `AUTO_TEMPLATE_VARIANTS` recognizes (e.g. `auto/best-coding`) are not in that
+ * set, so `parseAutoPrefix` returned `{ valid: false }` and every such request
+ * logged the misleading "Invalid auto prefix format: auto/best-*" warning even
+ * though the routing state was actually resolved correctly by
+ * `classifyAutoModel()` beforehand.
+ *
+ * The fix makes `applyAutoPrefix` skip the warning for any model that
+ * `AUTO_TEMPLATE_VARIANTS` already recognizes as a built-in variant.
+ *
+ * The routing variant was never actually broken (the early-return preserved
+ * the already-correct classified state); the regression here is purely the
+ * spurious warning log. We verify by writing to a known log file and asserting
+ * the warning text does NOT appear for a recognized built-in variant, while a
+ * genuinely unknown auto variant DOES still produce it (proving the probe works).
+ */
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { mkdtempSync, readFileSync, existsSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+// Configure file logging BEFORE importing the logger (the shared logger builds
+// its transports at import time).
+const dir = mkdtempSync(join(tmpdir(), "omniroute-auto-best-no-warn-"));
+const logFile = join(dir, "app.log");
+process.env.NODE_ENV = "production"; // JSON to file, simplest single-directory path
+process.env.APP_LOG_TO_FILE = "true";
+process.env.APP_LOG_FILE_PATH = logFile;
+process.env.APP_LOG_LEVEL = "debug";
+
+const { resolveAutoRoutingState } = await import("../../src/sse/handlers/autoRouting.ts");
+
+/** Poll the (worker-thread-written) log file until the predicate holds or timeout. */
+async function readLogWhen(
+  predicate: (contents: string) => boolean,
+  timeoutMs = 4000
+): Promise<string> {
+  const start = Date.now();
+  while (Date.now() - start < timeoutMs) {
+    if (existsSync(logFile)) {
+      const contents = readFileSync(logFile, "utf8");
+      if (predicate(contents)) return contents;
+    }
+    await new Promise((r) => setTimeout(r, 50));
+  }
+  return existsSync(logFile) ? readFileSync(logFile, "utf8") : "";
+}
+
+test(
+  "built-in auto/best-* variant does NOT log 'Invalid auto prefix format'",
+  async () => {
+    // Recognized built-in variant -> must resolve without the spurious warning.
+    const state = await resolveAutoRoutingState("auto/best-coding");
+    assert.equal(state.recognizedBuiltInAuto, true);
+    assert.equal(state.variant, "coding");
+    assert.equal(state.response, null);
+
+    // Known-good stream: the variant log line proves the request took the happy
+    // path (and flushed the worker before we read the file).
+    await readLogWhen((c) => c.includes("Zero-config routing variant"));
+    const contents = readFileSync(logFile, "utf8");
+
+    assert.doesNotMatch(
+      contents,
+      /Invalid auto prefix format: auto\/best-coding/,
+      "a recognized built-in auto/best-* variant must not trigger the misleading invalid-prefix warning"
+    );
+  }
+);
+
+test("genuinely unknown auto variant still logs 'Invalid auto prefix format' (probe has teeth)", async () => {
+  // A truly unrecognized prefix must still warn — proves this probe is able to
+  // detect the warning and that we're not simply missing it due to logging setup.
+  const state = await resolveAutoRoutingState("auto/not-a-real-variant");
+  assert.equal(state.recognizedBuiltInAuto, false);
+
+  const contents = await readLogWhen((c) =>
+    c.includes("Invalid auto prefix format: auto/not-a-real-variant")
+  );
+
+  assert.match(
+    contents,
+    /Invalid auto prefix format: auto\/not-a-real-variant/,
+    "an unknown auto variant must still produce the invalid-prefix warning"
+  );
+});
\ No newline at end of file