diff --git a/lib/constants.ts b/lib/constants.ts
index a6012d40..3ae8f722 100644
--- a/lib/constants.ts
+++ b/lib/constants.ts
@@ -55,7 +55,18 @@ export const API_CONFIG = {
 
   // OpenRouter
   OPENROUTER_API_URL: 'https://openrouter.ai/api/v1/chat/completions',
-  OPENROUTER_DEFAULT_MODEL: 'anthropic/claude-sonnet-5',
+  // Must be a FREE id. OpenRouter is the FALLBACK here — reached when Groq's
+  // free tier is spent, i.e. only when nobody is watching. This default was
+  // `anthropic/claude-sonnet-5`, a premium paid model, so "the free tier ran
+  // out" produced a bill rather than a degraded answer — a spending decision
+  // made by an outage instead of by a person. (It also broke the standing rule
+  // that Anthropic is never a primary or fallback provider in this fleet.)
+  //
+  // `openai/gpt-oss-20b:free` reports pricing.prompt = 0 in OpenRouter's own
+  // catalogue (checked 2026-08-16) and was probed live for tool support on
+  // 2026-08-15. Free catalogues rot — when it does, replace it with another
+  // `:free` id, never by dropping the suffix.
+  OPENROUTER_DEFAULT_MODEL: 'openai/gpt-oss-20b:free',
 
   // Token limits
   MAX_CONTEXT_CHARS: 8000,
diff --git a/tests/__tests__/lib/free-fallback.test.ts b/tests/__tests__/lib/free-fallback.test.ts
new file mode 100644
index 00000000..2f4e9632
--- /dev/null
+++ b/tests/__tests__/lib/free-fallback.test.ts
@@ -0,0 +1,27 @@
+/**
+ * The OpenRouter fallback must never be a paid model.
+ *
+ * Pinned as a test rather than trusted to review because this path is invisible
+ * in normal operation: OpenRouter is only reached when Groq's free tier is
+ * spent, so a paid id there bills precisely when nobody is watching. The
+ * previous default was `anthropic/claude-sonnet-5` — a premium model, and a
+ * breach of the standing fleet rule that Anthropic is never a primary or
+ * fallback provider.
+ *
+ * The rule mirrors `modelCost` in the shared `ai-ration` package: a routed id
+ * (`vendor/model`) is free only with the `:free` suffix. That suffix is the
+ * entire difference between free routing and a per-call charge for identical
+ * weights, which is why an id one token away from correct kept slipping through.
+ */
+import { API_CONFIG } from '@/lib/constants';
+
+describe('free-first fallback', () => {
+  it('uses a :free OpenRouter model as the fallback default', () => {
+    expect(API_CONFIG.OPENROUTER_DEFAULT_MODEL).toMatch(/:free$/);
+  });
+
+  it('never falls back to Anthropic', () => {
+    // Standing fleet rule: Anthropic is paid and is not a fallback anywhere.
+    expect(API_CONFIG.OPENROUTER_DEFAULT_MODEL).not.toMatch(/anthropic/i);
+  });
+});