diff --git a/package.json b/package.json
index f4d2105..f14526e 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "ai-ration",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "description": "Stay on free LLM tiers: a multi-vendor fallback chain, rate-limit classification that tells the three kinds of 429 apart, and fair-share rationing of a shared daily pool across users.",
   "license": "MIT",
   "author": "Mao Nakamoto",
diff --git a/src/chain.ts b/src/chain.ts
index 4e7f418..ebe93ea 100644
--- a/src/chain.ts
+++ b/src/chain.ts
@@ -162,6 +162,51 @@ export function freeChain(prefix = "AI"): Provider[] {
   ];
 }
 
+/** What a model id tells us about who pays. */
+export type CostVerdict = "free" | "paid" | "unknown";
+
+/**
+ * Does this model id cost money?
+ *
+ * Exists because the same mistake was found in THREE separate apps on one day,
+ * each a fallback that silently began spending when the free tier ran dry:
+ *
+ *   anthropic/claude-sonnet-5                  a premium model as the fallback
+ *   google/gemini-2.0-flash-001                the paid twin of a `:free` id
+ *   meta-llama/llama-3.3-70b-instruct          reads free; bills at 1e-7/token,
+ *                                              and its `:free` sibling has been
+ *                                              retired from the catalogue
+ *
+ * The decidable rule is narrow and stated as such. A routed id (`vendor/model`,
+ * the OpenRouter shape) is FREE only with the `:free` suffix, and PAID without
+ * it — that suffix is the entire difference between free routing and a per-call
+ * charge for the same weights. A bare id (`llama-3.1-8b-instant`) says nothing:
+ * whether it costs depends on the account's tier at that vendor, which no string
+ * can answer, so it returns "unknown" rather than guessing.
+ *
+ * Guessing "free" there would be the dangerous direction — it is what let three
+ * of these through code review.
+ */
+export function modelCost(id: string): CostVerdict {
+  const model = id.trim();
+  if (!model) return "unknown";
+  // OpenRouter's auto-router across the free catalogue.
+  if (model === "openrouter/free") return "free";
+  if (!model.includes("/")) return "unknown";
+  return model.endsWith(":free") ? "free" : "paid";
+}
+
+/**
+ * Assert every model in a chain is free, for apps that must never bill.
+ *
+ * Returns the offending ids rather than throwing: the caller knows whether a
+ * paid link is a bug or a deliberate, opted-in upgrade, and a library that
+ * throws on the second case forces people to route around it.
+ */
+export function paidModelsIn(chain: Provider[]): string[] {
+  return chain.flatMap((p) => p.models).filter((m) => modelCost(m) === "paid");
+}
+
 /**
  * The day's total budget: every provider we hold a key for.
  *
diff --git a/src/index.ts b/src/index.ts
index 880237e..133d3af 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -21,9 +21,12 @@ export {
   type Provider,
   type Env,
   type Link,
+  type CostVerdict,
   providerModels,
   withEnvPrefix,
   freeChain,
+  modelCost,
+  paidModelsIn,
   dayCapacityTokens,
   usableChain,
   chainFrom,
diff --git a/test/cost.test.js b/test/cost.test.js
new file mode 100644
index 0000000..7a6eda1
--- /dev/null
+++ b/test/cost.test.js
@@ -0,0 +1,37 @@
+/**
+ * `modelCost` exists because one mistake appeared in THREE apps on one day:
+ * a fallback that silently began spending the moment the free tier ran dry.
+ * These cases are the real ids that were found in production config.
+ */
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+import { modelCost, paidModelsIn, freeChain } from 'ai-ration';
+
+test('the three ids that were actually billing are all caught', () => {
+  assert.equal(modelCost('anthropic/claude-sonnet-5'), 'paid');
+  assert.equal(modelCost('google/gemini-2.0-flash-001'), 'paid');
+  // Reads free, bills at 1e-7/token, and its `:free` sibling has been retired.
+  assert.equal(modelCost('meta-llama/llama-3.3-70b-instruct'), 'paid');
+});
+
+test('the `:free` suffix is the whole difference', () => {
+  assert.equal(modelCost('openai/gpt-oss-20b:free'), 'free');
+  assert.equal(modelCost('openai/gpt-oss-20b'), 'paid');
+  assert.equal(modelCost('openrouter/free'), 'free', 'the free auto-router');
+});
+
+test('a bare vendor id is UNKNOWN, never assumed free', () => {
+  // Whether `llama-3.1-8b-instant` costs depends on the account tier at Groq —
+  // no string can answer that. Guessing "free" is the direction that let three
+  // of these through review, so it must not be the default.
+  assert.equal(modelCost('llama-3.1-8b-instant'), 'unknown');
+  assert.equal(modelCost('llama-3.3-70b-versatile'), 'unknown');
+  assert.equal(modelCost(''), 'unknown');
+  assert.equal(modelCost('   '), 'unknown');
+});
+
+test('the shipped free chain contains no paid model', () => {
+  // The package would have no standing to flag anyone else's chain otherwise.
+  assert.deepEqual(paidModelsIn(freeChain('TEST')), []);
+});