diff --git a/packages/loop-js/src/engine/claude.ts b/packages/loop-js/src/engine/claude.ts
index 64f46ff..b336464 100644
--- a/packages/loop-js/src/engine/claude.ts
+++ b/packages/loop-js/src/engine/claude.ts
@@ -130,6 +130,17 @@ export type TokenUsage = {
   cache_creation?: { ephemeral_5m_input_tokens?: number | null; ephemeral_1h_input_tokens?: number | null } | null
 }
 
+function hasTokenUsage(u: TokenUsage): boolean {
+  return (
+    (u.input_tokens ?? 0) !== 0 ||
+    (u.output_tokens ?? 0) !== 0 ||
+    (u.cache_creation_input_tokens ?? 0) !== 0 ||
+    (u.cache_read_input_tokens ?? 0) !== 0 ||
+    (u.cache_creation?.ephemeral_5m_input_tokens ?? 0) !== 0 ||
+    (u.cache_creation?.ephemeral_1h_input_tokens ?? 0) !== 0
+  )
+}
+
 export function stepUsage(u: TokenUsage, model: string): StepUsage {
   const inputTokens = u.input_tokens ?? 0
   const outputTokens = u.output_tokens ?? 0
@@ -146,7 +157,7 @@ export function stepUsage(u: TokenUsage, model: string): StepUsage {
         cachedInputTokens * p.input * CACHE_READ +
         outputTokens * p.output) /
       1e6
-    : 0 //  an unpriced model derives nothing; the result's `total_cost_usd` reconciles it
+    : 0 //  an unpriced model derives nothing; drainSession fails closed before this can bypass the guard
   return { inputTokens, outputTokens, cachedInputTokens, usd }
 }
 
@@ -181,6 +192,9 @@ export async function* drainSession(
         // repeating the turn's cumulative usage (contract §Mapping 1). Cost a turn once, on its id.
         if (m.message.id !== turn) {
           turn = m.message.id
+          if (!PRICES[model] && hasTokenUsage(m.message.usage)) {
+            throw new Interruption("budget", `unpriced model '${model}' produced token usage`)
+          }
           const usage = stepUsage(m.message.usage, model)
           derived += usage.usd
           yield { kind: "cost", usage }
diff --git a/packages/loop-js/src/engine/claude.unpriced.test.ts b/packages/loop-js/src/engine/claude.unpriced.test.ts
new file mode 100644
index 0000000..2ca0784
--- /dev/null
+++ b/packages/loop-js/src/engine/claude.unpriced.test.ts
@@ -0,0 +1,30 @@
+import { expect, test } from "bun:test"
+import type { SDKMessage } from "@anthropic-ai/claude-agent-sdk"
+import { drainSession, stepUsage } from "./claude.ts"
+import { Interruption } from "./executor.ts"
+
+async function* feed(...messages: SDKMessage[]): AsyncGenerator<SDKMessage, void> {
+  for (const message of messages) yield message
+}
+
+test("unpriced non-zero streamed usage fails closed at the cost boundary", async () => {
+  const assistant = {
+    type: "assistant",
+    message: { id: "turn_1", content: [{ type: "text", text: "partial" }], usage: { input_tokens: 1 } },
+  } as unknown as SDKMessage
+  const gen = drainSession(feed(assistant), "some-future-model")
+
+  expect(await gen.next()).toEqual({ done: false, value: { kind: "text", text: "partial" } })
+  try {
+    await gen.next()
+    throw new Error("expected unpriced usage to fail closed")
+  } catch (err) {
+    expect(err).toBeInstanceOf(Interruption)
+    expect((err as Interruption).cause).toBe("budget")
+    expect((err as Interruption).detail).toContain("unpriced model")
+  }
+})
+
+test("stepUsage keeps zero as its best-effort value for an unpriced model", () => {
+  expect(stepUsage({ input_tokens: 1 }, "some-future-model").usd).toBe(0)
+})