fix(anthropic): restore subscription OAuth billing routing

packages/ai/src/providers/anthropic.test.ts

@@ -149,6 +149,37 @@ describe("Anthropic provider", () => {
     expect(config.authToken).toBeNull();
   });
 
+  it("puts Claude subscription billing identity first for OAuth requests", async () => {
+    let capturedPayload: unknown;
+    const stream = streamSimpleAnthropic(
+      makeAnthropicModel(),
+      {
+        messages: [{ role: "user", content: "hello", timestamp: 1 }],
+      },
+      {
+        apiKey: "sk-ant-oat01-test-token",
+        onPayload: (payload) => {
+          capturedPayload = payload;
+        },
+      },
+    );
+
+    const result = await stream.result();
+
+    expect(result.stopReason).toBe("error");
+    expect((capturedPayload as { system?: unknown }).system).toEqual([
+      {
+        type: "text",
+        text: "x-anthropic-billing-header: cc_version=2.1.75; cc_entrypoint=sdk-cli;",
+      },
+      {
+        type: "text",
+        text: "You are Claude Code, Anthropic's official CLI for Claude.",
+        cache_control: { type: "ephemeral" },
+      },
+    ]);
+  });
+
   it("keeps aggregate cache billing buckets out of the context total", async () => {
     const client = {
       messages: {

packages/ai/src/providers/anthropic.ts

@@ -109,6 +109,7 @@ function getCacheControl(
 
 // Stealth mode: Mimic Claude Code's tool naming exactly
 const claudeCodeVersion = "2.1.75";
+const claudeCodeBillingSystemBlock = `x-anthropic-billing-header: cc_version=${claudeCodeVersion}; cc_entrypoint=sdk-cli;`;
 
 // Claude Code 2.x tool names (canonical casing)
 // Source: https://cchistory.mariozechner.at/data/prompts-2.1.11.md
@@ -1564,6 +1565,11 @@ function buildAnthropicSystemBlocks(
 ): TextBlockParam[] | undefined {
   const blocks: TextBlockParam[] = [];
   if (isOAuthTokenResult) {
+    // Anthropic uses this first system block to route Claude subscription OAuth billing.
+    blocks.push({
+      type: "text",
+      text: claudeCodeBillingSystemBlock,
+    });
     blocks.push({
       type: "text",
       text: "You are Claude Code, Anthropic's official CLI for Claude.",

src/agents/anthropic-transport-stream.test.ts

@@ -1618,6 +1618,9 @@ describe("anthropic transport stream", () => {
     expect(headers.get("user-agent")).toContain("claude-cli/");
     const firstCallParams = latestAnthropicRequest().payload;
     const system = requireArray(firstCallParams.system, "system");
+    expect(requireRecord(system[0], "billing system item").text).toBe(
+      "x-anthropic-billing-header: cc_version=2.1.75; cc_entrypoint=sdk-cli;",
+    );
     expect(
       system.some(
         (item) =>

src/agents/anthropic-transport-stream.ts

@@ -82,6 +82,7 @@ import {
 import type { ContextUsage } from "./usage.js";
 
 const CLAUDE_CODE_VERSION = "2.1.75";
+const CLAUDE_CODE_BILLING_SYSTEM_BLOCK = `x-anthropic-billing-header: cc_version=${CLAUDE_CODE_VERSION}; cc_entrypoint=sdk-cli;`;
 const ANTHROPIC_MESSAGES_ERROR_BODY_MAX_BYTES = 8 * 1024;
 const ANTHROPIC_MESSAGES_ERROR_BODY_MAX_CHARS = 400;
 const ANTHROPIC_MESSAGES_ERROR_BODY_READ_IDLE_TIMEOUT_MS = 10_000;
@@ -1006,6 +1007,11 @@ function buildAnthropicParams(
   }
   if (isOAuthToken) {
     params.system = [
+      // Anthropic requires this first block to route Claude subscription OAuth billing.
+      {
+        type: "text",
+        text: CLAUDE_CODE_BILLING_SYSTEM_BLOCK,
+      },
       {
         type: "text",
         text: "You are Claude Code, Anthropic's official CLI for Claude.",