🤖 fix: reduce flakiness in anthropic1MContext test (#1057)

Token limit errors fail immediately - the API rejects the request before
streaming starts. Instead of waiting for full response completion, we
now race between `stream-start` (accepted) vs `stream-error` (rejected).

## Changes
- Simplified test to check `stream-start` vs `stream-error` (not
`stream-end`)
- Reduced timeout from 60s to 30s
- Extracted `buildAnthropicHeaders()` into testable pure function
- Added unit tests for header building logic

_Generated with `mux`_

Author: ammar-agent

src/node/services/aiService.test.ts

@@ -3,7 +3,12 @@
 // For now, the commandProcessor tests demonstrate our testing approach
 
 import { describe, it, expect, beforeEach } from "bun:test";
-import { AIService, normalizeAnthropicBaseURL } from "./aiService";
+import {
+  AIService,
+  normalizeAnthropicBaseURL,
+  buildAnthropicHeaders,
+  ANTHROPIC_1M_CONTEXT_HEADER,
+} from "./aiService";
 import { HistoryService } from "./historyService";
 import { PartialService } from "./partialService";
 import { InitStateManager } from "./initStateManager";
@@ -76,3 +81,39 @@ describe("normalizeAnthropicBaseURL", () => {
     );
   });
 });
+
+describe("buildAnthropicHeaders", () => {
+  it("returns undefined when use1MContext is false and no existing headers", () => {
+    expect(buildAnthropicHeaders(undefined, false)).toBeUndefined();
+  });
+
+  it("returns existing headers unchanged when use1MContext is false", () => {
+    const existing = { "x-custom": "value" };
+    expect(buildAnthropicHeaders(existing, false)).toBe(existing);
+  });
+
+  it("returns existing headers unchanged when use1MContext is undefined", () => {
+    const existing = { "x-custom": "value" };
+    expect(buildAnthropicHeaders(existing, undefined)).toBe(existing);
+  });
+
+  it("adds 1M context header when use1MContext is true and no existing headers", () => {
+    const result = buildAnthropicHeaders(undefined, true);
+    expect(result).toEqual({ "anthropic-beta": ANTHROPIC_1M_CONTEXT_HEADER });
+  });
+
+  it("merges 1M context header with existing headers when use1MContext is true", () => {
+    const existing = { "x-custom": "value" };
+    const result = buildAnthropicHeaders(existing, true);
+    expect(result).toEqual({
+      "x-custom": "value",
+      "anthropic-beta": ANTHROPIC_1M_CONTEXT_HEADER,
+    });
+  });
+
+  it("overwrites existing anthropic-beta header when use1MContext is true", () => {
+    const existing = { "anthropic-beta": "other-beta" };
+    const result = buildAnthropicHeaders(existing, true);
+    expect(result).toEqual({ "anthropic-beta": ANTHROPIC_1M_CONTEXT_HEADER });
+  });
+});

src/node/services/aiService.ts

@@ -214,6 +214,26 @@ export function normalizeAnthropicBaseURL(baseURL: string): string {
   return `${trimmed}/v1`;
 }
 
+/** Header value for Anthropic 1M context beta */
+export const ANTHROPIC_1M_CONTEXT_HEADER = "context-1m-2025-08-07";
+
+/**
+ * Build headers for Anthropic provider, optionally including the 1M context beta header.
+ * Exported for testing.
+ */
+export function buildAnthropicHeaders(
+  existingHeaders: Record<string, string> | undefined,
+  use1MContext: boolean | undefined
+): Record<string, string> | undefined {
+  if (!use1MContext) {
+    return existingHeaders;
+  }
+  if (existingHeaders) {
+    return { ...existingHeaders, "anthropic-beta": ANTHROPIC_1M_CONTEXT_HEADER };
+  }
+  return { "anthropic-beta": ANTHROPIC_1M_CONTEXT_HEADER };
+}
+
 /**
  * Preload AI SDK provider modules to avoid race conditions in concurrent test environments.
  * This function loads @ai-sdk/anthropic, @ai-sdk/openai, and ollama-ai-provider-v2 eagerly
@@ -447,14 +467,10 @@ export class AIService extends EventEmitter {
           : configWithApiKey;
 
         // Add 1M context beta header if requested
-        const use1MContext = muxProviderOptions?.anthropic?.use1MContext;
-        const existingHeaders = normalizedConfig.headers;
-        const headers =
-          use1MContext && existingHeaders
-            ? { ...existingHeaders, "anthropic-beta": "context-1m-2025-08-07" }
-            : use1MContext
-              ? { "anthropic-beta": "context-1m-2025-08-07" }
-              : existingHeaders;
+        const headers = buildAnthropicHeaders(
+          normalizedConfig.headers,
+          muxProviderOptions?.anthropic?.use1MContext
+        );
 
         // Lazy-load Anthropic provider to reduce startup time
         const { createAnthropic } = await PROVIDER_REGISTRY.anthropic();