feat(pi-ai): support ANTHROPIC_BASE_URL env var for custom proxy endpoints (#4140)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
(cherry picked from commit e94cf668e2fdf28537aead642b4062cd3a22a8d3)

packages/pi-ai/src/providers/anthropic-auth.test.ts

@@ -4,7 +4,7 @@ import { readFileSync } from "node:fs";
 import { dirname, join } from "node:path";
 import { fileURLToPath } from "node:url";
 
-import { usesAnthropicBearerAuth } from "./anthropic.js";
+import { usesAnthropicBearerAuth, resolveAnthropicBaseUrl } from "./anthropic.js";
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 
@@ -31,3 +31,49 @@ test("createClient routes Bearer-auth providers through authToken (#3783)", () =
 		"Bearer-auth providers should send authToken instead",
 	);
 });
+
+// Minimal model stub — only the field resolveAnthropicBaseUrl cares about.
+const stubModel = { baseUrl: "https://api.anthropic.com" } as Parameters<typeof resolveAnthropicBaseUrl>[0];
+
+test("resolveAnthropicBaseUrl returns model.baseUrl when ANTHROPIC_BASE_URL is unset (#4140)", (t) => {
+	const saved = process.env.ANTHROPIC_BASE_URL;
+	t.after(() => {
+		if (saved === undefined) delete process.env.ANTHROPIC_BASE_URL;
+		else process.env.ANTHROPIC_BASE_URL = saved;
+	});
+
+	delete process.env.ANTHROPIC_BASE_URL;
+	assert.equal(resolveAnthropicBaseUrl(stubModel), "https://api.anthropic.com");
+});
+
+test("resolveAnthropicBaseUrl prefers ANTHROPIC_BASE_URL over model.baseUrl (#4140)", (t) => {
+	const saved = process.env.ANTHROPIC_BASE_URL;
+	t.after(() => {
+		if (saved === undefined) delete process.env.ANTHROPIC_BASE_URL;
+		else process.env.ANTHROPIC_BASE_URL = saved;
+	});
+
+	process.env.ANTHROPIC_BASE_URL = "https://proxy.example.com";
+	assert.equal(resolveAnthropicBaseUrl(stubModel), "https://proxy.example.com");
+});
+
+test("resolveAnthropicBaseUrl ignores whitespace-only ANTHROPIC_BASE_URL (#4140)", (t) => {
+	const saved = process.env.ANTHROPIC_BASE_URL;
+	t.after(() => {
+		if (saved === undefined) delete process.env.ANTHROPIC_BASE_URL;
+		else process.env.ANTHROPIC_BASE_URL = saved;
+	});
+
+	process.env.ANTHROPIC_BASE_URL = "   ";
+	assert.equal(resolveAnthropicBaseUrl(stubModel), "https://api.anthropic.com");
+});
+
+test("createClient uses resolveAnthropicBaseUrl for all auth paths (#4140)", () => {
+	const source = readFileSync(join(__dirname, "..", "..", "src", "providers", "anthropic.ts"), "utf-8");
+	const directUsages = (source.match(/baseURL:\s*model\.baseUrl/g) ?? []).length;
+	assert.equal(directUsages, 0, "createClient must not use model.baseUrl directly — use resolveAnthropicBaseUrl(model)");
+	assert.ok(
+		source.includes("baseURL: resolveAnthropicBaseUrl(model)"),
+		"all createClient branches should pass baseURL through resolveAnthropicBaseUrl",
+	);
+});

packages/pi-ai/src/providers/anthropic.ts

@@ -25,6 +25,24 @@ import {
 export type { AnthropicEffort, AnthropicOptions };
 export { extractRetryAfterMs };
 
+/**
+ * Resolve the base URL for Anthropic API requests.
+ *
+ * Resolution order:
+ * 1. ANTHROPIC_BASE_URL environment variable (if set and non-empty after trim)
+ * 2. model.baseUrl (from the model definition)
+ *
+ * This allows routing traffic through custom proxy endpoints (e.g. OpusMax,
+ * local mirrors, corporate gateways) without modifying model definitions.
+ */
+export function resolveAnthropicBaseUrl(model: Model<"anthropic-messages">): string {
+	const envBaseUrl = process.env.ANTHROPIC_BASE_URL?.trim();
+	if (envBaseUrl) {
+		return envBaseUrl;
+	}
+	return model.baseUrl;
+}
+
 let _AnthropicClass: typeof Anthropic | undefined;
 async function getAnthropicClass(): Promise<typeof Anthropic> {
 	if (!_AnthropicClass) {
@@ -75,7 +93,7 @@ async function createClient(
 		const client = new AnthropicClass({
 			apiKey: null,
 			authToken: apiKey,
-			baseURL: model.baseUrl,
+			baseURL: resolveAnthropicBaseUrl(model),
 			dangerouslyAllowBrowser: true,
 			defaultHeaders: mergeHeaders(
 				{