feat: allow injecting pre-built Anthropic client

Add optional `client` field to AnthropicOptions so callers can pass
alternative SDK clients (e.g. AnthropicVertex) that
share the same messages.stream() API. When omitted, behavior is unchanged.

Signed-off-by: sallyom <somalley@redhat.com>

packages/ai/src/providers/anthropic.ts

@@ -178,6 +178,12 @@ export interface AnthropicOptions extends StreamOptions {
 	effort?: AnthropicEffort;
 	interleavedThinking?: boolean;
 	toolChoice?: "auto" | "any" | "none" | { type: "tool"; name: string };
+	/**
+	 * Pre-built Anthropic client instance. When provided, skips internal client
+	 * construction entirely. Use this to inject alternative SDK clients such as
+	 * `AnthropicVertex` that shares the same messaging API.
+	 */
+	client?: Anthropic;
 }
 
 function mergeHeaders(...headerSources: (Record<string, string> | undefined)[]): Record<string, string> {
@@ -217,25 +223,35 @@ export const streamAnthropic: StreamFunction<"anthropic-messages", AnthropicOpti
 		};
 
 		try {
-			const apiKey = options?.apiKey ?? getEnvApiKey(model.provider) ?? "";
+			let client: Anthropic;
+			let isOAuth: boolean;
 
-			let copilotDynamicHeaders: Record<string, string> | undefined;
-			if (model.provider === "github-copilot") {
-				const hasImages = hasCopilotVisionInput(context.messages);
-				copilotDynamicHeaders = buildCopilotDynamicHeaders({
-					messages: context.messages,
-					hasImages,
-				});
+			if (options?.client) {
+				client = options.client;
+				isOAuth = false;
+			} else {
+				const apiKey = options?.apiKey ?? getEnvApiKey(model.provider) ?? "";
+
+				let copilotDynamicHeaders: Record<string, string> | undefined;
+				if (model.provider === "github-copilot") {
+					const hasImages = hasCopilotVisionInput(context.messages);
+					copilotDynamicHeaders = buildCopilotDynamicHeaders({
+						messages: context.messages,
+						hasImages,
+					});
+				}
+
+				const created = createClient(
+					model,
+					apiKey,
+					options?.interleavedThinking ?? true,
+					options?.headers,
+					copilotDynamicHeaders,
+				);
+				client = created.client;
+				isOAuth = created.isOAuthToken;
 			}
-
-			const { client, isOAuthToken } = createClient(
-				model,
-				apiKey,
-				options?.interleavedThinking ?? true,
-				options?.headers,
-				copilotDynamicHeaders,
-			);
-			let params = buildParams(model, context, isOAuthToken, options);
+			let params = buildParams(model, context, isOAuth, options);
 			const nextParams = await options?.onPayload?.(params, model);
 			if (nextParams !== undefined) {
 				params = nextParams as MessageCreateParamsStreaming;
@@ -290,7 +306,7 @@ export const streamAnthropic: StreamFunction<"anthropic-messages", AnthropicOpti
 						const block: Block = {
 							type: "toolCall",
 							id: event.content_block.id,
-							name: isOAuthToken
+							name: isOAuth
 								? fromClaudeCodeName(event.content_block.name, context.tools)
 								: event.content_block.name,
 							arguments: (event.content_block.input as Record<string, any>) ?? {},

packages/ai/test/anthropic-client-injection.test.ts

@@ -0,0 +1,54 @@
+import { describe, expect, it } from "vitest";
+import { getModel } from "../src/models.js";
+import type { Context } from "../src/types.js";
+
+describe("Anthropic client injection", () => {
+	const model = getModel("anthropic", "claude-sonnet-4-6");
+	const context: Context = {
+		systemPrompt: "Test",
+		messages: [{ role: "user", content: "Hello", timestamp: Date.now() }],
+	};
+
+	it("should use injected client and set isOAuth to false", async () => {
+		const { streamAnthropic } = await import("../src/providers/anthropic.js");
+
+		let capturedPayload: any = null;
+		const messagesStreamCalled = { value: false };
+
+		// Minimal mock: just enough to verify the injected client is used
+		const mockClient = {
+			messages: {
+				stream: () => {
+					messagesStreamCalled.value = true;
+					// Return async iterable that yields a stop event
+					const events = [
+						{ type: "message_start", message: { usage: { input_tokens: 0, output_tokens: 0 } } },
+						{ type: "message_delta", delta: { stop_reason: "end_turn" }, usage: {} },
+					];
+					return {
+						[Symbol.asyncIterator]: async function* () {
+							yield* events;
+						},
+					};
+				},
+			},
+		};
+
+		const s = streamAnthropic(model, context, {
+			client: mockClient as any,
+			onPayload: (payload) => {
+				capturedPayload = payload;
+			},
+		});
+
+		for await (const _event of s) {
+			// consume
+		}
+
+		expect(messagesStreamCalled.value).toBe(true);
+		expect(capturedPayload).not.toBeNull();
+		// isOAuth=false means no Claude Code system prompt identity is prepended
+		expect(capturedPayload.system).toHaveLength(1);
+		expect(capturedPayload.system[0].text).toBe("Test");
+	});
+});