feat(ai): expose routed model on openai-completions (#3968)

Adds `AssistantMessage.responseModel` on the openai-completions path:
surfaces the concrete `chunk.model` when it differs from the requested
id (e.g. OpenRouter `auto` -> `anthropic/...`).

packages/ai/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added `AssistantMessage.responseModel` on the openai-completions path: surfaces the concrete `chunk.model` when it differs from the requested id (e.g. OpenRouter `auto` -> `anthropic/...`).
+
 ### Fixed
 
 - Fixed DeepSeek V4 Flash `xhigh` thinking support so requests preserve `xhigh` and map it to DeepSeek's `max` reasoning effort ([#3944](https://github.com/badlogic/pi-mono/issues/3944)).

packages/ai/src/providers/openai-completions.ts

@@ -207,6 +207,9 @@ export const streamOpenAICompletions: StreamFunction<"openai-completions", OpenA
 				// OpenAI documents ChatCompletionChunk.id as the unique chat completion identifier,
 				// and each chunk in a streamed completion carries the same id.
 				output.responseId ||= chunk.id;
+				if (typeof chunk.model === "string" && chunk.model.length > 0 && chunk.model !== model.id) {
+					output.responseModel ||= chunk.model;
+				}
 				if (chunk.usage) {
 					output.usage = parseChunkUsage(chunk.usage, model);
 				}

packages/ai/src/types.ts

@@ -216,6 +216,7 @@ export interface AssistantMessage {
 	api: Api;
 	provider: Provider;
 	model: string;
+	responseModel?: string; // Concrete `chunk.model` when different from the requested `model` (e.g. OpenRouter `auto` -> `anthropic/...`)
 	responseId?: string; // Provider-specific response/message identifier when the upstream API exposes one
 	usage: Usage;
 	stopReason: StopReason;

packages/ai/test/openai-completions-response-model.test.ts

@@ -0,0 +1,140 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { complete } from "../src/stream.js";
+import type { Model } from "../src/types.js";
+
+// Router/virtual ids (e.g. OpenRouter `auto`) keep `model` pinned to the
+// requested id and surface the routed concrete id on `responseModel`.
+
+const mockState = vi.hoisted(() => ({
+	chunks: [] as unknown[],
+}));
+
+vi.mock("openai", () => {
+	class FakeOpenAI {
+		chat = {
+			completions: {
+				create: () => {
+					const chunks = mockState.chunks;
+					const stream = {
+						async *[Symbol.asyncIterator]() {
+							for (const chunk of chunks) yield chunk;
+						},
+					};
+					const promise = Promise.resolve(stream) as Promise<typeof stream> & {
+						withResponse: () => Promise<{
+							data: typeof stream;
+							response: { status: number; headers: Headers };
+						}>;
+					};
+					promise.withResponse = async () => ({
+						data: stream,
+						response: { status: 200, headers: new Headers() },
+					});
+					return promise;
+				},
+			},
+		};
+	}
+	return { default: FakeOpenAI };
+});
+
+function openRouterAuto(): Model<"openai-completions"> {
+	return {
+		id: "openrouter/auto",
+		name: "OpenRouter Auto",
+		api: "openai-completions",
+		provider: "openrouter",
+		baseUrl: "https://openrouter.ai/api/v1",
+		reasoning: false,
+		input: ["text"],
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+		contextWindow: 200_000,
+		maxTokens: 8192,
+	};
+}
+
+describe("openai-completions responseModel", () => {
+	beforeEach(() => {
+		mockState.chunks = [];
+	});
+
+	it("surfaces routed chunk.model on responseModel without changing model", async () => {
+		mockState.chunks = [
+			{ id: "chatcmpl-1", model: "anthropic/claude-opus-4.7", choices: [{ index: 0, delta: { content: "hi" } }] },
+			{
+				id: "chatcmpl-1",
+				model: "anthropic/claude-opus-4.7",
+				choices: [{ index: 0, delta: {}, finish_reason: "stop" }],
+				usage: {
+					prompt_tokens: 10,
+					completion_tokens: 5,
+					prompt_tokens_details: { cached_tokens: 0 },
+					completion_tokens_details: { reasoning_tokens: 0 },
+				},
+			},
+		];
+
+		const message = await complete(
+			openRouterAuto(),
+			{ messages: [{ role: "user", content: "hi", timestamp: Date.now() }] },
+			{ apiKey: "test" },
+		);
+
+		expect(message.model).toBe("openrouter/auto");
+		expect(message.responseModel).toBe("anthropic/claude-opus-4.7");
+		expect(message.provider).toBe("openrouter");
+		expect(message.stopReason).toBe("stop");
+	});
+
+	it("leaves responseModel undefined when chunks echo the requested id", async () => {
+		mockState.chunks = [
+			{ id: "chatcmpl-2", model: "openrouter/auto", choices: [{ index: 0, delta: { content: "hi" } }] },
+			{
+				id: "chatcmpl-2",
+				model: "openrouter/auto",
+				choices: [{ index: 0, delta: {}, finish_reason: "stop" }],
+				usage: {
+					prompt_tokens: 1,
+					completion_tokens: 1,
+					prompt_tokens_details: { cached_tokens: 0 },
+					completion_tokens_details: { reasoning_tokens: 0 },
+				},
+			},
+		];
+
+		const message = await complete(
+			openRouterAuto(),
+			{ messages: [{ role: "user", content: "hi", timestamp: Date.now() }] },
+			{ apiKey: "test" },
+		);
+
+		expect(message.model).toBe("openrouter/auto");
+		expect(message.responseModel).toBeUndefined();
+	});
+
+	it("ignores empty or missing chunk.model", async () => {
+		mockState.chunks = [
+			{ id: "chatcmpl-3", choices: [{ index: 0, delta: { content: "hi" } }] },
+			{ id: "chatcmpl-3", model: "", choices: [{ index: 0, delta: { content: "!" } }] },
+			{
+				id: "chatcmpl-3",
+				choices: [{ index: 0, delta: {}, finish_reason: "stop" }],
+				usage: {
+					prompt_tokens: 1,
+					completion_tokens: 2,
+					prompt_tokens_details: { cached_tokens: 0 },
+					completion_tokens_details: { reasoning_tokens: 0 },
+				},
+			},
+		];
+
+		const message = await complete(
+			openRouterAuto(),
+			{ messages: [{ role: "user", content: "hi", timestamp: Date.now() }] },
+			{ apiKey: "test" },
+		);
+
+		expect(message.model).toBe("openrouter/auto");
+		expect(message.responseModel).toBeUndefined();
+	});
+});