Fix Fable 5.1 Anthropic sampling compatibility

apps/server/src/agent/anthropic-provider.test.ts

@@ -5,6 +5,7 @@ import { mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 
+import { AnthropicRequestHandler } from "./anthropic-request";
 import { locate } from "./cli";
 import { plannerConfiguration, RUNTIME_ENV, verifyModel, workerConfiguration } from "./client";
 import { NAME } from "./planner";
@@ -65,11 +66,30 @@ async function runtime(
 	respond: (request: Request) => Promise<Response>,
 	run: (session: CopilotSession) => Promise<void>,
 ) {
-	let server = Bun.serve({ hostname: "127.0.0.1", port: 0, fetch: respond });
+	let server = Bun.serve({
+		hostname: "127.0.0.1",
+		port: 0,
+		async fetch(request) {
+			let body = await request.clone().json() as Record<string, unknown>;
+			for (let field of ["temperature", "top_p", "top_k"]) {
+				if (field in body) {
+					return Response.json({
+						type: "error",
+						error: {
+							type: "invalid_request_error",
+							message: `${field} is deprecated for this model.`,
+						},
+					}, { status: 400 });
+				}
+			}
+			return respond(request);
+		},
+	});
 	let directory = mkdtempSync(join(tmpdir(), "chopin-anthropic-test-"));
 	let cli = locate();
 	if (!cli.ok) throw new Error(cli.reason);
 	let client = new CopilotClient({
+		requestHandler: new AnthropicRequestHandler(server.url.origin),
 		mode: "empty",
 		workingDirectory: directory,
 		baseDirectory: directory,

apps/server/src/agent/anthropic-request.test.ts

@@ -0,0 +1,86 @@
+import { expect, it, spyOn } from "bun:test";
+
+import { AnthropicRequestHandler } from "./anthropic-request";
+
+import type { CopilotRequestContext } from "@github/copilot-sdk";
+
+class Handler extends AnthropicRequestHandler {
+	public override sendRequest(request: Request, context: CopilotRequestContext) {
+		return super.sendRequest(request, context);
+	}
+}
+
+let context: CopilotRequestContext = {
+	requestId: "fixture",
+	transport: "http",
+	url: "https://api.anthropic.com/v1/messages",
+	headers: {},
+	signal: new AbortController().signal,
+};
+
+it("preserves Anthropic messages, tools, credentials, streaming and cancellation", async () => {
+	let body = {
+		model: "claude-fable-5-1",
+		messages: [{ role: "user", content: "Read the document." }],
+		tools: [{ name: "read_plan", input_schema: { type: "object" } }],
+		max_tokens: 32000,
+		stream: true,
+		thinking: { type: "adaptive" },
+	};
+	let response = new Response("event: message_stop\ndata: {}\n\n", {
+		headers: { "content-type": "text/event-stream" },
+	});
+	let fetch = spyOn(globalThis, "fetch").mockImplementation(
+		(async (input, options) => {
+			let request = input as Request;
+			expect(await request.json()).toEqual(body);
+			expect(request.url).toBe(`${context.url}?beta=true`);
+			expect(request.headers.get("x-api-key")).toBe("fixture-key");
+			expect(request.headers.get("anthropic-version")).toBe("2023-06-01");
+			expect(request.headers.get("content-length")).toBeNull();
+			expect(options?.signal).toBe(context.signal);
+			return response;
+		}) as typeof globalThis.fetch,
+	);
+	try {
+		let request = new Request(`${context.url}?beta=true`, {
+			method: "POST",
+			headers: {
+				"x-api-key": "fixture-key",
+				"anthropic-version": "2023-06-01",
+				"content-type": "application/json",
+				"content-length": "9999",
+			},
+			body: JSON.stringify({ ...body, temperature: 0, top_p: 0.9, top_k: 40 }),
+		});
+		expect(await new Handler().sendRequest(request, context)).toBe(response);
+		expect(fetch).toHaveBeenCalledTimes(1);
+	} finally {
+		fetch.mockRestore();
+	}
+});
+
+it("passes Copilot and unrelated requests through without rewriting them", async () => {
+	let requests = [
+		new Request("https://api.githubcopilot.com/chat/completions", {
+			method: "POST",
+			body: '{ "temperature": 0.5 }',
+		}),
+		new Request("https://other.example/v1/messages", { method: "POST", body: "unchanged" }),
+		new Request("https://api.anthropic.com/v1/messages/count_tokens", {
+			method: "POST",
+			body: "unchanged",
+		}),
+		new Request("https://api.anthropic.com/v1/messages"),
+	];
+	let fetch = spyOn(globalThis, "fetch").mockResolvedValue(new Response("ok"));
+	try {
+		for (let request of requests) {
+			await new Handler().sendRequest(request, context);
+			expect(fetch.mock.calls.at(-1)?.[0]).toBe(request);
+			expect(request.bodyUsed).toBe(false);
+		}
+	} finally {
+		fetch.mockRestore();
+	}
+});

apps/server/src/agent/anthropic-request.ts

@@ -0,0 +1,29 @@
+import { CopilotRequestHandler } from "@github/copilot-sdk";
+
+import type { CopilotRequestContext } from "@github/copilot-sdk";
+
+/** Use Anthropic's sampling defaults; Fable rejects the CLI's temperature. */
+export class AnthropicRequestHandler extends CopilotRequestHandler {
+	constructor(private origin = "https://api.anthropic.com") {
+		super();
+	}
+
+	protected override async sendRequest(
+		request: Request,
+		context: CopilotRequestContext,
+	): Promise<Response> {
+		let url = new URL(request.url);
+		if (
+			request.method === "POST" && url.origin === this.origin && url.pathname === "/v1/messages"
+		) {
+			let body = await request.json() as Record<string, unknown>;
+			delete body.temperature;
+			delete body.top_p;
+			delete body.top_k;
+			let headers = new Headers(request.headers);
+			headers.delete("content-length");
+			request = new Request(request, { method: "POST", headers, body: JSON.stringify(body) });
+		}
+		return super.sendRequest(request, context);
+	}
+}

apps/server/src/agent/client.ts

@@ -11,6 +11,7 @@ import { mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 
+import { AnthropicRequestHandler } from "./anthropic-request";
 import { locate } from "./cli";
 import {
 	gate,
@@ -253,6 +254,7 @@ function connect() {
 	let home = mkdtempSync(join(tmpdir(), "chopin-copilot-"));
 	try {
 		let client = new CopilotClient({
+			requestHandler: new AnthropicRequestHandler(),
 			mode: "empty",
 			workingDirectory: home,
 			baseDirectory: home,

docs/hosted-agent.md

@@ -209,6 +209,11 @@ configured Anthropic model and refuses a mismatch. Runtime `assistant.usage`
 events log the model and token counts, without prompts or credentials. These
 logs provide evidence independent of an agent's self-description.
 
+The SDK request handler removes `temperature`, `top_p`, and `top_k` from direct
+Anthropic Messages requests. The pinned CLI adds a temperature that Fable 5.1
+rejects; these requests use Anthropic's sampling defaults. Other endpoints pass
+through unchanged, and responses retain streaming and cancellation support.
+
 Public research uses Anthropic's Messages API directly with only the disclosed
 query and the basic `web_search_20250305` server tool. It has no private document,
 repository, filesystem, or client tools. Search results and citations must agree;
@@ -223,6 +228,6 @@ GitHub's web-search MCP configuration is used only in Copilot mode. Private
 analysis and synthesis stay in separate no-web SDK sessions in both modes.
 
 `bun run test:anthropic` exercises the pinned CLI against a local mock Anthropic
-endpoint, including wire model/key routing, streaming, tool permission checks,
-and terminal results. It requires local socket access but no API key or Copilot
-login. It does not establish live model availability for a deployment's key.
+endpoint, including rejection of deprecated sampling parameters, wire model/key
+routing, streaming, tool permission checks, and terminal results. It requires
+local socket access but no API key or Copilot login. It does not establish live model availability for a deployment's key.