Preserve backend HTTP status in MCP errors

packages/choosekit-mcp/package.json

@@ -1,6 +1,6 @@
 {
   "name": "choosekit-mcp",
-  "version": "0.6.0",
+  "version": "0.6.1",
   "description": "Use choosekit as an MCP server.",
   "license": "Apache-2.0",
   "author": {
@@ -37,7 +37,7 @@
   },
   "dependencies": {
     "@modelcontextprotocol/server": "2.0.0",
-    "choosekit": "^0.9.0",
+    "choosekit": "^0.9.1",
     "zod": "^4.6.4"
   },
   "devDependencies": {

packages/choosekit-mcp/src/server.ts

@@ -91,9 +91,13 @@ function errorResult(error: unknown, signal: AbortSignal) {
     throw error;
   }
   if (error instanceof ScoringError) {
+    const status = error.httpStatus;
+    const text = typeof status === "number" && Number.isInteger(status) && status >= 100 && status <= 599
+      ? `The scoring backend returned HTTP ${status}.`
+      : "The model could not score the supplied choices.";
     return {
       isError: true as const,
-      content: [{ type: "text" as const, text: "The model could not score the supplied choices." }],
+      content: [{ type: "text" as const, text }],
     };
   }
   if (error instanceof ImageLoadError) {

packages/choosekit-mcp/tests/cli.test.mjs

@@ -357,3 +357,33 @@ test("serves a minimal-prefix choice using the configured llama.cpp endpoint", a
   assert.equal(many.result.structuredContent.followup.choice, "billing");
   for (const line of process_.lines) assert.doesNotThrow(() => JSON.parse(line));
 });
+
+test("CLI preserves adapter HTTP 429 in both tool errors", async (t) => {
+  const preload = `globalThis.fetch = async () => Response.json(
+    { error: "secret upstream details" }, { status: 429 });`;
+  const fixture = `data:text/javascript;base64,${Buffer.from(preload).toString("base64")}`;
+  const process_ = startCli(cliEnvironment({
+    CHOOSEKIT_BACKEND: "openrouter",
+    CHOOSEKIT_BASE_URL: "",
+    CHOOSEKIT_MODEL: "fixture/model",
+    OPENROUTER_API_KEY: "secret-api-key",
+  }), ["--import", fixture]);
+  t.after(() => process_.close());
+  await initialize(process_);
+
+  for (const [name, arguments_] of [
+    ["choose", request],
+    ["chooseMany", {
+      context: request.context,
+      questions: { route: { question: request.question, choices: request.choices } },
+    }],
+  ]) {
+    const response = await process_.rpc("tools/call", { name, arguments: arguments_ });
+    assert.equal(response.error, undefined);
+    assert.equal(response.result.isError, true);
+    assert.deepEqual(response.result.content,
+      [{ type: "text", text: "The scoring backend returned HTTP 429." }]);
+    assert.equal(response.result.structuredContent, undefined);
+    assert.doesNotMatch(JSON.stringify(response), /secret/);
+  }
+});

packages/choosekit-mcp/tests/server.test.mjs

@@ -484,3 +484,46 @@ test("preserves constructor as an ordinary choice key", async (t) => {
   assert.equal(response.result.structuredContent.choice, "constructor");
   assert.deepEqual(receivedChoices, arguments_.choices);
 });
+
+for (const [tool, arguments_] of [["choose", request], ["chooseMany", manyRequest]]) {
+  test(`${tool} reports only validated HTTP metadata and recovers after failure`, async (t) => {
+    const choose = createChooser(() => ({ logprobs: [-0.1, -2], boundaryTokens: 1 }));
+    let failure;
+    const connection = await connect(async (input) => {
+      if (failure) throw failure;
+      return choose(input);
+    });
+    t.after(() => connection.close());
+
+    const cases = [
+      ...[100, 200, 302, 401, 403, 429, 503, 599].map((httpStatus) => [
+        new ScoringError("secret upstream details", {
+          httpStatus, cause: new Error("secret cause"),
+        }),
+        `The scoring backend returned HTTP ${httpStatus}.`,
+      ]),
+      ...[undefined, null, "401", 99, 600, 401.5, NaN, Infinity, -Infinity,
+        { toString() { throw new Error("must not coerce"); } }].map((httpStatus) => [
+        new ScoringError("secret upstream details", { httpStatus }),
+        "The model could not score the supplied choices.",
+      ]),
+      [Object.assign(new Error("secret implementation detail"), { httpStatus: 401 }),
+        "The choice request failed unexpectedly."],
+    ];
+
+    for (const [error, text] of cases) {
+      failure = error;
+      const response = await connection.rpc("tools/call", { name: tool, arguments: arguments_ });
+      assert.equal(response.error, undefined);
+      assert.equal(response.result.isError, true);
+      assert.deepEqual(response.result.content, [{ type: "text", text }]);
+      assert.equal(response.result.structuredContent, undefined);
+      assert.doesNotMatch(JSON.stringify(response), /secret/);
+    }
+
+    failure = undefined;
+    const response = await connection.rpc("tools/call", { name: tool, arguments: arguments_ });
+    assert.equal(response.error, undefined);
+    assert.notEqual(response.result.isError, true);
+  });
+}