From 8523105459885c47b9594b81a3146a1073bc5b03 Mon Sep 17 00:00:00 2001
From: Rhys Sullivan <39114868+RhysSullivan@users.noreply.github.com>
Date: Sat, 10 Oct 2026 10:53:29 -0700
Subject: [PATCH] Test the MCP probe User-Agent

---
 .../src/sdk/probe-shape-user-agent.test.ts    | 98 +++++++++++++++++++
 1 file changed, 98 insertions(+)
 create mode 100644 packages/plugins/mcp/src/sdk/probe-shape-user-agent.test.ts

diff --git a/packages/plugins/mcp/src/sdk/probe-shape-user-agent.test.ts b/packages/plugins/mcp/src/sdk/probe-shape-user-agent.test.ts
new file mode 100644
index 0000000000..67087dc5b2
--- /dev/null
+++ b/packages/plugins/mcp/src/sdk/probe-shape-user-agent.test.ts
@@ -0,0 +1,98 @@
+import { describe, expect, it } from "@effect/vitest";
+import { Effect, Layer } from "effect";
+import { HttpClient, HttpClientRequest, HttpClientResponse } from "effect/unstable/http";
+
+import { probeMcpEndpointShape } from "./probe-shape";
+
+const endpoint = "https://mcp.example/mcp";
+const metadataUrl = "https://mcp.example/.well-known/oauth-protected-resource/mcp";
+
+interface RecordedRequest {
+  readonly method: string;
+  readonly url: string;
+  readonly mcpProtocolVersion: string | undefined;
+  readonly userAgent: string | undefined;
+}
+
+/** Records every request and answers with `respond`'s status. */
+const recordingClientLayer = (
+  recorded: Array<RecordedRequest>,
+  respond: (request: HttpClientRequest.HttpClientRequest) => number,
+) =>
+  Layer.succeed(HttpClient.HttpClient)(
+    HttpClient.make((request: HttpClientRequest.HttpClientRequest) => {
+      recorded.push({
+        method: request.method,
+        url: request.url,
+        mcpProtocolVersion: request.headers["mcp-protocol-version"],
+        userAgent: request.headers["user-agent"],
+      });
+      return Effect.succeed(
+        HttpClientResponse.fromWeb(request, new Response("", { status: respond(request) })),
+      );
+    }),
+  );
+
+/** POST → 405, so the probe falls back to GET; GET → 401 with no Bearer
+ *  challenge, so the probe reads RFC 9728 metadata, which is missing. */
+const challengeWithoutBearer = (request: HttpClientRequest.HttpClientRequest): number =>
+  request.method === "POST" ? 405 : request.url === endpoint ? 401 : 404;
+
+const summarize = (recorded: ReadonlyArray<RecordedRequest>) =>
+  recorded.map((request) => ({
+    request: `${request.method} ${request.url}${
+      request.mcpProtocolVersion ? ` (${request.mcpProtocolVersion})` : ""
+    }`,
+    userAgent: request.userAgent,
+  }));
+
+describe("MCP shape probe User-Agent", () => {
+  it.effect("sends a default User-Agent on the probe and its OAuth metadata request", () =>
+    Effect.gen(function* () {
+      const recorded: Array<RecordedRequest> = [];
+
+      yield* probeMcpEndpointShape(endpoint, {
+        httpClientLayer: recordingClientLayer(recorded, challengeWithoutBearer),
+      });
+
+      expect(summarize(recorded)).toEqual([
+        { request: `POST ${endpoint}`, userAgent: "executor" },
+        { request: `GET ${endpoint}`, userAgent: "executor" },
+        { request: `GET ${metadataUrl}`, userAgent: "executor" },
+      ]);
+    }),
+  );
+
+  it.effect("sends a default User-Agent on the discover request", () =>
+    Effect.gen(function* () {
+      const recorded: Array<RecordedRequest> = [];
+
+      yield* probeMcpEndpointShape(endpoint, {
+        httpClientLayer: recordingClientLayer(recorded, () => 404),
+      });
+
+      expect(summarize(recorded)).toEqual([
+        { request: `POST ${endpoint}`, userAgent: "executor" },
+        { request: `GET ${endpoint}`, userAgent: "executor" },
+        { request: `POST ${endpoint} (2026-07-28)`, userAgent: "executor" },
+      ]);
+    }),
+  );
+
+  it.effect("keeps a configured User-Agent on every probe request", () =>
+    Effect.gen(function* () {
+      const recorded: Array<RecordedRequest> = [];
+
+      yield* probeMcpEndpointShape(endpoint, {
+        headers: { "User-Agent": "custom-agent/1.0" },
+        httpClientLayer: recordingClientLayer(recorded, challengeWithoutBearer),
+      });
+
+      expect(summarize(recorded)).toEqual([
+        { request: `POST ${endpoint}`, userAgent: "custom-agent/1.0" },
+        { request: `GET ${endpoint}`, userAgent: "custom-agent/1.0" },
+        { request: `GET ${metadataUrl}`, userAgent: "custom-agent/1.0" },
+      ]);
+    }),
+  );
+});