diff --git a/.changeset/mcp-probe-user-agent.md b/.changeset/mcp-probe-user-agent.md
new file mode 100644
index 0000000000..bc264ea5da
--- /dev/null
+++ b/.changeset/mcp-probe-user-agent.md
@@ -0,0 +1,5 @@
+---
+"executor": patch
+---
+
+Send the default User-Agent on the MCP endpoint probe and its OAuth metadata request too, so adding a server that rejects requests without one no longer fails. A configured User-Agent still wins.
diff --git a/packages/plugins/mcp/src/sdk/connection.ts b/packages/plugins/mcp/src/sdk/connection.ts
index 94fed481d4..f8ddefca7e 100644
--- a/packages/plugins/mcp/src/sdk/connection.ts
+++ b/packages/plugins/mcp/src/sdk/connection.ts
@@ -21,6 +21,7 @@ import {
   McpOAuthReauthorizationRequired,
 } from "./errors";
 import { connectionHttpStatusFromCause, isStreamableHttpProtocolError } from "./http-status";
+import { withDefaultUserAgent } from "./user-agent";
 import { detectInsufficientScope } from "@executor-js/sdk/core";
 
 // ---------------------------------------------------------------------------
@@ -72,17 +73,6 @@ const buildEndpointUrl = (endpoint: string, queryParams: Record<string, string>)
   return url;
 };
 
-// Some upstreams (Cloudflare bot rules among them) reject requests without a
-// User-Agent, and Workers' fetch adds none. Both remote SDK transports apply
-// `requestInit.headers` to every request they send, including OAuth metadata
-// discovery, so the default goes there. A configured User-Agent wins.
-const DEFAULT_USER_AGENT = "executor";
-
-const withDefaultUserAgent = (headers: Record<string, string>): Record<string, string> =>
-  Object.keys(headers).some((name) => name.toLowerCase() === "user-agent")
-    ? headers
-    : { "User-Agent": DEFAULT_USER_AGENT, ...headers };
-
 type HttpMethod = Parameters<typeof HttpClientRequest.make>[0];
 const HTTP_METHODS = new Set<HttpMethod>([
   "DELETE",
@@ -636,7 +626,9 @@ export const createMcpConnector = (input: ConnectorInput): McpConnector => {
     });
   }
 
-  // Remote transport
+  // Remote transport. Both remote SDK transports apply `requestInit.headers`
+  // to every request they send, including OAuth metadata discovery, so the
+  // default User-Agent goes there.
   const headers = withDefaultUserAgent(input.headers ?? {});
   const remoteTransport = input.remoteTransport ?? "auto";
   const requestInit = { headers };
diff --git a/packages/plugins/mcp/src/sdk/probe-shape.ts b/packages/plugins/mcp/src/sdk/probe-shape.ts
index b64f791139..5fd6f192de 100644
--- a/packages/plugins/mcp/src/sdk/probe-shape.ts
+++ b/packages/plugins/mcp/src/sdk/probe-shape.ts
@@ -44,6 +44,7 @@
 
 import { Data, Duration, Effect, Layer, Option, Schema } from "effect";
 import { FetchHttpClient, HttpClient, HttpClientRequest } from "effect/unstable/http";
+import { userAgentFor } from "./user-agent";
 
 /** MCP initialize request body used as the shape probe. Any real MCP
  *  server either answers it (unauth-OK server) or returns the spec-
@@ -277,7 +278,13 @@ export const probeMcpEndpointShape = (
   Effect.gen(function* () {
     const timeoutMs = options.timeoutMs ?? 8_000;
     const outcome = yield* Effect.gen(function* () {
-      const client = yield* HttpClient.HttpClient;
+      // Every probe request — including the RFC 9728 metadata fallback, which
+      // carries none of the configured headers — sends the User-Agent.
+      const client = (yield* HttpClient.HttpClient).pipe(
+        HttpClient.mapRequest(
+          HttpClientRequest.setHeader("user-agent", userAgentFor(options.headers ?? {})),
+        ),
+      );
 
       const readBody = (response: {
         readonly text: Effect.Effect<string, unknown>;
diff --git a/packages/plugins/mcp/src/sdk/user-agent.ts b/packages/plugins/mcp/src/sdk/user-agent.ts
new file mode 100644
index 0000000000..11fd5fe58e
--- /dev/null
+++ b/packages/plugins/mcp/src/sdk/user-agent.ts
@@ -0,0 +1,16 @@
+// Some upstreams (Cloudflare bot rules among them) reject requests without a
+// User-Agent, and Workers' fetch adds none. Every request the MCP plugin sends
+// to an MCP server or its OAuth metadata carries this default. A configured
+// User-Agent wins.
+export const DEFAULT_USER_AGENT = "executor";
+
+const configuredUserAgent = (headers: Readonly<Record<string, string>>): string | undefined =>
+  Object.entries(headers).find(([name]) => name.toLowerCase() === "user-agent")?.[1];
+
+export const withDefaultUserAgent = (headers: Record<string, string>): Record<string, string> =>
+  configuredUserAgent(headers) === undefined
+    ? { "User-Agent": DEFAULT_USER_AGENT, ...headers }
+    : headers;
+
+export const userAgentFor = (headers: Readonly<Record<string, string>>): string =>
+  configuredUserAgent(headers) ?? DEFAULT_USER_AGENT;