Construct every MCP client through one factory that declares capabilties

src/lib/server/mcp/client.spec.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect, vi } from "vitest";
+
+const constructed = vi.hoisted(() => [] as Array<{ info: unknown; options: unknown }>);
+
+vi.mock("@modelcontextprotocol/sdk/client", () => ({
+	Client: class {
+		constructor(info: unknown, options: unknown) {
+			constructed.push({ info, options });
+		}
+	},
+}));
+
+const { createMcpClient, MCP_CLIENT_CAPABILITIES } = await import("./client");
+
+function infoAndOptionsOf(index: number) {
+	return constructed[index] as {
+		info: { name: string; version: string };
+		options: { capabilities: Record<string, unknown> };
+	};
+}
+
+describe("createMcpClient", () => {
+	it("declares capabilities on every client it builds", () => {
+		constructed.length = 0;
+
+		createMcpClient();
+		createMcpClient("health");
+
+		// Same declaration everywhere, or which client the server met decides the outcome.
+		expect(infoAndOptionsOf(0).options.capabilities).toBe(MCP_CLIENT_CAPABILITIES);
+		expect(infoAndOptionsOf(1).options.capabilities).toBe(MCP_CLIENT_CAPABILITIES);
+	});
+
+	it("keeps the session and health identities distinct", () => {
+		constructed.length = 0;
+
+		createMcpClient("session");
+		createMcpClient("health");
+
+		expect(infoAndOptionsOf(0).info).toEqual({ name: "chat-ui-mcp", version: "0.1.0" });
+		expect(infoAndOptionsOf(1).info).toEqual({ name: "chat-ui-health-check", version: "1.0.0" });
+	});
+
+	it("builds a session client by default", () => {
+		constructed.length = 0;
+
+		createMcpClient();
+
+		expect(infoAndOptionsOf(0).info).toEqual({ name: "chat-ui-mcp", version: "0.1.0" });
+	});
+
+	it("declares nothing it cannot yet answer", () => {
+		expect(MCP_CLIENT_CAPABILITIES).toEqual({});
+	});
+});

src/lib/server/mcp/client.ts

@@ -0,0 +1,26 @@
+import { Client } from "@modelcontextprotocol/sdk/client";
+import type { ClientCapabilities } from "@modelcontextprotocol/sdk/types.js";
+
+/**
+ * Empty on purpose: a capability declared without a handler makes servers issue a
+ * request the SDK can only answer with method-not-found. Add one only in the change
+ * that implements its handler.
+ */
+export const MCP_CLIENT_CAPABILITIES: ClientCapabilities = {};
+
+// Servers can tell these apart, so a health probe stays distinguishable from a session.
+const CLIENT_INFO = {
+	session: { name: "chat-ui-mcp", version: "0.1.0" },
+	health: { name: "chat-ui-health-check", version: "1.0.0" },
+} as const;
+
+export type McpClientKind = keyof typeof CLIENT_INFO;
+
+/**
+ * The one place an MCP client is constructed. Only the pooled client is alive while a
+ * tool runs, so a capability added to the listing client in `tools.ts` instead of here
+ * would look declared and never fire.
+ */
+export function createMcpClient(kind: McpClientKind = "session"): Client {
+	return new Client(CLIENT_INFO[kind], { capabilities: MCP_CLIENT_CAPABILITIES });
+}

src/lib/server/mcp/clientPool.ts

@@ -1,4 +1,5 @@
-import { Client } from "@modelcontextprotocol/sdk/client";
+import type { Client } from "@modelcontextprotocol/sdk/client";
+import { createMcpClient } from "./client";
 import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
 import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";
 import type { McpServerConfig } from "./httpClient";
@@ -73,7 +74,7 @@ export async function getClient(server: McpServerConfig, signal?: AbortSignal):
 	}
 
 	let firstError: unknown;
-	const client = new Client({ name: "chat-ui-mcp", version: "0.1.0" });
+	const client = createMcpClient();
 	const url = new URL(server.url);
 	// Pooled clients outlive the request that created them, so never bind the per-request
 	// abort signal to the transport. Per-call cancellation goes through RequestOptions instead.

src/lib/server/mcp/tools.ts

@@ -1,4 +1,4 @@
-import { Client } from "@modelcontextprotocol/sdk/client";
+import { createMcpClient } from "./client";
 import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
 import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";
 import type { McpServerConfig } from "./httpClient";
@@ -152,7 +152,7 @@ async function listServerTools(
 	opts: { signal?: AbortSignal } = {}
 ): Promise<ListedTool[]> {
 	const url = new URL(server.url);
-	const client = new Client({ name: "chat-ui-mcp", version: "0.1.0" });
+	const client = createMcpClient();
 	try {
 		try {
 			const transport = new StreamableHTTPClientTransport(url, {

src/routes/api/mcp/health/+server.ts

@@ -1,4 +1,5 @@
-import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import type { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { createMcpClient } from "$lib/server/mcp/client";
 import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
 import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";
 import type { KeyValuePair } from "$lib/types/Tool";
@@ -106,10 +107,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 		// Try Streamable HTTP transport first
 		try {
 			logger.info({}, `[MCP Health] Trying HTTP transport for ${url}`);
-			client = new Client({
-				name: "chat-ui-health-check",
-				version: "1.0.0",
-			});
+			client = createMcpClient("health");
 
 			const transport = new StreamableHTTPClientTransport(baseUrl, {
 				requestInit,
@@ -172,10 +170,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 			// Try SSE transport
 			try {
 				logger.info({}, `[MCP Health] Trying SSE transport for ${url}`);
-				client = new Client({
-					name: "chat-ui-health-check",
-					version: "1.0.0",
-				});
+				client = createMcpClient("health");
 
 				const sseTransport = new SSEClientTransport(baseUrl, {
 					requestInit,