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Allow multiple instances of the Inspector to connect to the same MCP server #428

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Merged
cliffhall merged 6 commits into from
May 30, 2025
Merged

Allow multiple instances of the Inspector to connect to the same MCP server #428

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f04470a
Allow multiple client connections to the MCP server, by making a uniq…
cliffhall May 21, 2025
bc90f11
Address [comment](https://github.com/modelcontextprotocol/inspector/p…
cliffhall May 28, 2025
9bc6bd7
Address [comment](https://github.com/modelcontextprotocol/inspector/p…
cliffhall May 28, 2025
0c1962c
prettier
cliffhall May 28, 2025
d0ba418
Merge branch 'main' into allow-multiple-client-connections
cliffhall May 29, 2025
a7aa726
Merge branch 'main' into allow-multiple-client-connections
cliffhall May 29, 2025
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58 changes: 28 additions & 30 deletions server/src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -48,7 +48,8 @@ app.use((req, res, next) => {
next();
});

const webAppTransports: Map<string, Transport> = new Map<string, Transport>(); // Transports by sessionId
const webAppTransports: Map<string, Transport> = new Map<string, Transport>(); // Web app transports by web app sessionId
const serverTransports: Map<string, Transport> = new Map<string, Transport>(); // Server Transports by web app sessionId

const createTransport = async (req: express.Request): Promise<Transport> => {
const query = req.query;
Expand Down Expand Up @@ -137,8 +138,6 @@ const createTransport = async (req: express.Request): Promise<Transport> => {
}
};

let backingServerTransport: Transport | undefined;

app.get("/mcp", async (req, res) => {
const sessionId = req.headers["mcp-session-id"] as string;
console.log(`Received GET message for sessionId ${sessionId}`);
Expand All @@ -161,12 +160,12 @@ app.get("/mcp", async (req, res) => {
app.post("/mcp", async (req, res) => {
const sessionId = req.headers["mcp-session-id"] as string | undefined;
console.log(`Received POST message for sessionId ${sessionId}`);
let serverTransport: Transport | undefined;
if (!sessionId) {
try {
console.log("New streamable-http connection");
try {
await backingServerTransport?.close();
backingServerTransport = await createTransport(req);
serverTransport = await createTransport(req);
} catch (error) {
if (error instanceof SseError && error.code === 401) {
console.error(
Expand All @@ -180,12 +179,13 @@ app.post("/mcp", async (req, res) => {
throw error;
}

console.log("Connected MCP client to backing server transport");
console.log("Connected MCP client to server transport");

const webAppTransport = new StreamableHTTPServerTransport({
sessionIdGenerator: randomUUID,
onsessioninitialized: (sessionId) => {
webAppTransports.set(sessionId, webAppTransport);
serverTransports.set(sessionId, serverTransport!);
console.log("Created streamable web app transport " + sessionId);
},
});
Expand All @@ -194,7 +194,7 @@ app.post("/mcp", async (req, res) => {

mcpProxy({
transportToClient: webAppTransport,
transportToServer: backingServerTransport,
transportToServer: serverTransport,
});

await (webAppTransport as StreamableHTTPServerTransport).handleRequest(
Expand Down Expand Up @@ -229,10 +229,9 @@ app.post("/mcp", async (req, res) => {
app.get("/stdio", async (req, res) => {
try {
console.log("New connection");

let serverTransport: Transport | undefined;
try {
await backingServerTransport?.close();
backingServerTransport = await createTransport(req);
serverTransport = await createTransport(req);
} catch (error) {
if (error instanceof SseError && error.code === 401) {
console.error(
Expand All @@ -250,26 +249,24 @@ app.get("/stdio", async (req, res) => {

const webAppTransport = new SSEServerTransport("/message", res);
webAppTransports.set(webAppTransport.sessionId, webAppTransport);

console.log("Created web app transport");
serverTransports.set(webAppTransport.sessionId, serverTransport);
console.log("Created client/server transports");

await webAppTransport.start();
(backingServerTransport as StdioClientTransport).stderr!.on(
"data",
(chunk) => {
webAppTransport.send({
jsonrpc: "2.0",
method: "notifications/stderr",
params: {
content: chunk.toString(),
},
});
},
);

(serverTransport as StdioClientTransport).stderr!.on("data", (chunk) => {
webAppTransport.send({
jsonrpc: "2.0",
method: "notifications/stderr",
params: {
content: chunk.toString(),
},
});
});

mcpProxy({
transportToClient: webAppTransport,
transportToServer: backingServerTransport,
transportToServer: serverTransport,
});

console.log("Set up MCP proxy");
Expand All @@ -284,10 +281,9 @@ app.get("/sse", async (req, res) => {
console.log(
"New SSE connection. NOTE: The sse transport is deprecated and has been replaced by streamable-http",
);

let serverTransport: Transport | undefined;
try {
await backingServerTransport?.close();
backingServerTransport = await createTransport(req);
serverTransport = await createTransport(req);
} catch (error) {
if (error instanceof SseError && error.code === 401) {
console.error(
Expand All @@ -305,13 +301,15 @@ app.get("/sse", async (req, res) => {

const webAppTransport = new SSEServerTransport("/message", res);
webAppTransports.set(webAppTransport.sessionId, webAppTransport);
console.log("Created web app transport");
console.log("Created client transport");
serverTransports.set(webAppTransport.sessionId, serverTransport);
console.log("Created server transport");

await webAppTransport.start();

mcpProxy({
transportToClient: webAppTransport,
transportToServer: backingServerTransport,
transportToServer: serverTransport,
});

console.log("Set up MCP proxy");
Expand Down