Support server returning only JSON on requests by ihrpr · Pull Request #299 · modelcontextprotocol/typescript-sdk

ihrpr · 2025-04-09T20:55:12Z

Spec states that servers must support either streaming or json response. Clients MUST support both.

If the input contains any number of JSON-RPC requests, the server MUST either return Content-Type: text/event-stream, to initiate an SSE stream, or Content-Type: application/json, to return one JSON object. The client MUST support both these cases.

We anticipate that TypeScript sdk will mostly be used with streaming, but for testing purpose adding support for returning JSON.
Returning JSON is more limited in functionality as we are loosing streaming tool notification/logging etc.

Example of a response calling to server that supports streaming:

Example of a response calling to server that support only JSON (loosing logging):

* always send headers specified in requestInit option * avoid doubled onerror call * use for-await to iterate SSE stream * remove outdated comments * simplify requestId tracking * throw error when response Content-Type is out of spec

StreamableHTTPClientTransport cleanup / fixes

jerome3o-anthropic

looks mostly good - some doc updates + remove accidental .swp file

some non-blocking questions/nits, feel free to ignore

jerome3o-anthropic · 2025-04-10T10:51:09Z

+
+```bash
+# Initialize the server and get the session ID from headers
+SESSION_ID=$(curl -X POST -H "Content-Type: application/json" -H "Accept: application/json" \


This doesn't work for me - I get:

❯ curl -X POST -H "Content-Type: application/json" -H "Accept: application/json" \ -d '{"jsonrpc":"2.0","method":"initialize","params":{"capabilities":{}},"id":"1"}' \ -i http://localhost:3000/mcp 2>&1 HTTP/1.1 406 Not Acceptable X-Powered-By: Express Date: Thu, 10 Apr 2025 10:50:08 GMT Connection: keep-alive Keep-Alive: timeout=5 Transfer-Encoding: chunked {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}%

This worked with the current implementation:

SESSION_ID=$(curl -X POST \ -H "Content-Type: application/json" \ -H "Accept: application/json" \ -H "Accept: text/event-stream" \ -d '{ "jsonrpc": "2.0", "method": "initialize", "params": { "capabilities": {}, "protocolVersion": "2025-03-26", "clientInfo": { "name": "test", "version": "1.0.0" } }, "id": "1" }' \ -i http://localhost:3000/mcp 2>&1 | grep -i "mcp-session-id" | cut -d' ' -f2 | tr -d '\r') echo "Session ID: $SESSION_ID"

jerome3o-anthropic · 2025-04-10T11:01:45Z

+curl -X POST -H "Content-Type: application/json" -H "Accept: application/json" \
+  -H "mcp-session-id: $SESSION_ID" \
+  -d '{"jsonrpc":"2.0","method":"mcp.call_tool","params":{"name":"greet","arguments":{"name":"World"}},"id":"2"}' \
+  http://localhost:3000/mcp


mcp.call_tool isn't correct, also I needed to have the Accept: text/event-stream header here too

curl -X POST \ -H "Content-Type: application/json" \ -H "Accept: application/json" \ -H "Accept: text/event-stream" \ -H "mcp-session-id: $SESSION_ID" \ -d '{ "jsonrpc": "2.0", "method": "tools/call", "params": { "name": "greet", "arguments": { "name": "World" } }, "id": "2" }' \ http://localhost:3000/mcp

jerome3o-anthropic · 2025-04-10T11:11:53Z

remove before merging

ugh, how this got it here!

jerome3o-anthropic · 2025-04-10T11:16:16Z

+app.listen(PORT, () => {
+  console.log(`MCP Streamable HTTP Server with JSON responses listening on port ${PORT}`);
+  console.log(`Server is running. Press Ctrl+C to stop.`);
+  console.log(`Initialize with: curl -X POST -H "Content-Type: application/json" -H "Accept: application/json" -d '{"jsonrpc":"2.0","method":"initialize","params":{"capabilities":{}},"id":"1"}' http://localhost:${PORT}/mcp`);


looks like this may need the same treatment mentioned prior

jerome3o-anthropic · 2025-04-10T11:22:03Z

+      await jsonResponseTransport.handleRequest(req, mockResponse);
+
+      // Wait for all promises to resolve
+      await new Promise(resolve => setTimeout(resolve, 50));


[non-blocking nit] is there a better way to do this?

seems like it would just slow down tests

jerome3o-anthropic · 2025-04-10T11:23:21Z

+    it("should return JSON response for a single request", async () => {
+      const requestMessage: JSONRPCMessage = {
+        jsonrpc: "2.0",
+        method: "test",


[non-blocking question] should this use real MCP messages?

yes, actually was thinking to use MCP server and rewrite tests, mocking request doesn't seem robust

jerome3o-anthropic · 2025-04-10T11:23:42Z

+      await jsonResponseTransport.handleRequest(req, mockResponse);
+
+      // Wait for all promises to resolve - give it enough time
+      await new Promise(resolve => setTimeout(resolve, 100));


same nit as prior

we actually don't even need it, was trying to make the test work, but the issues was in completely different place

ihrpr added 3 commits April 9, 2025 21:01

support for server returning json -- not stream

b37a2e1

add examples with notifications

71aeb9a

clean up somments

5b99e94

ihrpr requested a review from jspahrsummers April 9, 2025 20:55

Merge branch 'main' into ihrpr/response-returns-json

1aad768

ihrpr marked this pull request as ready for review April 9, 2025 21:14

beaulac reviewed Apr 9, 2025

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Comment thread src/server/streamableHttp.test.ts Outdated

beaulac mentioned this pull request Apr 10, 2025

StreamableHTTPClientTransport cleanup / fixes #300

Merged

9 tasks

beaulac and others added 3 commits April 9, 2025 21:18

StreamableHTTPClientTransport cleanup / fixes

a76004c

* always send headers specified in requestInit option * avoid doubled onerror call * use for-await to iterate SSE stream * remove outdated comments * simplify requestId tracking * throw error when response Content-Type is out of spec

remove order assumption for batch responses in test

632b836

Merge pull request #300 from prompteus-ai/streamable-http-client-fixes

77352db

StreamableHTTPClientTransport cleanup / fixes

ihrpr requested a review from jerome3o-anthropic April 10, 2025 09:31

jerome3o-anthropic requested changes Apr 10, 2025

View reviewed changes

ihrpr added 2 commits April 10, 2025 13:01

clean up and docs fixes

962a9c9

remove promise from test

569c928

jerome3o-anthropic approved these changes Apr 10, 2025

View reviewed changes

ihrpr merged commit 75ec9de into main Apr 10, 2025

ihrpr deleted the ihrpr/response-returns-json branch April 10, 2025 12:31

This was referenced Dec 16, 2025

docs: clarify that JSON response mode doesn't deliver notifications #1309

Closed

Notifications with StreamableHTTPServerTransport and enableJsonResponse = true #866

Closed

coderabbitai Bot mentioned this pull request Jul 27, 2026

MCP directory submissions: server.json and the listing playbook JaviMaligno/vitamind#22

Merged

maxisbey mentioned this pull request Jul 27, 2026

Fail fast on server-to-client requests in JSON-response mode instead of hanging modelcontextprotocol/python-sdk#3195

Merged

9 tasks

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ihrpr commented Apr 9, 2025

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ihrpr Apr 10, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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ihrpr Apr 10, 2025 •

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