fix: add Transfer-Encoding header to SSE responses for HTTP/2 compati…

[tinynakji]

…bility

Adds Transfer-Encoding: chunked header to all SSE response headers in WebStandardStreamableHTTPServerTransport to prevent HTTP/2 protocol errors.

Motivation and Context

Some HTTP adapters (e.g., @hono/node-server) buffer small SSE responses and automatically add a Content-Length header. When the SSE stream closes, this causes an HTTP/2 PROTOCOL_ERROR because HTTP/2 doesn't allow Content-Length with streaming responses.

Adding Transfer-Encoding: chunked prevents this buffering behavior and ensures SSE streams work correctly over HTTP/2.

Fixes #1619

How Has This Been Tested?

• Ran all existing streamableHttp tests across packages:
  • @modelcontextprotocol/server: 28 tests passed
  • @modelcontextprotocol/client: 38 tests passed
  • @modelcontextprotocol/node: 73 tests passed

Breaking Changes

None. This is an additive change to response headers.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Affected locations in packages/server/src/server/streamableHttp.ts:

• handleGetRequest method (~line 450)
• replayEvents method (~line 507)
• handlePostRequest method (~line 757)

Reference: https://github.com/honojs/node-server/blob/main/src/listener.ts#L463-L491

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7127af8

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/client

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1620

@modelcontextprotocol/server

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1620

@modelcontextprotocol/express

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1620

@modelcontextprotocol/hono

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1620

@modelcontextprotocol/node

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1620

commit: 07d76e4

[felixweinberger]

Thanks - did you run into this with an actual server or is this a speculative fix?

A regression test clearly demonstrating the issue would be required to be able to merge something like this.

[felixweinberger]

Thanks for this. Transfer-Encoding: chunked is forbidden in HTTP/2 (RFC 9113 §8.1.2.2) and Node throws ERR_HTTP2_INVALID_CONNECTION_HEADERS when sending it, so this fix would cause the error it's trying to prevent on actual HTTP/2 connections. The buffering is in @hono/node-server's responseViaResponseObject (reported upstream as honojs/node-server#266, honojs/hono#4041). If we need an SDK-side workaround, ensuring the SSE stream's first read is async bypasses the buffering without header hacks. Closing both this and #1723.