SSE streams cause HTTP/2 PROTOCOL_ERROR with some HTTP adapters (e.g., @hono/node-server)

[tinynakji]

Description

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.

Reproduction

1. Use WebStandardStreamableHTTPServerTransport with @hono/node-server over HTTP/2
2. Perform an MCP operation that returns a small SSE response
3. Observe PROTOCOL_ERROR on stream close

Expected Behavior

SSE streams should work correctly over HTTP/2 without protocol errors.

Proposed Fix

Add Transfer-Encoding: chunked header to all SSE responses in WebStandardStreamableHTTPServerTransport. This prevents HTTP adapters from buffering and adding Content-Length.

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

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

Additional Context

Reference: https://github.com/honojs/node-server/blob/main/src/listener.ts#L463-L491 (Hono's buffering logic)

[felixweinberger]

This is a @hono/node-server adapter issue, tracked upstream at honojs/node-server#266 and honojs/hono#4041. Adding Transfer-Encoding: chunked would cause ERR_HTTP2_INVALID_CONNECTION_HEADERS on HTTP/2 connections (the header is forbidden in HTTP/2 per RFC 9113 §8.1.2.2). Closing here, please follow the upstream issues.