fix(server): GET SSE stream crash on Netty in Streamable HTTP

[devcrocod]

appendSseHeaders() was called inside Ktor's sse {} block where response headers are already committed. On Netty this throws UnsupportedOperationException, killing the SSE stream and causing the client to retry in a loop

How Has This Been Tested?

unit, simple-streamable-server sample

Breaking Changes

none

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
• [x ] I have added appropriate error handling
• [ x] I have added or updated documentation as needed

[Copilot]

Pull request overview

Fixes a Netty crash that occurred when setting SSE response headers after Ktor’s sse {} handler had already committed headers, which could terminate the SSE stream and cause client retry loops.

Changes:

• Stop calling appendSseHeaders() inside Ktor’s sse {} blocks for GET SSE streams / replay paths.
• Set Mcp-Session-Id for GET SSE responses earlier in the Ktor pipeline (before headers are committed).
• Add a regression test ensuring the GET SSE stream returns Mcp-Session-Id and remains open.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransportTest.kt	Adds a test covering GET SSE header presence + connection staying open.
kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport.kt	Removes header mutation inside sse {} and relies on early header setting + flush.
kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/KtorServer.kt	Adds a route interceptor to attach Mcp-Session-Id on GET before SSE commits headers.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The GET interceptor reads the session ID via context.request.header(...) (single value), but the transport validation later rejects requests that contain multiple Mcp-Session-Id header values. In that case this interceptor can still echo a session ID onto a response that will be rejected. Consider using context.request.headers.getAll(MCP_SESSION_ID_HEADER) and only setting the response header when there is exactly one non-blank value (matching StreamableHttpServerTransport.validateSession).

Suggested change

	val sessionId = context.request.header(MCP_SESSION_ID_HEADER)
	val sessionIds = context.request.headers.getAll(MCP_SESSION_ID_HEADER)
	val sessionId =
	sessionIds
	?.singleOrNull()
	?.takeIf { it.isNotBlank() }

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...delcontextprotocol/kotlin/sdk/server/KtorServer.kt	80.00%	0 Missing and 1 partial ⚠️
...kotlin/sdk/server/StreamableHttpServerTransport.kt	50.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[e5l]

lgtm