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fix(server): leave SSE connection policy to adapters #2554

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fix(server): leave SSE connection policy to adapters #2554

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5 changes: 5 additions & 0 deletions .changeset/web-standard-connection-policy.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'@modelcontextprotocol/server': patch
---

Leave hop-by-hop connection policy for Streamable HTTP SSE responses to the HTTP adapter.
9 changes: 3 additions & 6 deletions packages/server/src/server/streamableHttp.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -480,8 +480,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {

const headers: Record<string, string> = {
'Content-Type': 'text/event-stream',
'Cache-Control': 'no-cache, no-transform',
Connection: 'keep-alive'
'Cache-Control': 'no-cache, no-transform'
};

// After initialization, always include the session ID if we have one
Expand Down Expand Up @@ -536,8 +535,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {

const headers: Record<string, string> = {
'Content-Type': 'text/event-stream',
'Cache-Control': 'no-cache, no-transform',
Connection: 'keep-alive'
'Cache-Control': 'no-cache, no-transform'
};

if (this.sessionId !== undefined) {
Expand Down Expand Up @@ -837,8 +835,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {

const headers: Record<string, string> = {
'Content-Type': 'text/event-stream',
'Cache-Control': 'no-cache',
Connection: 'keep-alive'
'Cache-Control': 'no-cache'
};

// After initialization, always include the session ID if we have one
Expand Down
3 changes: 3 additions & 0 deletions packages/server/test/server/streamableHttp.test.ts
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Expand Up @@ -166,6 +166,7 @@ describe('Zod v4', () => {
expect(response.status).toBe(200);
expect(response.headers.get('content-type')).toBe('text/event-stream');
expect(response.headers.get('mcp-session-id')).toBeDefined();
expect(response.headers.get('connection'), 'connection policy belongs to the HTTP adapter').toBeNull();
});

it('should reject second initialization request', async () => {
Expand Down Expand Up @@ -358,6 +359,7 @@ describe('Zod v4', () => {
expect(response.status).toBe(200);
expect(response.headers.get('content-type')).toBe('text/event-stream');
expect(response.headers.get('mcp-session-id')).toBe(sessionId);
expect(response.headers.get('connection'), 'connection policy belongs to the HTTP adapter').toBeNull();
});

it('should reject GET without Accept: text/event-stream', async () => {
Expand Down Expand Up @@ -857,6 +859,7 @@ describe('Zod v4', () => {
createRequest('GET', undefined, { sessionId, extraHeaders: { 'Last-Event-ID': primingId! } })
);
expect(reconnect.status).toBe(200);
expect(reconnect.headers.get('connection'), 'connection policy belongs to the HTTP adapter').toBeNull();
release();
const replayed = await readSSEEvent(reconnect);
expect(replayed).toContain('notifications/progress');
Expand Down
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