## Summary
- Add SSE (`text/event-stream`) as an alternative response format on
`POST /mcp` per the MCP streamable-http spec
- When clients send `Accept: text/event-stream`, the server responds
with SSE wire format; otherwise returns JSON as before (fully backwards
compatible)
- Emit a `notifications/progress` SSE event before tool execution to
signal long-running operations
- No sessions, no GET SSE, no protocol version bump — this is
transport-level only (Phase 2)
## Changes
- **New**: `write-sse-event.util.ts` — writes correctly formatted SSE
events to Express Response
- **New**: `mcp-progress-notification.const.ts` — constants for progress
notification method and token prefix
- **Modified**: `mcp-core.controller.ts` — checks `Accept` header,
branches into SSE vs JSON response path
- **Modified**: `mcp-protocol.service.ts` — passes optional `sseWriter`
callback to tool executor
- **Modified**: `mcp-tool-executor.service.ts` — emits progress
notification via `sseWriter` before tool execution
- **Tests**: Unit tests for SSE utility, controller SSE/JSON paths, tool
executor progress notifications, and integration tests for SSE streaming
## Test plan
- [x] Unit tests: `writeSseEvent` utility produces correct SSE wire
format
- [x] Unit tests: Controller returns SSE headers and writes events when
`Accept: text/event-stream`
- [x] Unit tests: Controller returns JSON when `Accept:
application/json` only
- [x] Unit tests: Notifications (no `id`) return 202 regardless of
Accept header
- [x] Unit tests: Tool executor emits progress notification via
sseWriter
- [x] Unit tests: Tool executor works without sseWriter (backwards
compatible)
- [x] Integration tests: SSE response for ping, JSON fallback, progress
notification before tool call
- [x] All 503 test suites pass, typecheck clean
https://claude.ai/code/session_01QrqjBUXePJkPMd6gBAoWaR
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Co-authored-by: Claude <noreply@anthropic.com>
