fix: avoid AlreadySentError on duplicate SSE GET#20
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dbernheisel merged 1 commit intodbernheisel:mainfrom Apr 28, 2026
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When a second `GET /mcp` arrives for an existing session, `maybe_track_session_stream/1` correctly responds with HTTP 409 and a JSON-RPC `-32000` error. However, the conn was already sent and halted at that point, so the surrounding `dispatch/2` clause then tried to call `put_resp_header` and `send_chunked` on the same conn, raising `Plug.Conn.AlreadySentError`. Branch on `conn.halted` after `maybe_track_session_stream/1` so the 409 response is returned as-is and SSE setup is skipped. Adds a regression test that performs two consecutive GETs on the same session id and asserts the second returns a clean 409 with the expected JSON-RPC error envelope.
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Fixes #19.
Summary
GET /mcparrives for a session that already has atracked SSE stream,
maybe_track_session_stream/1correctly respondswith HTTP 409 and a JSON-RPC
-32000error and halts the conn.dispatch(%Plug.Conn{method: \"GET\"}, opts)clausewas unconditionally calling
put_resp_header/send_chunkedonthe returned conn, which raised
Plug.Conn.AlreadySentError.conn.haltedaftermaybe_track_session_stream/1so the409 response is returned as-is and SSE setup is skipped.
Test plan
test/phantom/plug_test.exsopens twoconsecutive GETs with the same session id and asserts the
second returns
status == 409withcode: -32000and\"Only one SSE stream is allowed per session\".mainwithPlug.Conn.AlreadySentErrorraisedat
lib/phantom/plug.ex:285.mix test— 201 tests, 0 failures (1 pre-existingskip), Elixir 1.19.1 / OTP 28.1.
Notes for downstream users
While this lands, downstream apps can wrap
Phantom.Plugand rescuePlug.Conn.AlreadySentErrorto fall back to a 409 JSON-RPC errorthemselves. The wrapper is no longer necessary once this is released.