[Client] Resume interrupted SSE streams

[wWzZb]

Fixes #327.

When an SSE response ends before its JSON-RPC response arrived, the client now resumes it with a GET carrying Last-Event-ID, honouring the server's retry field - otherwise the request waited for its timeout.

• attempts and fallback backoff via ReconnectPolicy (default: 5 attempts, 1-10 s), scheduling via any PSR-20 clock
• a stream ending without event ID, a rejected reconnect (405, 401, ...) or 2026-07-28 fail the request at once
• 404 on the reconnect expires the session like any session-bound request ([Client] Invalidate expired HTTP sessions #560)
• sse-retry conformance scenario passes and is removed from the baseline

[chr-hertel]

Hi @wWzZb, I took this over to get it in after the recent client changes - rebased it on main and squashed it into one commit, you stay the author.

On top of your version:

• ReconnectPolicy and a PSR-20 clock instead of the four constructor arguments, SSE parsing extracted into SseParser
• fit in with the listening GET stream ([Client] Listen on the standalone GET stream over HTTP #543), cancellation & deadlines ([Client] Support cooperative tool-call cancellation and per-call deadlines #518), session expiry ([Client] Invalidate expired HTTP sessions #560) and the shared headers ([Client] Send MCP-Protocol-Version on every request after the handshake #425) - so the reconnect GET carries MCP-Protocol-Version now
• no resumption on 2026-07-28, which dropped it, and a rejected reconnect fails the request at once instead of retrying
• a reconnect no longer adopts a new session ID, sessions are only minted on initialize

sse-retry still passes 3/3. Thanks for your work on this! 🙏

[Copilot]

🟡 Changes recommended

Mixed SSE delimiters and invalid response-shaped events can prevent correct reconnection.

2 open findings

• Mark SSE request complete only after validating the JSON-RPC response · New
• Handle mixed SSE line endings when detecting event boundaries · New

What changed in this PR

Adds resumable SSE responses to the HTTP client with protocol-aware reconnection.

Changes:

• Adds SSE parsing, retry policy, and monotonic scheduling.
• Reconnects interrupted streams using Last-Event-ID.
• Adds unit and conformance coverage.

File	Description
src/​Client/​Transport/​HttpTransport.php	Implements SSE reconnection.
src/​Client/​Transport/​SseParser.php	Adds incremental SSE parsing.
src/​Client/​Transport/​ReconnectPolicy.php	Defines retry limits and backoff.
src/​Client/​MonotonicClock.php	Provides monotonic scheduling.
src/​Schema/​Enum/​ProtocolVersion.php	Gates resumption by protocol version.
tests/​Unit/​Client/​Transport/​HttpTransportTest.php	Covers reconnection behavior.
tests/​Unit/​Client/​Transport/​SseParserTest.php	Tests SSE parsing.
tests/​Unit/​Client/​Transport/​ReconnectPolicyTest.php	Tests backoff policy.
tests/​Unit/​Client/​MonotonicClockTest.php	Tests monotonic time.
tests/​Unit/​Schema/​Enum/​ProtocolVersionTest.php	Tests protocol gating.
tests/​Conformance/​client.php	Runs the SSE retry scenario.
tests/​Conformance/​conformance-baseline-2025-11-25.yml	Removes the passing scenario from baseline.
docs/​client/​transports.md	Documents reconnection configuration.
CHANGELOG.md	Records the feature.

🧠 Review effort: Balanced

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[Copilot]

🔵 Needs a closer look

Reconnect limits can be bypassed by replayed non-empty events, and standalone listening streams remain unresumable.

0 open findings

2 resolved since last review

• Handle mixed SSE line endings when detecting event boundaries
• Mark SSE request complete only after validating the JSON-RPC response

Previously missed (1)

In code that hasn't changed since last review

Preserve SSE resume metadata and reconnect listening streams

src/​Client/​Transport/​HttpTransport.php:604

The linked issue's acceptance criteria also require resumption while the session is active, but the standalone listening stream still discards every event's id/retry by using dispatchSseData() directly and is permanently closed on EOF below. With listen: true, a transient disconnect therefore silently stops future server-initiated messages for an otherwise connected session. Track its resume metadata and schedule its GET with Last-Event-ID as well.

This issue also appears on line 865 of the same file.

🧠 Review effort: Balanced