Channel plugin: duplicate message delivery from harness process respawn

Description

When using a custom MCP channel plugin (Matrix in our case), messages are delivered to the session multiple times — typically 2-3 deliveries of the same event.

Root Cause

The channel harness appears to spontaneously respawn the MCP plugin process ~3 minutes into a session with no preceding error. For ~10 seconds while both the old and new PIDs are alive, both processes receive the upstream event stream and both emit notifications/claude/channel for the same incoming message.

Since any in-memory deduplication state (e.g., a Set of delivered event IDs) is per-process, the newly spawned process has no knowledge of what the old process already delivered.

Reproduction

1. Configure a channel plugin MCP server (e.g., a Matrix bridge using matrix-bot-sdk)
2. Start Claude Code with --dangerously-load-development-channels server:matrix
3. Send a message from an external client to a room the bot is monitoring
4. Observe the message arriving 2-3 times — once as an inline <channel> tag and again as <system-reminder> notifications

Observed Behavior

Same message_id delivered through multiple paths:

• Inline channel tag (from original process)
• System-reminder (same notification, different delivery path)
• System-reminder (from duplicate spawned process — observable because in-memory state like display name aliasing may differ)

Expected Behavior

Each unique message_id from a channel plugin should be delivered exactly once to the session.

Workaround

We implemented server-side deduplication (event ID tracking + startup timestamp guard) which prevents duplicate emissions from our MCP server, but cannot prevent the harness from spawning a duplicate process that bypasses this.

Environment

• Claude Code version: latest (as of 2026-03-21)
• OS: NixOS (Linux 6.18.18)
• Channel: Custom Matrix MCP plugin using matrix-bot-sdk + @modelcontextprotocol/sdk
• Flag: --dangerously-load-development-channels server:matrix

Possibly related to #36800.