🤖 fix: cancel active stream on message edit to prevent history corruption

[ethanndickson]

Problem

While trying to edit the token count of a running compaction request, the entire chat history was replaced with [truncated].

Root Cause

When editing a compacting message while the first compaction stream is still running:

1. The edit truncates history and starts a new compaction stream
2. The old stream continues running and isn't cancelled
3. When the old stream completes, handleCompletion reads the current history
4. It finds the NEW compaction request (from the edit), not the old one
5. Since the new request isn't in processedCompactionRequestIds, it proceeds to perform compaction
6. This clears ALL history and replaces it with the old stream's partial summary → entire chat becomes [truncated]

Solution

Cancel any active stream before processing message edits in AgentSession.sendMessage:

// Cancel any active stream when editing a message to prevent race conditions
if (options?.editMessageId && this.aiService.isStreaming(this.workspaceId)) {
  const stopResult = await this.interruptStream(/* abandonPartial */ true);
  if (!stopResult.success) {
    return Err(createUnknownSendMessageError(stopResult.error));
  }
}

This ensures:

• Only one stream runs at a time
• Edits truly discard the old stream (aligns with user intent)
• No orphaned stream completions that corrupt history

Changes

• src/node/services/agentSession.ts: Added stream cancellation check before processing edits (9 lines)

Testing

• ✅ Typecheck passes
• ✅ Graceful error handling (interruptStream is idempotent)
• ✅ Fixes the specific scenario: editing compaction token count mid-stream

Generated with mux

[ethanndickson]

@codex review