Scaling from 4 to 8 Claude Code subagents pushed my error rate from 0.8% to 4.3%. The bottleneck wasn't the model.
The culprit was a stateful MCP tool called analytics_query that held pagination cursors, mid-aggregation values, and filter chains in instance memory between calls. Cloudflare Workers routes each request to whichever PoP instance is handy — no guarantees you land on the same one twice. At 4 subagents, collisions were rare enough that sessions accidentally stayed sticky. At 8, the distribution spread out and context misses went nonlinear. The error looked like this:
Error: Tool call failed — session context not found
session_id: "sess_7f3a9b"
worker_instance: "worker-11"
