Two AI agents (two sessions, or an agent and a script) read the same file and both write it back. The later write silently clobbers the first: no error, the work is just gone. agent-coherence is a drop-in guard that catches the stale read before the overwrite lands, so the stale writer is denied and re-reads first. It ships as an MCP server, a LangGraph store drop-in, and a Python library. Single-host today (2+ agents on one machine); cross-host is opt-in, demo-grade. Open source, TLA+-checked.