The race that nobody warns you about
Run two Claude Code sessions against the same repo — one refactoring a module, one fixing a typo three files away that happens to also touch a shared config file — and eventually both sessions decide to write the same file within a few hundred milliseconds of each other. Nothing stops that. The second write wins, silently. The first agent's changes are gone, and neither session has any idea it happened.
The same thing happens one layer down, with MCP servers instead of native editor tools: an MCP filesystem server invoked by two agent processes, or a run_migration tool exposed by a database server, called twice in close succession by two unrelated sessions. Heimdall MCP already had tool-name policies (v1.2) and argument-level policies (v1.4) — you could say "never call write_file on /etc/passwd" — but neither of those answers a different question: what happens when two calls to the same resource land at the same time?
That's the failure mode 1.5.0 is built to close.
Repo: https://github.com/enmanuelmag/heimdall-mcp
