AgentRoom: Concurrent Multi-Agent Coding in a CRDT-Backed Shared Workspace

Concurrent multi-agent coding promises division of labor across modules, robustness through redundancy, and parallel exploration at the natural granularity of multi-file projects. Realtime collaborative editing protocols solve this coordination problem for human teams via Conflict-free Replicated Data Types (CRDTs), but the LLMs underneath generate one token at a time and existing multi-agent coding systems inherit this serial limit: they either sequence agents through phase handoffs or pool independent samples without coordination, and a single agent abandons up to 70% of hard tasks with a one-file stub-and-exit. AgentRoom is a realtime collaborative editing protocol for concurrent coding agents: a runtime layer exposing file-level claim, status, and broadcast as MCP tools on top of a CRDT-merged shared filesystem. Across five frontier coding-CLI models on four backend coding tasks, alongside Python DevBench and Rust+axum cross-language checks, AgentRoom ×2 suppresses Solo abandonment on the CLI-stable models and tightens run-to-run variance. Two matched-compute contrasts isolate the cause: parallel-merge vs AgentRoom shows a positive mean LLM-judge contrast, and a bundle probe attributes the gain primarily to the MCP coordination layer rather than to parallelism or substrate convergence alone. Coordination, not parallelism or CRDT-merge, is the load-bearing engineering.