MCP-Injector: protocol-level fault injection for tool-calling LLM agents

Artifact for the NeurIPS 2026 paper "Tool-Calling LLM Agents Under Realistic MCP Faults: Scale Does Not Purchase Fault Tolerance".

MCP-Injector is a protocol-aware, drop-in fault-injection middleware for the Model Context Protocol (MCP). It interposes on the tool-call path with controlled persistence (permanent, transient, intermittent) and seedable fault schedules, and requires no modification to the agent runtime, the MCP client, or the servers.

This record archives the full artifact: the injector and its fault operators; the null and calibrated fault profiles; the calibration pipeline over 57,240 issue and pull-request threads from 1,774 open-source MCP repositories, with the per-thread classifier outputs (labels only, no raw narratives); the runners for the 1,350-cell experiment crossing two backbone sizes, three agent stacks and three fault conditions; the merged per-task results for all 1,350 cells and the raw per-round campaign dumps; and the analysis pipeline that produced the figures and tables of the paper.

Source code is released under Apache-2.0 and data artifacts under CC-BY-4.0. See NOTICE, DATA-LICENSE, and LICENSES-README.md in the archive for the path-by-path map. Portions are derived from MCP-Bench (https://github.com/accenture/mcp-bench).