Adding usage instructions to MCP tool descriptions triggers AI agent calls, study finds

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A developer built a parallel-execution MCP tool that assigns independent tasks to AI subagents and tested whether three coding agents — OpenHands, OpenCode, and Qwen Code — would call it on their own. None of the agents called the tool until a single sentence describing when to use it was added directly to the tool's description field. Across 24 controlled runs, the tool was called in 7 of 12 runs with the sentence present, versus 0 of 12 without it, yielding a p-value of 0.0046 by Fisher's exact test. However, when only methodologically consistent runs were compared, the p-value rose to 0.182, falling short of statistical significance. A separate lighter-weight consultation tool showed the opposite pattern, responding reliably to rule-file instructions rather than tool descriptions, suggesting that the optimal way to prompt AI agents may depend on how resource-intensive the tool's processing is.

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