Five cheap patterns to make MCP tools tell agents what they didn't check

Honest_Traffic_8613 · reddit · 2026-10-05

Core problem: agents can't distinguish "nothing exists" from "I didn't look" — a tool silently returning partial answers leads to confident wrong edits.
The author made every tool in his MCP server state its own error bars. Five cheap patterns:
Empty answers explain the gap: list what is indexed and what isn't (macros, gitignored/build dirs) and how to reach them, so the agent picks the right fallback.
Every result states its coverage: e.g. impact follows imports two levels deep and says so, plus what it misses (dynamic dispatch, reflection, string routes, config refs).
Flag the file, not just the answer: outline notes when a file looks generated (.pb., DO NOT EDIT) or uses macros.
Never answer refactor questions from a stale index: incremental refresh on every call, 50 ms on a mid-size repo.
Say "unchanged" instead of repeating when the agent re-reads, with a fresh: true opt-out.
Rule of thumb: a tool's answer should carry its own error bars. Patterns 1/3/4/5 ship in Sweepspace 0.2.1.

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