Screening Files Before Reading with MCP
Due_Resolve6229 · reddit · 2026-07-11
The author proposed an MCP server for pre-screening files before reading: letting the agent pass through an observation layer immune to prompt injection before deciding whether to actually read the file.
The core design treats files as pure byte objects for deterministic pre-screening: checking type, size, structure, metadata, and directory listings first, without letting the LLM directly interpret file contents. This way, even if a file contains malicious instructions, it is treated as ordinary bytes and won't enter model inference directly.
They also pointed out an MCP-specific pitfall: passing sensitive word lists as tool arguments to the model actually exposes them in the model's context. Therefore, the lexicon must be loaded at server startup and not constructed by the caller. The author clarifies that this is strictly a pre-screen and not an absolute security boundary.
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