Harnis: an open-source call-level seatbelt for local stdio MCP servers
L00pLegend · reddit · 2026-11-02
Unlike sandbox-based agent security tools, Harnis wraps a local stdio MCP server and inspects tools/call and path-like resources/read before the child process sees them; denials are returned to the client and never forwarded.
Usage: npx -y harnis mcp --preset github -- github-mcp-server stdio, or npx -y harnis init; requires Python 3.11+.
In scope: credential paths in any tool args (renamed tools included), opaque/destructive shell, GitHub preset deletes and default-branch writes, optional budgets and denytools.
Out of scope: URL/SSE MCP, removing the wrap, renamed tools with no secret args. Ships with an explicit threat model doc.
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