mcp-slim: Zero-Config MCP Proxy Cuts Tool Schema/Output Tokens 50–80%, Redacts Secrets
Leading_Strain3682 · reddit · 2026-11-11
A developer released open-source mcp-slim on Reddit: a zero-dependency Python stdio JSON-RPC 2.0 proxy that sits transparently between MCP clients (Claude Code, Cursor, Windsurf) and upstream MCP servers, tackling two pain points when connecting multiple MCP servers:
- Token bloat: auto-generated JSON Schema boilerplate in tools/list ($schema, title, nullable anyOf wrappers, essay-length descriptions) and uncompacted tool outputs eat 25k–60k+ tokens per turn.
- Secret leakage: tools reading logs, .env files, or git remotes can echo live API keys (sk-ant-..., ghp..., AKIA...) straight into the LLM context.
Three core modules:
- SchemaSlimmer: strips schema boilerplate, simplifies nullable anyOf unions, drops self-repeating descriptions; supports --allow-tools/--deny-tools filtering (52% schema token reduction).
- OutputSlimmer: compacts JSON outputs, prunes null/empty fields, strips ANSI codes, caps runaway arrays (43% output reduction).
- SecretGuard: bidirectional regex firewall redacting 12 categories of secrets in real time.
One-command setup wraps existing servers in place: mcp-slim wrap -i /.cursor/mcp.json. The author is asking for benchmarks and feedback from real MCP setups.
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