TACIT: Tracked Agent Capabilities In Types MCP Server

What is TACIT?

TACIT is a safety harness for AI agents that uses Scala 3's capture checking type system to statically track capabilities and enforce safety. It provides an MCP interface for use by MCP-compatible agents like Claude Code, OpenCode, and GitHub Copilot.

How to use TACIT?

Download the prebuilt release JARs (or build from source with sbt), then configure your agent to run the server with java -jar TACIT.jar --library-jar /path/to/TACIT-library.jar. Optionally disable the agent's built-in tools to route all operations through TACIT. For stateful tasks, use session tools (create_repl_session, execute_in_session, etc.).

Key features of TACIT

• Scala 3 compiler with capture checking in safe mode
• Local REPL for stateless or stateful code execution
• Capability-safe library for file, process, and network access
• Classified[T] type enforces pure-only access to sensitive data
• Dual LLM design: trusted local model processes classified content
• Tools: execute_scala, session management, show_interface

Use cases of TACIT

• Sandbox agent code execution with compile-time capability enforcement
• Granularly control file system, process, and network access per session
• Prevent exfiltration of classified data (API keys, secrets) to cloud LLMs
• Run multi-step agent tasks with persistent state across interactions