What is Arbitype?

Arbitype is an MCP-native typed decision layer for AI agents, powered by TypeSafe Jev. It turns probabilistic judgments into structured decision primitives that an agent or program can consume directly.

Arbitype is available on PyPI and the official MCP Registry.

Quick Start

The fastest way to connect an MCP host is a local STDIO server launched by uvx:

export TYPESAFE_API_KEY="your-key"
uvx arbitype

For a pinned, reproducible launch:

uvx --from 'arbitype==0.7.0' arbitype

Or install the package into the current environment:

python -m pip install arbitype
arbitype

The API key stays in the process environment. It is not an MCP argument and is never printed to standard output.

Host setup

Inspect detected hosts before writing anything:

arbitype setup --detect --dry-run

Then apply a reviewed plan:

arbitype setup codex
arbitype doctor --json

The setup flow is plan → unified diff → confirmation → backup → apply. Use claude, cursor, or vscode in place of codex. Detailed host formats, secret forwarding, --yes, --remove, ownership, and non-interactive behavior are documented in docs/HOST_SETUP.md.

Configuration

At minimum, set TYPESAFE_API_KEY. To select a model explicitly:

export TYPESAFE_API_KEY="your-key"
export TYPESAFE_MODEL="jev-latest"

See the full configuration reference for endpoints, timeouts, retries, and request/response limits.

Compatibility

arbitype is the canonical distribution, Python package, and CLI. Existing TypeSafe MCP users should migrate to Arbitype; see docs/REGISTRY_MIGRATION.md.

For the safe migration from the historical distribution:

python -m pip uninstall typesafe-mcp
python -m pip install arbitype

Why Arbitype?

Arbitype is designed around contracts and bounded decisions rather than free-form prose:

The decision flow is:

free-form state
      ↓
  TypeSafe Jev
      ↓
probabilistic judgment
      ↓
    Arbitype
      ↓
typed decision + probability
      ↓
agent / code branch

Real Use Cases

Start with one of the small, runnable fixtures:

The example outputs are illustrative fixtures. They are not live model results, accuracy claims, or authorization decisions.

Tools

Arbitype advertises nine read-only, idempotent MCP tools:

Tool Selection Guide

Tool descriptions also state USE WHEN and DO NOT USE WHEN boundaries so hosts can select a primitive without hidden prompt conventions.

Probabilities and confidence are model signals, not proof. gate and review are advisory decision transformations, not authorization systems, security boundaries, or approval engines.

Architecture

flowchart LR
    host["AI Agent / MCP Host<br/>Codex · Claude · Cursor · VS Code"]
    arbitype["Arbitype<br/>Typed decision tools"]
    jev["TypeSafe Jev<br/>System One Model"]
    env["TYPESAFE_API_KEY<br/>process environment"]

    host -->|MCP| arbitype
    arbitype -->|validated HTTPS| jev
    jev -->|typed probabilistic judgment| arbitype
    arbitype -->|structured decision| host
    env -. credential .-> arbitype

The public product is Arbitype; TypeSafe Jev is the current provider. Provider configuration intentionally keeps the TYPESAFE_* names because the credential and endpoint belong to TypeSafe.

Benchmark Snapshot

Recorded on the public 120-case Jev-mediated tool-selection evaluation:

This is a Jev-mediated evaluation over Arbitype's advertised MCP tool catalog. It is not a Codex, Claude, or Cursor host benchmark and is not a general model-performance guarantee.

Details and reproducible assets:

• Benchmark methodology
• Tool-selection dataset
• Recorded reports

Security Boundaries

Arbitype is a local MCP adapter and typed decision layer. It is not:

• a sandbox for untrusted code;
• an authorization or identity system;
• a prompt-injection firewall;
• a security approval boundary; or
• an official TypeSafe AI product.

It does not execute actions suggested by route, edit files, run shell commands, or treat model probabilities as proof. Read SECURITY.md before using live credentials.

Documentation

• Host setup
• Configuration reference
• MCP Registry migration
• Supply-chain posture
• Observability proposal
• Testing
• Benchmarks
• Contributing
• Security
• Official MCP Registry entry
• Glama listing (indexing and metadata refresh may lag publication)

Development

The test suite uses local fakes and does not need an API key:

python3 -m unittest discover -s tests -v
python3 -m compileall -q .
make check
make release-check

The official MCP Python SDK interoperability smoke test is in scripts/official_sdk_smoke.py. A live Jev check is opt-in and paid:

TYPESAFE_API_KEY="your-key" arbitype doctor --live

Normal CI does not call the provider or run paid benchmarks.