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.

Quick start

The shortest path is a local STDIO server launched by uvx:

export TYPESAFE_API_KEY="your-key"
uvx arbitype

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

Install a pinned release with:

uvx --from 'arbitype==0.6.0' arbitype

Or run the repository checkout:

git clone https://github.com/Renwang-Huang/arbitype.git
cd arbitype
export TYPESAFE_API_KEY="your-key"
python3 server.py

Why Arbitype?

Generative models are excellent at prose, code, and open-ended generation. Agent workflows also need bounded decisions that software can branch on:

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

Arbitype provides that decision layer over MCP. It exposes short, host-neutral primitives for classification, scoring, verification, routing, review, and fail-closed gate signals. The result is structured data, not a paragraph that an agent must interpret again.

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.

Tools

Arbitype advertises nine read-only, idempotent MCP tools:

For raw Noul questions, provide non-empty instructions or at least one non-empty true/false criterion. Score levels must be non-null structured values. These provider-level constraints are validated locally before a paid request.

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

Host setup

Arbitype uses standard MCP STDIO. For a host that accepts an installed command:

[mcp_servers.arbitype]
command = "arbitype"
env_vars = ["TYPESAFE_API_KEY"]
startup_timeout_sec = 10
tool_timeout_sec = 60
default_tools_approval_mode = "prompt"

For a checkout:

[mcp_servers.arbitype]
command = "python3"
args = ["/absolute/path/to/arbitype/server.py"]
env_vars = ["TYPESAFE_API_KEY"]
startup_timeout_sec = 10
tool_timeout_sec = 60

The same process can be registered by Claude, Cursor, VS Code, Codex, or any other MCP host using its native configuration format. Keep the key out of host configuration files; use the host's environment forwarding mechanism.

CLI and Python

The canonical CLI and package are arbitype:

arbitype --version
arbitype doctor --json
cat request.json | arbitype evaluate
arbitype evaluate --input request.json

The Python API is intentionally small:

from arbitype import TypeSafeClient

client = TypeSafeClient()
result = client.evaluate({
    "state": "A payment failed twice.",
    "questions": {
        "urgent": {
            "type": "noul",
            "instructions": "Does this require urgent handling?",
        }
    },
})

Package and import compatibility

The canonical wheel contains the one implementation plus the legacy import shims. The old PyPI project is not deleted, yanked, or released under a new identity.

CLI compatibility

Tool compatibility

The historical typesafe-mcp PyPI project remains intact. A metadata-only replacement with the same distribution name was tested and rejected because pip can remove legacy console-script files while replacing the old distribution. Therefore no typesafe-mcp==0.6.0 migration package will be published.

Existing users should use this explicit, safe migration:

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

New installations should use arbitype directly. This leaves one distribution owning the canonical implementation, compatibility shims, and all three CLI entry points.

Discovery and Registry

The canonical MCP Registry identity is:

io.github.Renwang-Huang/arbitype

The intended package entry is:

uvx arbitype

The PyPI, MCP Registry, and Glama links are prepared before publication, but their badges remain explicitly marked pending until the external listings are verified. Release ordering and the legacy Registry migration procedure are documented in docs/REGISTRY_MIGRATION.md.

The former identity io.github.Renwang-Huang/typesafe-mcp is a legacy identity. It must remain available for existing users and should be marked deprecated through the Registry publisher when that mutation is supported. New installations should use the Arbitype identity.

Configuration

Custom provider endpoints must use HTTPS. Plain HTTP is accepted only for loopback hosts such as localhost, 127.0.0.1, and ::1. Redirects are disabled so a bearer credential is never forwarded to a redirect target.

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.

Development

python3 -m unittest discover -s tests -v
python3 -m compileall -q .
python3 -m pip wheel --no-deps . --wheel-dir /tmp/arbitype-dist

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

See TESTING.md, CONTRIBUTING.md, and BENCHMARK.md for the full engineering checks and comparison. Maintainer migration details are in docs/REGISTRY_MIGRATION.md.

Release identity

Arbitype is currently released as 0.6.0 because it remains Beta while its canonical public identity moves to the new package, CLI, and Registry name. The old package history remains intact; the brand migration does not rewrite Git history or delete the former PyPI project.