Call-owning middleware · PyPI 0.1.0 · Alpha · Apache-2.0

fastmcp-acgs for FastMCP (MCP Server).

Intercepts MCP server tool calls through on_call_tool middleware before and around call_next dispatch, serving any connected MCP client.

PyPI 0.1.0 GitHub repository Inspect src/fastmcp_acgs/middleware.py#L92

Package Specification · Inspected HEAD 853ab3a

Framework maintainer
Prefect · gofastmcp.com
Upstream repository
PrefectHQ/fastmcp
PyPI release
fastmcp-acgs==0.1.0
Python requirement
>=3.10
Framework constraint
fastmcp-slim[server]>=4.1.0,<5
Governance engine pin
acgs-lite>=3.0.1,<4
Integration mechanic
Call-owning middleware (wraps tool call)
Hook entry point
ACGSMiddleware.on_call_tool

01 · Local Setup & Quickstart

Attach governance to one tool-calling agent.

Install the pinned release from PyPI alongside acgs-lite>=3.0.1,<4. Rule evaluation uses deterministic keyword and regex matching locally in Python—no external model or network call is made by the governance check.

Caveat: Evaluates acgs-lite policy rules (not gove-zone receipts). FastMCP 2.x and 3.x are unsupported; requires fastmcp-slim[server]>=4.1.0,<5.

Shell · PyPI installation

pip install "fastmcp-acgs==0.1.0"

Python · FastMCP (MCP Server) integration

src/fastmcp_acgs/middleware.py#L92
from fastmcp import FastMCP
from fastmcp_acgs import ACGSMiddleware

mcp = FastMCP("support", middleware=[ACGSMiddleware()])


@mcp.tool
def send_email(to: str, body: str) -> str:
    """Send an email."""
    return f"sent to {to}"

02 · Verdict & Review Mapping

How ACGS-Lite verdicts map to FastMCP (MCP Server).

When a tool call is proposed, the adapter formats the tool name and arguments via format_tool_call and evaluates them against the configured Constitution. Blocking rules always take precedence over review rules.

Verdict.ALLOW

Calls matching no blocking or review rules pass through to call_next(context) unchanged.

Verdict.DENY

Blocking rule violations or evaluation exceptions raise fastmcp.exceptions.ToolError before the tool executes.

Verdict.REVIEW

Denied by default; with on_review='elicit', requests MCP client elicitation approval before running the tool.

03 · Public API Surface

Exported symbols and source entry points.

Each adapter keeps its public surface minimal and re-exports the shared GovernanceDecision, Verdict, and ToolCallFormatter types alongside the framework-specific hook or middleware class.

__all__ exports · fastmcp-acgs

src/fastmcp_acgs/middleware.py#L92 ↗
ACGSMiddlewareOnReviewREVIEW_INPUT_KEYGovernanceDecisionToolCallFormatterVerdictformat_tool_call

Primary hook surface: ACGSMiddleware.on_call_tool. Custom ToolCallFormatter functions may be supplied if your tool arguments require custom canonical serialization before rule matching.

04 · Scope & Bypass Boundaries

What this adapter does and does not cover.

Verify these framework-specific boundaries when integrating fastmcp-acgs into an evaluation workflow.

All 13 adaptersReceipts, grants & EffectStore
  1. 01

    Place ACGSMiddleware last in the middleware list so it inspects the final arguments right before tool execution.

  2. 02

    Governs MCP tool calls arriving at the server; direct Python calls to the underlying function bypass MCP middleware.

  3. 03

    Evaluates acgs-lite keyword/regex rules; does not mint SignedReceipt, ExecutionGrant, or EffectStore reservations.

  4. 04

    In the current 0.1.x adapter series, rule evaluation runs through acgs-lite's ToolCallGovernor path. If your workflow requires SignedReceipt, ExecutionGrant, or SQLite EffectStore reservations, wire those contracts explicitly at the tool executor boundary.

Other call-owning middleware packages

langchain-acgs0.1.1

LangChain & LangGraph · LangChain

agno-acgs0.1.0

Agno · Agno

agent-framework-acgs0.1.0

Microsoft Agent Framework · Microsoft

ag2-acgs0.1.0

AG2 (AutoGen) · AG2

Put one real workflow under your control.

Choose one tool. Define what it may do. Inspect what happens when a request crosses the boundary.

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