mcp-assure

Deny-by-default assurance runtime for MCP-style tool calls.

The model proposes. The gate decides. Receipts remember.

mcp-assure is a plug-in control plane you put in front of tool execution: policy catalog, argument constraints, optional resource/audience binding checks, velocity and blast limits, freeze mode, and hash-chained decision receipts. It is not a full SOC, not a scanner, and not a claim that all agent misuse is impossible.

Built by Alex Price / StellarRequiem. Apache-2.0. Zero runtime dependencies (core).
Version 0.2.1 adds policy packs, host/decorator integration, FastMCP on_call_tool middleware, purple stress fixtures, and an optional verity hook.

Public page: xclusivexo.com/mcp-assurance/#mcp-assure

Why this exists

MCP hosts give agents tools. Tools are power. Under MCP 2026-07-28, more security responsibility sits with implementers (stateless core, auth expectations, extensions). Scanners help at build time; runtime authorization is still required at the moment of tools/call.

Install

pip install mcp-assure
# optional FastMCP middleware:
pip install "mcp-assure[fastmcp]"
# from git (development tip):
pip install "git+https://github.com/StellarRequiem/mcp-assure"
# from a checkout:
pip install -e ".[dev,fastmcp]"

Real host demo (local)

Simulates an MCP host tools/call path with AssuredToolDispatcher:

python examples/host_demo.py

Expect: ALLOW for allowlisted tools, DENY for unknown tools / smuggled args, receipts verify.

60-second integration

from mcp_assure import (
    AssureEngine,
    AssuredRunner,
    ToolCall,
    ToolPolicy,
    ToolPolicyRegistry,
)

registry = ToolPolicyRegistry([
    ToolPolicy(
        name="read_file",
        required_args=("path",),
        allowed_args=("path",),
        forbidden_args=("token", "password"),
        max_blast=1,
    ),
])

engine = AssureEngine(registry, receipts_path="./mcp-assure-receipts.jsonl")

def read_file(args):
    # your real implementation
    return open(args["path"], encoding="utf-8").read()

runner = AssuredRunner(engine, handlers={"read_file": read_file})

out = runner.invoke(ToolCall(tool="read_file", arguments={"path": "README.md"}))
# out["executed"] is True only if the gate ALLOWed
# out["verdict"]["receipt_hash"] is the audit seal for this decision

Property: on DENY / DRY_RUN, the handler is never called.

What it enforces (tested)

ID	Property
P1	Unknown tool → DENY
P2	Empty catalog → DENY
P3–P4	DENY/DRY_RUN never invoke handlers
P5–P6	Velocity / blast limits
P7	lab_only tools require lab_mode
P8	Model notes cannot flip DENY→ALLOW
P9	Receipt chain verifies; tamper fails
P10	Freeze mode blocks non-allowlisted tools
P11	Forbidden / disallowed args → DENY
P12	Resource/audience mismatch → DENY when configured

See THREAT_MODEL.md and CLAIMS.md.

Policy from JSON

import json
from mcp_assure import AssureEngine, ToolPolicyRegistry

with open("examples/policy.example.json") as f:
    reg = ToolPolicyRegistry.from_mapping(json.load(f))
engine = AssureEngine(reg)

MCP-shaped payloads

from mcp_assure.mcp_types import tool_call_from_mcp
from mcp_assure import AssureEngine, ToolPolicy, ToolPolicyRegistry

engine = AssureEngine(ToolPolicyRegistry([ToolPolicy(name="echo")]))
call = tool_call_from_mcp({"name": "echo", "arguments": {"text": "hi"}})
print(engine.evaluate(call).as_dict())

Policy packs

from mcp_assure import AssureEngine, load_pack

engine = AssureEngine(load_pack("baseline"))
# also: mcp_authz_boundaries, strict_local

python -m mcp_assure packs

mcp_authz_boundaries encodes runtime gates for resource/audience-style failures (the class of bugs mcp-bench measures in scanners) — call-time enforcement, not a scanner replacement.

Host / FastMCP integration

from mcp_assure.integrations import AssuredToolDispatcher, assure_callable
# See mcp_assure/integrations/fastmcp_notes.py for patterns.

Dispatcher is the usual host hook for tools/call. Decorators wrap kwargs-style tool functions before registration.

FastMCP middleware (on_call_tool)

Requires pip install "mcp-assure[fastmcp]" (FastMCP ≥2.9):

from fastmcp import FastMCP
from mcp_assure import AssureEngine
from mcp_assure.packs import load_pack
from mcp_assure.integrations import build_assure_middleware

engine = AssureEngine(load_pack("baseline"), receipts_path="receipts.jsonl")
mcp = FastMCP("secured")
mcp.add_middleware(build_assure_middleware(engine))

@mcp.tool
def echo(text: str) -> dict:
    return {"echo": text}

On DENY the middleware raises ToolError and does not call the tool handler.
Demo: python examples/fastmcp_assured.py

Purple stress suite

Synthetic adversarial sequences (no network):

python -m mcp_assure purple

Optional verity hook

If verity-core is installed, claim-like tool results can be soft-checked:

from mcp_assure.verity_hook import maybe_verify_tool_result
maybe_verify_tool_result({"accuracy": 0.99, "sample_size": 5})

Not required; no-ops cleanly when verity is absent.

CLI

python -m mcp_assure demo
python -m mcp_assure packs
python -m mcp_assure purple
python -m mcp_assure verify-receipts ./mcp-assure-receipts.jsonl

Adversarial stance

This package is designed to survive hostile review:

• Explicit threat model and claim gate
• Properties P1–P12 locked to unit tests
• No runtime deps in the TCB surface
• Residual risk documented (host must not bypass the runner)

pytest -q

What this is not

• Not a replacement for OAuth authorization servers
• Not host EDR / network IDS
• Not “stops all prompt injection”
• Not a full enterprise SOC platform

Related work (StellarRequiem)

• mcp-bench — do scanners catch authz-logic bugs?
• scope-gate — deny-by-default research authorization
• verity-core — refuse bad claims; audit chains

License

Apache-2.0. Copyright 2026 Alex Price / StellarRequiem.