ToolBoundary

Runtime boundary enforcement for AI agents — as a library, not a service.

ToolBoundary answers one question, fast and locally, every time your agent tries to call a tool: "is this exact call allowed, right now?"

No separate web app. No database to stand up. No dashboard to log into. No subscription. Your policy is plain Python, version-controlled with the rest of your code.

pip install toolboundary

What's new in v1.0.0

ToolBoundary v1.0.0 introduces a provider-neutral external authorization and evidence layer — the foundation for integrating external authorization providers (like AgentKey) while keeping ToolBoundary as the local enforcement authority.

Key additions

• EvidenceProvider protocol — a clean interface any external provider can implement to add authorization and execution evidence without coupling to a specific vendor
• authorize_call() / record_execution() — a centralized orchestration flow that freezes the exact tool call, computes a cryptographic call digest (SHA-256 of canonical JSON), consults an optional provider, and records execution evidence
• Observe / Enforce modes — ProviderMode.OBSERVE logs provider decisions without blocking; ProviderMode.ENFORCE fails closed on provider denial or unavailability
• evaluate() method — returns a structured LocalDecision instead of raise-on-deny, enabling richer programmatic integration
• Deterministic canonicalization — equivalent dictionaries ({"a":1,"b":2} vs {"b":2,"a":1}) always produce identical digests, binding authorization to the exact call
• Replay prevention — consumed authorizations cannot be reused for a second dispatch
• Post-dispatch resilience — if a provider fails to record execution evidence, the local record is preserved

Core invariant

ToolBoundary remains the local enforcement authority. An external provider can add a stricter gate or external evidence, but it can never turn a local deny into an allow.

Why this exists

Enterprise AI-governance platforms (agent registries, policy engines, approval dashboards) make sense when a large organization has dozens of AI agents built by different teams and needs a compliance layer to track all of them. That's real infrastructure for a real problem — but it's disproportionate for the much more common case: one developer or a small team building one to a handful of agents, who just need to make sure a tool-calling agent can't do something catastrophic.

ToolBoundary is built for that second case. It costs nothing, requires no infrastructure, and takes minutes to add to an existing agent.

Quickstart

from toolboundary import Boundary, ToolPermission, AutonomyLevel, AccessMode

boundary = Boundary(
    agent_name="support-agent",
    autonomy=AutonomyLevel.LIMITED_AUTONOMOUS,
    permissions=[
        ToolPermission("read_ticket_db", access_mode=AccessMode.READ_ONLY),
        ToolPermission(
            "send_reply_email",
            access_mode=AccessMode.EXECUTE,
            max_calls_per_hour=30,
        ),
    ],
    blocked_operations=frozenset({"delete_ticket"}),
    max_actions_per_hour=100,
    kill_switch_env="TOOLBOUNDARY_KILL_SWITCH",
)

# Somewhere in your agent's tool-calling code:
boundary.check("read_ticket_db", access_mode=AccessMode.READ_ONLY)   # passes silently
boundary.check("delete_ticket", operation="delete_ticket")           # raises BoundaryViolation

If a call is denied, boundary.check(...) raises BoundaryViolation (or a more specific subclass like KillSwitchActive or RateLimitExceeded). If a call needs a human before it can proceed, it raises ApprovalRequired. Every decision — allow, deny, or approval-required — is written to a structured audit log automatically.

Emergency stop

export TOOLBOUNDARY_KILL_SWITCH=1

Set the environment variable your Boundary was configured with, and every future call for that agent is denied immediately — no restart required, no code change, no separate dashboard to log into.

Two ways to enforce the boundary

1. Decorator (plain Python functions)

from toolboundary import guarded_tool, AccessMode

@guarded_tool(boundary, access_mode=AccessMode.EXECUTE, value_arg="amount")
def wire_transfer(account_id: str, amount: float) -> str:
    return f"transferred {amount} to {account_id}"

wire_transfer(account_id="acct_1", amount=250_000)
# raises BoundaryViolation if 250_000 exceeds the permission's max_value —
# the function body never executes.

Once a function is decorated, calling it is calling through ToolBoundary. There is no code path to the real implementation that skips the check.

2. LangChain tools

from toolboundary.integrations.langchain import guard_tools
from toolboundary import AccessMode

guarded_tools = guard_tools(
    [read_db_tool, send_email_tool, wire_transfer_tool],
    boundary,
    default_access_mode=AccessMode.READ_ONLY,
    overrides={
        "send_email_tool": {"access_mode": AccessMode.EXECUTE},
        "wire_transfer_tool": {"access_mode": AccessMode.EXECUTE, "value_arg": "amount"},
    },
)

agent_executor = AgentExecutor(agent=agent, tools=guarded_tools)

This wraps the LangChain BaseTool objects themselves — the objects your AgentExecutor actually invokes when the LLM decides to call a tool — so the boundary check runs inside LangChain's own tool-execution path, not as a step the agent's reasoning loop has to remember to call.

Install with the LangChain extra: pip install toolboundary[langchain]

External authorization providers (new in v1.0.0)

ToolBoundary can optionally consult an external authorization provider before dispatching a tool call. The provider adds a second gate — it can never weaken a local policy decision.

Without a provider (default — unchanged from v0.1.0)

boundary = Boundary(
    agent_name="support-agent",
    ...
)
# Works exactly as before. No external service needed.

With a provider (observe mode)

from toolboundary import Boundary, ProviderMode

boundary = Boundary(
    agent_name="support-agent",
    ...,
    provider=my_provider,
    provider_mode=ProviderMode.OBSERVE,
)
# Provider decisions are logged but don't block locally-allowed actions.
# Provider unavailability is gracefully degraded.

With a provider (enforce mode)

boundary = Boundary(
    agent_name="support-agent",
    ...,
    provider=my_provider,
    provider_mode=ProviderMode.ENFORCE,
)
# Provider must explicitly allow the action.
# Provider denial or unavailability blocks execution before dispatch.

Implementing a custom provider

Any class that implements the EvidenceProvider protocol can serve as a provider:

from toolboundary import EvidenceProvider, FrozenToolCall, LocalDecision, ProviderGrant
from toolboundary import ExecutionRecord, ProviderReceipt

class MyProvider:
    def authorize(self, call: FrozenToolCall, local: LocalDecision) -> ProviderGrant:
        # Your authorization logic here
        return ProviderGrant(allowed=True, provider="my-provider")

    def record(self, grant: ProviderGrant, execution: ExecutionRecord) -> ProviderReceipt:
        # Your evidence recording logic here
        return ProviderReceipt(recorded=True, provider="my-provider")

Authorization flow

Local policy check → DENY? → stop (provider never consulted)
                   → ALLOW? → freeze call → compute digest → provider.authorize()
                                                            → execute exact call
                                                            → provider.record()

What a Boundary can enforce

Control	Example
Which tools an agent may use at all	permissions=[ToolPermission("read_db", ...)]
Operation-level allow/block lists	blocked_operations=frozenset({"delete_customer"})
Access mode (READ_ONLY / WRITE / EXECUTE / ADMIN)	access_mode=AccessMode.EXECUTE
Transaction value ceilings	ToolPermission(..., max_value=500_000)
Record-count ceilings	ToolPermission(..., max_records=100)
Rate limits (global or per-tool)	max_actions_per_hour=60
Autonomy level	AutonomyLevel.RECOMMEND_ONLY / HUMAN_APPROVAL_REQUIRED / LIMITED_AUTONOMOUS / AUTONOMOUS / QUARANTINED
Time-bounded validity	valid_from=, valid_to=
Environment restriction	allowed_environments=frozenset({"DEV", "TEST"})
Emergency kill switch	in-process flag or environment variable
Custom policy logic	policy_hooks=[my_custom_check]
External authorization provider	provider=my_provider, provider_mode=ProviderMode.ENFORCE
Exact-call binding with cryptographic digest	Automatic when a provider is configured

Full field reference: see docs/API.md.

Audit trail

Every decision produces a structured event. By default it goes to Python's standard logging module under the logger name toolboundary.audit, so it flows into whatever logging pipeline you already have (stdout, a file, CloudWatch, Datadog, etc.) with zero extra code.

from toolboundary.audit import AuditTrail, JSONLFileSink

boundary = Boundary(
    agent_name="support-agent",
    ...,
    audit=AuditTrail(sinks=[JSONLFileSink("toolboundary-audit.jsonl")]),
)

When a provider is configured, audit events automatically include evidence metadata: call digests, provider decisions, authorization IDs, and result digests.

A WebhookSink is also included if you want to forward events to a self-hosted dashboard or a centralized governance platform. Audit delivery is always best-effort — a network hiccup in your audit pipeline can never block or crash your agent, because the ALLOW/DENY decision has already been enforced locally before the sink is invoked.

Design philosophy

• Fail closed. Anything ambiguous, misconfigured, or erroring is treated as denied by default. See fail_closed_on_hook_error for the one place this is configurable.
• No infrastructure required. No database, no server, no login. The whole thing is a Python object you construct alongside your agent code.
• Version-controlled policy. Your boundary is code, reviewed in the same pull requests as everything else — not a setting buried in a web UI that drifts silently out of sync with what the agent actually does.
• Loud by default. Denials raise exceptions, not silent False returns that are easy to accidentally ignore.
• Framework-agnostic core, framework-specific adapters. The core Boundary has zero dependencies. Framework integrations (LangChain today; more welcome via PR) are optional extras.
• Local authority, optional extension. External providers add evidence and stricter gates but never override local policy. ToolBoundary works identically with or without a provider configured.

Known limitations — please read this

ToolBoundary is an in-process, application-layer library. Being explicit about what it does not do is more important than what it does:

• It cannot stop an agent that bypasses it entirely. If your agent's code has any path that calls a tool's real implementation directly — instead of through a @guarded_tool-wrapped function or a guard_tool-wrapped LangChain tool — that call is not evaluated. ToolBoundary governs the doors you route through it; it is not a network firewall.
• It is not a substitute for credential scoping. If the underlying API key or database credential your tool uses has broader permissions than ToolBoundary's policy allows, a determined attacker who obtains that credential directly bypasses ToolBoundary entirely. Scope your actual credentials as tightly as you can — ToolBoundary is a second layer, not a replacement for the first.
• It is not a compliance/audit system of record for large organizations. If you have dozens of agents across multiple teams and need human governance workflows, cross-team registries, and formal approval routing, look at enterprise AI governance platforms — ToolBoundary is intentionally not trying to be that.
• The in-memory rate limiter is per-process. If you run multiple replicas of your agent, each process has its own rate-limit counters unless you supply a shared backing store (see Boundary's internals / open an issue if you need this — a Redis-backed limiter is a natural community contribution).
• Provider evidence is only as trustworthy as the provider. An SDK-reported result does not itself prove that an external side effect occurred — it proves the SDK reported it. See the provider documentation for what guarantees each provider makes.

If your threat model requires guaranteeing that a compromised agent physically cannot reach a tool's network endpoint except through an approved path, you need a network-layer control (a sidecar proxy, egress firewall rule, or service mesh policy) in addition to ToolBoundary, not instead of it.

Installation

pip install toolboundary                # core, zero dependencies
pip install toolboundary[langchain]     # + LangChain integration

Contributing

Issues and PRs are welcome. See CONTRIBUTING.md.

Ideas that would make great first contributions:

• Custom EvidenceProvider implementations for popular platforms
• Redis-backed rate limiter for multi-process deployments
• CrewAI / AutoGen / LangGraph integrations (mirroring integrations/langchain.py)
• A minimal read-only local dashboard that tails a JSONLFileSink log

License

MIT — see LICENSE.