We implemented the `GuardrailProvider` protocol from this proposal in [Aegis](https://github.com/Acacian/aegis) (open-source AI agent governance framework). Some notes from building against the spec:

### The `GuardrailRequest` / `GuardrailDecision` contract works well

Having a typed request/response instead of raw hook args made it straightforward to build a provider that could be swapped out. Our implementation ([source](https://github.com/Acacian/aegis/blob/main/src/aegis/adapters/crewai.py)):

```python
from aegis.adapters.crewai import AegisGuardrailProvider, GuardrailRequest

provider = AegisGuardrailProvider(policy=my_policy, fail_closed=True)

decision = provider.evaluate(GuardrailRequest(
    tool_name="web_search",
    tool_input={"query": "sensitive data"},
    agent_role="researcher",
))

if not decision.allow:
    print(decision.reason)   # "Blocked by policy rule: pii_search_block"
    print(decision.metadata) # {"risk_level": "HIGH", "matched_rule": "pii_search_block"}
```

The `before_tool_call()` method maps directly to `BeforeToolCallHook` — returns `True`/`False`.

### On the async approval discussion

@uchibeke your instinct is right — suspend/resolve doesn't fit CrewAI's autonomy model cleanly. Our approach: when a policy rule requires human approval, the provider returns `allow=False` with `approval_required: True` in `GuardrailDecision.metadata`. The agent receives a clear denial reason and can report it or try an alternative tool. Approval happens out-of-band, not by pausing the agent loop.

This keeps the provider contract synchronous (evaluate → decision) while still distinguishing "blocked by policy" from "needs human sign-off" at the metadata level.

### `fail_closed` as a first-class parameter

+1 on this being essential. When the policy engine errors, you need a defined default. We default to blocking. The two failure modes produce different `GuardrailDecision.metadata` so the caller can distinguish "intentionally blocked" from "errored and failed safe":

```python
# fail_closed=True (default): errors → deny
# fail_closed=False: errors → allow + audit
provider = AegisGuardrailProvider(policy=my_policy, fail_closed=True)
```

### On @lowkey-divine's charter point

`agent_role` in `GuardrailRequest` enables this. Our provider passes it through as `Action.agent_id`, which means policy rules can scope by agent identity — not just tool name. A researcher agent and an admin agent hitting the same tool can get different verdicts. The proposed interface already supports this without changes.

### Test coverage

41 tests covering allow, block, approval-required, fail-closed, fail-open, policy hot-swap, health checks, audit logging, and the hook protocol. ([tests](https://github.com/Acacian/aegis/blob/main/tests/test_crewai_guardrail.py))

Happy to coordinate if this moves toward a PR.
