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Rust Safety Kernel

The safety kernel (crates/effero-safety-kernel/) is a small, dependency-minimal Rust process that runs independently of the Python runtime. This isolation is deliberate: no LLM-side prompt injection can modify the guardrail logic because it runs in a separate OS process.

Architecture

  • Transport: Newline-delimited JSON over TCP (127.0.0.1:9400)
  • Dependencies: Minimal — serde, serde_yaml, globset, tokio
  • Condition parser: ~150-line hand-rolled recursive descent parser supporting boolean (&&, ||) and numeric comparisons (<, <=, >, >=, ==, !=). Fails safe toward false on any parse error.

Protocol

Request: 

{"skill": "iot.locks.front_door", "facts": {"time_hour": 23.5}}

Response: 

{"decision": "require_approval", "matched_rule": "no-exterior-unlock-at-night", "reason": "Exterior lock actions require approval at night."}

Building and running

cd crates
cargo build -p effero-safety-kernel
cargo test -p effero-safety-kernel    # 11 tests

# Run the daemon
cargo run -p effero-safety-kernel --bin effero-safety-kerneld

Python integration

The Python side connects via SafetyClient in src/effero/safety/client.py:

from effero.safety import SafetyClient

client = SafetyClient(host="127.0.0.1", port=9400)
result = await client.check("iot.locks.front_door", {"time_hour": 23.5})
print(result.decision)  # "require_approval"