Rust SDK#

Use the Rust SDK at a runtime boundary you own. This guide needs Rust 1.88 or newer and the quickstart policy.

Rust returns Result for fallible operations. HushGuard::check returns a GuardDecision; branch on allowed(), or use enforce and handle Denied before dispatch.

Install and run the complete example#

Create an empty directory. Download these files, preserving the listed relative paths, and place policy.yaml at the top of that directory:

• Cargo.toml
• src/main.rs
• src/trust.rs

The example enables features = ["signing"]. Basic parsing and evaluation do not need that feature. HTTPS providers require http; OTLP requires otlp.

cargo run -- policy.yaml

Expected output: PASS: deny=0 dispatches; unconfirmed warn=0; confirmed warn=1; invalid policy refused. All effects are in a fresh temporary directory. No model API or credentials are required, and the program cleans up its own synthetic output.

Enforce before the effect#

The example parses and validates the policy, resolves it, and compiles it once. It then attaches a consistent actor and callback receipt sink to the guard. The synthetic handler creates one file only after enforcement permits dispatch.

use hushspec::{
    Actor, CallbackSink, CompiledPolicy, Decision, EvaluationAction, HushGuard, HushSpec, Policy,
    resolve_from_path, validate,
};
use std::{
    fs,
    io::Write,
    sync::{Arc, Mutex},
};
mod trust;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let policy_path = std::env::args()
        .nth(1)
        .unwrap_or_else(|| "policy.yaml".into());
    let document = HushSpec::parse(&fs::read_to_string(&policy_path)?)?;
    assert!(validate(&document).is_valid());
    let resolved = resolve_from_path(&policy_path)?;
    let compiled = CompiledPolicy::compile(&resolved)?;
    trust::check_trust(&policy_path, &resolved)?;
    let action = |target: &str| EvaluationAction {
        action_type: "tool_call".into(),
        target: Some(target.into()),
        ..Default::default()
    };
    assert_eq!(
        compiled.evaluate(&action("search")).decision,
        Decision::Allow
    );
    let receipts = Arc::new(Mutex::new(Vec::new()));
    let builder = || {
        let recorded = Arc::clone(&receipts);
        HushGuard::builder()
            .actor(Actor {
                agent_id: Some("docs-agent".into()),
                session_id: Some("docs-session".into()),
                principal: Some("docs-user".into()),
                runtime: Some("docs/1.0.0".into()),
            })
            .sink(Box::new(CallbackSink::new(move |receipt| {
                recorded.lock().unwrap().push(receipt.clone());
                Ok(())
            })))
    };
    let guard = builder().build_from_policy(Policy::from_path(&policy_path)?)?;
    let directory = tempfile::tempdir()?;
    let output = directory.path().join("effect.txt");
    let dispatches = std::cell::Cell::new(0);
    let dispatch = |guard: &HushGuard, tool: &str| -> Result<bool, std::io::Error> {
        if !guard.check(&action(tool)).allowed() {
            return Ok(false);
        }
        let mut file = fs::OpenOptions::new()
            .write(true)
            .create_new(true)
            .open(&output)?;
        file.write_all(b"confirmed once\n")?;
        dispatches.set(dispatches.get() + 1);
        Ok(true)
    };
    assert!(!dispatch(&guard, "deploy")?);
    assert!(!dispatch(&guard, "write_file")?);
    assert_eq!(dispatches.get(), 0);
    assert!(!output.exists());
    let confirmed = builder()
        .on_warn(|_, _| true)
        .build_from_policy(Policy::from_path(&policy_path)?)?;
    assert!(dispatch(&confirmed, "write_file")?);
    assert_eq!(dispatches.get(), 1);
    assert_eq!(fs::read_to_string(output)?, "confirmed once\n");
    let receipts = receipts.lock().unwrap();
    let last = receipts.last().unwrap();
    assert_eq!(
        last.enforcement.outcome,
        hushspec::EnforcementOutcome::Confirmed
    );
    assert_eq!(
        last.actor.as_ref().unwrap().agent_id.as_deref(),
        Some("docs-agent")
    );
    assert!(HushSpec::parse("hushspec: \"1.0.0\"\nunknown_rule: true\n").is_err());
    println!(
        "PASS: deny=0 dispatches; unconfirmed warn=0; confirmed warn=1; invalid policy refused"
    );
    Ok(())
}

The on_warn / onWarn callback in this demonstration approves one synthetic test action. A real integration must obtain approval from an authenticated channel and bind it to the pending action. Do not replace approval with unconditional true in a production agent.

Signing and keyrings#

The companion src/trust.rs program is called by the main example. It creates an ephemeral test key, signs the resolved document, verifies with an explicit keyring, and rejects a different keyring. It prints no private key and does not persist one. Production signing identities belong to the operator.

Signing and verification must refer to the same resolved policy used for evaluation. See signing, bundles, and the exact signing APIs. A successful signature proves authenticity relative to your trust roots, not that a tool obeyed the policy.

Providers and reload#

FileProvider::load() returns a Resolution; constructing a provider does not start a watcher. Use PolicyWatcher or PolicyPoller to follow changes. Share an Arc<HushGuard> across threads when needed. The ordinary guard waits through in-flight checks and receipt delivery before committing a reload.

Initial policy-load errors cannot fall back to a nonexistent policy. Ordinary guard reload preserves the last good policy on a rejected update and reports the error. It is not nonblocking: reload can wait for confirmation and receipt delivery. See hot reload for ordering and the distinct experimental coordinator refusal contract.

Receipts and failure handling#

The callback sink makes receipt contents visible to the test. It is not durable storage. Configure a chained file sink or another reviewed delivery path for operational evidence, and handle sink.error through an observer. A failed ordinary sink does not change the decision; if dispatch must require a durable permit, use the separately bounded experimental invocation workflow.

Receipts record actor fields, policy hash, decision, trace and enforcement outcome. They do not carry raw action content. The confirmed outcome distinguishes an approved warning from a plain allow.

API map and next steps#

The SDK API contract covers parse/validate, resolve/merge, compile/evaluate, actors, receipts, sinks, signing, keyrings, providers, panic mode and error conventions. For mapping effects, read MCP and runtime integration. Use conformance to understand what a test corpus establishes; this example is an integration regression, not an independent conformance certificate.