feat(agent): control judge — LLM recognize stage (temp 0, verbatim, fail-closed)

ControlJudge trait + LlmControlJudge: judges one (control, region) with a closed
temperature-0 prompt that must quote the offending code VERBATIM; parsing fails
closed to non-violation (never a fabricated finding). Behind a trait so the
checker stays stub-testable. Its output is re-checked by the core grounding gate,
never trusted directly. 3 lib tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

compliance-agent/src/controls/judge.rs

@@ -0,0 +1,167 @@
+//! The "recognize" stage: judge whether a code region violates a control.
+//!
+//! Behind the [`ControlJudge`] trait so the grounded checker can be driven by a
+//! deterministic stub in tests. The real [`LlmControlJudge`] runs the model at
+//! temperature 0 with a closed prompt — it must quote the offending code VERBATIM,
+//! and everything it returns is then re-checked by the grounding gate
+//! ([`compliance_core::control_check::ground`]). The judge is allowed to be
+//! smart; it is never trusted.
+
+use std::sync::Arc;
+
+use serde::Deserialize;
+
+use compliance_core::control_check::{CandidateRegion, ControlCheckSpec, LlmVerdict};
+
+use crate::llm::LlmClient;
+
+/// Prompt/logic version — part of the verdict cache key, bump on any change here.
+pub const PROMPT_VERSION: &str = "control-judge-v1";
+
+const SYSTEM_PROMPT: &str = "You are a precise security & compliance code auditor. \
+You are given ONE compliance control (a requirement) and ONE code region. Decide \
+ONLY whether the code region VIOLATES the control. Rules: (1) Judge only the code \
+shown — never assume code that is not present. (2) If and only if it violates, copy \
+the EXACT offending code VERBATIM into `snippet`, character-for-character from the \
+region — do not paraphrase, reformat, or reconstruct it. (3) If it does not clearly \
+violate, set violates=false and leave snippet empty. (4) Prefer false over guessing. \
+Respond with STRICT JSON only, no prose: \
+{\"violates\": bool, \"snippet\": \"<verbatim code or empty>\", \"cwe\": \"CWE-NNN or null\", \"confidence\": 0.0-1.0}";
+
+/// Judges one (control, region). Async-in-trait so a stub can drive tests.
+#[allow(async_fn_in_trait)]
+pub trait ControlJudge: Send + Sync {
+    async fn judge(&self, spec: &ControlCheckSpec, region: &CandidateRegion) -> LlmVerdict;
+}
+
+/// The real judge: the LLM at temperature 0 with the closed, verbatim-snippet prompt.
+pub struct LlmControlJudge {
+    llm: Arc<LlmClient>,
+}
+
+impl LlmControlJudge {
+    pub fn new(llm: Arc<LlmClient>) -> Self {
+        Self { llm }
+    }
+}
+
+impl ControlJudge for LlmControlJudge {
+    async fn judge(&self, spec: &ControlCheckSpec, region: &CandidateRegion) -> LlmVerdict {
+        let user = build_user_prompt(spec, region);
+        match self.llm.chat(SYSTEM_PROMPT, &user, Some(0.0)).await {
+            Ok(response) => parse_verdict(&response),
+            Err(e) => {
+                // Fail closed: a transient model error yields no finding, never a
+                // fabricated one.
+                tracing::warn!(control = %spec.control_id, error = %e, "control judge call failed");
+                no_violation()
+            }
+        }
+    }
+}
+
+fn build_user_prompt(spec: &ControlCheckSpec, region: &CandidateRegion) -> String {
+    format!(
+        "CONTROL {id} — {title}\nRequirement: {req}\n\nCODE ({file}, first line = {line}):\n```\n{code}\n```\n\nReturn the JSON verdict.",
+        id = spec.control_id,
+        title = spec.title,
+        req = spec.requirement,
+        file = region.file,
+        line = region.start_line,
+        code = region.content,
+    )
+}
+
+#[derive(Debug, Default, Deserialize)]
+struct RawVerdict {
+    #[serde(default)]
+    violates: bool,
+    #[serde(default)]
+    snippet: String,
+    #[serde(default)]
+    cwe: Option<String>,
+    #[serde(default)]
+    confidence: f64,
+}
+
+/// Parse the model's JSON verdict, tolerant of ```json fencing. Any parse failure
+/// degrades to a non-violation (never a fabricated finding).
+fn parse_verdict(response: &str) -> LlmVerdict {
+    let cleaned = response
+        .trim()
+        .trim_start_matches("```json")
+        .trim_start_matches("```")
+        .trim_end_matches("```")
+        .trim();
+    match serde_json::from_str::<RawVerdict>(cleaned) {
+        Ok(raw) => LlmVerdict {
+            violates: raw.violates,
+            snippet: raw.snippet,
+            cwe: raw.cwe.filter(|c| !c.trim().is_empty()),
+            confidence: raw.confidence,
+        },
+        Err(e) => {
+            tracing::debug!(error = %e, "failed to parse control verdict; treating as non-violation");
+            no_violation()
+        }
+    }
+}
+
+fn no_violation() -> LlmVerdict {
+    LlmVerdict {
+        violates: false,
+        snippet: String::new(),
+        cwe: None,
+        confidence: 0.0,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use compliance_core::models::finding::Severity;
+
+    fn spec() -> ControlCheckSpec {
+        ControlCheckSpec {
+            control_id: "cra-ai-8".into(),
+            title: "No default passwords".into(),
+            requirement: "Products must not ship default credentials".into(),
+            default_cwe: Some("CWE-798".into()),
+            severity: Severity::High,
+        }
+    }
+
+    #[test]
+    fn parses_plain_and_fenced_json() {
+        let plain = r#"{"violates": true, "snippet": "PASSWORD = \"x\"", "cwe": "CWE-798", "confidence": 0.9}"#;
+        let v = parse_verdict(plain);
+        assert!(v.violates);
+        assert_eq!(v.snippet, "PASSWORD = \"x\"");
+        assert_eq!(v.cwe.as_deref(), Some("CWE-798"));
+
+        let fenced = "```json\n{\"violates\": false, \"snippet\": \"\", \"cwe\": null, \"confidence\": 0.1}\n```";
+        assert!(!parse_verdict(fenced).violates);
+    }
+
+    #[test]
+    fn garbage_and_empty_cwe_are_safe() {
+        assert!(!parse_verdict("not json at all").violates); // fail closed
+        let no_cwe =
+            parse_verdict(r#"{"violates": true, "snippet": "x", "cwe": "  ", "confidence": 0.5}"#);
+        assert!(no_cwe.cwe.is_none()); // blank CWE normalised away
+    }
+
+    #[test]
+    fn user_prompt_carries_control_and_code() {
+        let region = CandidateRegion {
+            file: "src/auth.py".into(),
+            start_line: 10,
+            content: "PASSWORD = \"admin\"".into(),
+        };
+        let p = build_user_prompt(&spec(), &region);
+        assert!(p.contains("cra-ai-8"));
+        assert!(p.contains("Products must not ship default credentials"));
+        assert!(p.contains("PASSWORD = \"admin\""));
+        assert!(p.contains("src/auth.py"));
+    }
+}

compliance-agent/src/controls/mod.rs

@@ -5,6 +5,8 @@
 //! [`OscalControlsProvider`], which pulls breakpilot-compliance's OSCAL catalog
 //! and snapshots it locally.
 
+mod judge;
 mod oscal_provider;
 
+pub use judge::{ControlJudge, LlmControlJudge, PROMPT_VERSION};
 pub use oscal_provider::OscalControlsProvider;