diff --git a/src/domain.rs b/src/domain.rs
index c07d685..e3991b2 100644
--- a/src/domain.rs
+++ b/src/domain.rs
@@ -1 +1,217 @@
-// Placeholder for upcoming domain module.
+use serde::{Deserialize, Serialize};
+use std::fmt;
+use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+pub const POLICY_SCHEMA_VERSION: u16 = 1;
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub enum CommitmentSource {
+    Manual,
+    Planner,
+    Recurring,
+    Recovery,
+    Template,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub enum CommitmentState {
+    Draft,
+    Active,
+    Paused,
+    Completed,
+    Abandoned,
+    Violated,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub enum RuntimeState {
+    Locked,
+    Planning,
+    Active,
+    Transition,
+    Review,
+    AdminOverride,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub enum EnforcementLevel {
+    Observe,
+    Warn,
+    Block,
+    Locked,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub enum EvidenceHealth {
+    Available,
+    Degraded(String),
+    Unavailable(String),
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub struct AllowedContext {
+    pub window_classes: Vec<String>,
+    pub window_title_substrings: Vec<String>,
+    pub domains: Vec<String>,
+    pub repos: Vec<String>,
+    pub commands: Vec<String>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct Commitment {
+    pub id: String,
+    pub created_at_unix_secs: u64,
+    pub source: CommitmentSource,
+    pub project_id: Option<String>,
+    pub template_id: Option<String>,
+    pub next_action: String,
+    pub success_condition: String,
+    pub timebox_secs: u64,
+    pub transition_policy_id: Option<String>,
+    pub state: CommitmentState,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PolicySnapshot {
+    pub id: String,
+    pub commitment_id: String,
+    pub schema_version: u16,
+    pub created_at_unix_secs: u64,
+    pub runtime_state: RuntimeState,
+    pub enforcement_level: EnforcementLevel,
+    pub allowed_context: AllowedContext,
+    pub required_monitors: Vec<String>,
+    pub violation_actions: Vec<String>,
+    pub expires_at_unix_secs: Option<u64>,
+    pub generated_by_agent_version: String,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum CommitmentValidationError {
+    MissingNextAction,
+    MissingSuccessCondition,
+    MissingTimebox,
+}
+
+impl fmt::Display for CommitmentValidationError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            CommitmentValidationError::MissingNextAction => write!(f, "next action is required"),
+            CommitmentValidationError::MissingSuccessCondition => {
+                write!(f, "success condition is required")
+            }
+            CommitmentValidationError::MissingTimebox => write!(f, "timebox must be nonzero"),
+        }
+    }
+}
+
+impl Commitment {
+    pub fn new_manual(
+        next_action: impl Into<String>,
+        success_condition: impl Into<String>,
+        timebox: Duration,
+    ) -> Result<Self, CommitmentValidationError> {
+        let next_action = next_action.into().trim().to_string();
+        let success_condition = success_condition.into().trim().to_string();
+
+        if next_action.is_empty() {
+            return Err(CommitmentValidationError::MissingNextAction);
+        }
+        if success_condition.is_empty() {
+            return Err(CommitmentValidationError::MissingSuccessCondition);
+        }
+        if timebox.is_zero() {
+            return Err(CommitmentValidationError::MissingTimebox);
+        }
+
+        let created_at_unix_secs = unix_secs_now();
+        Ok(Self {
+            id: format!("commitment-{created_at_unix_secs}"),
+            created_at_unix_secs,
+            source: CommitmentSource::Manual,
+            project_id: None,
+            template_id: None,
+            next_action,
+            success_condition,
+            timebox_secs: timebox.as_secs(),
+            transition_policy_id: None,
+            state: CommitmentState::Draft,
+        })
+    }
+}
+
+impl PolicySnapshot {
+    pub fn for_runtime(
+        commitment_id: String,
+        runtime_state: RuntimeState,
+        enforcement_level: EnforcementLevel,
+        allowed_context: AllowedContext,
+    ) -> Self {
+        let created_at_unix_secs = unix_secs_now();
+        Self {
+            id: format!("policy-{created_at_unix_secs}"),
+            commitment_id,
+            schema_version: POLICY_SCHEMA_VERSION,
+            created_at_unix_secs,
+            runtime_state,
+            enforcement_level,
+            allowed_context,
+            required_monitors: Vec::new(),
+            violation_actions: Vec::new(),
+            expires_at_unix_secs: None,
+            generated_by_agent_version: env!("CARGO_PKG_VERSION").to_string(),
+        }
+    }
+}
+
+pub fn unix_secs_now() -> u64 {
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_secs()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::time::Duration;
+
+    #[test]
+    fn commitment_requires_concrete_action_and_success_condition() {
+        let err = Commitment::new_manual("", "tests pass", Duration::from_secs(1500))
+            .expect_err("empty next action must be rejected");
+        assert_eq!(err, CommitmentValidationError::MissingNextAction);
+
+        let err = Commitment::new_manual("Refactor state", "", Duration::from_secs(1500))
+            .expect_err("empty success condition must be rejected");
+        assert_eq!(err, CommitmentValidationError::MissingSuccessCondition);
+    }
+
+    #[test]
+    fn commitment_requires_nonzero_timebox() {
+        let err = Commitment::new_manual("Refactor state", "tests pass", Duration::ZERO)
+            .expect_err("zero timebox must be rejected");
+        assert_eq!(err, CommitmentValidationError::MissingTimebox);
+    }
+
+    #[test]
+    fn policy_snapshot_carries_runtime_contract() {
+        let commitment = Commitment::new_manual(
+            "Implement domain types",
+            "domain tests pass",
+            Duration::from_secs(1500),
+        )
+        .unwrap();
+        let policy = PolicySnapshot::for_runtime(
+            commitment.id.clone(),
+            RuntimeState::Active,
+            EnforcementLevel::Warn,
+            AllowedContext::default(),
+        );
+
+        assert_eq!(policy.commitment_id, commitment.id);
+        assert_eq!(policy.runtime_state, RuntimeState::Active);
+        assert_eq!(policy.enforcement_level, EnforcementLevel::Warn);
+        assert_eq!(policy.schema_version, 1);
+    }
+}