diff --git a/src/session.rs b/src/session.rs
index c380728..2afaf7a 100644
--- a/src/session.rs
+++ b/src/session.rs
@@ -1 +1,286 @@
-// Placeholder for upcoming session module.
+use crate::domain::{AllowedContext, Commitment, EnforcementLevel, PolicySnapshot, RuntimeState};
+use crate::event_log::{EventLog, EventType};
+use crate::state_machine::{
+    self, transition_commitment, transition_runtime, CommitmentAction, RuntimeAction,
+};
+use anyhow::{anyhow, Context, Result};
+use serde_json::json;
+use std::path::Path;
+use std::time::Duration;
+
+#[derive(Debug)]
+pub struct SessionController {
+    runtime_state: RuntimeState,
+    active_commitment: Option<Commitment>,
+    active_policy: Option<PolicySnapshot>,
+    event_log: EventLog,
+}
+
+impl SessionController {
+    pub fn new(event_log_path: impl AsRef<Path>) -> Result<Self> {
+        let event_log = EventLog::open(event_log_path.as_ref()).with_context(|| {
+            format!(
+                "open session event log at {}",
+                event_log_path.as_ref().display()
+            )
+        })?;
+
+        Ok(Self {
+            runtime_state: RuntimeState::Locked,
+            active_commitment: None,
+            active_policy: None,
+            event_log,
+        })
+    }
+
+    pub fn runtime_state(&self) -> RuntimeState {
+        self.runtime_state
+    }
+
+    pub fn active_policy(&self) -> Option<&PolicySnapshot> {
+        self.active_policy.as_ref()
+    }
+
+    pub fn active_commitment(&self) -> Option<&Commitment> {
+        self.active_commitment.as_ref()
+    }
+
+    pub fn enter_planning(&mut self) -> Result<()> {
+        let previous_runtime_state = self.runtime_state;
+        let next_runtime_state =
+            transition_runtime(previous_runtime_state, RuntimeAction::EnterPlanning)
+                .context("enter planning runtime transition")?;
+
+        self.event_log
+            .append(
+                EventType::RuntimeTransition,
+                next_runtime_state,
+                self.active_commitment
+                    .as_ref()
+                    .map(|commitment| commitment.id.clone()),
+                json!({
+                    "from": previous_runtime_state,
+                    "to": next_runtime_state,
+                    "action": "enter_planning",
+                }),
+            )
+            .context("log runtime transition into planning")?;
+
+        self.runtime_state = next_runtime_state;
+        Ok(())
+    }
+
+    pub fn start_manual_commitment(
+        &mut self,
+        next_action: impl Into<String>,
+        success_condition: impl Into<String>,
+        timebox: Duration,
+    ) -> Result<()> {
+        let previous_runtime_state = self.runtime_state;
+        let next_runtime_state = transition_runtime(
+            previous_runtime_state,
+            RuntimeAction::Activate {
+                policy_accepted: true,
+            },
+        )
+        .context("activate runtime for manual commitment")?;
+
+        let mut commitment = Commitment::new_manual(next_action, success_condition, timebox)
+            .context("create manual commitment")?;
+        commitment.state =
+            transition_commitment(commitment.state.clone(), CommitmentAction::Activate)
+                .context("activate manual commitment")?;
+
+        let policy = PolicySnapshot::try_for_runtime(
+            commitment.id.clone(),
+            next_runtime_state,
+            EnforcementLevel::Warn,
+            AllowedContext::default(),
+        )
+        .context("create active policy snapshot")?;
+
+        self.event_log
+            .append(
+                EventType::CommitmentCreated,
+                next_runtime_state,
+                Some(commitment.id.clone()),
+                serde_json::to_value(&commitment).context("serialize commitment payload")?,
+            )
+            .context("log commitment creation")?;
+        self.event_log
+            .append(
+                EventType::PolicyApplied,
+                next_runtime_state,
+                Some(commitment.id.clone()),
+                serde_json::to_value(&policy).context("serialize policy payload")?,
+            )
+            .context("log policy application")?;
+
+        self.runtime_state = next_runtime_state;
+        self.active_commitment = Some(commitment);
+        self.active_policy = Some(policy);
+        Ok(())
+    }
+
+    pub fn start_transition(
+        &mut self,
+        reason: impl Into<String>,
+        return_target: impl Into<String>,
+        expected_duration: Duration,
+    ) -> Result<()> {
+        let reason = reason.into().trim().to_string();
+        if reason.is_empty() {
+            return Err(anyhow!("transition reason is required"));
+        }
+
+        let return_target = return_target.into().trim().to_string();
+        if return_target.is_empty() {
+            return Err(anyhow!("transition return target is required"));
+        }
+
+        if expected_duration.is_zero() {
+            return Err(anyhow!("transition expected duration must be nonzero"));
+        }
+
+        let active_commitment = self
+            .active_commitment
+            .as_ref()
+            .ok_or_else(|| anyhow!("active commitment is required to start transition"))?;
+        let composite =
+            state_machine::start_transition(self.runtime_state, active_commitment.state.clone())
+                .context("start transition state change")?;
+
+        let previous_commitment_state = active_commitment.state.clone();
+        let next_commitment_state = composite.commitment_state.clone();
+        let mut next_commitment = active_commitment.clone();
+        next_commitment.state = next_commitment_state.clone();
+
+        self.event_log
+            .append(
+                EventType::TransitionStarted,
+                composite.runtime_state,
+                Some(next_commitment.id.clone()),
+                json!({
+                    "reason": reason,
+                    "return_target": return_target,
+                    "expected_duration_secs": expected_duration.as_secs(),
+                    "runtime_state": {
+                        "from": self.runtime_state,
+                        "to": composite.runtime_state,
+                    },
+                    "commitment_state": {
+                        "from": previous_commitment_state,
+                        "to": next_commitment_state,
+                    },
+                }),
+            )
+            .context("log transition start")?;
+
+        self.runtime_state = composite.runtime_state;
+        self.active_commitment = Some(next_commitment);
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::domain::{EnforcementLevel, RuntimeState};
+    use crate::event_log::{EventRecord, EventType};
+    use crate::session::SessionController;
+    use std::fs;
+    use std::path::PathBuf;
+    use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+    fn temp_log_path(name: &str) -> PathBuf {
+        let nonce = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap()
+            .as_nanos();
+        std::env::temp_dir().join(format!(
+            "antidrift-session-{name}-{}-{nonce}.jsonl",
+            std::process::id()
+        ))
+    }
+
+    fn read_records(path: &PathBuf) -> Vec<EventRecord> {
+        fs::read_to_string(path)
+            .unwrap()
+            .lines()
+            .map(|line| serde_json::from_str(line).unwrap())
+            .collect()
+    }
+
+    #[test]
+    fn starts_commitment_and_emits_policy() {
+        let path = temp_log_path("start");
+        let mut session = SessionController::new(&path).unwrap();
+
+        session.enter_planning().unwrap();
+        session
+            .start_manual_commitment(
+                "Implement session controller",
+                "session tests pass",
+                Duration::from_secs(1500),
+            )
+            .unwrap();
+
+        assert_eq!(session.runtime_state(), RuntimeState::Active);
+        assert_eq!(
+            session.active_policy().unwrap().enforcement_level,
+            EnforcementLevel::Warn
+        );
+        assert!(session.active_commitment().is_some());
+
+        let records = read_records(&path);
+        assert!(records
+            .iter()
+            .any(|record| record.event_type == EventType::CommitmentCreated));
+        assert!(records
+            .iter()
+            .any(|record| record.event_type == EventType::PolicyApplied));
+
+        fs::remove_file(path).unwrap();
+    }
+
+    #[test]
+    fn transition_requires_reason_and_return_target() {
+        let path = temp_log_path("transition");
+        let mut session = SessionController::new(&path).unwrap();
+        session.enter_planning().unwrap();
+        session
+            .start_manual_commitment(
+                "Implement session controller",
+                "session tests pass",
+                Duration::from_secs(1500),
+            )
+            .unwrap();
+
+        let err = session
+            .start_transition("", "review test results", Duration::from_secs(300))
+            .expect_err("empty reason must fail");
+        assert!(err.to_string().contains("reason"));
+        assert_eq!(session.runtime_state(), RuntimeState::Active);
+
+        let err = session
+            .start_transition("Need to inspect failures", "", Duration::from_secs(300))
+            .expect_err("empty return target must fail");
+        assert!(err.to_string().contains("return target"));
+        assert_eq!(session.runtime_state(), RuntimeState::Active);
+
+        session
+            .start_transition(
+                "Need to inspect failures",
+                "Return to session controller",
+                Duration::from_secs(300),
+            )
+            .unwrap();
+
+        assert_eq!(session.runtime_state(), RuntimeState::Transition);
+        assert_eq!(
+            session.active_commitment().unwrap().state,
+            crate::domain::CommitmentState::Paused
+        );
+
+        fs::remove_file(path).unwrap();
+    }
+}