diff --git a/crates/loopd/src/lib.rs b/crates/loopd/src/lib.rs
index 52391fb..b8f7bc0 100644
--- a/crates/loopd/src/lib.rs
+++ b/crates/loopd/src/lib.rs
@@ -5,5 +5,6 @@ pub mod config;
 pub mod http;
 pub mod llama;
 pub mod selftest;
+pub mod session;
 pub mod sse;
 pub mod tools;
diff --git a/crates/loopd/src/session.rs b/crates/loopd/src/session.rs
new file mode 100644
index 0000000..af5dfcb
--- /dev/null
+++ b/crates/loopd/src/session.rs
@@ -0,0 +1,236 @@
+//! A session on disk: `sessions/<id>/0.baseline.json` and `0.jsonl`. The log is the state —
+//! create, open, append and resume all live here, and nothing is durable until `append` syncs it.
+
+use std::fs::{File, OpenOptions};
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+use crate::baseline::{Baseline, BaselineError};
+use proto::{CallId, Epoch, LogRecord, SessionId, Timestamp};
+
+/// Creating, opening, appending and resuming a session, and the errors each can raise.
+#[derive(Debug)]
+pub enum SessionError {
+    Exists(SessionId),
+    NotFound(SessionId),
+    Io(PathBuf, std::io::Error),
+    Torn {
+        path: PathBuf,
+        line: usize,
+        why: String,
+    },
+    Baseline(BaselineError),
+    Encode(serde_json::Error),
+}
+
+impl std::fmt::Display for SessionError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            SessionError::Exists(id) => write!(f, "session already exists: {}", id.as_str()),
+            SessionError::NotFound(id) => write!(f, "session not found: {}", id.as_str()),
+            SessionError::Io(path, err) => write!(f, "{}: {err}", path.display()),
+            SessionError::Torn { path, line, why } => write!(f, "{}:{line}: {why}", path.display()),
+            SessionError::Baseline(err) => write!(f, "{err}"),
+            SessionError::Encode(err) => write!(f, "{err}"),
+        }
+    }
+}
+
+impl std::error::Error for SessionError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            SessionError::Io(_, err) => Some(err),
+            SessionError::Baseline(err) => Some(err),
+            SessionError::Encode(err) => Some(err),
+            _ => None,
+        }
+    }
+}
+
+/// A live session: its id, the directory holding the baseline and log, the replayed records, the
+/// appended log file, and the next call id.
+#[derive(Debug)]
+pub struct Session {
+    id: SessionId,
+    dir: PathBuf,
+    baseline: Baseline,
+    records: Vec<LogRecord>,
+    log: File,
+    next_call: CallId,
+}
+
+impl Session {
+    pub fn dir_for(home: &Path, id: &SessionId) -> PathBuf {
+        home.join("sessions").join(id.as_str())
+    }
+
+    pub fn create(
+        home: &Path,
+        id: SessionId,
+        baseline: Baseline,
+        slot: u32,
+    ) -> Result<Session, SessionError> {
+        let dir = Session::dir_for(home, &id);
+        if dir.exists() {
+            return Err(SessionError::Exists(id));
+        }
+        std::fs::create_dir_all(&dir).map_err(|e| SessionError::Io(dir.clone(), e))?;
+
+        let baseline_path = dir.join("0.baseline.json");
+        let json = baseline.to_json().map_err(SessionError::Encode)?;
+        std::fs::write(&baseline_path, json)
+            .map_err(|e| SessionError::Io(baseline_path.clone(), e))?;
+
+        let log_path = dir.join("0.jsonl");
+        let log = OpenOptions::new()
+            .create_new(true)
+            .append(true)
+            .open(&log_path)
+            .map_err(|e| SessionError::Io(log_path.clone(), e))?;
+
+        let mut session = Session {
+            id,
+            dir,
+            baseline,
+            records: Vec::new(),
+            log,
+            next_call: CallId(1),
+        };
+        session.persist(LogRecord::SessionStart {
+            time: Timestamp::now(),
+            session: session.id.clone(),
+            epoch: Epoch(0),
+            slot,
+            baseline: session.baseline.hash().map_err(SessionError::Baseline)?,
+        })?;
+        Ok(session)
+    }
+
+    pub fn open(home: &Path, id: SessionId) -> Result<Session, SessionError> {
+        let dir = Session::dir_for(home, &id);
+        let log_path = dir.join("0.jsonl");
+        if !log_path.exists() {
+            return Err(SessionError::NotFound(id));
+        }
+        let baseline_path = dir.join("0.baseline.json");
+        let baseline = Baseline::load(&baseline_path).map_err(SessionError::Baseline)?;
+
+        let text = std::fs::read_to_string(&log_path)
+            .map_err(|e| SessionError::Io(log_path.clone(), e))?;
+        let mut records = Vec::new();
+        if !text.is_empty() {
+            for (idx, line) in text.split_inclusive('\n').enumerate() {
+                let line_no = idx + 1;
+                if !line.ends_with('\n') {
+                    return Err(SessionError::Torn {
+                        path: log_path.clone(),
+                        line: line_no,
+                        why: "last line does not end in a newline".to_string(),
+                    });
+                }
+                match serde_json::from_str::<LogRecord>(line) {
+                    Ok(record) => records.push(record),
+                    Err(why) => {
+                        return Err(SessionError::Torn {
+                            path: log_path.clone(),
+                            line: line_no,
+                            why: why.to_string(),
+                        });
+                    }
+                }
+            }
+        }
+
+        let mut highest = 0u64;
+        for record in &records {
+            if let LogRecord::ToolResult { call, .. } = record {
+                highest = highest.max(call.0);
+            }
+        }
+
+        let log = OpenOptions::new()
+            .append(true)
+            .open(&log_path)
+            .map_err(|e| SessionError::Io(log_path.clone(), e))?;
+
+        Ok(Session {
+            id,
+            dir,
+            baseline,
+            records,
+            log,
+            next_call: CallId(highest + 1),
+        })
+    }
+
+    pub fn id(&self) -> &SessionId {
+        &self.id
+    }
+
+    pub fn dir(&self) -> &Path {
+        &self.dir
+    }
+
+    pub fn baseline(&self) -> &Baseline {
+        &self.baseline
+    }
+
+    pub fn records(&self) -> &[LogRecord] {
+        &self.records
+    }
+
+    pub fn append(&mut self, record: LogRecord) -> Result<(), SessionError> {
+        self.persist(record)
+    }
+
+    pub fn next_call(&mut self) -> CallId {
+        let id = self.next_call;
+        self.next_call = CallId(self.next_call.0 + 1);
+        id
+    }
+
+    pub fn last_usage(&self) -> Option<proto::Usage> {
+        for record in self.records.iter().rev() {
+            if let LogRecord::Usage {
+                cache_n,
+                prompt_n,
+                predicted_n,
+                reasoning_tokens,
+                thinking_capped,
+                ..
+            } = record
+            {
+                return Some(proto::Usage {
+                    cache_n: *cache_n,
+                    prompt_n: *prompt_n,
+                    predicted_n: *predicted_n,
+                    reasoning_tokens: *reasoning_tokens,
+                    thinking_capped: *thinking_capped,
+                });
+            }
+        }
+        None
+    }
+
+    /// Encode one record, write it and its newline, sync the data to disk, and only then keep it in
+    /// memory — so a record is never in memory without being durable.
+    fn persist(&mut self, record: LogRecord) -> Result<(), SessionError> {
+        let line = serde_json::to_string(&record).map_err(SessionError::Encode)?;
+        let path = self.log_path();
+        self.log
+            .write_all(line.as_bytes())
+            .map_err(|e| SessionError::Io(path.clone(), e))?;
+        self.log
+            .write_all(b"\n")
+            .map_err(|e| SessionError::Io(path.clone(), e))?;
+        self.log
+            .sync_data()
+            .map_err(|e| SessionError::Io(path.clone(), e))?;
+        self.records.push(record);
+        Ok(())
+    }
+
+    fn log_path(&self) -> PathBuf {
+        self.dir.join("0.jsonl")
+    }
+}
diff --git a/crates/loopd/tests/session.rs b/crates/loopd/tests/session.rs
new file mode 100644
index 0000000..70ee75f
--- /dev/null
+++ b/crates/loopd/tests/session.rs
@@ -0,0 +1,198 @@
+//! Tests for the session store. Do not edit.
+
+mod support;
+
+use loopd::baseline::Baseline;
+use loopd::session::{Session, SessionError};
+use proto::{CallId, DataClass, Epoch, LogRecord, SessionId, Timestamp};
+use support::Home;
+
+fn baseline() -> Baseline {
+    Baseline {
+        system: "sys".to_string(),
+        tools: vec![],
+    }
+}
+
+fn id(s: &str) -> SessionId {
+    SessionId::new(s).unwrap()
+}
+
+fn ts() -> Timestamp {
+    Timestamp::parse("2026-09-18T08:00:00.000Z").unwrap()
+}
+
+#[test]
+fn create_writes_the_baseline_file_and_the_start_record() {
+    let home = Home::new();
+    let s = Session::create(&home.dir, id("a"), baseline(), 3).unwrap();
+    assert_eq!(s.dir(), home.dir.join("sessions").join("a"));
+    assert_eq!(
+        Baseline::from_json(&home.read("sessions/a/0.baseline.json")).unwrap(),
+        baseline()
+    );
+    let records = home.records("a");
+    assert_eq!(records.len(), 1);
+    match &records[0] {
+        LogRecord::SessionStart {
+            session,
+            epoch,
+            slot,
+            baseline: hash,
+            ..
+        } => {
+            assert_eq!(session, &id("a"));
+            assert_eq!(*epoch, Epoch(0));
+            assert_eq!(*slot, 3);
+            assert_eq!(*hash, baseline().hash().unwrap());
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(s.records(), &records[..]);
+    assert!(matches!(
+        Session::create(&home.dir, id("a"), baseline(), 0),
+        Err(SessionError::Exists(_))
+    ));
+}
+
+#[test]
+fn append_is_visible_on_disk_at_once_and_after_reopen() {
+    let home = Home::new();
+    let mut s = Session::create(&home.dir, id("a"), baseline(), 0).unwrap();
+    s.append(LogRecord::User {
+        time: ts(),
+        content: "hi\nthere \"quoted\" caf\u{e9}".to_string(),
+    })
+    .unwrap();
+    assert_eq!(
+        home.records("a").len(),
+        2,
+        "written and synced before append returns"
+    );
+    drop(s);
+    let s = Session::open(&home.dir, id("a")).unwrap();
+    assert_eq!(s.records().len(), 2);
+    assert_eq!(s.baseline(), &baseline());
+    assert!(
+        matches!(s.records()[1], LogRecord::User { ref content, .. } if content.contains("caf\u{e9}"))
+    );
+}
+
+#[test]
+fn resume_uses_the_baseline_file_not_system_md() {
+    let home = Home::new();
+    let s = Session::create(&home.dir, id("a"), baseline(), 0).unwrap();
+    drop(s);
+    home.write("system.md", "A different prompt.\n");
+    let s = Session::open(&home.dir, id("a")).unwrap();
+    assert_eq!(s.baseline().system, "sys");
+}
+
+#[test]
+fn opening_a_missing_session_is_not_found() {
+    let home = Home::new();
+    assert!(matches!(
+        Session::open(&home.dir, id("nope")),
+        Err(SessionError::NotFound(_))
+    ));
+}
+
+#[test]
+fn a_torn_log_is_refused_with_the_line_number() {
+    let home = Home::new();
+    let mut s = Session::create(&home.dir, id("a"), baseline(), 0).unwrap();
+    s.append(LogRecord::User {
+        time: ts(),
+        content: "hi".to_string(),
+    })
+    .unwrap();
+    drop(s);
+    let path = "sessions/a/0.jsonl";
+    let good = home.read(path);
+    // A last line cut in the middle.
+    home.write(path, &good[..good.len() - 5]);
+    match Session::open(&home.dir, id("a")).map(|_| ()) {
+        Err(SessionError::Torn { line, .. }) => assert_eq!(line, 2),
+        other => panic!("{other:?}"),
+    }
+    // A complete line that is not a record.
+    home.write(path, &format!("{good}{{\"type\":\"zz\"}}\n"));
+    match Session::open(&home.dir, id("a")).map(|_| ()) {
+        Err(SessionError::Torn { line, path, .. }) => {
+            assert_eq!(line, 3);
+            assert!(path.ends_with("0.jsonl"));
+        }
+        other => panic!("{other:?}"),
+    }
+    let e: Box<dyn std::error::Error> =
+        Box::new(Session::open(&home.dir, id("a")).map(|_| ()).unwrap_err());
+    assert!(e.to_string().contains("0.jsonl:3"), "{e}");
+}
+
+#[test]
+fn call_ids_continue_across_a_reopen() {
+    let home = Home::new();
+    let mut s = Session::create(&home.dir, id("a"), baseline(), 0).unwrap();
+    assert_eq!(s.next_call(), CallId(1));
+    assert_eq!(s.next_call(), CallId(2));
+    s.append(LogRecord::ToolResult {
+        time: ts(),
+        call: CallId(2),
+        tool_call_id: "x".to_string(),
+        content: String::new(),
+        class: DataClass::Public,
+        untrusted: false,
+        truncated: false,
+    })
+    .unwrap();
+    drop(s);
+    let mut s = Session::open(&home.dir, id("a")).unwrap();
+    assert_eq!(
+        s.next_call(),
+        CallId(3),
+        "one more than the highest call in the log"
+    );
+    let mut fresh = Session::create(&home.dir, id("b"), baseline(), 0).unwrap();
+    assert_eq!(fresh.next_call(), CallId(1));
+}
+
+#[test]
+fn last_usage_is_the_latest_usage_record() {
+    let home = Home::new();
+    let mut s = Session::create(&home.dir, id("a"), baseline(), 0).unwrap();
+    assert_eq!(s.last_usage(), None);
+    s.append(LogRecord::Usage {
+        time: ts(),
+        cache_n: 1,
+        prompt_n: 2,
+        predicted_n: 3,
+        reasoning_tokens: 0,
+        thinking_capped: false,
+    })
+    .unwrap();
+    s.append(LogRecord::User {
+        time: ts(),
+        content: "x".to_string(),
+    })
+    .unwrap();
+    s.append(LogRecord::Usage {
+        time: ts(),
+        cache_n: 6,
+        prompt_n: 7,
+        predicted_n: 8,
+        reasoning_tokens: 4,
+        thinking_capped: true,
+    })
+    .unwrap();
+    let u = s.last_usage().unwrap();
+    assert_eq!(
+        (
+            u.cache_n,
+            u.prompt_n,
+            u.predicted_n,
+            u.reasoning_tokens,
+            u.thinking_capped
+        ),
+        (6, 7, 8, 4, true)
+    );
+}
diff --git a/docs/implementer-log.md b/docs/implementer-log.md
index b16c864..5483d03 100644
--- a/docs/implementer-log.md
+++ b/docs/implementer-log.md
@@ -34,6 +34,7 @@ reviewer adds findings under "Reviews" once per milestone.
 | M2b/02-loopd-config | 2026-09-18 | done | 2 | fail | none | Added Paths/Channel/Loop/Baseline structs to config.rs with `#[serde(deny_unknown_fields, default)]` and Debug/Clone/PartialEq/Eq/Deserialize (Default derived for Channel, hand-written for the other three: home is $BOXMAKER_HOME else /var/lib/boxmaker, loop is 8/true/16384, baseline system is system.md); Config gained the four `#[serde(default)]` fields and `channel_socket()` fills the default `<home>/run/loop/loop.sock` when the socket is empty. `load` joins `baseline.system` to the config file's directory via `parent.join` (which replaces an already-absolute path); `parse` leaves it. 9 config tests pass, `deny_unknown_fields` count is 10. Two clippy fixes on the first (failing) gate run: the nested if in `load` collapsed by relying on Path::join replacing absolute paths instead of a 1.98 let-chain, and `Path::is_empty` (stable 1.98) replaced with `as_os_str().is_empty()`. | ? |
 | M2b/03-loopd-tools | 2026-09-18 | done | 1 | pass | none | Added `pub mod tools;` to lib.rs and `serde::Serialize`/`serde::Deserialize`/`deny_unknown_fields` to ToolSchema; wrote crates/loopd/src/tools.rs with ToolPort, Entry, Registry (m2b/config), core_schemas/get/find, FIND_TOOL/CALL_TOOL constants, clock_schema/echo_schema, Dispatch with dispatch (find_tool/call_tool/local dispatch rows), cap_result via floor_char_boundary, and FakeTools recording calls and answering clock/echo/denying others with `unwrap_or_else(|p| p.into_inner())` on Mutex::lock. 7 tools tests pass, `make gate` prints `gate: ok`, no `unwrap()` in tools.rs. | ? |
 | M2b/04-loopd-baseline | 2026-09-18 | done | 2 | fail | none | Wrote crates/loopd/src/baseline.rs: `Baseline` (system prompt + core tool schemas, `deny_unknown_fields`), `BaselineError` (Read/Parse name the file, plus Hash) with Display/std::error::Error, `assemble` (system prompt trimmed of trailing whitespace, core memory appended with a blank line when its trimmed content is non-empty), `to_json`/`from_json`, `load`, and `hash` (sha256 of the canonical JSON). `messages` prepends the system message and replays every `LogRecord` variant explicitly named, so a new one is a compile error. The `\n\n` separator between system prompt and core memory had to be two newlines (a blank line), not one. All 6 baseline tests pass and all loopd tests pass with the new support module; first gate run failed on a rustfmt import-order diff, fixed with `cargo fmt`. | ? |
+| M2b/05-loopd-session | 2026-09-18 | done | 2 | fail | none | Copied the given test byte-identical and wrote crates/loopd/src/session.rs: `Session` (id, dir, baseline, records, appended log file, next_call) and `SessionError` (Exists/NotFound/Io/Torn/Baseline/Encode) with derived Debug, Display and std::error::Error::source. `create` refuses an existing dir, writes `0.baseline.json`, opens `0.jsonl` with `create_new`+`append`, and appends a `SessionStart` (`Timestamp::now()`, epoch 0, the slot, `baseline.hash()`). `open` reads the baseline from the file (not `system.md`), requires every log line to end in `\n` and parse as a `LogRecord` else `Torn` with the 1-based line and reason, and sets `next_call` to one past the highest `ToolResult` call. `append` encodes, writes, `sync_data()`, then pushes to memory. All 7 session tests pass. First gate failed on clippy: split the `source()` arm that bound three different error types into three arms, removed the redundant `.write(true)` (implied by `append`), and used `path.display()` for the `Torn` path. | ? |
 
 
 ## Reviews