diff --git a/services/flysim/crates/flysim/src/journal.rs b/services/flysim/crates/flysim/src/journal.rs
new file mode 100644
index 0000000..472c2b7
--- /dev/null
+++ b/services/flysim/crates/flysim/src/journal.rs
@@ -0,0 +1,161 @@
+//! The sugar journal: every admitted audience input, stamped with the frame it was applied before.
+//!
+//! `sugar-journal.jsonl` in `[paths] hot_dir`, one JSON object per line, append-only. It is not
+//! part of a checkpoint and nothing reads it back into the fly: it is the record a shadow run
+//! (the session framework's CUT-01, `docs/design/session-framework/legacy-gameboy-v1.md` section
+//! 15) replays audience input from. The legacy loop applies an admitted sugar at once, in the
+//! command drain at the top of a frame, so an input is fully placed by the transition it precedes:
+//!
+//! ```json
+//! {"frame":"6465126","brainMs":108246189,"kind":"sugar","durationMs":400,"by":"viewer","source":"twitch","eventId":81234,"wallMs":1790000000000}
+//! ```
+//!
+//! - `frame` is the frame counter when the input was applied, which is the `step` of the next
+//!   transition in the frame trace (`crate::trace`): replay applies it before that transition's
+//!   ticks. A restore carries the frame counter, so stamps continue across restarts.
+//! - `kind` is `sugar` (a `reward-pulse` stimulation of `durationMs`, after the admission rules
+//!   and the clamp) or `reward` (an operator's `POST /reward`, one reinforcement of `value`).
+//! - `brainMs` cross-checks the stamp; `eventId` joins the event log; `wallMs` is for people.
+//!
+//! Refused requests are not journalled: admission is wall-clock policy, and a replay applies what
+//! was admitted. A write that fails is a warning, never a refusal: the input has already reached
+//! the fly.
+
+use std::fs::{File, OpenOptions};
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+use serde_json::{Value, json};
+
+/// The journal's file name inside the hot directory.
+pub const FILE_NAME: &str = "sugar-journal.jsonl";
+
+/// What was applied.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Input {
+    Sugar { duration_ms: f64 },
+    Reward { value: f64 },
+}
+
+/// One journal line.
+#[derive(Debug, Clone, PartialEq)]
+pub struct Entry<'a> {
+    pub frame: u64,
+    pub brain_ms: f64,
+    pub input: Input,
+    pub by: &'a str,
+    pub source: &'a str,
+    pub event_id: u64,
+    pub wall_ms: u64,
+}
+
+impl Entry<'_> {
+    pub fn to_json(&self) -> Value {
+        let mut line = json!({ "frame": self.frame.to_string(), "brainMs": self.brain_ms });
+        let map = line.as_object_mut().expect("an object");
+        match self.input {
+            Input::Sugar { duration_ms } => {
+                map.insert("kind".into(), "sugar".into());
+                map.insert("durationMs".into(), json!(duration_ms));
+            }
+            Input::Reward { value } => {
+                map.insert("kind".into(), "reward".into());
+                map.insert("value".into(), json!(value));
+            }
+        }
+        map.insert("by".into(), self.by.into());
+        map.insert("source".into(), self.source.into());
+        map.insert("eventId".into(), self.event_id.into());
+        map.insert("wallMs".into(), self.wall_ms.into());
+        line
+    }
+}
+
+/// The append-only journal. Opened on the first input, so a run nobody feeds writes no file.
+pub struct SugarJournal {
+    path: PathBuf,
+    file: Option<File>,
+}
+
+impl SugarJournal {
+    pub fn new(hot_dir: &Path) -> Self {
+        Self { path: hot_dir.join(FILE_NAME), file: None }
+    }
+
+    pub fn path(&self) -> &Path {
+        &self.path
+    }
+
+    /// Append one line. Each line is one `write` of the whole line, so a crash leaves at most a
+    /// torn last line, which a reader skips.
+    pub fn record(&mut self, entry: &Entry<'_>) {
+        if self.file.is_none() {
+            match OpenOptions::new().create(true).append(true).open(&self.path) {
+                Ok(file) => self.file = Some(file),
+                Err(error) => {
+                    tracing::warn!(%error, path = %self.path.display(), "could not open the sugar journal");
+                    return;
+                }
+            }
+        }
+        let mut line = entry.to_json().to_string();
+        line.push('\n');
+        if let Some(file) = self.file.as_mut()
+            && let Err(error) = file.write_all(line.as_bytes())
+        {
+            tracing::warn!(%error, path = %self.path.display(), "could not append to the sugar journal");
+            // Reopen on the next input rather than write through a broken handle.
+            self.file = None;
+        }
+    }
+}
+
+/// Read a journal back: every whole line, in order. A torn or unreadable line is skipped.
+pub fn read(path: &Path) -> std::io::Result<Vec<Value>> {
+    let text = std::fs::read_to_string(path)?;
+    Ok(text
+        .lines()
+        .filter_map(|line| serde_json::from_str::<Value>(line).ok())
+        .filter(|value| value.get("frame").is_some())
+        .collect())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn inputs_are_appended_with_their_frame_and_survive_a_reopen() {
+        let dir = tempfile::tempdir().expect("a temp dir");
+        let mut journal = SugarJournal::new(dir.path());
+        assert!(!journal.path().exists(), "nothing is written before an input");
+        let sugar = Entry {
+            frame: 42,
+            brain_ms: 703.0,
+            input: Input::Sugar { duration_ms: 400.0 },
+            by: "viewer",
+            source: "test",
+            event_id: 7,
+            wall_ms: 1,
+        };
+        journal.record(&sugar);
+        drop(journal);
+        let mut journal = SugarJournal::new(dir.path());
+        journal.record(&Entry { frame: 43, input: Input::Reward { value: 0.5 }, event_id: 8, ..sugar });
+        // A torn tail from a crash is skipped, not fatal.
+        std::fs::OpenOptions::new()
+            .append(true)
+            .open(journal.path())
+            .and_then(|mut file| file.write_all(b"{\"frame\":\"44\",\"kin"))
+            .expect("appending a torn line");
+        let lines = read(journal.path()).expect("the journal reads back");
+        assert_eq!(lines.len(), 2);
+        assert_eq!(lines[0]["frame"], "42");
+        assert_eq!(lines[0]["kind"], "sugar");
+        assert_eq!(lines[0]["durationMs"], 400.0);
+        assert_eq!(lines[1]["frame"], "43");
+        assert_eq!(lines[1]["kind"], "reward");
+        assert_eq!(lines[1]["value"], 0.5);
+        assert_eq!(lines[1]["eventId"], 8);
+    }
+}
diff --git a/services/flysim/crates/flysim/src/lib.rs b/services/flysim/crates/flysim/src/lib.rs
index 410742a..80adc21 100644
--- a/services/flysim/crates/flysim/src/lib.rs
+++ b/services/flysim/crates/flysim/src/lib.rs
@@ -28,6 +28,7 @@ pub mod eventlog;
 pub mod feed;
 pub mod feedbus;
 pub mod frame;
+pub mod journal;
 pub mod macros;
 pub mod metrics;
 pub mod pacing;
diff --git a/services/flysim/crates/flysim/src/simloop.rs b/services/flysim/crates/flysim/src/simloop.rs
index 097f540..ff04d30 100644
--- a/services/flysim/crates/flysim/src/simloop.rs
+++ b/services/flysim/crates/flysim/src/simloop.rs
@@ -45,6 +45,7 @@ use tokio::sync::{mpsc, oneshot, watch};
 use crate::chat::{ChatLimiter, ChatRefusal, ChatRing, DenyList, RejectReason};
 use crate::config::Config;
 use crate::eventlog::{EventLog, EventRing, NewEvent, now_wall_ms, utc_day};
+use crate::journal::{Input, SugarJournal};
 use crate::frame::{FrameObserver, FramePhase, LegacyFrame, Parts, RollbackTrigger};
 use crate::macros::{MacroEvent, MacroLayer, macro_layer};
 use crate::metrics::Metrics;
@@ -375,6 +376,9 @@ pub struct Sim {
     sugar_last_by: Option<String>,
     sugar_today: u64,
     sugar_day: String,
+    /// Every admitted sugar and operator pulse, frame-stamped, in the hot directory
+    /// (`crate::journal`): what a shadow run replays. Not checkpointed.
+    journal: SugarJournal,
 
     /// The chat path. None of it is wired to the agent, the emulator or plasticity.
     chat_ring: ChatRing,
@@ -577,6 +581,7 @@ impl Sim {
             sugar_last_by: None,
             sugar_today: 0,
             sugar_day: utc_day(now_wall_ms()),
+            journal: SugarJournal::new(&config.paths.hot_dir),
             chat_ring: ChatRing::new(config.chat.ring),
             chat_limits: ChatLimiter::default(),
             deny_list: if config.chat.enabled {
@@ -1093,6 +1098,15 @@ impl Sim {
         let event = self.emit(NewEvent::new(FeedEventKind::Sugar, sugar_label(by))
             .by(by)
             .value(duration));
+        self.journal.record(&crate::journal::Entry {
+            frame: self.frame.frame_counter,
+            brain_ms: self.agent.network.ms,
+            input: Input::Sugar { duration_ms: duration },
+            by,
+            source,
+            event_id: event.id,
+            wall_ms: event.wall_ms,
+        });
         tracing::info!(by, source, duration_ms = duration, "sugar accepted");
         Ok(event.id)
     }
@@ -1106,12 +1120,21 @@ impl Sim {
             trace.reward_pulse(value);
         }
         tracing::info!(by, source, value, "reward pulse applied");
-        self.emit(
+        let event = self.emit(
             NewEvent::new(FeedEventKind::Reward, format!("{by} sent a reward pulse ({value})"))
                 .by(by)
                 .value(value),
-        )
-        .id
+        );
+        self.journal.record(&crate::journal::Entry {
+            frame: self.frame.frame_counter,
+            brain_ms: ms,
+            input: Input::Reward { value },
+            by,
+            source,
+            event_id: event.id,
+            wall_ms: event.wall_ms,
+        });
+        event.id
     }
 
     /// `POST /chat`: the on-screen chat path, enforced here rather than trusted from the bridge.