Show a compaction happening, and what it recovered

The Compacting status had been declared, rendered in four places, and
never once emitted: no driver produced it, and the app had no control to
ask for a compaction in the first place. Pressing nothing for two
minutes and then quietly having less context was the whole experience.

The CLI turns out to announce all of it, which was worth measuring
rather than guessing at. Driven through /compact against 2.1.237 it
emits a `status: "compacting"` line at the start, a `status: null`
carrying `compact_result` at the end -- `"failed"` with a sentence
saying why, when it does -- and then a `compact_boundary` with the token
counts. The same records appear in the CLI's own transcript file with
camelCase keys, which is the obvious place to read the shape off and
gets every field name wrong.

So none of it is inferred here. The driver writes the line and says
nothing; the translator reports what the CLI reports. A failed
compaction surfaces the CLI's own sentence, which is specific enough to
act on.

The counts are the part worth keeping afterwards, so they land in the
transcript rather than only in a status that vanishes: a session that
went from 128,402 tokens to 9,617 has just been given its context back.
They are optional throughout, because a compaction whose size nobody
reported has to be able to say so -- a zero would read as "recovered
nothing".

Also here, all found on the way:

- `rename_all` renames variants; fields need `rename_all_fields`. Every
  field in Event was a single word until `pre_tokens`, which went out as
  snake_case, was not found by the app, and rendered as the "no counts
  reported" case -- a state it is allowed to be in, so nothing looked
  wrong. There is now a test on the wire names.
- The unparseable-line warning sliced bytes, not chars, on output that
  is full of em dashes. A panic there kills the task reading the
  session's stdout, and the session goes deaf with nothing on screen.
  The other three truncations in the tree already did this correctly.
- Echo compacts too, with invented numbers and a real shape, so this
  screen can be looked at without spending two minutes of somebody's
  account to reach the state.

server/src/session/claude.rs

@@ -482,7 +482,16 @@ impl Driver for ClaudeDriver {
     }
 
     fn compact(&self) {
-        // Slash commands ride the normal user-message channel.
+        // A turn is in flight from here. The CLI answers `/compact` like
+        // any other message -- a status line, a boundary, then a `result`
+        // -- so a message sent meanwhile belongs in the queue's "written,
+        // announce when it has been read" path rather than being reported
+        // as read the moment it is typed.
+        self.queue.lock().unwrap().running = true;
+        // Slash commands ride the normal user-message channel. Nothing is
+        // emitted here: the CLI announces the compaction itself, and
+        // saying so first would be this side's guess standing in for its
+        // measurement.
         self.send_line(
             json!({"type": "user", "message": {"role": "user", "content": [
                 {"type": "text", "text": "/compact"}
@@ -680,10 +689,14 @@ fn translate_line(
     queue: &Arc<Mutex<Queue>>,
 ) -> bool {
     let Ok(message) = serde_json::from_str::<Value>(line) else {
-        tracing::warn!(
-            "unparseable claude output line: {}",
-            &line[..line.len().min(200)]
-        );
+        // By characters, not bytes: the CLI emits plenty of non-ASCII, and
+        // a byte slice that lands mid-character panics -- inside `follow`,
+        // which is the task reading this session's output, so the session
+        // would go permanently deaf with nothing on screen to say so. The
+        // other three truncations in this codebase (`setups.rs`,
+        // `translate.rs`, `import.rs`) already do it this way.
+        let shown: String = line.chars().take(200).collect();
+        tracing::warn!("unparseable claude output line: {shown}");
         return true;
     };
     let (events, new_session_id) = {

server/src/session/claude/translate.rs

@@ -69,14 +69,7 @@ impl Translator {
             return Vec::new();
         }
         match message.get("type").and_then(Value::as_str) {
-            Some("system") => {
-                if message.get("subtype").and_then(Value::as_str) == Some("init")
-                    && let Some(id) = message.get("session_id").and_then(Value::as_str)
-                {
-                    self.session_id = Some(id.to_string());
-                }
-                Vec::new()
-            }
+            Some("system") => self.translate_system(message),
             Some("stream_event") => self.translate_stream_event(&message["event"]),
             Some("assistant") => self.translate_assistant(&message["message"]),
             Some("user") => self.translate_user(message),
@@ -131,6 +124,88 @@ impl Translator {
         }
     }
 
+    /// The CLI's own notices: which session this is, and what it is doing
+    /// that is not a turn.
+    ///
+    /// Compaction is the whole of that second kind, and it is announced
+    /// rather than inferred. Measured against CLI 2.1.237 (2026-08-29) by
+    /// driving a session through `/compact`, one produces in order:
+    ///
+    /// - `{"subtype":"status","status":"compacting"}` -- the start;
+    /// - `{"subtype":"status","status":null,"compact_result":"success"}`,
+    ///   or `"failed"` with a `compact_error` saying why -- the end;
+    /// - a fresh `init` carrying the same `session_id`;
+    /// - `{"subtype":"compact_boundary","compact_metadata":{…}}` with the
+    ///   token counts, and only when it succeeded;
+    /// - the turn's ordinary `result`, which is what returns it to idle.
+    ///
+    /// The keys are snake_case here and camelCase in the CLI's own
+    /// transcript file, which records the same events. Reading the shape
+    /// off that file -- the obvious place to find one, since it is on
+    /// disk -- gets every field name wrong and silently yields a
+    /// compaction with no numbers in it.
+    fn translate_system(&mut self, message: &Value) -> Vec<Event> {
+        match message.get("subtype").and_then(Value::as_str) {
+            Some("init") => {
+                if let Some(id) = message.get("session_id").and_then(Value::as_str) {
+                    self.session_id = Some(id.to_string());
+                }
+                Vec::new()
+            }
+            Some("status") => self.translate_status(message),
+            Some("compact_boundary") => {
+                let meta = &message["compact_metadata"];
+                vec![Event::Compacted {
+                    pre_tokens: meta.get("pre_tokens").and_then(Value::as_u64),
+                    post_tokens: meta.get("post_tokens").and_then(Value::as_u64),
+                    trigger: meta
+                        .get("trigger")
+                        .and_then(Value::as_str)
+                        .map(str::to_string),
+                }]
+            }
+            _ => Vec::new(),
+        }
+    }
+
+    /// A `system/status` line: the CLI entering or leaving a state that is
+    /// not a turn.
+    ///
+    /// A null `status` is the leaving edge, and it carries how the thing
+    /// went. Whatever it was, the turn it happened inside is still going
+    /// when it ends -- the `result` has not arrived yet -- so leaving says
+    /// `Running`, which is also the only place in this file that does. A
+    /// state this build does not recognise is left alone rather than
+    /// mapped onto the nearest one we do.
+    fn translate_status(&self, message: &Value) -> Vec<Event> {
+        if let Some(status) = message.get("status").and_then(Value::as_str) {
+            return match status {
+                "compacting" => vec![Event::Status {
+                    state: SessionStatus::Compacting,
+                }],
+                _ => Vec::new(),
+            };
+        }
+        let Some(result) = message.get("compact_result").and_then(Value::as_str) else {
+            return Vec::new();
+        };
+        let mut events = Vec::new();
+        if result != "success" {
+            // The CLI's own sentence, because it is specific enough to act
+            // on: "Not enough messages to compact." is a complete answer.
+            events.push(Event::Error {
+                message: match message.get("compact_error").and_then(Value::as_str) {
+                    Some(why) => format!("compaction failed: {why}"),
+                    None => format!("compaction {result}"),
+                },
+            });
+        }
+        events.push(Event::Status {
+            state: SessionStatus::Running,
+        });
+        events
+    }
+
     /// Raw API streaming: only text deltas become events. Consolidated
     /// blocks arriving later re-carry the same text, so those are skipped
     /// in `translate_assistant` -- one source per fact.
@@ -628,6 +703,86 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_compaction_reports_its_start_and_what_it_recovered() {
+        // Real lines (trimmed) from a 2.1.237 session driven through
+        // `/compact`. Note the snake_case keys -- the CLI's transcript
+        // file writes the same records in camelCase.
+        let dir = tempfile::tempdir().expect("tempdir");
+        let mut translator = Translator::new(dir.path().to_path_buf());
+        let events = translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"system","subtype":"status","status":"compacting","session_id":"s","uuid":"u1"}"#,
+                r#"{"type":"system","subtype":"status","status":null,"compact_result":"success","session_id":"s","uuid":"u2"}"#,
+                r#"{"type":"system","subtype":"compact_boundary","session_id":"s","uuid":"u3","compact_metadata":{"trigger":"manual","pre_tokens":28719,"post_tokens":1125,"duration_ms":17130}}"#,
+            ],
+        );
+        assert_eq!(
+            events,
+            vec![
+                Event::Status {
+                    state: SessionStatus::Compacting
+                },
+                Event::Status {
+                    state: SessionStatus::Running
+                },
+                Event::Compacted {
+                    pre_tokens: Some(28719),
+                    post_tokens: Some(1125),
+                    trigger: Some("manual".to_string()),
+                },
+            ]
+        );
+    }
+
+    #[test]
+    fn a_failed_compaction_says_why_and_leaves_the_turn_running() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let mut translator = Translator::new(dir.path().to_path_buf());
+        let events = translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"system","subtype":"status","status":"compacting","session_id":"s","uuid":"u1"}"#,
+                r#"{"type":"system","subtype":"status","status":null,"compact_result":"failed","compact_error":"Not enough messages to compact.","session_id":"s","uuid":"u2"}"#,
+            ],
+        );
+        assert_eq!(
+            events,
+            vec![
+                Event::Status {
+                    state: SessionStatus::Compacting
+                },
+                Event::Error {
+                    message: "compaction failed: Not enough messages to compact.".to_string()
+                },
+                Event::Status {
+                    state: SessionStatus::Running
+                },
+            ]
+        );
+    }
+
+    #[test]
+    fn a_boundary_without_counts_says_so_rather_than_inventing_them() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let mut translator = Translator::new(dir.path().to_path_buf());
+        let events = translate_lines(
+            &mut translator,
+            &[
+                r#"{"type":"system","subtype":"compact_boundary","session_id":"s","compact_metadata":{"trigger":"auto"}}"#,
+            ],
+        );
+        assert_eq!(
+            events,
+            vec![Event::Compacted {
+                pre_tokens: None,
+                post_tokens: None,
+                trigger: Some("auto".to_string()),
+            }]
+        );
+    }
+
     #[test]
     fn an_error_result_surfaces_the_message() {
         let dir = tempfile::tempdir().expect("tempdir");

server/src/session/driver.rs

@@ -21,7 +21,19 @@ pub type ImageRef = String;
 /// reconnecting is just "events after seq N" -- no separate history path
 /// to drift from the live one.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
-#[serde(tag = "type", rename_all = "camelCase")]
+// `rename_all` renames the variants; `rename_all_fields` renames what is
+// inside them. Both are needed and only the first is obvious: every field
+// here was a single lowercase word until `pre_tokens` arrived, so a
+// multi-word field went out as snake_case, the app looked for camelCase and
+// found nothing, and the event still rendered -- as the "no counts were
+// reported" case, which is a state it is allowed to be in. A wire mismatch
+// that lands on a plausible state is invisible; anything added below with a
+// two-word field would have hit the same thing.
+#[serde(
+    tag = "type",
+    rename_all = "camelCase",
+    rename_all_fields = "camelCase"
+)]
 pub enum Event {
     /// What the user sent, written into the transcript by the manager (not
     /// by drivers) so every device renders the full conversation from the
@@ -106,6 +118,29 @@ pub enum Event {
     UsageDelta {
         tokens: u64,
     },
+    /// A compaction that finished, and how much context it recovered.
+    ///
+    /// The counts are the point, and a spinner is not: what a reader wants
+    /// afterwards is that the session went from a million tokens to ten
+    /// thousand, which is measured rather than estimated. They are
+    /// optional because the record has shipped without them, and "the
+    /// compaction happened, we don't know by how much" is a state this
+    /// has to be able to say -- filling in a plausible number would make
+    /// it indistinguishable from one that was counted.
+    Compacted {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        pre_tokens: Option<u64>,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        post_tokens: Option<u64>,
+        /// What asked for it, in the dialect's own word -- `auto` when the
+        /// session compacted on its own. Carried rather than reduced to a
+        /// bool so an unrecognised trigger stays unrecognised: an
+        /// automatic compaction is the one worth naming, because it
+        /// explains a wait nobody asked for, and defaulting the unknown
+        /// case to "you asked for this" would explain it away.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        trigger: Option<String>,
+    },
     Error {
         message: String,
     },
@@ -180,3 +215,32 @@ pub trait Driver: Send + Sync {
     /// [`detach`]: Driver::detach
     fn stop(&self);
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A tripwire for the wire format, not for serde.
+    ///
+    /// The app reads these names, and getting one wrong does not fail
+    /// loudly: a field the app cannot find reads as a field the server
+    /// chose not to send, which several of them are allowed to be.
+    #[test]
+    fn multi_word_fields_go_out_in_camel_case() {
+        let json = serde_json::to_value(Event::Compacted {
+            pre_tokens: Some(28719),
+            post_tokens: Some(1125),
+            trigger: Some("manual".to_string()),
+        })
+        .expect("serialize");
+        assert_eq!(
+            json,
+            serde_json::json!({
+                "type": "compacted",
+                "preTokens": 28719,
+                "postTokens": 1125,
+                "trigger": "manual",
+            })
+        );
+    }
+}

server/src/session/echo.rs

@@ -1,6 +1,6 @@
 //! The phase-1 fake driver: no child process, just events. It exists to
 //! prove the whole pipe -- spawn, transcript, SSE cursors, questions,
-//! interrupts -- before any AI is involved, and stays useful afterwards as
+//! interrupts, compaction -- before any AI is involved, and stays useful afterwards as
 //! a connectivity check that costs no tokens.
 //!
 //! Behavior: every message is echoed back as a few streamed text deltas.
@@ -36,6 +36,12 @@ use super::driver::{Driver, Event, EventSink, ImageRef, SessionStatus};
 /// stay fast.
 const DELTA_DELAY: Duration = Duration::from_millis(50);
 
+/// How long a fake compaction takes. A real one runs for a minute or two,
+/// which is too long to sit through when what is being checked is what the
+/// screen does; this is long enough that the state is visible and short
+/// enough to wait for.
+const COMPACT_TIME: Duration = Duration::from_secs(3);
+
 pub struct EchoDriver {
     sink: EventSink,
     /// Whether a turn is in flight, and what arrived during it.
@@ -74,6 +80,28 @@ impl EchoDriver {
     }
 }
 
+/// Ending a turn is also when anything held during it is taken up -- the
+/// moment a real CLI would have injected it. One place, because a turn has
+/// several ways to end (a reply, an interrupt, a compaction) and every one
+/// of them owes the same answer.
+fn finish_turn(sink: &EventSink, queued: &Mutex<Vec<String>>, busy: &AtomicBool) {
+    let held = std::mem::take(&mut *queued.lock().unwrap());
+    for text in held {
+        // Announced before it is answered, in that order: a phone showing
+        // the message as pending needs the signal that it has been read,
+        // and the answer is meaningless above a message still drawn as
+        // waiting.
+        let _ = sink.send(Event::MessageTaken { text: text.clone() });
+        let _ = sink.send(Event::AssistantText {
+            delta: format!("\n(taken from the queue) You said: {text}"),
+        });
+    }
+    busy.store(false, Ordering::SeqCst);
+    let _ = sink.send(Event::Status {
+        state: SessionStatus::Idle,
+    });
+}
+
 impl Driver for EchoDriver {
     fn send_user_message(&self, text: String, _images: Vec<ImageRef>) {
         let sink = self.sink.clone();
@@ -141,27 +169,7 @@ impl Driver for EchoDriver {
             let send = |event: Event| {
                 let _ = sink.send(event);
             };
-            // Ending a turn is also when anything held during it is taken
-            // up -- the moment a real CLI would have injected it. One
-            // place, because a turn has several ways to end and every one
-            // of them owes the same answer.
-            let finish = || {
-                let held = std::mem::take(&mut *queued.lock().unwrap());
-                for text in held {
-                    // Announced before it is answered, in that order: a
-                    // phone showing the message as pending needs the
-                    // signal that it has been read, and the answer is
-                    // meaningless above a message still drawn as waiting.
-                    send(Event::MessageTaken { text: text.clone() });
-                    send(Event::AssistantText {
-                        delta: format!("\n(taken from the queue) You said: {text}"),
-                    });
-                }
-                busy.store(false, Ordering::SeqCst);
-                send(Event::Status {
-                    state: SessionStatus::Idle,
-                });
-            };
+            let finish = || finish_turn(&sink, &queued, &busy);
             // Echo takes a message the instant it gets one, but it says so
             // anyway: a driver that skips this leaves the phone holding a
             // message it thinks is still queued, and the point of an echo
@@ -295,9 +303,30 @@ impl Driver for EchoDriver {
         });
     }
 
+    /// A compaction with nothing to compact.
+    ///
+    /// The counts are invented, like everything else this driver says --
+    /// what is real is the shape and the order: busy, a pause long enough
+    /// to see, then the result. `Compacting` and `Compacted` are states a
+    /// screen has to draw, and the only other way to reach them is to fill
+    /// a real session's context and spend two minutes of somebody's
+    /// account getting it back.
     fn compact(&self) {
-        self.emit(Event::Error {
-            message: "echo sessions have nothing to compact".to_string(),
+        let sink = self.sink.clone();
+        let queued = Arc::clone(&self.queued);
+        let busy = Arc::clone(&self.busy);
+        busy.store(true, Ordering::SeqCst);
+        tokio::spawn(async move {
+            let _ = sink.send(Event::Status {
+                state: SessionStatus::Compacting,
+            });
+            tokio::time::sleep(COMPACT_TIME).await;
+            let _ = sink.send(Event::Compacted {
+                pre_tokens: Some(128_402),
+                post_tokens: Some(9_617),
+                trigger: Some("manual".to_string()),
+            });
+            finish_turn(&sink, &queued, &busy);
         });
     }