Never let one unreadable line take a transcript down

Removing `Event::TaskNote` hours after adding it made every transcript that
had recorded one unreadable. `Transcript::open` parses every line, so `launch`
failed for those sessions and `SessionManager::new` logged
"couldn't relaunch session <id>" and skipped them -- and a skipped session has
no pump and no driver. On the phone that is no status, no history and nothing
sendable, for every live session that had run a background task. One
unfamiliar word took down every conversation it appeared in.

A transcript is append-only and permanent, so the set of kinds one can hold
only ever grows: what this build writes is not what it may have to read. A
line can come from a newer server, or from an older one that wrote a kind
since dropped, and neither may be able to end the file.

`Indexed::parse_at` degrades a line it cannot make sense of to
`Event::Unreadable { kind }` instead of failing the whole read. It keeps the
line's seq -- the cursors, the page bisection and the next-seq counter are all
addressed by it, and dropping the line would hand out a seq the file already
contains -- and carries the word the line called itself, so the phone can say
what is missing rather than that something is. A line with no readable seq is
still an error: that one cannot be placed at all.

`Event::TaskNote` comes back retired rather than deleted: deserializable,
never constructed, dated, with the reason on it. The phone folds it to no row,
which is the point -- an unreadable line correctly draws a placeholder, and
one per background task is the wall the row was removed for in the first
place.

Found while diagnosing a report that live sessions had lost their status and
could not be sent to. 173 server tests pass, including the new one, which
fails on the old code within a second.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -296,6 +296,19 @@ written, and the fold uses that same predicate to decide a reply is settled.
 
 ## Things that have bitten
 
+- **A transcript outlives the enum.** Removing `Event::TaskNote` hours after
+  adding it made every transcript that had recorded one unreadable, so
+  `launch` failed for those sessions and `SessionManager::new` skipped them —
+  no status, nothing sendable, no new messages, for every live session that
+  had run a background task. **The set of kinds a transcript can hold only
+  ever grows**: a line may come from a newer server or from an older one that
+  wrote a kind since dropped, and one unfamiliar word must never be able to
+  end the file. `Indexed::parse_at` degrades a line it cannot read to
+  `Event::Unreadable { kind }`, keeping its seq — which is what everything
+  downstream is addressed by — and the phone draws it as a placeholder saying
+  which kind. Never delete a variant instead of retiring it; `Event::TaskNote`
+  is what retiring looks like, and the phone folds it to no row.
+
 Project-specific only — a lesson that would bite any project on this machine
 belongs in `~/.claude/TOOLCHAIN.md` or `~/.claude/MACHINE.md` instead.
 

PLAN.md

@@ -706,6 +706,34 @@ those, and `open_tasks` covers the backgrounded command, which has no subagent
 to be found in the registry at all. Both are needed and neither subsumes the
 other.
 
+### A transcript outlives this enum (2026-09-06)
+
+**The set of event kinds a transcript can hold only ever grows.** It is
+append-only and permanent, so what this build *writes* is not what it may have
+to *read*: a line can come from a newer server, or from an older one that wrote
+a kind since dropped.
+
+That was learned the expensive way. `Event::TaskNote` was added and removed
+again within hours, and every transcript that had recorded one became
+unreadable — `Transcript::open` parses every line, so `launch` failed for those
+sessions and `SessionManager::new` skipped them. On the phone that is a session
+with no status, no history and nothing sendable: one unfamiliar word took down
+every live conversation it appeared in.
+
+Two rules now. `Indexed::parse_at` degrades a line it cannot make sense of to
+`Event::Unreadable { kind }` rather than failing the file, keeping its seq —
+which is what the cursors, the bisection and the next-seq counter are all
+addressed by — and carrying the word the line called itself, so the reader is
+told what they are missing rather than that something is. The seq is still
+required: a line that cannot say where it sits is not one this file can hold,
+and dropping it silently would hand out a seq the file already contains.
+
+And a variant is **retired, not deleted**: kept deserializable, never
+constructed, with the date and the reason on it. `Event::TaskNote` is the
+example, and the phone folds it to no row — which is the point, since an
+unreadable line correctly draws a placeholder and one per background task is
+the wall the row was removed for.
+
 ### A limit a subagent hits is the session's (2026-09-06)
 
 A background Task runs on long after its parent's turn ended, so **the account

app/androidApp/src/main/kotlin/com/example/aiapp/Events.kt

@@ -108,6 +108,26 @@ sealed class SessionEvent {
         val turnStart: Long? = null,
     ) : SessionEvent()
 
+    /**
+     * A line in the transcript this build cannot read: a kind a newer server wrote, or one an older
+     * server wrote that has since been dropped.
+     *
+     * [kind] is the word the line called itself, so the row can say what is missing rather than
+     * that something is. The server makes these when reading; no driver sends one.
+     */
+    data class Unreadable(val kind: String) : SessionEvent()
+
+    /**
+     * Retired on 2026-09-06, hours after it was added: a background task finishing, which turned
+     * out to be a screenful of notices about work nobody was asking after.
+     *
+     * Kept because a transcript is append-only -- the sessions that ran a background task in that
+     * window have these lines for ever. It draws no row, which is the whole reason it is still
+     * named here rather than left to fall through to [Unknown]: that would draw a placeholder per
+     * background task, which is the same wall the row was removed for.
+     */
+    object RetiredTaskNote : SessionEvent()
+
     /**
      * A command the session was asked to run on itself and cannot run yet. Resolved by
      * [CommandSent] with the same id; a command that ran straight away has only that one.
@@ -252,6 +272,8 @@ fun parseSeqEvent(json: String): SeqEvent {
                     body.getString("text"),
                     if (body.has("turnStart")) body.getLong("turnStart") else null,
                 )
+            "unreadable" -> SessionEvent.Unreadable(body.getString("kind"))
+            "taskNote" -> SessionEvent.RetiredTaskNote
             "commandQueued" ->
                 SessionEvent.CommandQueued(body.getString("id"), body.getString("text"))
             "commandSent" -> SessionEvent.CommandSent(body.getString("id"), body.getString("text"))

app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptItems.kt

@@ -515,6 +515,12 @@ fun foldEvent(items: List<TranscriptItem>, entry: SeqEvent): List<TranscriptItem
         is SessionEvent.Compacted ->
             items + TranscriptItem.CompactedNote(entry.seq, event.preTokens, event.postTokens)
         is SessionEvent.Unknown -> items + TranscriptItem.Note(entry.seq, "[${event.type}]")
+        // Said rather than skipped: a line the server could not read is a hole in the conversation,
+        // and one that draws nothing is a hole nothing on screen ever mentions.
+        is SessionEvent.Unreadable ->
+            items + TranscriptItem.Note(entry.seq, "[unreadable: ${event.kind}]")
+        // No row: see [SessionEvent.RetiredTaskNote].
+        is SessionEvent.RetiredTaskNote -> items
         // Screen-level state, not transcript rows -- see SessionScreen.
         is SessionEvent.UsageDelta -> items
     }

server/src/session/driver.rs

@@ -208,6 +208,48 @@ pub enum Event {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         turn_start: Option<u64>,
     },
+    /// **Retired on 2026-09-06, hours after it was added.** Kept only so the
+    /// transcripts written while it existed still read: a session that ran a
+    /// background task in that window has these lines for ever, and a
+    /// transcript is append-only, so there is no pass that could remove them.
+    ///
+    /// Never constructed. It reported a background task finishing, and drawing
+    /// a row per one turned out to be a screen of notices about work the
+    /// reader was not asking after -- see PLAN.md's "Two turns must never be
+    /// drawn as one". The phone folds it to no row at all, which is what makes
+    /// keeping it cheap.
+    ///
+    /// Deleting the variant instead is what broke every live session, and
+    /// [`Event::Unreadable`] is the reason that cannot happen again. This is
+    /// still here rather than left to that: an unreadable line draws a
+    /// placeholder, correctly, and one per background task is the same wall
+    /// the row was removed for.
+    TaskNote {
+        about: String,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        title: Option<String>,
+        status: String,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        summary: Option<String>,
+    },
+    /// A line in a transcript that this build cannot read: a kind a newer
+    /// server wrote, a kind an older one wrote that has since been dropped, or
+    /// a line whose contents do not fit the kind it names.
+    ///
+    /// **Only ever made when reading, never sent by a driver**, and it is the
+    /// reason a transcript can outlive a change to this enum. See
+    /// `Indexed::parse_at` for the incident: removing a variant after
+    /// transcripts had recorded it made every read of those files fail, so
+    /// every session in them lost its status, its history and its ability to
+    /// be sent to.
+    ///
+    /// It carries the word the line called itself so a reader is told what
+    /// they are missing rather than that something is missing. `kind` is not
+    /// an enum for the obvious reason: the whole point of this variant is the
+    /// words that are not in one.
+    Unreadable {
+        kind: String,
+    },
     /// The manager's record of a question being answered, so a rendered
     /// question card resolves on every device rather than only the one that
     /// answered.

server/src/session/transcript.rs

@@ -309,18 +309,70 @@ impl<'a> Indexed<'a> {
     }
 
     fn parse(&self, range: Range<usize>) -> Result<Vec<SeqEvent>> {
-        self.lines[range]
+        self.lines[range.clone()]
             .iter()
-            .map(|at| {
+            .enumerate()
-                serde_json::from_str(&self.text[at.clone()])
+            .map(|(offset, at)| self.parse_at(range.start + offset, at.clone()))
-                    .with_context(|| format!("bad transcript line in {}", self.path.display()))
-            })
             .collect()
     }
 
     fn parse_one(&self, index: usize) -> Result<SeqEvent> {
-        serde_json::from_str(&self.text[self.lines[index].clone()])
+        self.parse_at(index, self.lines[index].clone())
-            .with_context(|| format!("bad transcript line in {}", self.path.display()))
+    }
+
+    /// One line, degrading to [`Event::Unreadable`] rather than failing when
+    /// this build cannot make sense of the event on it.
+    ///
+    /// **A transcript is append-only and permanent, so the set of kinds that
+    /// can appear in one only ever grows.** What this build writes is not what
+    /// it may have to read: a line may come from a newer server, or from an
+    /// older one that wrote a kind since dropped. Refusing the whole file for
+    /// one such line is what happened on 2026-09-06 -- a kind was removed after
+    /// transcripts had recorded it, every read of those files failed, and the
+    /// sessions in them could not be opened, listed, paged or sent to. One
+    /// unfamiliar word took down every conversation it appeared in.
+    ///
+    /// So the line survives as a line. It keeps its seq, which is the part
+    /// everything downstream is addressed by, and says what it was rather than
+    /// pretending to be something -- there is a row for that on the phone
+    /// already.
+    ///
+    /// The seq itself is still required, and this still fails without one: a
+    /// line that cannot say where it sits in the sequence is not a line this
+    /// file can hold, and quietly dropping it would hand out a seq the file
+    /// already contains.
+    fn parse_at(&self, index: usize, at: Range<usize>) -> Result<SeqEvent> {
+        let line = &self.text[at];
+        match serde_json::from_str(line) {
+            Ok(entry) => Ok(entry),
+            Err(err) => {
+                #[derive(Deserialize)]
+                struct JustPlace {
+                    seq: u64,
+                    ts: f64,
+                    #[serde(rename = "type")]
+                    kind: Option<String>,
+                }
+                let place: JustPlace = serde_json::from_str(line).with_context(|| {
+                    format!("bad transcript line in {}", self.path.display())
+                })?;
+                // Debug rather than a warning: a transcript written against a
+                // newer build has one of these per line it wrote, and the row
+                // on the phone is where this is actually reported.
+                tracing::debug!(
+                    "transcript line {index} of {} (seq {}) is not one this build can read: {err}",
+                    self.path.display(),
+                    place.seq,
+                );
+                Ok(SeqEvent {
+                    seq: place.seq,
+                    ts: place.ts,
+                    event: Event::Unreadable {
+                        kind: place.kind.unwrap_or_else(|| "no kind".to_string()),
+                    },
+                })
+            }
+        }
     }
 
     /// The newest `limit` *rows* ending at line `end`, with each run of
@@ -396,6 +448,51 @@ mod tests {
         }
     }
 
+    /// The failure that took every live session down on 2026-09-06: an event
+    /// kind was removed from the enum after transcripts had already recorded
+    /// it, so every read of those files failed and the sessions in them could
+    /// not be opened, listed, paged or sent to.
+    ///
+    /// A transcript is append-only and permanent, so **the set of kinds that
+    /// can appear in one only ever grows** -- what this build writes is not
+    /// what it may have to read. A line it cannot make sense of has to be a
+    /// line, not the end of the file.
+    #[test]
+    fn a_line_of_a_kind_this_build_does_not_know_does_not_break_the_file() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join("transcript.jsonl");
+        std::fs::write(
+            &path,
+            concat!(
+                r#"{"seq":1,"ts":1.0,"type":"status","state":"running"}"#,
+                "\n",
+                r#"{"seq":2,"ts":2.0,"type":"assistantText","delta":"hi"}"#,
+                "\n",
+                r#"{"seq":3,"ts":3.0,"type":"taskNote","about":"t","title":"h","status":"completed"}"#,
+                "\n",
+                r#"{"seq":4,"ts":4.0,"type":"somethingFromTheFuture","whatever":[1,2]}"#,
+                "\n",
+                r#"{"seq":5,"ts":5.0,"type":"status","state":"idle"}"#,
+                "\n",
+            ),
+        )
+        .expect("write");
+
+        let entries = read_after(&path, 0).expect("a strange line is not a broken file");
+        assert_eq!(entries.len(), 5, "every line is still a line: {entries:?}");
+        assert_eq!(entries[4].seq, 5);
+
+        let transcript = Transcript::open(&path).expect("open");
+        assert_eq!(transcript.last_status(), Some(SessionStatus::Idle));
+        // The next seq is counted from the newest *line*, whatever kind it is.
+        // Skipping the ones this build cannot read would hand out a seq the
+        // file already contains.
+        assert_eq!(transcript.next_seq, 6);
+
+        let window = read_window(&path, None, None, 80, false).expect("window");
+        assert_eq!(window.len(), 5, "{window:?}");
+    }
+
     #[test]
     fn assigns_increasing_seqs_and_replays_after_a_cursor() {
         let dir = tempfile::tempdir().expect("tempdir");