diff --git a/src/ids/things_id.rs b/src/ids/things_id.rs
index 596b05b..02269bb 100644
--- a/src/ids/things_id.rs
+++ b/src/ids/things_id.rs
@@ -11,7 +11,8 @@ use uuid::Uuid;
 /// A Things 3 entity identifier.
 ///
 /// Internally stored as canonical 16 bytes (SHA1-truncated UUID digest).
-/// Hyphenated UUIDs and compact base58 IDs are accepted at parse-time.
+/// Hyphenated UUIDs, historical `ACTIONGROUP-<UUID>` IDs, and compact base58
+/// IDs are accepted at parse-time.
 #[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Default)]
 pub struct ThingsId([u8; 16]);
 
@@ -79,7 +80,13 @@ impl FromStr for ThingsId {
         if s.is_empty() {
             return Err(ParseThingsIdError(s.to_owned()));
         }
-        if let Ok(uuid) = Uuid::parse_str(s) {
+
+        // Early Things clients stored project-heading IDs with an
+        // `ACTIONGROUP-` discriminator in both object keys and task
+        // relationships. The UUID suffix identifies the same entity and can
+        // be canonicalized through the normal legacy UUID path.
+        let uuid_candidate = s.strip_prefix("ACTIONGROUP-").unwrap_or(s);
+        if let Ok(uuid) = Uuid::parse_str(uuid_candidate) {
             return Ok(ThingsId(uuid_to_bytes(&uuid)));
         }
         if s.len() > 22 {
@@ -233,6 +240,7 @@ mod tests {
 
     const LEGACY_UUID: &str = "3C6BBD49-8D11-4FFF-8B0E-B8F33FA9C00A";
     const LEGACY_UUID_LOWER: &str = "3c6bbd49-8d11-4fff-8b0e-b8f33fa9c00a";
+    const LEGACY_ACTION_GROUP_ID: &str = "ACTIONGROUP-3C6BBD49-8D11-4FFF-8B0E-B8F33FA9C00A";
     fn compact_for_legacy() -> String {
         ThingsId::from_str(LEGACY_UUID).unwrap().to_string()
     }
@@ -251,6 +259,20 @@ mod tests {
         assert_eq!(upper, lower, "UUID parsing must be case-insensitive");
     }
 
+    #[test]
+    fn parse_legacy_action_group_id_as_its_uuid() {
+        let uuid: ThingsId = LEGACY_UUID.parse().unwrap();
+        let action_group: ThingsId = LEGACY_ACTION_GROUP_ID.parse().unwrap();
+        assert_eq!(action_group, uuid);
+    }
+
+    #[test]
+    fn serde_deserialize_legacy_action_group_id() {
+        let parsed: ThingsId = serde_json::from_str(&format!(r#""{LEGACY_ACTION_GROUP_ID}""#))
+            .expect("deserialize legacy action-group ID");
+        assert_eq!(parsed, LEGACY_UUID.parse().unwrap());
+    }
+
     #[test]
     fn parse_compact_preserved() {
         let compact = compact_for_legacy();
diff --git a/src/log_cache.rs b/src/log_cache.rs
index 1d9e022..5587be5 100644
--- a/src/log_cache.rs
+++ b/src/log_cache.rs
@@ -197,6 +197,7 @@ pub fn fold_state_from_append_log(cache_dir: &Path) -> Result<RawState> {
     let mut safe_offset = byte_offset;
 
     loop {
+        let entry_offset = reader.stream_position()?;
         line.clear();
         let read = reader.read_line(&mut line)?;
         if read == 0 {
@@ -212,8 +213,14 @@ pub fn fold_state_from_append_log(cache_dir: &Path) -> Result<RawState> {
             safe_offset = reader.stream_position()?;
             continue;
         }
-        let item: WireItem = serde_json::from_str(stripped)
-            .with_context(|| format!("Corrupt log entry at {}", log_path.display()))?;
+        let item: WireItem = serde_json::from_str(stripped).map_err(|error| {
+            anyhow!(
+                "Corrupt log entry at {} byte {}: {}",
+                log_path.display(),
+                entry_offset,
+                error
+            )
+        })?;
         fold_item(item, &mut state);
         new_lines += 1;
         safe_offset = reader.stream_position()?;
@@ -301,6 +308,41 @@ mod tests {
         assert_eq!(offset, log.len() as u64);
     }
 
+    #[test]
+    fn fold_state_accepts_legacy_action_group_ids() {
+        let temp_dir = tempfile::tempdir().expect("tempdir");
+        let cache_dir = temp_dir.path();
+        let action_group_id = "ACTIONGROUP-11111111-2222-4333-8444-555555555555";
+        let task_id = "3C6BBD49-8D11-4FFF-8B0E-B8F33FA9C00A";
+        let log = format!(
+            r#"{{"{action_group_id}":{{"t":0,"e":"Task3","p":{{"tt":"Heading","ss":0,"tp":2,"st":1}}}},"{task_id}":{{"t":0,"e":"Task3","p":{{"tt":"Legacy child","ss":0,"tp":0,"st":1,"agr":["{action_group_id}"]}}}}}}"#
+        ) + "\n";
+        fs::write(cache_dir.join("things.log"), log).expect("seed legacy log");
+
+        let state = fold_state_from_append_log(cache_dir).expect("fold legacy action-group IDs");
+        let store = crate::store::ThingsStore::from_raw_state(&state);
+        let task = store.get_task(task_id).expect("legacy child task");
+
+        assert_eq!(
+            task.action_group,
+            Some(action_group_id.parse().expect("action-group ID"))
+        );
+    }
+
+    #[test]
+    fn fold_state_reports_the_offset_and_parse_error() {
+        let temp_dir = tempfile::tempdir().expect("tempdir");
+        let cache_dir = temp_dir.path();
+        fs::write(cache_dir.join("things.log"), "{not-json}\n").expect("seed corrupt log");
+
+        let error = fold_state_from_append_log(cache_dir)
+            .expect_err("corrupt log must fail")
+            .to_string();
+
+        assert!(error.contains("byte 0"));
+        assert!(error.contains("key must be a string"));
+    }
+
     #[test]
     fn fold_state_ignores_trailing_partial_line() {
         let temp_dir = tempfile::tempdir().expect("tempdir");