diff --git a/docs/SIDEBAR.md b/docs/SIDEBAR.md
index 1ac2442..cab0444 100644
--- a/docs/SIDEBAR.md
+++ b/docs/SIDEBAR.md
@@ -41,7 +41,7 @@ Otherwise line 2 is absent — never an empty placeholder row.
 When present, line 2 reads left to right:
 
 ```
-<dot> <duration> [<blocked work> | exited] [<command>]
+<dot> <duration> [<detail>] [<blocked work> | exited] [<command>]
 ```
 
 `<dot>` is a single filled glyph (`●` on both clients) carrying the activity
@@ -52,6 +52,11 @@ and duration are unconditional parts of line 2; the bracketed fragments appear
 only when their trigger holds. A payload that predates #102 (no usable
 `activity_since`) contributes no duration — the dot still renders.
 
+`<detail>` is an agent-reported string (#105) — e.g. why a session is in
+`Error`. It sits right after the duration, in the muted style, and shows only
+while an agent-reported state is in force (see Precedence): when the report
+lapses at its TTL or is replaced, the detail goes with it.
+
 `<command>` is optional and muted: the session's command line, shown only when
 the rendered name came from `metadata.name` and differs from the command (the
 web has always surfaced this; the TUI may omit it).
@@ -94,9 +99,10 @@ cutover points land within one tick.
 ## Precedence
 
 An agent-reported `ActivityState` (#105) overrides the daemon's inference for
-the TTL the report carries; when the TTL expires, inference resumes. Clients
-never blend the two: whichever source is in force supplies both the colour and
-the clock for `activity_since`.
+**five minutes** (the TTL is fixed by the daemon; re-reporting extends it).
+When the TTL expires, inference resumes and the session's reported detail —
+if any — clears with the report. Clients never blend the two: whichever source
+is in force supplies both the colour and the clock for `activity_since`.
 
 If a client receives an `ActivityState` variant it does not know, it must not
 fail rendering: treat it as `Idle` (neutral/dim, line-2 rules apply as usual)
diff --git a/web/src/types.ts b/web/src/types.ts
index c03aebd..3731779 100644
--- a/web/src/types.ts
+++ b/web/src/types.ts
@@ -26,6 +26,10 @@ export interface Session {
   foreground?: string | null;
   /** Inferred activity. Defaults to `Idle` on the wire when omitted. */
   activity?: ActivityState;
+  /** Short human string an agent reported alongside its state (#105) — e.g.
+   * why it is in `Error`. Shown on a sidebar card's meta line while the
+   * report is in force; cleared when the report lapses or is replaced. */
+  activity_detail?: string | null;
   /** Wall-clock of the most recent observed PTY output (NAP-340). */
   last_activity_at?: IsoTimestamp;
   /** Wall-clock of when the session entered its current `activity` state