diff --git a/docs/web-ui/agent.md b/docs/web-ui/agent.md
index 3d93cfef..1b521206 100644
--- a/docs/web-ui/agent.md
+++ b/docs/web-ui/agent.md
@@ -393,7 +393,15 @@ hit-rate** (`cache_read` over all input-side tokens) and
 **tokens/turn**. When `reminder_stats` is present (fetched via
 `ReminderRollup` RPC and merged into the snapshot in
 `web_ui.rs::api_stats`) three more chips appear: **reminders
-scheduled / delivered / pending** for the window.
+scheduled / delivered / pending** for the window. When the
+per-session capture has data, a **first-turn ctx** chip shows the
+input tokens of the most recent fresh claude session's first turn —
+a proxy for system-prompt + CLAUDE.md sprawl (a fresh session's first
+turn pays the full static prefix uncached). It's derived in `stats.rs`
+(`first_turn_ctx`: the first turn, `ORDER BY started_at LIMIT 1`, of
+the latest `sessions` row in the window) and is omitted from the JSON
+until the `sessions` / `turn_stats.session_id` capture has rows — so
+the chip stays hidden on older dbs (inert-until-capture).
 `stats.rs` opens the sqlite db read-only and degrades to an
 empty snapshot on any error — the page is decorative, never
 authoritative.