diff --git a/CLAUDE.md b/CLAUDE.md
index 50ec101..3d58eaf 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -66,6 +66,10 @@ See `docs/INSTALL.md` for end-user install instructions.
 collisions, what blocks a kill, what `--force` retains, and what reclaim
 touches. Read it before changing anything that creates or removes one.
 
+`docs/SIDEBAR.md` is the contract for session cards in the web and TUI
+sidebars — line layout, activity colours, duration format, state precedence.
+Read it before changing sidebar rendering.
+
 `nix run .#dev` (or `just dev`) brings up the whole local stack under
 process-compose: a debug daemon on 127.0.0.1:7071, then Vite on :5173 once the
 daemon answers `/api/info`. The SPA proxies `/api/*` back to the daemon, so only
diff --git a/docs/SIDEBAR.md b/docs/SIDEBAR.md
new file mode 100644
index 0000000..c3e5eb4
--- /dev/null
+++ b/docs/SIDEBAR.md
@@ -0,0 +1,92 @@
+# SIDEBAR.md — the session card contract
+
+One rule for how a session is rendered as a card in the sidebar, so the web
+sidebar (#107) and the TUI sidebar (#108) describe a session the same way.
+Read this before changing either. Both clients implement exactly what is
+written here; if it needs to change, change it here first.
+
+A card has **line 1** always and **line 2** only when line 2 has something to
+say.
+
+## Line 1 — name + actions
+
+- Left: the session name — `metadata.name` if present, else the command line
+  (same fallback both clients already use).
+- Right: two actions anchored to the top-right corner: **spawn similar**
+  (`⧉`) and **kill** (`✕`). In the TUI these are keyboard affordances shown on
+  the highlighted row; in the web they are buttons. The glyphs are shared so
+  muscle memory transfers.
+- Actions reserve their fixed width; the name truncates with `…` before it can
+  ever reach them. **Overlap between the name and the actions is a defect** —
+  that overlap is what started this milestone.
+
+Line 1 carries no badges. The `⚠ blocked work` badge (#83) and the `exited`
+badge move off this row into line 2.
+
+## Line 2 — meta line (adaptive)
+
+Rendered only when at least one of these holds:
+
+1. The session has been held in its current activity state longer than
+   **30 seconds** (`activity_since` from #102). A freshly-active session
+   stays one line.
+2. Its worktree holds blocked work: `blocked_work` is `Some` and not clear
+   (`InTheWay::is_clear()`), rendered in `formatInTheWay`'s phrasing on web
+   and as `⚠ <summary>` on the TUI.
+3. It has exited: `status == Dead`.
+4. An agent-reported state (#105) of `Error` is in force (see Precedence).
+
+Otherwise line 2 is absent — never an empty placeholder row.
+
+When present, line 2 reads left to right:
+
+```
+<duration> [<state badge>] [<blocked work> | exited]
+```
+
+## Colour per activity state
+
+The state word is never spelled out — the colour carries it.
+
+| Activity state   | Web token | ratatui colour    |
+| ---------------- | --------- | ----------------- |
+| `Active`         | `--live`  | `Color::Green`    |
+| `Idle`           | `--muted` | `Color::DarkGray` |
+| `AwaitingInput`  | `--warn`  | `Color::Yellow`   |
+| `Error`          | `--danger`| `Color::Red`      |
+
+Web tokens resolve per theme automatically (`styles.css` defines dark, light,
+and `prefers-color-scheme` variants for each). The TUI uses the plain ratatui
+named colours above; do not introduce RGB values or a new palette module for
+this.
+
+The duration text inherits its state's colour; the blocked-work and exited
+fragments use the muted colour (`--faint` / default style), since the ⚠ glyph
+already signals severity.
+
+## Duration format
+
+Time spent in the **current activity state**, sourced from `activity_since`
+(#102) — *not* time since spawn, and *not* time since last PTY output
+(`last_activity_at`). Both clients compute it identically:
+
+- `< 60s` → whole seconds, floored: `30s`
+- `< 60m` → whole minutes, floored: `4m`
+- `≥ 60m` → whole hours, floored: `2h`
+
+No days unit, no seconds inside minutes, no rounding up. Floor everywhere so
+the counter never shows a duration the state hasn't actually reached yet.
+Clock granularity: re-render at least once every 30s while visible so the
+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`.
+
+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)
+until the daemon sends a variant it does know. Rust clients deserialize
+activity leniently for the same reason.