swarm-ui: show how recent each subagent is in the details panel

The subagent list endpoint now returns {name, last_row_unix} per
subagent, the publish time of its newest terminal row, read from the
queue controller-side so agents publish nothing new. The panel sorts
newest first, shows a ticking age beside each name, and dims one quiet
for 15 minutes.

Closes #4928

docs/swarm/ui.md

@@ -66,7 +66,8 @@ The agent's subagent daemon publishes each subagent's terminal rows on
 into a stream named `term-sub-<agent>` that swarm-controller creates for every
 declared agent. The stream keeps rows for 24 hours. swarm-controller serves the list as
 `GET /api/agents/<name>/subagents`, the subagents named by the subjects in that
-stream, and relays one subagent's rows as SSE on
+stream, each with `last_row_unix`, the stream's publish time of its newest row,
+and relays one subagent's rows as SSE on
 `GET /api/agents/<name>/subagents/<subagent>/term/stream`. An agent with no
 store identity, or no queue address, publishes no subagent terminals.
 

frontend/packages/swarm-ui/src/pages/agents/SubagentTerms.css

@@ -11,6 +11,21 @@
   gap: 0.5em;
 }
 
+.ui-subagent-terms-item {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.35em;
+}
+
+.ui-subagent-terms-item-stale {
+  opacity: 0.6;
+}
+
+.ui-subagent-terms-age {
+  color: var(--muted);
+  font-size: 0.85em;
+}
+
 .ui-subagent-terms-heading {
   color: var(--muted);
 }

frontend/packages/swarm-ui/src/pages/agents/SubagentTerms.tsx

@@ -1,9 +1,11 @@
 // <SubagentTerms> — an agent's subagents, under its own terminal preview in
 // `AgentsPage`'s detail panel. The list is `GET /api/agents/{name}/subagents`:
-// the subagents the swarm queue holds terminal rows for. Subagents are
-// spawned on demand, so it is refetched every `REFRESH_MS`. Picking one
-// shows its terminal in `AgentTermPreview`, expand-to-tab included. Read-only
-// like the agent's own: subagents take no input.
+// the subagents the swarm queue holds terminal rows for, each with the
+// publish time of its newest row. Newest first, with a ticking age beside
+// the name; one quiet for `STALE_S` is dimmed. Subagents are spawned on
+// demand, so it is refetched every `REFRESH_MS`. Picking one shows its
+// terminal in `AgentTermPreview`, expand-to-tab included. Read-only like the
+// agent's own: subagents take no input.
 //
 // Mount it keyed by agent name: the refresh timer is armed once per mount.
 import { useState } from "preact/hooks";
@@ -12,12 +14,27 @@ import { readApiError, type ProblemDetails } from "@hive/shared/api-error.js";
 import { Badge } from "@hive/shared/badge.js";
 import { AgentTermPreview } from "./AgentTermPreview.js";
 import { useRefreshInterval } from "../../ui/refresh-interval/RefreshInterval.js";
+import { RelativeTime } from "../../ui/relative-time/RelativeTime.js";
 import "./SubagentTerms.css";
 
 const REFRESH_MS = 10_000;
+const STALE_S = 15 * 60;
+
+interface Subagent {
+  name: string;
+  last_row_unix: number | null;
+}
+
+// Newest row first; a subagent whose time could not be read sorts last.
+function byRecency(a: Subagent, b: Subagent): number {
+  const ta = a.last_row_unix ?? -Infinity;
+  const tb = b.last_row_unix ?? -Infinity;
+  if (ta !== tb) return tb > ta ? 1 : -1;
+  return a.name.localeCompare(b.name);
+}
 
 export function SubagentTerms({ agentName }: { agentName: string }) {
-  const [names, setNames] = useState<string[] | null>(null);
+  const [subagents, setSubagents] = useState<Subagent[] | null>(null);
   const [error, setError] = useState<ProblemDetails | null>(null);
   const [selected, setSelected] = useState<string | null>(null);
 
@@ -29,7 +46,7 @@ export function SubagentTerms({ agentName }: { agentName: string }) {
       setError(await readApiError(res));
       return;
     }
-    setNames((await res.json()) as string[]);
+    setSubagents(((await res.json()) as Subagent[]).sort(byRecency));
     setError(null);
   }
 
@@ -40,26 +57,41 @@ export function SubagentTerms({ agentName }: { agentName: string }) {
   // An agent with no subagents shows nothing here — no heading, no
   // "none" line — and so does the first fetch while it is in flight. A
   // failed fetch still shows, so the error is not swallowed.
-  if (!error && (names === null || names.length === 0)) return null;
+  if (!error && (subagents === null || subagents.length === 0)) return null;
+
+  const nowS = Date.now() / 1000;
 
   return (
     <section class="ui-subagent-terms" aria-label={`${agentName}'s subagents`}>
       <div class="ui-subagent-terms-list">
         <span class="ui-subagent-terms-heading">subagents</span>
-        {names?.map((name) => (
-          <Badge
-            key={name}
-            variant={name === selected ? "default" : "quiet"}
-            value={name}
-            expanded={name === selected}
-            title={
-              name === selected
-                ? `hide ${name}'s terminal`
-                : `show ${name}'s terminal`
-            }
-            onClick={() => setSelected(name === selected ? null : name)}
-          />
-        ))}
+        {subagents?.map(({ name, last_row_unix }) => {
+          const stale =
+            last_row_unix !== null && nowS - last_row_unix > STALE_S;
+          return (
+            <span
+              key={name}
+              class={`ui-subagent-terms-item${stale ? " ui-subagent-terms-item-stale" : ""}`}
+            >
+              <Badge
+                variant={name === selected ? "default" : "quiet"}
+                value={name}
+                expanded={name === selected}
+                title={
+                  name === selected
+                    ? `hide ${name}'s terminal`
+                    : `show ${name}'s terminal`
+                }
+                onClick={() => setSelected(name === selected ? null : name)}
+              />
+              {last_row_unix !== null && (
+                <span class="ui-subagent-terms-age">
+                  <RelativeTime epochMs={last_row_unix * 1000} />
+                </span>
+              )}
+            </span>
+          );
+        })}
       </div>
       {error && (
         <ApiErrorPanel

swarm-controller/src/subagent_term.rs

@@ -8,6 +8,8 @@
 //! `swarm_queue_client::subagent_term` spells. Subagents are spawned on demand
 //! under caller-chosen names, so the list is the set of subjects that stream
 //! holds rows for, read with `STREAM.INFO`. A missing stream lists nothing.
+//! Each subagent carries the stream's own publish time of its newest row, so
+//! a reader can tell a subagent that is running from one that has gone quiet.
 //!
 //! **No hive lookup.** Unlike [`crate::term_stream`], there is no
 //! hive-scoped subject to follow: only the agent's own credential is granted
@@ -28,7 +30,9 @@ use axum::extract::{Path, State};
 use axum::http::StatusCode;
 use axum::response::sse::{Event, KeepAlive, Sse};
 use futures_util::{Stream, StreamExt as _, TryStreamExt as _};
+use serde::Serialize;
 use swarm_queue_client::subagent_term;
+use utoipa::ToSchema;
 
 use crate::AppState;
 use crate::wanted::WantedWriter;
@@ -84,14 +88,25 @@ async fn live_agents(wanted: &WantedWriter, hives: &[String]) -> BTreeSet<String
     crate::agent_renewal::live_agents(&declarations)
 }
 
+/// One subagent of an agent, as the list endpoint reports it.
+#[derive(Debug, PartialEq, Eq, Serialize, ToSchema)]
+pub(crate) struct SubagentEntry {
+    /// The subagent's name, as it appears in its stream subject.
+    pub name: String,
+    /// When its newest row in the stream was published, unix seconds, by the
+    /// stream's own clock. `None` when that row could not be read.
+    pub last_row_unix: Option<i64>,
+}
+
 #[utoipa::path(
     get,
     path = "/api/agents/{name}/subagents",
     params(("name" = String, Path, description = "agent whose subagents to list")),
     responses(
-        (status = 200, description = "names of the agent's subagents with terminal rows \
-            in the swarm queue, sorted; empty when the agent has published none",
-            body = Vec<String>),
+        (status = 200, description = "the agent's subagents with terminal rows in the \
+            swarm queue, sorted by name, each with the publish time of its newest row; \
+            empty when the agent has published none",
+            body = Vec<SubagentEntry>),
         (status = 400, description = "the agent name is not shaped like an identifier \
             (problem+json)", body = String),
         (status = 503, description = "no swarm queue is configured on this host, or the \
@@ -104,7 +119,7 @@ async fn live_agents(wanted: &WantedWriter, hives: &[String]) -> BTreeSet<String
 pub(crate) async fn list_subagents(
     State(state): State<AppState>,
     Path(agent): Path<String>,
-) -> Result<Json<Vec<String>>, problem_details::ProblemDetails> {
+) -> Result<Json<Vec<SubagentEntry>>, problem_details::ProblemDetails> {
     let agent = ident(&agent).map_err(bad_request)?;
     let client = connected_client(&state).map_err(|e| unavailable(&e))?;
     let js = async_nats::jetstream::new(client);
@@ -121,10 +136,41 @@ pub(crate) async fn list_subagents(
         .try_collect()
         .await
         .map_err(|e| read_failed(&agent, &e))?;
-    Ok(Json(subagent_term::subagent_names(
-        &agent,
-        subjects.iter().map(|(s, _)| s.as_str()),
-    )))
+    let names = subagent_term::subagent_names(&agent, subjects.iter().map(|(s, _)| s.as_str()));
+    let mut entries = Vec::with_capacity(names.len());
+    for name in names {
+        let last_row_unix = last_row_unix(&stream, &agent, &name).await;
+        entries.push(SubagentEntry {
+            name,
+            last_row_unix,
+        });
+    }
+    Ok(Json(entries))
+}
+
+/// The publish time of a subagent's newest row. A row that cannot be read,
+/// aged out between the listing and this lookup say, is a missing time and
+/// not a failed list.
+async fn last_row_unix(
+    stream: &async_nats::jetstream::stream::Stream,
+    agent: &str,
+    subagent: &str,
+) -> Option<i64> {
+    match stream
+        .get_last_raw_message_by_subject(&subagent_term::subject(agent, subagent))
+        .await
+    {
+        Ok(row) => Some(row.time.unix_timestamp()),
+        Err(e) => {
+            tracing::debug!(
+                %agent,
+                %subagent,
+                error = %swarm_queue_client::chain(&e),
+                "listing subagents: reading the newest row failed"
+            );
+            None
+        }
+    }
 }
 
 #[utoipa::path(