diff --git a/AGENTS.md b/AGENTS.md
index 0f6f3b5..2f9dd13 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -208,6 +208,14 @@ directory's name unless `BOARD_TITLE` in `local/.env` overrides it. The
 server renders it into the page, so it is right on first paint; switching
 view swaps only the tail (`<project> · sessions`).
 
+While agents are running the title leads with how many — `2◌ · <project>
+· bench` — because a backgrounded tab is the only place that state can
+still be read, and the reasoning that put the project first applies
+harder to it: a tab narrowed to a few characters still shows the count.
+It is the same list the header's live chip counts, so the two can never
+disagree, it rides every view, and a board with nothing running has
+exactly the plain title above, byte for byte.
+
 All settings live in `manager/core/.env.example` with their defaults documented —
 the port, the binaries agents launch with, the commands agents may run,
 the worktrees directory, whether moves claim and commit themselves,
diff --git a/manager/core/board.html b/manager/core/board.html
index a97927f..f7b1a54 100644
--- a/manager/core/board.html
+++ b/manager/core/board.html
@@ -818,8 +818,14 @@ function scheduleRender() {
 
 /* ── header ───────────────────────────────────────────────────────────── */
 
+/* One reading of "an agent is working", shared by the header chip and the
+   tab title, so the two can never disagree about how many. */
+function runningAgents() {
+  return (S.state?.agents || []).filter(a => a.status === 'running');
+}
+
 function renderChip() {
-  const agents = (S.state?.agents || []).filter(a => a.status === 'running');
+  const agents = runningAgents();
   const liveYou = sessionsOf(m => isLiveSession(m) && !m.agentId);
   const el = $('#livechip');
   if (S.sseDown) {
@@ -867,12 +873,33 @@ function setView(view) {
 
 /* The tab says which bench this is: the project first (tab truncation eats
    the tail, and the tail is the same in every bench tab), then the view.
-   Without a project in state the server-rendered title stands. */
+   Without a project in state the server-rendered title stands.
+
+   Ahead of even the project, while and only while agents run, the count —
+   the one state you want most from a tab you are not looking at, and the
+   only place a backgrounded window can still say it. The same reasoning
+   that put the project first applies harder to it, so it leads: a tab
+   narrowed to a few characters still shows it. It is the in-flight glyph
+   the CI and copilot chips already use, not an emoji, and a quiet board
+   is the plain title, byte for byte. */
 const VIEW_TITLES = { board: 'bench', flight: 'sessions', focus: 'focus' };
+const WORKING_MARK = '◌';
+
+function tabTitle(project, view, working) {
+  return (working ? working + WORKING_MARK + ' · ' : '') +
+    project + ' · ' + (VIEW_TITLES[view] || 'bench');
+}
+
+/* render() runs on every SSE frame, so the title is written only when it
+   actually changed rather than dozens of times a second. */
+let shownTitle = null;
 
 function renderTitle() {
   if (!S.state?.project) return;
-  document.title = S.state.project + ' · ' + (VIEW_TITLES[S.view] || 'bench');
+  const title = tabTitle(S.state.project, S.view, runningAgents().length);
+  if (title === shownTitle) return;
+  shownTitle = title;
+  document.title = title;
 }
 
 function render() {
diff --git a/tests/test_board_title.py b/tests/test_board_title.py
index c274165..c56a1cc 100644
--- a/tests/test_board_title.py
+++ b/tests/test_board_title.py
@@ -127,13 +127,18 @@ class PageInvariants(unittest.TestCase):
         self.assertNotIn("task board", self.html.lower())
 
     def test_the_title_is_only_ever_written_from_the_project(self):
-        """One writer, and it starts with the project — so no code path can
-        put the view name first or revert to the generic string."""
+        """One writer, and the string it writes is built by tabTitle() from
+        the project — so no code path can revert to the generic string. That
+        the project leads everything but the running count (task 39) is
+        tested in test_tab_agents.py."""
         writes = re.findall(r"document\.title\s*=\s*([^\n;]+)", self.html)
         self.assertEqual(len(writes), 1,
                          f"expected one document.title assignment, got {writes}")
-        self.assertTrue(writes[0].startswith("S.state.project"),
-                        f"the project must lead the title, got {writes[0]!r}")
+        body = re.search(r"function renderTitle\(\)\s*\{(.*?)\n\}",
+                         self.html, re.DOTALL)
+        self.assertIsNotNone(body, "board.html lost renderTitle()")
+        self.assertIn("tabTitle(S.state.project", body.group(1),
+                      "the title must be built from the project")
 
     def test_a_stateless_page_keeps_the_served_title(self):
         """Before the first state load there is nothing better to say than
diff --git a/tests/test_tab_agents.py b/tests/test_tab_agents.py
new file mode 100644
index 0000000..8e965a3
--- /dev/null
+++ b/tests/test_tab_agents.py
@@ -0,0 +1,241 @@
+"""The tab title carries how many agents are working (task 39).
+
+A running agent was visible on the board and nowhere else, so the moment
+you switched tabs — the normal thing to do while an agent works for
+minutes — bench said the same string whether three agents ran or none.
+The count now leads the title, ahead of the project, because a truncated
+tab must still show it.
+
+Two halves, as elsewhere for board.html. The behaviour — what the string
+is for 0, 1 and N agents, which set is counted, and that document.title
+is written only when it changed — is exercised for real: the pieces are
+lifted out of the page and run in node, skipped where node is absent.
+The wiring (both readers of "running" going through one function) is a
+source-level invariant, board.html being a single file with inline JS and
+no frontend test runner.
+
+    python3 -m unittest discover -s tests -v
+"""
+
+from __future__ import annotations
+
+import json
+import re
+import shutil
+import subprocess
+import unittest
+from pathlib import Path
+
+BOARD = Path(__file__).resolve().parents[1] / "manager" / "core" / "board.html"
+
+# The pieces the title is made of, lifted from the page as written.
+PARTS = (
+    r"function runningAgents\(\) \{.*?\n\}",
+    r"const VIEW_TITLES = \{[^\n]*\};",
+    r"const WORKING_MARK = '[^']+';",
+    r"function tabTitle\(project, view, working\) \{.*?\n\}",
+    r"let shownTitle = null;",
+    r"function renderTitle\(\) \{.*?\n\}",
+)
+
+
+def _harness() -> str:
+    html = BOARD.read_text(encoding="utf-8")
+    out = []
+    for pattern in PARTS:
+        m = re.search(pattern, html, re.S)
+        if m is None:
+            raise AssertionError(f"board.html no longer defines {pattern!r}")
+        out.append(m.group(0))
+    return "\n".join(out)
+
+
+def _running(n: int) -> list:
+    """n agent records the board would call running, plus noise that is not:
+    a finished run and a failed one."""
+    agents = [{"status": "running", "started": 1000 + i} for i in range(n)]
+    return agents + [{"status": "done"}, {"status": "failed"}]
+
+
+class TitleBehaviour(unittest.TestCase):
+    """What the tab actually says, run as the browser would run it."""
+
+    @classmethod
+    def setUpClass(cls):
+        cls.node = shutil.which("node")
+        if not cls.node:
+            raise unittest.SkipTest("node not available — tab title unrun")
+        cls.src = _harness()
+
+    def titles(self, frames: list) -> list:
+        """document.title after each frame — a frame being the state and
+        view the page would render — with every write recorded, so a title
+        assigned twice with the same string shows up as two entries."""
+        script = (self.src + """
+var writes = [];
+var document = { set title(v) { writes.push(v); }, get title() {
+  return writes.length ? writes[writes.length - 1] : null; } };
+var S = {};
+for (const frame of """ + json.dumps(frames) + """) {
+  S = { state: frame.state, view: frame.view || 'board' };
+  renderTitle();
+}
+console.log(JSON.stringify(writes));
+""")
+        out = subprocess.run([self.node, "-e", script],
+                             capture_output=True, text=True)
+        self.assertEqual(out.returncode, 0, out.stderr)
+        return json.loads(out.stdout)
+
+    def title(self, state: dict, view: str = "board") -> str:
+        """The one title a single frame produces."""
+        writes = self.titles([{"state": state, "view": view}])
+        self.assertEqual(len(writes), 1, f"expected one write, got {writes}")
+        return writes[0]
+
+    def test_a_quiet_board_is_the_plain_title(self):
+        """Byte for byte what it was before this existed — no prefix, no
+        separator, no space. A board with nothing running looks untouched."""
+        self.assertEqual(self.title({"project": "bench", "agents": []}),
+                         "bench · bench")
+        self.assertEqual(
+            self.title({"project": "bench", "agents": _running(0)}),
+            "bench · bench")
+        self.assertEqual(self.title({"project": "bench"}), "bench · bench")
+
+    def test_the_count_leads_the_title(self):
+        """Ahead of the project, because a tab truncated to a few characters
+        must still answer the question this card exists for."""
+        title = self.title({"project": "bench", "agents": _running(3)})
+        self.assertTrue(title.startswith("3"),
+                        f"the count must lead the title, got {title!r}")
+        self.assertEqual(title, "3◌ · bench · bench")
+
+    def test_one_agent_reads_as_one(self):
+        """A count and a mark rather than a word, so "1 agents" is not a
+        string this code can produce."""
+        title = self.title({"project": "bench", "agents": _running(1)})
+        self.assertEqual(title, "1◌ · bench · bench")
+        self.assertNotIn("agents", title)
+
+    def test_only_running_agents_are_counted(self):
+        """The same filter the header chip applies — see the wiring test
+        below for why they cannot drift apart."""
+        state = {"project": "bench",
+                 "agents": [{"status": "done"}, {"status": "failed"},
+                            {"status": "running"}]}
+        self.assertEqual(self.title(state), "1◌ · bench · bench")
+
+    def test_the_project_still_names_which_bench_this_is(self):
+        """The thing the tab already did well survives the prefix."""
+        a = self.title({"project": "projectA", "agents": _running(2)})
+        b = self.title({"project": "projectB", "agents": _running(2)})
+        self.assertEqual(a, "2◌ · projectA · bench")
+        self.assertNotEqual(a, b)
+
+    def test_every_view_carries_the_count(self):
+        """It describes the board, not the view, so switching views changes
+        only the view word."""
+        state = {"project": "bench", "agents": _running(2)}
+        self.assertEqual(
+            [self.title(state, v) for v in ("board", "flight", "focus")],
+            ["2◌ · bench · bench", "2◌ · bench · sessions",
+             "2◌ · bench · focus"])
+
+    def test_a_finished_run_returns_the_tab_to_the_plain_title(self):
+        """No reload: the frame after the last agent stops is a plain
+        title again."""
+        busy = {"project": "bench", "agents": _running(1)}
+        quiet = {"project": "bench", "agents": _running(0)}
+        self.assertEqual(
+            self.titles([{"state": busy}, {"state": quiet}]),
+            ["1◌ · bench · bench", "bench · bench"])
+
+    def test_a_stateless_page_keeps_the_served_title(self):
+        """Before state arrives there is nothing better to say than what the
+        server rendered, so nothing is written at all."""
+        self.assertEqual(self.titles([{"state": None}, {"state": {}}]), [])
+
+    def test_the_title_is_written_only_when_it_changed(self):
+        """render() runs on every SSE frame; a burst with nothing moving
+        must not touch document.title once."""
+        busy = {"project": "bench", "agents": _running(2)}
+        frames = [{"state": busy} for _ in range(20)]
+        self.assertEqual(self.titles(frames), ["2◌ · bench · bench"])
+
+    def test_a_changed_count_is_written_again(self):
+        """The cache is a cache, not a latch: every distinct title lands,
+        including a return to one seen before."""
+        def frame(n, view="board"):
+            return {"state": {"project": "bench", "agents": _running(n)},
+                    "view": view}
+        self.assertEqual(
+            self.titles([frame(1), frame(1), frame(2), frame(2),
+                         frame(2, "focus"), frame(0), frame(1)]),
+            ["1◌ · bench · bench", "2◌ · bench · bench",
+             "2◌ · bench · focus", "bench · bench", "1◌ · bench · bench"])
+
+
+class Wiring(unittest.TestCase):
+    """board.html's own half: what keeps the tab and the header chip from
+    counting two different things, and the mark from becoming an emoji."""
+
+    @classmethod
+    def setUpClass(cls):
+        cls.html = BOARD.read_text(encoding="utf-8")
+
+    def test_one_filter_answers_how_many_are_running(self):
+        """Both counters go through runningAgents(); a second inline filter
+        is how the tab and the chip would start disagreeing. (Asking which
+        agent holds one card is a different question and stays where it
+        is.)"""
+        filters = re.findall(r"\.filter\(a => a\.status === 'running'\)",
+                             self.html)
+        self.assertEqual(len(filters), 1,
+                         "only runningAgents() may filter for running agents, "
+                         f"found {len(filters)} such filters")
+        body = re.search(r"function runningAgents\(\) \{(.*?)\n\}",
+                         self.html, re.DOTALL)
+        self.assertIsNotNone(body, "board.html lost runningAgents()")
+        self.assertIn("a.status === 'running'", body.group(1))
+
+    def test_the_header_chip_counts_that_same_list(self):
+        chip = re.search(r"function renderChip\(\) \{(.*?)\n\}",
+                         self.html, re.DOTALL)
+        self.assertIsNotNone(chip, "board.html lost renderChip()")
+        self.assertIn("runningAgents()", chip.group(1))
+
+    def test_the_tab_counts_that_same_list(self):
+        title = re.search(r"function renderTitle\(\) \{(.*?)\n\}",
+                          self.html, re.DOTALL)
+        self.assertIsNotNone(title, "board.html lost renderTitle()")
+        self.assertIn("runningAgents()", title.group(1))
+
+    def test_the_mark_is_the_boards_own_in_flight_glyph(self):
+        """Not an emoji: the same ◌ the CI and copilot chips wear while
+        something is in flight."""
+        mark = re.search(r"const WORKING_MARK = '([^']+)';", self.html)
+        self.assertIsNotNone(mark, "board.html lost WORKING_MARK")
+        self.assertEqual(mark.group(1), "◌")
+        self.assertGreater(len(re.findall("◌", self.html)), 1,
+                           "the mark should already be in use on the chips")
+
+    def test_the_prefix_is_the_only_thing_ahead_of_the_project(self):
+        """One writer, and everything it can put before the project name is
+        the count and the mark — no view name, no generic string."""
+        writes = re.findall(r"document\.title\s*=\s*([^\n;]+)", self.html)
+        self.assertEqual(len(writes), 1,
+                         f"expected one document.title assignment, got {writes}")
+        built = re.search(r"function tabTitle\(project, view, working\) \{"
+                          r"(.*?)\n\}", self.html, re.DOTALL)
+        self.assertIsNotNone(built, "board.html lost tabTitle()")
+        before, _, after = built.group(1).partition("project +")
+        self.assertTrue(after, "tabTitle must put the project in the title")
+        self.assertNotIn("VIEW_TITLES", before,
+                         "the view name may never precede the project")
+        self.assertIn("working", before,
+                      "only the running count precedes the project")
+
+
+if __name__ == "__main__":
+    unittest.main()