diff --git a/ui/src/tbd/ui/db.cljs b/ui/src/tbd/ui/db.cljs
index 1cc37cb..62f55c4 100644
--- a/ui/src/tbd/ui/db.cljs
+++ b/ui/src/tbd/ui/db.cljs
@@ -1,4 +1,5 @@
-(ns tbd.ui.db)
+(ns tbd.ui.db
+  (:require [tbd.ui.tictactoe :as tictactoe]))
 
 (def default-db
   {:calc {:x "" :y ""
@@ -20,4 +21,7 @@
            :error nil}
    ;; The "Add this MCP server to Claude Code" section: :copied? is true right after the
    ;; command was copied to the clipboard.
-   :mcp {:copied? false}})
+   :mcp {:copied? false}
+   ;; Tic-tac-toe, entirely client side: the board (see tbd.ui.tictactoe) and whose turn it is.
+   :ttt {:board tictactoe/new-board
+         :turn :x}})
diff --git a/ui/src/tbd/ui/events.cljs b/ui/src/tbd/ui/events.cljs
index ce5b94f..3d618af 100644
--- a/ui/src/tbd/ui/events.cljs
+++ b/ui/src/tbd/ui/events.cljs
@@ -2,7 +2,8 @@
   (:require [cljs.reader :as reader]
             [clojure.string :as str]
             [re-frame.core :as rf]
-            [tbd.ui.db :as db]))
+            [tbd.ui.db :as db]
+            [tbd.ui.tictactoe :as tictactoe]))
 
 ;; Effect: request `uri` (GET unless `method` is given), then dispatch
 ;; `(conj on-response {:ok? bool :status int-or-nil :body string})`. A `body` is sent as EDN.
@@ -188,3 +189,19 @@
   ::mcp-copied-timeout
   (fn [db _]
     (assoc-in db [:mcp :copied?] false)))
+
+;; --- Tic-tac-toe ---
+
+;; An illegal move (taken cell, game over) changes nothing.
+(rf/reg-event-db
+  ::ttt-move
+  (fn [db [_ i]]
+    (let [{:keys [board turn]} (:ttt db)]
+      (if-let [board' (tictactoe/move board turn i)]
+        (assoc db :ttt {:board board' :turn (tictactoe/next-player turn)})
+        db))))
+
+(rf/reg-event-db
+  ::ttt-reset
+  (fn [db _]
+    (assoc db :ttt (:ttt db/default-db))))
diff --git a/ui/src/tbd/ui/subs.cljs b/ui/src/tbd/ui/subs.cljs
index 6dd1d92..66b338d 100644
--- a/ui/src/tbd/ui/subs.cljs
+++ b/ui/src/tbd/ui/subs.cljs
@@ -1,8 +1,21 @@
 (ns tbd.ui.subs
-  (:require [re-frame.core :as rf]))
+  (:require [re-frame.core :as rf]
+            [tbd.ui.tictactoe :as tictactoe]))
 
 (rf/reg-sub ::operand (fn [db [_ k]] (get-in db [:calc k])))
 (rf/reg-sub ::result (fn [db _] (get-in db [:calc :result])))
 (rf/reg-sub ::user (fn [db _] (:user db)))
 (rf/reg-sub ::todos (fn [db _] (:todos db)))
 (rf/reg-sub ::mcp-copied? (fn [db _] (get-in db [:mcp :copied?])))
+
+(rf/reg-sub ::ttt (fn [db _] (:ttt db)))
+
+;; What the board view needs: the cells, whose turn, and how the game stands.
+;; :outcome is nil while the game is on, or {:player :x-or-:o :line [..]} for a win, or :draw.
+(rf/reg-sub
+  ::ttt-game
+  :<- [::ttt]
+  (fn [{:keys [board turn]} _]
+    {:board board
+     :turn turn
+     :outcome (tictactoe/outcome board)}))
diff --git a/ui/src/tbd/ui/tictactoe.cljs b/ui/src/tbd/ui/tictactoe.cljs
new file mode 100644
index 0000000..792822e
--- /dev/null
+++ b/ui/src/tbd/ui/tictactoe.cljs
@@ -0,0 +1,39 @@
+(ns tbd.ui.tictactoe
+  "Tic-tac-toe rules as plain functions of the board, called by the re-frame events and subs
+  (tbd.ui.events, tbd.ui.subs). The board is a vector of 9 cells, row by row, each nil, :x or :o.")
+
+(def new-board (vec (repeat 9 nil)))
+
+(def ^:private lines
+  [[0 1 2] [3 4 5] [6 7 8]
+   [0 3 6] [1 4 7] [2 5 8]
+   [0 4 8] [2 4 6]])
+
+(defn winner
+  "{:player :x-or-:o :line [i j k]} when someone has three in a row, else nil."
+  [board]
+  (some (fn [[a b c :as line]]
+          (let [player (board a)]
+            (when (and player (= player (board b) (board c)))
+              {:player player :line line})))
+        lines))
+
+(defn draw?
+  "The board is full and nobody won."
+  [board]
+  (and (every? some? board) (nil? (winner board))))
+
+(defn outcome
+  "How the game stands: nil while it's on, :draw, or the winner (see `winner`)."
+  [board]
+  (or (winner board) (when (draw? board) :draw)))
+
+(defn next-player [player]
+  (if (= :x player) :o :x))
+
+(defn move
+  "`player` takes cell `i`. Returns the new board, or nil when the move isn't allowed (the cell
+  is taken or off the board, or the game is already over)."
+  [board player i]
+  (when (and (contains? board i) (nil? (board i)) (nil? (winner board)))
+    (assoc board i player)))
diff --git a/ui/src/tbd/ui/views.cljs b/ui/src/tbd/ui/views.cljs
index 22133ec..60bff8c 100644
--- a/ui/src/tbd/ui/views.cljs
+++ b/ui/src/tbd/ui/views.cljs
@@ -1,5 +1,6 @@
 (ns tbd.ui.views
-  (:require [re-frame.core :as rf]
+  (:require [clojure.string :as str]
+            [re-frame.core :as rf]
             [tbd.ui.events :as events]
             [tbd.ui.subs :as subs]
             [tbd.ui.webmcp :as webmcp]))
@@ -69,6 +70,35 @@
      (when error [:p {:role "alert" :style {:color "crimson"}} error])
      [todo-list state]]))
 
+(defn- ttt-status [{:keys [turn outcome]}]
+  (cond
+    (= :draw outcome) "It's a draw."
+    outcome (str (str/upper-case (name (:player outcome))) " wins!")
+    :else (str (str/upper-case (name turn)) "'s turn")))
+
+(defn- ttt-cell [i mark winning? over?]
+  [:button {:type "button"
+            :aria-label (str "Cell " (inc i) (when mark (str ", " (str/upper-case (name mark)))))
+            :disabled (or over? (some? mark))
+            :style {:aspect-ratio "1" :font-size "2.5rem" :font-weight "bold" :cursor "pointer"
+                    :background (when winning? "gold") :color (when winning? "black")}
+            :on-click #(rf/dispatch [::events/ttt-move i])}
+   (some-> mark name str/upper-case)])
+
+(defn tic-tac-toe
+  "Two players take turns on one board. Nothing goes to the server."
+  []
+  (let [{:keys [board outcome] :as game} @(rf/subscribe [::subs/ttt-game])
+        winning (set (:line outcome))]
+    [:section
+     [:h2 "Tic-tac-toe"]
+     [:p {:role "status" :aria-live "polite"} (ttt-status game)]
+     [:div {:style {:display "grid" :grid-template-columns "repeat(3, 4.5rem)" :gap "0.25rem"
+                    :width "fit-content"}}
+      (for [[i mark] (map-indexed vector board)]
+        ^{:key i} [ttt-cell i mark (contains? winning i) (some? outcome)])]
+     [:p [:button {:type "button" :on-click #(rf/dispatch [::events/ttt-reset])} "New game"]]]))
+
 (defn- mcp-command
   "The command that adds this server to Claude Code, for the address the page was loaded from
   (which is what BASE_URL has to be for the MCP login to work)."
@@ -110,7 +140,7 @@
   client to them. The token is pasted into the WebMCP widget in the corner of the page."
   []
   [collapsible "Browser tools (WebMCP)"
-   [:p "This page offers " [:strong (count webmcp/tools)] " tools of its own (the calculator and the todos) to "
+   [:p "This page offers " [:strong (count webmcp/tools)] " tools of its own (the calculator, the todos and the tic-tac-toe game) to "
     "your MCP client, over " [:a {:href "https://webmcp.dev" :target "_blank" :rel "noreferrer"} "WebMCP"]
     ", and you see them work on the page. To connect:"]
    [:ol
@@ -145,6 +175,7 @@
      [calculator]
      (when (= :logged-in (:status user))
        [todos])
+     [tic-tac-toe]
      [mcp-setup]
      [browser-tools]
      [:p [:a {:href "/api-docs"} "API docs"] " · " [:a {:href "/swagger.json"} "swagger.json"]
diff --git a/ui/src/tbd/ui/webmcp.cljs b/ui/src/tbd/ui/webmcp.cljs
index 1c0f738..0be9aa1 100644
--- a/ui/src/tbd/ui/webmcp.cljs
+++ b/ui/src/tbd/ui/webmcp.cljs
@@ -13,7 +13,8 @@
   (:require ["@jason.today/webmcp/src/webmcp.js" :as WebMCP]
             [re-frame.core :as rf]
             [re-frame.db :as rf-db]
-            [tbd.ui.events :as events]))
+            [tbd.ui.events :as events]
+            [tbd.ui.tictactoe :as tictactoe]))
 
 ;; --- Reading and waiting for the app-db ---
 
@@ -174,6 +175,48 @@
                              "the todo to go")))
       (.then (fn [_] "deleted"))))
 
+;; Tic-tac-toe is moves taken in turns: X, then O, on the same board. The agent can move for
+;; either side, so every result says whose turn it is.
+
+(defn- tic-tac-toe-state
+  "The game as the page shows it: the 9 cells (row by row, cell 0 is the top left; null when
+  empty, else \"x\" or \"o\"), `turn` (whose move it is, while the game is on), and `status`
+  (in_progress, won or draw; with `winner` and `line` when won)."
+  []
+  (let [{:keys [board turn]} (:ttt (db))
+        {:keys [player line] :as outcome} (tictactoe/outcome board)]
+    (cond-> {:board board}
+      (nil? outcome) (assoc :status "in_progress" :turn turn)
+      (= :draw outcome) (assoc :status "draw")
+      player (assoc :status "won" :winner player :line line))))
+
+(defn- get-tic-tac-toe [_]
+  (tic-tac-toe-state))
+
+(defn- move-tic-tac-toe
+  "Presses cell `cell`, which moves for whoever's turn it is. With `player`, only if it is that
+  player's turn: a guard against moving for the wrong side."
+  [{:keys [cell player]}]
+  (when-not (and (number? cell) (js/Number.isInteger cell) (<= 0 cell 8))
+    (fail! "invalid arguments: cell must be an integer from 0 to 8"))
+  (when-not (contains? #{nil "x" "o"} player)
+    (fail! "invalid arguments: player must be \"x\" or \"o\""))
+  (let [{:keys [board turn]} (:ttt (db))]
+    (when (tictactoe/outcome board)
+      (fail! "the game is over: start a new game"))
+    (when (and player (not= player (name turn)))
+      (fail! (str "it is " (name turn) "'s turn, not " player "'s")))
+    (when (board cell)
+      (fail! (str "cell " cell " is taken")))
+    (rf/dispatch-sync [::events/ttt-move cell])
+    (tic-tac-toe-state)))
+
+(defn- new-tic-tac-toe-game
+  "Presses New game."
+  [_]
+  (rf/dispatch-sync [::events/ttt-reset])
+  (tic-tac-toe-state))
+
 (defn- object-schema [properties required]
   {:type "object" :properties properties :required required})
 
@@ -226,7 +269,31 @@
    {:name "delete_todo"
     :description "Presses a todo's Delete button"
     :input-schema (object-schema {:id todo-id} ["id"])
-    :handler delete-todo}])
+    :handler delete-todo}
+
+   {:name "get_tic_tac_toe"
+    :description (str "The tic-tac-toe game on this page (JSON): board (9 cells, row by row, cell 0 the top left "
+                      "and cell 8 the bottom right; null when empty, else \"x\" or \"o\"), turn (whose move it "
+                      "is, \"x\" or \"o\": X goes first, and they alternate) and status (in_progress, won or draw; "
+                      "when won, also winner and line). A person moves on the same board, so check the turn "
+                      "before you move")
+    :input-schema (object-schema {} []) :handler get-tic-tac-toe}
+
+   {:name "move_tic_tac_toe"
+    :description (str "Presses a cell of the tic-tac-toe board, which moves for whoever's turn it is, and returns "
+                      "the game as get_tic_tac_toe does (including whose turn is next). Fails if the cell is "
+                      "taken or the game is over")
+    :input-schema (object-schema {:cell {:type "integer" :minimum 0 :maximum 8
+                                         :description "0 is the top left, 2 the top right, 8 the bottom right"}
+                                  :player {:type "string" :enum ["x" "o"]
+                                           :description (str "Optional: the side you are moving for. The move is "
+                                                             "refused if it is the other side's turn")}}
+                                 ["cell"])
+    :handler move-tic-tac-toe}
+
+   {:name "new_tic_tac_toe_game"
+    :description "Presses New game: clears the tic-tac-toe board, X to move (JSON, as get_tic_tac_toe)"
+    :input-schema (object-schema {} []) :handler new-tic-tac-toe-game}])
 
 ;; --- Registering them with WebMCP ---
 
diff --git a/ui/test/tbd/ui/tictactoe_test.cljs b/ui/test/tbd/ui/tictactoe_test.cljs
new file mode 100644
index 0000000..e76ab2b
--- /dev/null
+++ b/ui/test/tbd/ui/tictactoe_test.cljs
@@ -0,0 +1,72 @@
+(ns tbd.ui.tictactoe-test
+  (:require [cljs.test :refer [deftest is testing]]
+            [re-frame.core :as rf]
+            [re-frame.db :as rf-db]
+            [tbd.ui.events :as events]
+            [tbd.ui.tictactoe :as ttt]))
+
+(defn- board
+  "A board from a string of 9 characters: x, o or anything else for an empty cell."
+  [s]
+  (mapv {\x :x \o :o} s))
+
+(deftest winner-test
+  (testing "every row, column and diagonal"
+    (doseq [line [[0 1 2] [3 4 5] [6 7 8] [0 3 6] [1 4 7] [2 5 8] [0 4 8] [2 4 6]]
+            player [:x :o]]
+      (let [b (reduce #(assoc %1 %2 player) ttt/new-board line)]
+        (is (= {:player player :line line} (ttt/winner b))))))
+  (testing "no winner"
+    (is (nil? (ttt/winner ttt/new-board)))
+    (is (nil? (ttt/winner (board "xo.ox.xo."))))
+    (is (nil? (ttt/winner (board "xxo.oxxox"))))))
+
+(deftest draw-test
+  (is (ttt/draw? (board "xoxxoooxx")))
+  (is (not (ttt/draw? (board "xxxooxoxo"))) "a full board with a winner is not a draw")
+  (is (not (ttt/draw? (board "xoxxooox.")))))
+
+(deftest outcome-test
+  (is (nil? (ttt/outcome ttt/new-board)))
+  (is (= :draw (ttt/outcome (board "xoxxoooxx"))))
+  (is (= {:player :o :line [0 4 8]} (ttt/outcome (board "oxxxoxxxo")))))
+
+(deftest move-test
+  (is (= :x (get (ttt/move ttt/new-board :x 4) 4)))
+  (is (nil? (ttt/move (board "x........") :o 0)) "taken cell")
+  (is (nil? (ttt/move ttt/new-board :x 9)) "off the board")
+  (is (nil? (ttt/move ttt/new-board :x -1)) "off the board")
+  (is (nil? (ttt/move (board "xxxoo....") :o 8)) "game already won"))
+
+(defn- move! [& cells]
+  (doseq [i cells] (rf/dispatch-sync [::events/ttt-move i])))
+
+(deftest game-flow-test
+  (rf/dispatch-sync [::events/initialize])
+  (let [game #(let [{:keys [board turn]} (:ttt @rf-db/app-db)]
+                {:board board :turn turn :outcome (ttt/outcome board)})]
+    (testing "a new game"
+      (is (= :x (:turn (game))))
+      (is (nil? (:outcome (game)))))
+    (testing "players alternate"
+      (move! 0)
+      (is (= :o (:turn (game))))
+      (move! 3)
+      (is (= [:x nil nil :o] (subvec (:board (game)) 0 4))))
+    (testing "a taken cell is ignored and doesn't pass the turn"
+      (move! 0)
+      (is (= :x (:turn (game))))
+      (is (= :o (get (:board (game)) 3))))
+    (testing "X wins on the top row; the game then ignores moves"
+      (move! 1 4 2)
+      (is (= {:player :x :line [0 1 2]} (:outcome (game))))
+      (let [before (:board (game))]
+        (move! 8)
+        (is (= before (:board (game))))))
+    (testing "new game"
+      (rf/dispatch-sync [::events/ttt-reset])
+      (is (= ttt/new-board (:board (game))))
+      (is (= :x (:turn (game)))))
+    (testing "a draw"
+      (move! 0 1 2 4 3 5 7 6 8)
+      (is (= :draw (:outcome (game)))))))
diff --git a/ui/test/tbd/ui/webmcp_test.cljs b/ui/test/tbd/ui/webmcp_test.cljs
index 4ab9bc4..987e1e4 100644
--- a/ui/test/tbd/ui/webmcp_test.cljs
+++ b/ui/test/tbd/ui/webmcp_test.cljs
@@ -228,3 +228,52 @@
                  (is (re-find #"Invalid body" (:text r)))
                  (is (empty? (get-in @rf-db/app-db [:todos :items])))
                  (done))))))
+
+(deftest tic-tac-toe
+  (open-page!)
+  (async done
+    (-> (call "get_tic_tac_toe" {})
+        (.then (fn [r]
+                 (is (false? (:error? r)))
+                 (is (= {:board (vec (repeat 9 nil)) :status "in_progress" :turn "x"} (json (:text r))))
+                 (call "move_tic_tac_toe" {:cell 4})))
+        (.then (fn [r]
+                 ;; It pressed the cell, as a user would, and says it is O's turn now.
+                 (is (= :x (get-in @rf-db/app-db [:ttt :board 4])))
+                 (is (= {:board [nil nil nil nil "x" nil nil nil nil] :status "in_progress" :turn "o"}
+                        (json (:text r))))
+                 (call "move_tic_tac_toe" {:cell 4})))
+        (.then (fn [r]
+                 (is (= {:text "cell 4 is taken" :error? true} r))
+                 (call "move_tic_tac_toe" {:cell 0 :player "x"})))
+        (.then (fn [r]
+                 (is (= {:text "it is o's turn, not x's" :error? true} r))
+                 (is (nil? (get-in @rf-db/app-db [:ttt :board 0])) "nothing was moved")
+                 (call "move_tic_tac_toe" {:cell 9})))
+        (.then (fn [r]
+                 (is (:error? r))
+                 (is (re-find #"0 to 8" (:text r)))
+                 (call "move_tic_tac_toe" {:cell 1.5})))
+        (.then (fn [r]
+                 (is (:error? r))
+                 (call "move_tic_tac_toe" {:cell 0 :player "z"})))
+        (.then (fn [r]
+                 (is (:error? r))
+                 (call "move_tic_tac_toe" {:cell 0 :player "o"})))
+        (.then (fn [r]
+                 (is (= "x" (:turn (json (:text r)))))
+                 ;; The person moves X to 3 and O to 1; the agent's X then completes the middle row.
+                 (rf/dispatch-sync [::events/ttt-move 3])
+                 (rf/dispatch-sync [::events/ttt-move 1])
+                 (call "move_tic_tac_toe" {:cell 5 :player "x"})))
+        (.then (fn [r]
+                 (is (= {:board ["o" "o" nil "x" "x" "x" nil nil nil]
+                         :status "won" :winner "x" :line [3 4 5]}
+                        (json (:text r))))
+                 (call "move_tic_tac_toe" {:cell 8})))
+        (.then (fn [r]
+                 (is (= {:text "the game is over: start a new game" :error? true} r))
+                 (call "new_tic_tac_toe_game" {})))
+        (.then (fn [r]
+                 (is (= {:board (vec (repeat 9 nil)) :status "in_progress" :turn "x"} (json (:text r))))
+                 (done))))))