diff --git a/web/harness-reactive.sx b/web/harness-reactive.sx
new file mode 100644
index 00000000..0a934912
--- /dev/null
+++ b/web/harness-reactive.sx
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+;; ==========================================================================
+;; web/harness-reactive.sx — Signal and reactive testing extensions
+;;
+;; Extends spec/harness.sx with assertions for the reactive signal system.
+;; Depends on spec/signals.sx (core reactive primitives).
+;; No DOM dependency — works on any host.
+;; ==========================================================================
+
+
+;; --------------------------------------------------------------------------
+;; Signal assertions
+;; --------------------------------------------------------------------------
+
+;; Assert a signal has a specific value
+(define assert-signal-value :effects []
+  (fn ((sig :as any) expected)
+    (let ((actual (signal-value sig)))
+      (assert= actual expected
+        (str "Expected signal value " expected ", got " actual)))))
+
+;; Assert a signal has subscribers (i.e., something is watching it)
+(define assert-signal-has-subscribers :effects []
+  (fn ((sig :as any))
+    (assert (> (len (signal-subscribers sig)) 0)
+      "Expected signal to have subscribers")))
+
+;; Assert a signal has no subscribers
+(define assert-signal-no-subscribers :effects []
+  (fn ((sig :as any))
+    (assert (= (len (signal-subscribers sig)) 0)
+      "Expected signal to have no subscribers")))
+
+;; Assert a signal has exactly N subscribers
+(define assert-signal-subscriber-count :effects []
+  (fn ((sig :as any) (n :as number))
+    (let ((actual (len (signal-subscribers sig))))
+      (assert= actual n
+        (str "Expected " n " subscribers, got " actual)))))
+
+
+;; --------------------------------------------------------------------------
+;; Signal simulation
+;; --------------------------------------------------------------------------
+
+;; Set a signal value directly (like a user action would)
+(define simulate-signal-set! :effects [mutation]
+  (fn ((sig :as any) value)
+    (reset! sig value)))
+
+;; Swap a signal value (like a button click handler would)
+(define simulate-signal-swap! :effects [mutation]
+  (fn ((sig :as any) (f :as lambda) &rest args)
+    (apply swap! (cons sig (cons f args)))))
+
+
+;; --------------------------------------------------------------------------
+;; Computed assertions
+;; --------------------------------------------------------------------------
+
+;; Assert a computed signal tracks the expected number of dependencies
+(define assert-computed-dep-count :effects []
+  (fn ((sig :as any) (n :as number))
+    (let ((actual (len (signal-deps sig))))
+      (assert= actual n
+        (str "Expected " n " deps, got " actual)))))
+
+;; Assert a computed signal depends on a specific signal
+(define assert-computed-depends-on :effects []
+  (fn ((computed-sig :as any) (dep-sig :as any))
+    (assert (contains? (signal-deps computed-sig) dep-sig)
+      "Expected computed to depend on the given signal")))
+
+
+;; --------------------------------------------------------------------------
+;; Effect tracking
+;; --------------------------------------------------------------------------
+
+;; Run a function and count how many times an effect fires
+(define count-effect-runs :effects [mutation]
+  (fn ((thunk :as lambda))
+    (let ((count (signal 0)))
+      (effect (fn () (deref count)))  ;; subscribe to count changes
+      (let ((run-count 0)
+            (tracker (effect (fn ()
+              (set! run-count (+ run-count 1))
+              (cek-call thunk nil)))))
+        run-count))))
+
+;; Create a signal + effect pair for testing, returns dict with :signal, :history
+(define make-test-signal :effects [mutation]
+  (fn (initial-value)
+    (let ((sig (signal initial-value))
+          (history (list)))
+      (effect (fn ()
+        (append! history (deref sig))))
+      {:signal sig :history history})))
+
+
+;; --------------------------------------------------------------------------
+;; Batch assertions
+;; --------------------------------------------------------------------------
+
+;; Assert that a batch of signal writes only triggers N subscriber notifications
+(define assert-batch-coalesces :effects [mutation]
+  (fn ((thunk :as lambda) (expected-notify-count :as number))
+    (let ((notify-count 0)
+          (sig (signal 0)))
+      (effect (fn ()
+        (deref sig)
+        (set! notify-count (+ notify-count 1))))
+      ;; Initial effect run counts as 1
+      (set! notify-count 0)
+      (batch thunk)
+      (assert= notify-count expected-notify-count
+        (str "Expected " expected-notify-count " notifications, got " notify-count)))))
diff --git a/web/harness-web.sx b/web/harness-web.sx
new file mode 100644
index 00000000..52d90194
--- /dev/null
+++ b/web/harness-web.sx
@@ -0,0 +1,154 @@
+;; ==========================================================================
+;; web/harness-web.sx — Web platform testing extensions
+;;
+;; Extends spec/harness.sx with DOM mocking, event simulation, and
+;; web-specific assertions. Depends on web/signals.sx for reactive features.
+;;
+;; Mock DOM: lightweight element stubs that record operations.
+;; No real browser needed — runs on any host.
+;; ==========================================================================
+
+
+;; --------------------------------------------------------------------------
+;; Mock DOM elements
+;; --------------------------------------------------------------------------
+
+;; Create a mock element with tag name, attrs dict, children list, and event log
+(define mock-element :effects []
+  (fn ((tag :as string) &key class id)
+    {:tag tag
+     :attrs (merge {} (if class {:class class} {}) (if id {:id id} {}))
+     :children (list)
+     :text ""
+     :event-log (list)
+     :listeners {}}))
+
+;; Set text content on mock element
+(define mock-set-text! :effects [mutation]
+  (fn (el (text :as string))
+    (dict-set! el "text" text)))
+
+;; Append child to mock element
+(define mock-append-child! :effects [mutation]
+  (fn (parent child)
+    (append! (get parent "children") child)))
+
+;; Set attribute on mock element
+(define mock-set-attr! :effects [mutation]
+  (fn (el (name :as string) value)
+    (dict-set! (get el "attrs") name value)))
+
+;; Get attribute from mock element
+(define mock-get-attr :effects []
+  (fn (el (name :as string))
+    (get (get el "attrs") name)))
+
+;; Add event listener to mock element
+(define mock-add-listener! :effects [mutation]
+  (fn (el (event-name :as string) (handler :as lambda))
+    (let ((listeners (get el "listeners")))
+      (when (not (has-key? listeners event-name))
+        (dict-set! listeners event-name (list)))
+      (append! (get listeners event-name) handler))))
+
+
+;; --------------------------------------------------------------------------
+;; Event simulation
+;; --------------------------------------------------------------------------
+
+;; Simulate a click event on a mock element
+(define simulate-click :effects [mutation]
+  (fn (el)
+    (let ((handlers (get (get el "listeners") "click")))
+      (when handlers
+        (for-each (fn (h) (cek-call h (list {:type "click" :target el})))
+          handlers))
+      (append! (get el "event-log") {:type "click"}))))
+
+;; Simulate an input event with a value
+(define simulate-input :effects [mutation]
+  (fn (el (value :as string))
+    (mock-set-attr! el "value" value)
+    (let ((handlers (get (get el "listeners") "input")))
+      (when handlers
+        (for-each (fn (h) (cek-call h (list {:type "input" :target el})))
+          handlers))
+      (append! (get el "event-log") {:type "input" :value value}))))
+
+;; Simulate a custom event (for lake→island bridge)
+(define simulate-event :effects [mutation]
+  (fn (el (event-name :as string) detail)
+    (let ((handlers (get (get el "listeners") event-name)))
+      (when handlers
+        (for-each (fn (h) (cek-call h (list {:type event-name :detail detail :target el})))
+          handlers))
+      (append! (get el "event-log") {:type event-name :detail detail}))))
+
+
+;; --------------------------------------------------------------------------
+;; DOM assertions
+;; --------------------------------------------------------------------------
+
+;; Assert mock element has specific text content
+(define assert-text :effects []
+  (fn (el (expected :as string))
+    (let ((actual (get el "text")))
+      (assert= actual expected
+        (str "Expected text \"" expected "\", got \"" actual "\"")))))
+
+;; Assert mock element has an attribute with expected value
+(define assert-attr :effects []
+  (fn (el (name :as string) expected)
+    (let ((actual (mock-get-attr el name)))
+      (assert= actual expected
+        (str "Expected attr " name "=\"" expected "\", got \"" actual "\"")))))
+
+;; Assert mock element has a CSS class
+(define assert-class :effects []
+  (fn (el (class-name :as string))
+    (let ((classes (or (mock-get-attr el "class") "")))
+      (assert (contains? (split classes " ") class-name)
+        (str "Expected class \"" class-name "\" in \"" classes "\"")))))
+
+;; Assert mock element does NOT have a CSS class
+(define assert-no-class :effects []
+  (fn (el (class-name :as string))
+    (let ((classes (or (mock-get-attr el "class") "")))
+      (assert (not (contains? (split classes " ") class-name))
+        (str "Expected no class \"" class-name "\" but found in \"" classes "\"")))))
+
+;; Assert mock element has N children
+(define assert-child-count :effects []
+  (fn (el (n :as number))
+    (let ((actual (len (get el "children"))))
+      (assert= actual n
+        (str "Expected " n " children, got " actual)))))
+
+;; Assert an event was fired on mock element
+(define assert-event-fired :effects []
+  (fn (el (event-name :as string))
+    (assert (some (fn (e) (= (get e "type") event-name)) (get el "event-log"))
+      (str "Expected event \"" event-name "\" to have been fired"))))
+
+;; Assert an event was NOT fired on mock element
+(define assert-no-event :effects []
+  (fn (el (event-name :as string))
+    (assert (not (some (fn (e) (= (get e "type") event-name)) (get el "event-log")))
+      (str "Expected event \"" event-name "\" to NOT have been fired"))))
+
+;; Count how many times an event was fired
+(define event-fire-count :effects []
+  (fn (el (event-name :as string))
+    (len (filter (fn (e) (= (get e "type") event-name)) (get el "event-log")))))
+
+
+;; --------------------------------------------------------------------------
+;; Web harness constructor — extends make-harness with DOM mock state
+;; --------------------------------------------------------------------------
+
+(define make-web-harness :effects []
+  (fn (&key platform)
+    (let ((h (make-harness :platform platform)))
+      (harness-set! h "dom" {:root (mock-element "div" :id "root")
+                             :elements {}})
+      h)))