diff --git a/moli-benchmark/wpt-cross-current/passed-cases.txt b/moli-benchmark/wpt-cross-current/passed-cases.txt
index 18484d775e..1868e1b84f 100644
--- a/moli-benchmark/wpt-cross-current/passed-cases.txt
+++ b/moli-benchmark/wpt-cross-current/passed-cases.txt
@@ -4541,8 +4541,13 @@ dom/observable/tentative/observable-event-target.any.js?moli-wpt-any=window
 dom/observable/tentative/observable-event-target.window.js?moli-wpt-script=window
 dom/observable/tentative/observable-first.any.js?moli-wpt-any=dedicatedworker
 dom/observable/tentative/observable-first.any.js?moli-wpt-any=window
+dom/observable/tentative/observable-forEach.any.js?moli-wpt-any=dedicatedworker
+dom/observable/tentative/observable-forEach.any.js?moli-wpt-any=window
+dom/observable/tentative/observable-forEach.window.js?moli-wpt-script=window
 dom/observable/tentative/observable-last.any.js?moli-wpt-any=dedicatedworker
 dom/observable/tentative/observable-last.any.js?moli-wpt-any=window
+dom/observable/tentative/observable-reduce.any.js?moli-wpt-any=dedicatedworker
+dom/observable/tentative/observable-reduce.any.js?moli-wpt-any=window
 dom/observable/tentative/observable-toArray.any.js?moli-wpt-any=dedicatedworker
 dom/observable/tentative/observable-toArray.any.js?moli-wpt-any=window
 dom/ranges/Range-adopt-test.html
diff --git a/moli-renderer-v8/src/observable.rs b/moli-renderer-v8/src/observable.rs
index d02b2306aa..d7f11986ac 100644
--- a/moli-renderer-v8/src/observable.rs
+++ b/moli-renderer-v8/src/observable.rs
@@ -5,6 +5,7 @@
 
 mod callbacks;
 mod collect;
+mod consume;
 mod event_target;
 mod first;
 mod from;
@@ -33,6 +34,10 @@ struct ObservablePrototype {
     last: (),
     #[webapi(method = "toArray", length = 0, returns_promise, callback = collect::to_array)]
     to_array: (),
+    #[webapi(method = "forEach", length = 1, returns_promise, callback = consume::for_each)]
+    for_each: (),
+    #[webapi(method, length = 1, returns_promise, callback = consume::reduce)]
+    reduce: (),
 }
 
 #[derive(WebApiFunctionTemplate)]
diff --git a/moli-renderer-v8/src/observable/callbacks.rs b/moli-renderer-v8/src/observable/callbacks.rs
index f7342adced..4eaecadcf4 100644
--- a/moli-renderer-v8/src/observable/callbacks.rs
+++ b/moli-renderer-v8/src/observable/callbacks.rs
@@ -42,17 +42,33 @@ pub(super) fn invoke<'s>(
     carrier: v8::Local<'s, v8::Object>,
     arguments: &[v8::Local<'s, v8::Value>],
 ) -> Option<v8::Local<'s, v8::Value>> {
+    invoke_value(scope, carrier, arguments).err()
+}
+
+/// The same typed boundary, retaining the raw result for any-returning
+/// callbacks such as reducers. This does not assimilate returned thenables.
+pub(super) fn invoke_value<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    carrier: v8::Local<'s, v8::Object>,
+    arguments: &[v8::Local<'s, v8::Value>],
+) -> Result<v8::Local<'s, v8::Value>, v8::Local<'s, v8::Value>> {
     let callback = V8TracedWebIdlCallbackFunction::from_object(carrier).prepare(scope);
     let context = callback.relevant_context(scope);
     if !context_is_current(scope, context) {
-        return None;
+        return Ok(v8::undefined(scope).into());
     }
     v8::tc_scope!(let scope, scope);
     let receiver = v8::undefined(scope).into();
-    invoke_synchronous_webidl_callback_function(scope, &callback, receiver, arguments);
+    let result = invoke_synchronous_webidl_callback_function(scope, &callback, receiver, arguments);
     let exception = scope.exception();
     scope.reset();
-    exception
+    if let Some(exception) = exception {
+        Err(exception)
+    } else {
+        Ok(result
+            .map(|value| v8::Local::new(scope, value))
+            .unwrap_or_else(|| v8::undefined(scope).into()))
+    }
 }
 
 pub(super) fn report_callback_exception<'s>(
diff --git a/moli-renderer-v8/src/observable/consume.rs b/moli-renderer-v8/src/observable/consume.rs
new file mode 100644
index 0000000000..1f13a3e6ef
--- /dev/null
+++ b/moli-renderer-v8/src/observable/consume.rs
@@ -0,0 +1,212 @@
+//! Callback-driven Promise consumers use traced Web IDL callbacks and cancel
+//! only their own observer when a visitor or reducer throws.
+
+use super::{
+    callbacks,
+    observer::{self, Notification},
+    promise, signal_arg,
+    state::{object_slot, set_callback},
+    subscribe_internal,
+};
+use crate::{
+    abort_signal_route::ResolvedAbortSignal,
+    util::{get_private_value, set_private_value, v8str},
+    webidl,
+};
+
+const CALLBACK: &str = "__moliObservableConsumerCallback";
+const INDEX: &str = "__moliObservableConsumerIndex";
+const HAS_ACCUMULATOR: &str = "__moliObservableHasAccumulator";
+// Unlike terminal collection state, a reducer's accumulator and callback remain
+// observable through a next() snapshot already being dispatched at cancellation.
+// The observer retains them until that snapshot is released, without Rust roots.
+const ACCUMULATOR: &str = "__moliObservableAccumulator";
+
+#[derive(webidl::WebIdlArgs)]
+#[webidl(prefix = "Observable.forEach")]
+struct ForEachArgs<'scope> {
+    #[webidl(required, converter = "callback_function")]
+    callback: webidl::WebIdlCallbackFunction,
+    #[webidl(with = signal_arg)]
+    signal: Option<ResolvedAbortSignal<'scope>>,
+}
+
+#[derive(webidl::WebIdlArgs)]
+#[webidl(prefix = "Observable.reduce")]
+struct ReduceArgs<'scope> {
+    #[webidl(required, converter = "callback_function")]
+    callback: webidl::WebIdlCallbackFunction,
+    #[webidl(converter = "raw")]
+    initial: Option<v8::Local<'scope, v8::Value>>,
+    #[webidl(with = signal_arg)]
+    signal: Option<ResolvedAbortSignal<'scope>>,
+}
+
+pub(super) fn for_each<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    args: v8::FunctionCallbackArguments<'s>,
+    mut rv: v8::ReturnValue<'_, v8::Value>,
+) {
+    let Some(parsed) = webidl::parse_args::<ForEachArgs<'s>>(scope, &args) else {
+        return;
+    };
+    if let Some(promise) = consume(
+        scope,
+        args.this(),
+        parsed.callback,
+        None,
+        parsed.signal,
+        observer::FOR_EACH,
+    ) {
+        rv.set(promise.into());
+    }
+}
+
+pub(super) fn reduce<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    args: v8::FunctionCallbackArguments<'s>,
+    mut rv: v8::ReturnValue<'_, v8::Value>,
+) {
+    let Some(parsed) = webidl::parse_args::<ReduceArgs<'s>>(scope, &args) else {
+        return;
+    };
+    // Web IDL treats undefined for an optional argument without a default as
+    // missing. Option<raw> preserves null and other actual seed values.
+    if let Some(promise) = consume(
+        scope,
+        args.this(),
+        parsed.callback,
+        parsed.initial,
+        parsed.signal,
+        observer::REDUCE,
+    ) {
+        rv.set(promise.into());
+    }
+}
+
+fn consume<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    source: v8::Local<'s, v8::Object>,
+    callback: webidl::WebIdlCallbackFunction,
+    initial: Option<v8::Local<'s, v8::Value>>,
+    signal: Option<ResolvedAbortSignal<'s>>,
+    kind: i32,
+) -> Option<v8::Local<'s, v8::Promise>> {
+    let resolver = v8::PromiseResolver::new(scope)?;
+    let promise = resolver.get_promise(scope);
+    let Some((observer, signal)) = promise::new_controlled_observer(scope, kind, resolver, signal)
+    else {
+        return Some(promise);
+    };
+    set_callback(scope, observer, CALLBACK, callback);
+    if let Some(initial) = initial {
+        set_accumulator(scope, observer, initial);
+    }
+    subscribe_internal(scope, source, observer, Some(signal));
+    Some(promise)
+}
+
+fn index<'s>(scope: &mut v8::PinScope<'s, '_>, observer: v8::Local<'s, v8::Object>) -> u64 {
+    get_private_value(scope, observer, INDEX)
+        .and_then(|value| v8::Local::<v8::BigInt>::try_from(value).ok())
+        .map_or(0, |value| value.u64_value().0)
+}
+
+fn increment_index<'s>(scope: &mut v8::PinScope<'s, '_>, observer: v8::Local<'s, v8::Object>) {
+    let next = index(scope, observer).wrapping_add(1);
+    set_private_value(
+        scope,
+        observer,
+        INDEX,
+        v8::BigInt::new_from_u64(scope, next).into(),
+    );
+}
+
+fn set_accumulator<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    observer: v8::Local<'s, v8::Object>,
+    value: v8::Local<'s, v8::Value>,
+) {
+    set_private_value(scope, observer, ACCUMULATOR, value);
+    set_private_value(
+        scope,
+        observer,
+        HAS_ACCUMULATOR,
+        v8::Boolean::new(scope, true).into(),
+    );
+}
+
+pub(super) fn notify<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    observer: v8::Local<'s, v8::Object>,
+    notification: Notification<'s>,
+    kind: i32,
+) {
+    let has_accumulator =
+        get_private_value(scope, observer, HAS_ACCUMULATOR).is_some_and(|value| value.is_true());
+    match notification {
+        Notification::Next(value) => {
+            // Do not skip a snapshot notification after settlement: a prior
+            // observer can cancel/complete the source during this same next().
+            if kind == observer::REDUCE && !has_accumulator {
+                set_accumulator(scope, observer, value);
+                increment_index(scope, observer);
+                return;
+            }
+            let callback =
+                object_slot(scope, observer, CALLBACK).expect("Observable consumer callback");
+            let accumulator = get_private_value(scope, observer, ACCUMULATOR)
+                .unwrap_or_else(|| v8::undefined(scope).into());
+            let idx = index(scope, observer);
+            let arguments = [
+                accumulator,
+                value,
+                v8::Number::new(scope, idx as f64).into(),
+            ];
+            let arguments = if kind == observer::REDUCE {
+                &arguments[..]
+            } else {
+                &arguments[1..]
+            };
+            let result = callbacks::invoke_value(scope, callback, arguments);
+            if let Err(error) = result {
+                if let Some(resolver) = promise::start_settlement(scope, observer) {
+                    resolver.reject(scope, error);
+                }
+                promise::abort_controller(scope, observer, error);
+            }
+            // The draft increments after invocation; a reentrant next() sees
+            // the current index. Read it again to preserve nested increments.
+            increment_index(scope, observer);
+            if kind == observer::REDUCE
+                && let Ok(value) = result
+            {
+                set_accumulator(scope, observer, value);
+            }
+        }
+        Notification::Error(error) => {
+            if let Some(resolver) = promise::start_settlement(scope, observer) {
+                resolver.reject(scope, error);
+            }
+        }
+        Notification::Complete => {
+            let value = if kind == observer::REDUCE {
+                get_private_value(scope, observer, ACCUMULATOR)
+                    .unwrap_or_else(|| v8::undefined(scope).into())
+            } else {
+                v8::undefined(scope).into()
+            };
+            if let Some(resolver) = promise::start_settlement(scope, observer) {
+                if kind == observer::REDUCE && !has_accumulator {
+                    let error = v8::Exception::type_error(
+                        scope,
+                        v8str(scope, "Reduce of empty Observable with no initial value"),
+                    );
+                    resolver.reject(scope, error);
+                } else {
+                    resolver.resolve(scope, value);
+                }
+            }
+        }
+    }
+}
diff --git a/moli-renderer-v8/src/observable/first.rs b/moli-renderer-v8/src/observable/first.rs
index 560f2a58e8..9adc2b9f3b 100644
--- a/moli-renderer-v8/src/observable/first.rs
+++ b/moli-renderer-v8/src/observable/first.rs
@@ -1,16 +1,8 @@
 use super::{
     observer::{self, Notification},
-    promise, signal_arg,
-    state::object_slot,
-    subscribe_internal,
+    promise, signal_arg, subscribe_internal,
 };
-use crate::{
-    abort_signal_route::ResolvedAbortSignal,
-    util::{set_private_value, v8str},
-    webidl,
-};
-
-const CONTROLLER_SIGNAL: &str = "__moliObservableFirstControllerSignal";
+use crate::{abort_signal_route::ResolvedAbortSignal, util::v8str, webidl};
 
 #[derive(webidl::WebIdlArgs)]
 #[webidl(prefix = "Observable.first")]
@@ -32,29 +24,11 @@ pub(super) fn first<'s>(
     };
     let promise = resolver.get_promise(scope);
     rv.set(promise.into());
-    let Some(controller) = ResolvedAbortSignal::new(scope) else {
+    let Some((observer, signal)) =
+        promise::new_controlled_observer(scope, observer::FIRST, resolver, parsed.signal)
+    else {
         return;
     };
-    let mut sources = vec![controller];
-    sources.extend(parsed.signal);
-    let Some(signal) = ResolvedAbortSignal::dependent(scope, &sources) else {
-        return;
-    };
-    let Some(observer) = promise::new_observer(
-        scope,
-        observer::FIRST,
-        resolver,
-        Some(signal),
-        parsed.signal.is_some(),
-    ) else {
-        return;
-    };
-    set_private_value(
-        scope,
-        observer,
-        CONTROLLER_SIGNAL,
-        controller.value().into(),
-    );
     subscribe_internal(scope, args.this(), observer, Some(signal));
 }
 
@@ -70,12 +44,7 @@ pub(super) fn notify<'s>(
             }
             // Abort after resolving, including when a then getter reenters
             // next(). This removes just this observer from a shared producer.
-            if let Some(signal) = object_slot(scope, observer, CONTROLLER_SIGNAL)
-                .and_then(|signal| ResolvedAbortSignal::resolve(scope, signal))
-            {
-                let reason = crate::native_bridge::abort::abort_error_value(scope);
-                signal.abort(scope, reason);
-            }
+            promise::abort_controller(scope, observer, v8::undefined(scope).into());
         }
         Notification::Error(error) => {
             if let Some(resolver) = promise::start_settlement(scope, observer) {
diff --git a/moli-renderer-v8/src/observable/observer.rs b/moli-renderer-v8/src/observable/observer.rs
index e00a46a375..97f4693d05 100644
--- a/moli-renderer-v8/src/observable/observer.rs
+++ b/moli-renderer-v8/src/observable/observer.rs
@@ -1,13 +1,15 @@
 //! Internal observer steps share notification ordering with script observers,
 //! while script callbacks keep their typed Web IDL invocation boundary.
 
-use super::{callbacks, collect, first, invoke_and_report, state::*};
+use super::{callbacks, collect, consume, first, invoke_and_report, state::*};
 use crate::util::get_private_value;
 
 pub(super) const NATIVE_KIND: &str = "__moliObservableNativeObserver";
 pub(super) const FIRST: i32 = 1;
 pub(super) const LAST: i32 = 2;
 pub(super) const TO_ARRAY: i32 = 3;
+pub(super) const FOR_EACH: i32 = 4;
+pub(super) const REDUCE: i32 = 5;
 pub(super) const SUBSCRIBER: &str = "__moliObservableNativeSubscriber";
 
 #[derive(Clone, Copy)]
@@ -45,6 +47,7 @@ pub(super) fn notify<'s>(
             match kind {
                 FIRST => first::notify(scope, observer, notification),
                 LAST | TO_ARRAY => collect::notify(scope, observer, notification, kind),
+                FOR_EACH | REDUCE => consume::notify(scope, observer, notification, kind),
                 _ => unreachable!("unknown native Observable observer"),
             }
             let exception = scope.exception();
diff --git a/moli-renderer-v8/src/observable/promise.rs b/moli-renderer-v8/src/observable/promise.rs
index 2c61f67276..b08883a8fe 100644
--- a/moli-renderer-v8/src/observable/promise.rs
+++ b/moli-renderer-v8/src/observable/promise.rs
@@ -13,6 +13,7 @@ const REJECTION_SIGNAL: &str = "__moliObservableRejectionSignal";
 const REJECTION_ALGORITHM: &str = "__moliObservableRejectionAlgorithm";
 const SETTLED: &str = "__moliObservablePromiseSettled";
 const PROMISE_OBSERVER: &str = "__moliObservablePromiseObserver";
+const CONTROLLER_SIGNAL: &str = "__moliObservableOperatorController";
 pub(super) const VALUE: &str = "__moliObservablePromiseValue";
 
 #[derive(WebApiObject)]
@@ -26,6 +27,46 @@ struct PromiseObserver<'scope> {
     settled: bool,
 }
 
+/// Operators that may stop their own subscription use a private controller
+/// and the existing AbortSignal dependency graph, preserving caller abort order.
+pub(super) fn new_controlled_observer<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    kind: i32,
+    resolver: v8::Local<'s, v8::PromiseResolver>,
+    caller_signal: Option<ResolvedAbortSignal<'s>>,
+) -> Option<(v8::Local<'s, v8::Object>, ResolvedAbortSignal<'s>)> {
+    let controller = ResolvedAbortSignal::new(scope)?;
+    let mut sources = vec![controller];
+    sources.extend(caller_signal);
+    let signal = ResolvedAbortSignal::dependent(scope, &sources)?;
+    let observer = new_observer(scope, kind, resolver, Some(signal), caller_signal.is_some())?;
+    set_private_value(
+        scope,
+        observer,
+        CONTROLLER_SIGNAL,
+        controller.value().into(),
+    );
+    Some((observer, signal))
+}
+
+pub(super) fn abort_controller<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    observer: v8::Local<'s, v8::Object>,
+    reason: v8::Local<'s, v8::Value>,
+) {
+    if let Some(signal) = object_slot(scope, observer, CONTROLLER_SIGNAL)
+        .and_then(|signal| ResolvedAbortSignal::resolve(scope, signal))
+    {
+        // Signal-abort defaults an undefined reason, including `throw undefined`.
+        let reason = if reason.is_undefined() {
+            crate::native_bridge::abort::abort_error_value(scope)
+        } else {
+            reason
+        };
+        signal.abort(scope, reason);
+    }
+}
+
 pub(super) fn new_observer<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     kind: i32,
diff --git a/moli-renderer-v8/src/script_vm/tests/observable.rs b/moli-renderer-v8/src/script_vm/tests/observable.rs
index ffbc72941f..8c1c2d6ee5 100644
--- a/moli-renderer-v8/src/script_vm/tests/observable.rs
+++ b/moli-renderer-v8/src/script_vm/tests/observable.rs
@@ -1,5 +1,119 @@
 use super::*;
 
+#[test]
+fn observable_callback_consumers_conversion_reentrancy_cancellation_and_exception_identity() {
+    let mut vm = new_storage_test_vm("https://observable-consumers.test/");
+    vm.eval(&format!(
+        "({}).then(value => {{ globalThis.consumerResult = JSON.stringify(value); }});",
+        include_str!("../../../tests/fixtures/observable-callback-consumers.js")
+    ))
+    .expect("Observable callback consumers fixture should evaluate");
+    let result = vm
+        .eval("consumerResult")
+        .expect("Observable consumers fixture should settle");
+    let result: serde_json::Value = serde_json::from_str(&result).unwrap();
+    assert_eq!(result["failures"], serde_json::json!([]), "{result}");
+    assert!(result["checks"].as_u64().unwrap() >= 132, "{result}");
+}
+
+#[test]
+fn observable_callback_consumers_preserve_callback_realms_and_callee_promises() {
+    let mut vm = new_storage_test_vm("https://observable-consumers-realms.test/");
+    vm.eval("document.appendChild(document.createElement('iframe'))")
+        .unwrap();
+    materialize_single_child_default_realm_for_test(&mut vm, "Observable callback consumers realm");
+    vm.eval(r#"
+(async () => {
+  const child = document.querySelector('iframe').contentWindow, checks = [];
+  const callback = child.Function('a', 'b', 'globalThis.consumerThis = this; return a + b;');
+  for (const name of ['forEach', 'reduce']) {
+    const method = child.Observable.prototype[name];
+    const call = (method, source, cb) => Reflect.apply(method, source, name === 'reduce' ? [cb, 10] : [cb]);
+    const promise = call(method, Observable.from([5]), callback);
+    checks.push(promise instanceof child.Promise, !(promise instanceof Promise));
+    checks.push(await promise === (name === 'reduce' ? 15 : undefined), child.consumerThis === child);
+    const local = call(Observable.prototype[name], child.Observable.from([7]), (a, b) => a + b);
+    checks.push(local instanceof Promise, !(local instanceof child.Promise), await local === (name === 'reduce' ? 17 : undefined));
+    const invalid = call(method, {}, () => {});
+    checks.push(invalid instanceof child.Promise);
+    await invalid.catch(e => checks.push(e instanceof child.TypeError, !(e instanceof TypeError)));
+    const marker = new child.Error('callback error');
+    await call(method, Observable.from([1]), () => { throw marker; }).catch(e => checks.push(e === marker, e instanceof child.Error));
+  }
+  await child.Observable.prototype.reduce.call(Observable.from([]), () => {}).catch(e => checks.push(e instanceof child.TypeError, !(e instanceof TypeError)));
+  globalThis.consumerRealms = JSON.stringify(checks);
+})();
+"#).unwrap();
+    let result: Vec<bool> = serde_json::from_str(&vm.eval("consumerRealms").unwrap()).unwrap();
+    assert_eq!(result.len(), 26);
+    assert!(result.iter().all(|value| *value), "{result:?}");
+}
+
+#[test]
+fn observable_callback_consumers_trace_callbacks_and_release_abandoned_and_cancelled_state() {
+    let mut vm = new_storage_test_vm("https://observable-consumers-gc.test/");
+    vm.eval(r#"
+globalThis.consumerCases = [];
+globalThis.abandonedConsumers = [];
+for (const name of ['forEach', 'reduce']) {
+  for (const kept of [false, true]) (() => {
+    let subscriber;
+    const source = new Observable(s => { subscriber = s; });
+    const token = {}, seed = {}, callback = () => token;
+    const promise = name === 'reduce' ? source.reduce(callback, seed) : source.forEach(callback);
+    const refs = {source: new WeakRef(source), subscriber: new WeakRef(subscriber), callback: new WeakRef(callback), token: new WeakRef(token), seed: new WeakRef(seed)};
+    if (kept) consumerCases.push({name, promise, refs});
+    else abandonedConsumers.push(refs);
+  })();
+}
+"#).unwrap();
+    let collect = |vm: &mut StandaloneScriptVmHarness| {
+        vm.renderer_document_isolate
+            .clone()
+            .with_entered_renderer_document_isolate(|isolate| {
+                isolate.clear_kept_objects();
+                isolate.low_memory_notification();
+                Ok(())
+            })
+            .unwrap();
+    };
+    collect(&mut vm);
+    assert_eq!(vm.eval(r#"JSON.stringify([
+abandonedConsumers.every(refs => Object.values(refs).every(ref => ref.deref() === undefined)),
+consumerCases.every(c => c.refs.source.deref() === undefined && c.refs.subscriber.deref() !== undefined && c.refs.callback.deref() !== undefined && c.refs.token.deref() !== undefined),
+consumerCases[0].refs.seed.deref() === undefined, consumerCases[1].refs.seed.deref() !== undefined
+])"#).unwrap(), "[true,true,true,true]");
+    vm.eval(r#"
+for (const c of consumerCases) {
+  c.promise.then(value => { c.correct = value === (c.name === 'reduce' ? c.refs.seed.deref() : undefined); });
+  c.refs.subscriber.deref().complete(); delete c.promise;
+}
+"#).unwrap();
+    assert_eq!(
+        vm.eval("consumerCases.every(c => c.correct)").unwrap(),
+        "true"
+    );
+    collect(&mut vm);
+    assert_eq!(vm.eval("consumerCases.every(c => Object.values(c.refs).every(ref => ref.deref() === undefined))").unwrap(), "true");
+    vm.eval(r#"
+globalThis.cancelledConsumers = [];
+for (const name of ['forEach', 'reduce']) (() => {
+  const ac = new AbortController(), token = {}, seed = {}, callback = () => token;
+  let subscriber;
+  const source = new Observable(s => { subscriber = s; });
+  const promise = name === 'reduce' ? source.reduce(callback, seed, {signal: ac.signal}) : source.forEach(callback, {signal: ac.signal});
+  promise.catch(() => {}); ac.abort('cancelled');
+  cancelledConsumers.push({promise, signal: ac.signal, refs: [new WeakRef(token), new WeakRef(seed), new WeakRef(callback), new WeakRef(subscriber)]});
+})();
+"#).unwrap();
+    collect(&mut vm);
+    assert_eq!(
+        vm.eval("cancelledConsumers.every(c => c.refs.every(ref => ref.deref() === undefined))")
+            .unwrap(),
+        "true"
+    );
+}
+
 #[test]
 fn observable_collect_values_abort_order_and_native_promise_observers() {
     let mut vm = new_storage_test_vm("https://observable-collect.test/");
diff --git a/moli-renderer-v8/src/worker/thread/tests/postmessage.rs b/moli-renderer-v8/src/worker/thread/tests/postmessage.rs
index 73994b671c..3a82cf53c2 100644
--- a/moli-renderer-v8/src/worker/thread/tests/postmessage.rs
+++ b/moli-renderer-v8/src/worker/thread/tests/postmessage.rs
@@ -1,5 +1,22 @@
 use super::*;
 
+#[tokio::test]
+async fn worker_observable_callback_consumers_conversion_reentrancy_cancellation_and_exception_identity()
+ {
+    ensure_v8();
+    let mut handle = spawn_worker(
+        format!(
+            "({}).then(value => {{ postMessage(value); close(); }});",
+            include_str!("../../../../tests/fixtures/observable-callback-consumers.js")
+        ),
+        "https://observable-consumers.test/worker.js".into(),
+    );
+    let message = timeout(TIMEOUT, handle.recv()).await.unwrap().unwrap();
+    let result: serde_json::Value = serde_json::from_str(&expect_post_json(message)).unwrap();
+    assert_eq!(result["failures"], serde_json::json!([]), "{result}");
+    assert!(result["checks"].as_u64().unwrap() >= 132, "{result}");
+}
+
 #[tokio::test]
 async fn worker_observable_collect_values_abort_order_and_native_promise_observers() {
     ensure_v8();
diff --git a/moli-renderer-v8/tests/fixtures/observable-callback-consumers.js b/moli-renderer-v8/tests/fixtures/observable-callback-consumers.js
new file mode 100644
index 0000000000..bcc55f0e3b
--- /dev/null
+++ b/moli-renderer-v8/tests/fixtures/observable-callback-consumers.js
@@ -0,0 +1,193 @@
+(async () => {
+  'use strict';
+  const failures = [];
+  let checks = 0;
+  const check = (value, label) => { checks++; if (!value) failures.push(label); };
+  const same = (a, b, label) => check(JSON.stringify(a) === JSON.stringify(b), label);
+  const rejected = p => p.then(() => { throw new Error('expected rejection'); }, e => e);
+  const thrown = fn => { try { fn(); } catch (e) { return e; } };
+  const test = async (label, fn) => { try { await fn(); } catch (e) { check(false, label + ': ' + e); } };
+  const names = ['forEach', 'reduce'];
+  for (const name of names) check(typeof Observable.prototype[name] === 'function', name + ' exposed');
+  if (failures.length) return {checks, failures};
+
+  for (const name of names) {
+    const method = Observable.prototype[name];
+    const call = (source, callback, options) => Reflect.apply(method, source,
+      name === 'reduce' ? [callback, 10, options] : [callback, options]);
+    await test(name + ' conversion', async () => {
+      const desc = Object.getOwnPropertyDescriptor(Observable.prototype, name);
+      check(method.name === name && method.length === 1, name + ' name and length');
+      check(desc.enumerable && desc.writable && desc.configurable, name + ' descriptor');
+      check(thrown(() => new method(() => {})) instanceof TypeError, name + ' non-constructor');
+      const source = Observable.from([1]);
+      let reads = 0, traps = 0;
+      const options = {get signal() { reads++; }};
+      const revoked = Proxy.revocable(source, {}); revoked.revoke();
+      for (const value of [undefined, null, false, 1, Symbol(), {}, Object.create(source),
+        Object.create(Observable.prototype), new Proxy(source, {get() { traps++; }}), revoked.proxy]) {
+        const promise = call(value, () => {}, options);
+        check(promise instanceof Promise && await rejected(promise) instanceof TypeError, name + ' receiver rejects Promise');
+      }
+      for (const callback of [undefined, null, {}, {handleEvent() {}}, 1, Object.create(Function.prototype)]) {
+        check(await rejected(call(source, callback, options)) instanceof TypeError, name + ' invalid callback');
+      }
+      check(await rejected(method.call(source)) instanceof TypeError, name + ' required callback');
+      check(reads === 0 && traps === 0, name + ' receiver and callback validation precede options');
+      for (const options of [1, true, {signal: null}, {signal: {}}, {signal: new Proxy(new AbortController().signal, {})}]) {
+        check(await rejected(call(source, () => {}, options)) instanceof TypeError, name + ' invalid options');
+      }
+      const marker = {};
+      check(await rejected(call(source, () => {}, {get signal() { throw marker; }})) === marker, name + ' getter error identity');
+      Object.setPrototypeOf(source, null);
+      await call(source, () => {}, options);
+      check(reads === 1, name + ' genuine brand with changed prototype and one signal read');
+      const proxy = Proxy.revocable(() => {}, {});
+      const promise = call(Observable.from([1]), proxy.proxy, {get signal() { proxy.revoke(); }});
+      check(await rejected(promise) instanceof TypeError, name + ' callback revoked after conversion rejects');
+      check(await rejected(call(Observable.from([1]), class Visitor {})) instanceof TypeError, name + ' class callback fails at invocation');
+    });
+
+    await test(name + ' invocation', async () => {
+      const log = [], marker = {}, returned = {get then() { throw marker; }};
+      let subscriber;
+      const source = new Observable(s => { subscriber = s; });
+      const callback = new Proxy(function (...args) {
+        check(this === undefined, name + ' strict callback this is undefined');
+        log.push(args);
+        return name === 'reduce' ? args[0] + args[1] : returned;
+      }, {});
+      const promise = call(source, callback);
+      subscriber.next(1); subscriber.next(2);
+      same(log, name === 'reduce' ? [[10, 1, 0], [11, 2, 1]] : [[1, 0], [2, 1]], name + ' synchronous arguments');
+      check(subscriber.active, name + ' success keeps producer active');
+      subscriber.complete();
+      check(await promise === (name === 'reduce' ? 13 : undefined), name + ' completion result');
+      check(await rejected(call(new Observable(s => s.error(marker)), () => {})) === marker, name + ' source error identity');
+      check(await rejected(call(new Observable(() => { throw marker; }), () => {})) === marker, name + ' initializer error identity');
+    });
+
+    await test(name + ' callback errors', async () => {
+      for (const marker of [{}, null, undefined]) {
+        let subscriber, visits = 0, teardowns = 0;
+        const source = new Observable(s => {
+          subscriber = s; s.addTeardown(() => teardowns++);
+          s.next(1); s.next(2); s.complete();
+        });
+        const error = await rejected(call(source, () => { visits++; throw marker; }));
+        check(error === marker && visits === 1 && teardowns === 1 && !subscriber.active, name + ' callback throw rejects and cancels once');
+        check(marker === undefined ? subscriber.signal.reason.name === 'AbortError' : subscriber.signal.reason === marker, name + ' callback throw abort reason');
+      }
+      const ac = new AbortController(), reason = {}, later = {};
+      const promise = call(Observable.from([1]), () => { ac.abort(reason); throw later; }, {signal: ac.signal});
+      check(await rejected(promise) === reason, name + ' reentrant caller abort wins before callback throw');
+    });
+
+    await test(name + ' cancellation timing', async () => {
+      const reason = {}, preaborted = AbortSignal.abort(reason);
+      let starts = 0, calls = 0, subscriber;
+      const source = new Observable(s => { starts++; subscriber = s; });
+      check(await rejected(call(source, () => calls++, {signal: preaborted})) === reason && starts === 0 && calls === 0, name + ' pre-abort skips producer');
+      const ac = new AbortController(), log = [];
+      ac.signal.addEventListener('abort', () => { log.push('outer'); queueMicrotask(() => log.push('outer job')); });
+      const promise = call(source, () => {}, {signal: ac.signal}).catch(e => { log.push('reject'); return e; });
+      subscriber.signal.addEventListener('abort', () => { log.push('inner'); queueMicrotask(() => log.push('inner job')); });
+      subscriber.addTeardown(() => log.push('teardown'));
+      ac.abort(reason);
+      same(log, ['outer', 'inner', 'teardown'], name + ' dependent signal abort order');
+      check(await promise === reason, name + ' caller reason identity');
+      same(log, ['outer', 'inner', 'teardown', 'outer job', 'reject', 'inner job'], name + ' dependent signal microtask order');
+    });
+
+    await test(name + ' reentrant dispatch', async () => {
+      let subscriber;
+      const source = new Observable(s => { subscriber = s; }), indices = [], values = [];
+      const promise = call(source, (...args) => {
+        const value = args[name === 'reduce' ? 1 : 0], idx = args.at(-1);
+        indices.push(idx); values.push(value);
+        if (value === 1) subscriber.next(2);
+        return name === 'reduce' ? args[0] + value : undefined;
+      });
+      subscriber.next(1); subscriber.next(3); subscriber.complete();
+      same(values, [1, 2, 3], name + ' nested emission order');
+      // The draft increments after callback invocation; Chromium currently
+      // increments before it and returns [0, 1, 2] in this case.
+      same(indices, [0, 0, 2], name + ' draft reentrant index order');
+      check(await promise === (name === 'reduce' ? 14 : undefined), name + ' outer reducer result replaces nested result');
+      for (const terminal of ['abort', 'complete']) {
+        const ac = new AbortController(), marker = {}, received = [];
+        const source = new Observable(s => { subscriber = s; });
+        source.subscribe(() => { if (terminal === 'abort') ac.abort(marker); else subscriber.complete(); });
+        const promise = call(source, (...args) => { received.push(args); return 99; }, {signal: ac.signal});
+        const handled = terminal === 'abort' ? rejected(promise) : promise;
+        subscriber.next(1);
+        same(received, name === 'reduce' ? [[10, 1, 0]] : [[1, 0]], name + ' already captured next runs after ' + terminal);
+        check(await handled === (terminal === 'abort' ? marker : name === 'reduce' ? 10 : undefined), name + ' snapshot cannot replace settled result');
+        subscriber.complete();
+      }
+    });
+  }
+
+  await test('reduce seeds and raw results', async () => {
+    const empty = Observable.from([]);
+    check(await rejected(empty.reduce(() => {})) instanceof TypeError, 'empty reduction without seed rejects');
+    check(await rejected(empty.reduce(() => {}, undefined)) instanceof TypeError, 'optional undefined seed is missing under Web IDL');
+    check(await Observable.from([undefined]).reduce(() => { throw 1; }) === undefined, 'emitted undefined is a real accumulator');
+    check(await empty.reduce(() => {}, null) === null, 'null is a real seed');
+    const reason = {};
+    check(await rejected(empty.reduce(() => {}, undefined, {signal: AbortSignal.abort(reason)})) === reason, 'optional missing seed still converts later options');
+    const seed = {}, values = [];
+    check(await empty.reduce(() => { throw 1; }, seed) === seed, 'empty reduction preserves seed identity');
+    const sum = await Observable.from([1, 2, 3]).reduce((a, v, i) => { values.push([a, v, i]); return a + v; });
+    check(sum === 6, 'seedless reduction result');
+    same(values, [[1, 2, 1], [3, 3, 2]], 'first emitted value seeds without callback');
+    const thenable = {get then() { throw new Error('intermediate result assimilated'); }};
+    let calls = 0;
+    check(await Observable.from([1, 2]).reduce((a, v) => { calls++; if (v === 1) return thenable; check(a === thenable, 'raw intermediate accumulator'); return 42; }, 0) === 42 && calls === 2, 'intermediate thenables not assimilated');
+    let subscriber, reads = 0, resolveValue;
+    const ac = new AbortController(), value = {get then() { reads++; ac.abort('late'); return resolve => { resolveValue = resolve; }; }};
+    const promise = new Observable(s => { subscriber = s; }).reduce(() => value, 0, {signal: ac.signal});
+    subscriber.next(1); check(reads === 0, 'final thenable not read before completion');
+    subscriber.complete(); await Promise.resolve(); resolveValue(8);
+    check(await promise === 8 && reads === 1, 'final assimilation locks before then getter abort');
+    const ignored = Promise.reject('visitor promise'); ignored.catch(() => {});
+    check(await Observable.from([1]).forEach(() => ignored) === undefined, 'visitor return Promise is ignored');
+  });
+
+  await test('shared subscription and iterator close', async () => {
+    let subscriber;
+    const source = new Observable(s => { subscriber = s; }), marker = {};
+    const all = source.toArray(), completion = rejected(source.forEach(() => { throw marker; }));
+    subscriber.next(1); check(subscriber.active && await completion === marker, 'callback error removes only its own observer');
+    subscriber.next(2); subscriber.complete();
+    same(await all, [1, 2], 'other observer continues after visitor error');
+    const closeError = {}, errors = [];
+    const onerror = e => { if (e.error === closeError) { errors.push(e.error); e.preventDefault(); } };
+    addEventListener('error', onerror);
+    try {
+      for (const name of names) {
+        const iterator = {next: () => ({value: 1}), return() { throw closeError; }};
+        const source = Observable.from({[Symbol.iterator]: () => iterator});
+        const promise = name === 'reduce' ? source.reduce(() => { throw marker; }, 0) : source.forEach(() => { throw marker; });
+        check(await rejected(promise) === marker, name + ' close error cannot replace callback error');
+      }
+      check(errors.length === 2, 'iterator close errors reported once per consumer');
+    } finally { removeEventListener('error', onerror); }
+  });
+
+  await test('intrinsic operations', async () => {
+    const source = Observable.from([1, 2]), forEach = Observable.prototype.forEach, reduce = Observable.prototype.reduce;
+    const saved = [globalThis.Promise, globalThis.AbortController, Observable.prototype.subscribe, AbortSignal.any];
+    const poison = () => { throw new Error('public native implementation consulted'); };
+    let each, reduced;
+    try {
+      globalThis.Promise = globalThis.AbortController = Observable.prototype.subscribe = AbortSignal.any = poison;
+      each = forEach.call(source, () => {}); reduced = reduce.call(source, (a, v) => a + v, 0);
+    } finally {
+      [globalThis.Promise, globalThis.AbortController, Observable.prototype.subscribe, AbortSignal.any] = saved;
+    }
+    check(each instanceof Promise && await each === undefined, 'forEach bypasses mutable globals and subscribe');
+    check(reduced instanceof Promise && await reduced === 3, 'reduce bypasses mutable globals and subscribe');
+  });
+  return {checks, failures};
+})()