diff --git a/moli-renderer-v8/src/context_bootstrap/stream_adapter/utils.rs b/moli-renderer-v8/src/context_bootstrap/stream_adapter/utils.rs
index 47dda2f1bd..bd2634eac6 100644
--- a/moli-renderer-v8/src/context_bootstrap/stream_adapter/utils.rs
+++ b/moli-renderer-v8/src/context_bootstrap/stream_adapter/utils.rs
@@ -121,7 +121,7 @@ pub(in crate::context_bootstrap) fn publish_required_stream_promise_reactions<'s
     publish_required_stream_value(scope, None, "promise reaction attachment", role)
 }
 
-fn publish_required_stream_value<T>(
+pub(in crate::context_bootstrap::stream_adapter) fn publish_required_stream_value<T>(
     scope: &mut v8::PinScope<'_, '_>,
     value: Option<T>,
     operation: &'static str,
diff --git a/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable.rs b/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable.rs
index cb172ff12f..ee2ae57711 100644
--- a/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable.rs
+++ b/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable.rs
@@ -1,6 +1,8 @@
 use super::*;
+mod finish_context;
 use crate::text_codec::{TextCodecStore, TextDecodeError};
 use crate::util::get_private_value;
+use finish_context::{TransformFinishContext, TransformFinishWithReason};
 use moli_streams::queue::{QueueBounds, QueueRemainderPlan};
 use moli_streams::strategy::StrategySnapshot;
 use moli_streams::transform::{
@@ -760,7 +762,14 @@ pub(in crate::context_bootstrap::stream_adapter) fn transform_stream_readable_ca
         rv.set(finish_promise.into());
         return;
     };
-    attach_transform_source_cancel_reactions(scope, cancel_promise, writable, residence, reason);
+    attach_transform_source_cancel_reactions(
+        scope,
+        cancel_promise,
+        writable,
+        readable,
+        residence,
+        reason,
+    );
     rv.set(finish_promise.into());
 }
 
@@ -902,13 +911,21 @@ fn attach_transform_source_cancel_reactions<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     cancel_promise: v8::Local<'s, v8::Promise>,
     writable: v8::Local<'s, v8::Object>,
+    readable: v8::Local<'s, v8::Object>,
     residence: v8::Local<'s, v8::Object>,
     reason: v8::Local<'s, v8::Value>,
 ) {
-    let data = v8::Array::new(scope, 3);
-    let _ = data.set_index(scope, 0, writable.into());
-    let _ = data.set_index(scope, 1, residence.into());
-    let _ = data.set_index(scope, 2, reason);
+    let StreamOwnerPublication::Published(data) = (TransformFinishWithReason {
+        finish: TransformFinishContext {
+            writable,
+            readable,
+            residence,
+        },
+        reason,
+    })
+    .into_callback_data(scope) else {
+        return;
+    };
     publish_required_stream_promise_reactions(
         scope,
         cancel_promise,
@@ -926,20 +943,19 @@ fn transform_source_cancel_fulfilled_callback<'s>(
     args: v8::FunctionCallbackArguments<'s>,
     mut rv: v8::ReturnValue<'_, v8::Value>,
 ) {
-    let Some((writable, residence, reason)) =
-        transform_source_cancel_reaction_values(scope, args.data())
+    let StreamOwnerPublication::Published(TransformFinishWithReason { finish, reason }) =
+        TransformFinishWithReason::from_callback_data(scope, args.data())
     else {
         rv.set_undefined();
         return;
     };
-    let Some(readable) = stream_slot_object(scope, writable, WRITABLE_STREAM_TARGET_READABLE_SLOT)
-        .filter(|readable| !readable.is_null_or_undefined())
-    else {
-        reject_pending_read(scope, residence, reason);
-        rv.set_undefined();
-        return;
-    };
-    apply_transform_source_cancel_fulfillment(scope, writable, readable, residence, reason);
+    apply_transform_source_cancel_fulfillment(
+        scope,
+        finish.writable,
+        finish.readable,
+        finish.residence,
+        reason,
+    );
     rv.set_undefined();
 }
 
@@ -970,20 +986,18 @@ fn transform_source_cancel_rejected_callback<'s>(
     args: v8::FunctionCallbackArguments<'s>,
     mut rv: v8::ReturnValue<'_, v8::Value>,
 ) {
-    let Some((writable, residence, _)) =
-        transform_source_cancel_reaction_values(scope, args.data())
+    let StreamOwnerPublication::Published(context) =
+        TransformFinishWithReason::from_callback_data(scope, args.data())
     else {
         rv.set_undefined();
         return;
     };
+    let TransformFinishContext {
+        writable,
+        readable,
+        residence,
+    } = context.finish;
     let error = args.get(0);
-    let Some(readable) = stream_slot_object(scope, writable, WRITABLE_STREAM_TARGET_READABLE_SLOT)
-        .filter(|readable| !readable.is_null_or_undefined())
-    else {
-        reject_pending_read(scope, residence, error);
-        rv.set_undefined();
-        return;
-    };
     match transform_stream_snapshot(scope, writable, readable)
         .plan_finish_settlement(FinishOperation::ReadableCancel, AlgorithmOutcome::Rejected)
     {
@@ -996,27 +1010,6 @@ fn transform_source_cancel_rejected_callback<'s>(
     rv.set_undefined();
 }
 
-fn transform_source_cancel_reaction_values<'s>(
-    scope: &mut v8::PinScope<'s, '_>,
-    data: v8::Local<'s, v8::Value>,
-) -> Option<(
-    v8::Local<'s, v8::Object>,
-    v8::Local<'s, v8::Object>,
-    v8::Local<'s, v8::Value>,
-)> {
-    let data = v8::Local::<v8::Array>::try_from(data).ok()?;
-    let writable = data
-        .get_index(scope, 0)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    let residence = data
-        .get_index(scope, 1)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    let reason = data
-        .get_index(scope, 2)
-        .unwrap_or_else(|| v8::undefined(scope).into());
-    Some((writable, residence, reason))
-}
-
 fn transform_stream_sink_abort_algorithm<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     writable: v8::Local<'s, v8::Object>,
@@ -1069,11 +1062,17 @@ fn transform_stream_sink_abort_algorithm_in_relevant_realm<'s>(
         );
         return Some(finish_promise.into());
     };
-    let data = v8::Array::new(scope, 4);
-    let _ = data.set_index(scope, 0, writable.into());
-    let _ = data.set_index(scope, 1, readable.into());
-    let _ = data.set_index(scope, 2, residence.into());
-    let _ = data.set_index(scope, 3, reason);
+    let StreamOwnerPublication::Published(data) = (TransformFinishWithReason {
+        finish: TransformFinishContext {
+            writable,
+            readable,
+            residence,
+        },
+        reason,
+    })
+    .into_callback_data(scope) else {
+        return Some(finish_promise.into());
+    };
     if matches!(
         publish_required_stream_promise_reactions(
             scope,
@@ -1096,18 +1095,19 @@ fn transform_sink_abort_fulfilled_callback<'s>(
     args: v8::FunctionCallbackArguments<'s>,
     mut rv: v8::ReturnValue<'_, v8::Value>,
 ) {
-    let Some((readable, residence, reason)) =
-        transform_sink_abort_reaction_values(scope, args.data())
+    let StreamOwnerPublication::Published(TransformFinishWithReason { finish, reason }) =
+        TransformFinishWithReason::from_callback_data(scope, args.data())
     else {
         rv.set_undefined();
         return;
     };
-    let Some(writable) = transform_sink_abort_reaction_writable(scope, args.data()) else {
-        reject_pending_read(scope, residence, reason);
-        rv.set_undefined();
-        return;
-    };
-    apply_transform_sink_abort_fulfillment(scope, writable, readable, residence, reason);
+    apply_transform_sink_abort_fulfillment(
+        scope,
+        finish.writable,
+        finish.readable,
+        finish.residence,
+        reason,
+    );
     rv.set_undefined();
 }
 
@@ -1138,17 +1138,18 @@ fn transform_sink_abort_rejected_callback<'s>(
     args: v8::FunctionCallbackArguments<'s>,
     mut rv: v8::ReturnValue<'_, v8::Value>,
 ) {
-    let Some((readable, residence, _)) = transform_sink_abort_reaction_values(scope, args.data())
+    let StreamOwnerPublication::Published(context) =
+        TransformFinishWithReason::from_callback_data(scope, args.data())
     else {
         rv.set_undefined();
         return;
     };
+    let TransformFinishContext {
+        writable,
+        readable,
+        residence,
+    } = context.finish;
     let error = args.get(0);
-    let Some(writable) = transform_sink_abort_reaction_writable(scope, args.data()) else {
-        reject_pending_read(scope, residence, error);
-        rv.set_undefined();
-        return;
-    };
     match transform_stream_snapshot(scope, writable, readable)
         .plan_finish_settlement(FinishOperation::WritableAbort, AlgorithmOutcome::Rejected)
     {
@@ -1161,36 +1162,6 @@ fn transform_sink_abort_rejected_callback<'s>(
     rv.set_undefined();
 }
 
-fn transform_sink_abort_reaction_values<'s>(
-    scope: &mut v8::PinScope<'s, '_>,
-    data: v8::Local<'s, v8::Value>,
-) -> Option<(
-    v8::Local<'s, v8::Object>,
-    v8::Local<'s, v8::Object>,
-    v8::Local<'s, v8::Value>,
-)> {
-    let data = v8::Local::<v8::Array>::try_from(data).ok()?;
-    let readable = data
-        .get_index(scope, 1)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    let residence = data
-        .get_index(scope, 2)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    let reason = data
-        .get_index(scope, 3)
-        .unwrap_or_else(|| v8::undefined(scope).into());
-    Some((readable, residence, reason))
-}
-
-fn transform_sink_abort_reaction_writable<'s>(
-    scope: &mut v8::PinScope<'s, '_>,
-    data: v8::Local<'s, v8::Value>,
-) -> Option<v8::Local<'s, v8::Object>> {
-    let data = v8::Local::<v8::Array>::try_from(data).ok()?;
-    data.get_index(scope, 0)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())
-}
-
 fn process_transform_writable_queue<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     writable: v8::Local<'s, v8::Object>,
@@ -2690,10 +2661,14 @@ fn attach_transform_sink_close_reactions<'s>(
     readable: v8::Local<'s, v8::Object>,
     residence: v8::Local<'s, v8::Object>,
 ) {
-    let data = v8::Array::new(scope, 3);
-    let _ = data.set_index(scope, 0, writable.into());
-    let _ = data.set_index(scope, 1, readable.into());
-    let _ = data.set_index(scope, 2, residence.into());
+    let StreamOwnerPublication::Published(data) = (TransformFinishContext {
+        writable,
+        readable,
+        residence,
+    })
+    .into_callback_data(scope) else {
+        return;
+    };
     publish_required_stream_promise_reactions(
         scope,
         flush_promise,
@@ -2711,8 +2686,11 @@ fn transform_sink_close_fulfilled_callback<'s>(
     args: v8::FunctionCallbackArguments<'s>,
     mut rv: v8::ReturnValue<'_, v8::Value>,
 ) {
-    let Some((writable, readable, residence)) =
-        transform_sink_close_reaction_values(scope, args.data())
+    let StreamOwnerPublication::Published(TransformFinishContext {
+        writable,
+        readable,
+        residence,
+    }) = TransformFinishContext::from_callback_data(scope, args.data())
     else {
         rv.set_undefined();
         return;
@@ -2742,8 +2720,11 @@ fn transform_sink_close_rejected_callback<'s>(
     args: v8::FunctionCallbackArguments<'s>,
     mut rv: v8::ReturnValue<'_, v8::Value>,
 ) {
-    let Some((writable, readable, residence)) =
-        transform_sink_close_reaction_values(scope, args.data())
+    let StreamOwnerPublication::Published(TransformFinishContext {
+        writable,
+        readable,
+        residence,
+    }) = TransformFinishContext::from_callback_data(scope, args.data())
     else {
         rv.set_undefined();
         return;
@@ -2761,27 +2742,6 @@ fn transform_sink_close_rejected_callback<'s>(
     rv.set_undefined();
 }
 
-fn transform_sink_close_reaction_values<'s>(
-    scope: &mut v8::PinScope<'s, '_>,
-    data: v8::Local<'s, v8::Value>,
-) -> Option<(
-    v8::Local<'s, v8::Object>,
-    v8::Local<'s, v8::Object>,
-    v8::Local<'s, v8::Object>,
-)> {
-    let data = v8::Local::<v8::Array>::try_from(data).ok()?;
-    let writable = data
-        .get_index(scope, 0)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    let readable = data
-        .get_index(scope, 1)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    let residence = data
-        .get_index(scope, 2)
-        .and_then(|value| v8::Local::<v8::Object>::try_from(value).ok())?;
-    Some((writable, readable, residence))
-}
-
 fn attach_transform_writable_close_settlement<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     writable: v8::Local<'s, v8::Object>,
diff --git a/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable/finish_context.rs b/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable/finish_context.rs
new file mode 100644
index 0000000000..bf30a0577b
--- /dev/null
+++ b/moli-renderer-v8/src/context_bootstrap/stream_adapter/writable/finish_context.rs
@@ -0,0 +1,169 @@
+//! GC-visible data shared by the two reactions of a transform finish operation.
+//! These Rust views never outlive their HandleScope; the V8 callback owns the
+//! private-slot carrier across promise jobs.
+
+use moli_webapi_declare::WebApiObject;
+
+use super::super::utils::{StreamOwnerPublication, publish_required_stream_value};
+use crate::util::{get_private_object, private_key};
+
+const WRITABLE_SLOT: &str = "__moliTransformFinishWritable";
+const READABLE_SLOT: &str = "__moliTransformFinishReadable";
+const RESIDENCE_SLOT: &str = "__moliTransformFinishResidence";
+const REASON_SLOT: &str = "__moliTransformFinishReason";
+
+#[derive(WebApiObject)]
+#[webapi(plain)]
+struct FinishDeclaration<'s> {
+    #[webapi(slot = WRITABLE_SLOT)]
+    writable: v8::Local<'s, v8::Object>,
+    #[webapi(slot = READABLE_SLOT)]
+    readable: v8::Local<'s, v8::Object>,
+    #[webapi(slot = RESIDENCE_SLOT)]
+    residence: v8::Local<'s, v8::Object>,
+}
+
+#[derive(WebApiObject)]
+#[webapi(fragment)]
+struct ReasonDeclaration<'s> {
+    #[webapi(slot = REASON_SLOT)]
+    reason: v8::Local<'s, v8::Value>,
+}
+
+pub(super) struct TransformFinishContext<'s> {
+    pub(super) writable: v8::Local<'s, v8::Object>,
+    pub(super) readable: v8::Local<'s, v8::Object>,
+    pub(super) residence: v8::Local<'s, v8::Object>,
+}
+
+impl<'s> TransformFinishContext<'s> {
+    pub(super) fn into_callback_data(
+        self,
+        scope: &mut v8::PinScope<'s, '_>,
+    ) -> StreamOwnerPublication<v8::Local<'s, v8::Object>> {
+        let data = FinishDeclaration::new(self.writable, self.readable, self.residence)
+            .bind(scope)
+            .ok();
+        publish_required_stream_value(scope, data, "callback data creation", "transform finish")
+    }
+
+    fn decode(scope: &mut v8::PinScope<'s, '_>, data: v8::Local<'s, v8::Value>) -> Option<Self> {
+        let data = v8::Local::<v8::Object>::try_from(data).ok()?;
+        Some(Self {
+            writable: get_private_object(scope, data, WRITABLE_SLOT)?,
+            readable: get_private_object(scope, data, READABLE_SLOT)?,
+            residence: get_private_object(scope, data, RESIDENCE_SLOT)?,
+        })
+    }
+
+    pub(super) fn from_callback_data(
+        scope: &mut v8::PinScope<'s, '_>,
+        data: v8::Local<'s, v8::Value>,
+    ) -> StreamOwnerPublication<Self> {
+        let context = Self::decode(scope, data);
+        publish_required_stream_value(scope, context, "callback data decoding", "transform finish")
+    }
+}
+
+pub(super) struct TransformFinishWithReason<'s> {
+    pub(super) finish: TransformFinishContext<'s>,
+    pub(super) reason: v8::Local<'s, v8::Value>,
+}
+
+impl<'s> TransformFinishWithReason<'s> {
+    pub(super) fn into_callback_data(
+        self,
+        scope: &mut v8::PinScope<'s, '_>,
+    ) -> StreamOwnerPublication<v8::Local<'s, v8::Object>> {
+        let StreamOwnerPublication::Published(data) = self.finish.into_callback_data(scope) else {
+            return StreamOwnerPublication::OwnerTerminating;
+        };
+        let data = ReasonDeclaration::new(self.reason)
+            .initialize(scope, data)
+            .ok()
+            .map(|_| data);
+        publish_required_stream_value(
+            scope,
+            data,
+            "callback data creation",
+            "transform finish reason",
+        )
+    }
+
+    pub(super) fn from_callback_data(
+        scope: &mut v8::PinScope<'s, '_>,
+        data: v8::Local<'s, v8::Value>,
+    ) -> StreamOwnerPublication<Self> {
+        let context = (|| {
+            let finish = TransformFinishContext::decode(scope, data)?;
+            let data = v8::Local::<v8::Object>::try_from(data).ok()?;
+            // get_private_value treats undefined as absent. A cancel/abort
+            // reason may be undefined, but the slot itself must be present.
+            let key = private_key(scope, REASON_SLOT)?;
+            if data.has_private(scope, key) != Some(true) {
+                return None;
+            }
+            let reason = data.get_private(scope, key)?;
+            Some(Self { finish, reason })
+        })();
+        publish_required_stream_value(
+            scope,
+            context,
+            "callback data decoding",
+            "transform finish with reason",
+        )
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn finish<'s>(scope: &mut v8::PinScope<'s, '_>) -> TransformFinishContext<'s> {
+        TransformFinishContext {
+            writable: v8::Object::new(scope),
+            readable: v8::Object::new(scope),
+            residence: v8::Object::new(scope),
+        }
+    }
+
+    #[test]
+    fn undefined_cancel_reason_survives_callback_data() {
+        moli_v8_test_util::ensure_v8();
+        let mut isolate = v8::Isolate::new(Default::default());
+        let scope = std::pin::pin!(v8::HandleScope::new(&mut isolate));
+        let scope = &mut scope.init();
+        let context = v8::Context::new(scope, Default::default());
+        let scope = &mut v8::ContextScope::new(scope, context);
+        let context = TransformFinishWithReason {
+            finish: finish(scope),
+            reason: v8::undefined(scope).into(),
+        };
+        let StreamOwnerPublication::Published(data) = context.into_callback_data(scope) else {
+            panic!("live context must publish callback data")
+        };
+        let StreamOwnerPublication::Published(context) =
+            TransformFinishWithReason::from_callback_data(scope, data.into())
+        else {
+            panic!("undefined is a valid cancel reason")
+        };
+        assert!(context.reason.is_undefined());
+    }
+
+    #[test]
+    #[should_panic(expected = "callback data decoding for `transform finish with reason`")]
+    fn close_context_cannot_supply_a_missing_cancel_reason() {
+        moli_v8_test_util::ensure_v8();
+        let mut isolate = v8::Isolate::new(Default::default());
+        let scope = std::pin::pin!(v8::HandleScope::new(&mut isolate));
+        let scope = &mut scope.init();
+        let context = v8::Context::new(scope, Default::default());
+        let scope = &mut v8::ContextScope::new(scope, context);
+        let StreamOwnerPublication::Published(data) = finish(scope).into_callback_data(scope)
+        else {
+            panic!("live context must publish callback data")
+        };
+        TransformFinishWithReason::from_callback_data(scope, data.into())
+            .finish_at_owner_boundary();
+    }
+}
diff --git a/moli-renderer-v8/src/script_vm/tests/streams/transform_finish.rs b/moli-renderer-v8/src/script_vm/tests/streams/transform_finish.rs
index 5a26c7eae1..4df2a609ab 100644
--- a/moli-renderer-v8/src/script_vm/tests/streams/transform_finish.rs
+++ b/moli-renderer-v8/src/script_vm/tests/streams/transform_finish.rs
@@ -1,5 +1,66 @@
 use super::*;
 
+#[test]
+fn transform_finish_reactions_keep_their_context_alive_across_gc() {
+    for operation in ["cancel", "abort", "close"] {
+        let mut vm = stream_test_vm();
+        vm.eval(&format!("globalThis.__finishOperation = {operation:?}"))
+            .expect("finish operation");
+        vm.eval(
+            r#"
+(() => {
+  const state = globalThis.__finishState = { calls: 0, closed: false, finished: false };
+  const gate = new Promise(resolve => { globalThis.__releaseFinish = resolve; });
+  const stream = new TransformStream({
+    cancel() { state.calls++; return gate; },
+    flush() { state.calls++; return gate; }
+  });
+  const reason = { marker: 'original reason' };
+  const expectedReason = new WeakRef(reason);
+  const operation = __finishOperation;
+  const closed = operation === 'cancel'
+    ? stream.writable.getWriter().closed
+    : stream.readable.getReader().closed;
+  closed.then(
+    () => { state.closed = operation === 'close'; },
+    error => { state.closed = error === expectedReason.deref() && error.marker === 'original reason'; }
+  );
+  const finish = operation === 'cancel' ? stream.readable.cancel(reason)
+    : operation === 'abort' ? stream.writable.abort(reason) : stream.writable.close();
+  finish.then(
+    () => { state.finished = true; },
+    error => { state.finished = String(error); }
+  );
+})()
+"#,
+        )
+        .expect("pending finish setup");
+        assert_eq!(
+            vm.eval("JSON.stringify(__finishState)")
+                .expect("pending finish state"),
+            r#"{"calls":1,"closed":false,"finished":false}"#,
+            "{operation} must wait for its callback promise"
+        );
+
+        vm.renderer_document_isolate
+            .clone()
+            .with_entered_renderer_document_isolate(|isolate| {
+                isolate.low_memory_notification();
+                Ok(())
+            })
+            .expect("collect while only the pending reactions retain their context");
+
+        vm.eval("__releaseFinish()")
+            .expect("release finish callback promise");
+        assert_eq!(
+            vm.eval("JSON.stringify(__finishState)")
+                .expect("finish after GC"),
+            r#"{"calls":1,"closed":true,"finished":true}"#,
+            "{operation} must retain its streams, residence and original reason"
+        );
+    }
+}
+
 #[test]
 fn transform_finish_pending_cancel_observes_writable_error_at_fulfillment() {
     let mut vm = stream_test_vm();