diff --git a/apps/web/src/components/ComposerPromptEditorTiptap.tsx b/apps/web/src/components/ComposerPromptEditorTiptap.tsx
index abc9549ee3d00..911674fe91c00 100644
--- a/apps/web/src/components/ComposerPromptEditorTiptap.tsx
+++ b/apps/web/src/components/ComposerPromptEditorTiptap.tsx
@@ -1013,7 +1013,7 @@ function ComposerPromptEditorTiptapInner(
 
   const handleEditorChange = (updated: TiptapEditor) => {
     const map = serializeEditorDoc(updated.state.doc);
-    const contextIds = literalText ? [] : map.contextIds;
+    const contextIds = literalText ? [] : [...map.contextIds];
     const { from, to } = updated.state.selection;
     const fromFlat = pmToFlat(map, from);
     const toFlat = pmToFlat(map, to);
@@ -1468,7 +1468,7 @@ function ComposerPromptEditorTiptapInner(
       value: map.value,
       cursor: collapseExpandedComposerCursor(map.value, expandedCursor, literalText),
       expandedCursor,
-      contextIds: literalText ? [] : map.contextIds,
+      contextIds: literalText ? [] : [...map.contextIds],
     };
     const toFlat = pmToFlat(map, to);
     selectionRangeRef.current = {
diff --git a/apps/web/src/composer-rich-text-doc.ts b/apps/web/src/composer-rich-text-doc.ts
index c0e0a653016e3..2a2a3d3b9af42 100644
--- a/apps/web/src/composer-rich-text-doc.ts
+++ b/apps/web/src/composer-rich-text-doc.ts
@@ -811,11 +811,12 @@ export interface RichRun {
   nodeName?: string;
 }
 
+// Shared by every caller that serializes the same doc, so it must not be mutated.
 export interface RichDocMap {
-  value: string;
-  runs: RichRun[];
-  docLength: number;
-  contextIds: string[];
+  readonly value: string;
+  readonly runs: ReadonlyArray<Readonly<RichRun>>;
+  readonly docLength: number;
+  readonly contextIds: ReadonlyArray<string>;
 }
 
 function readAtomSource(node: ProseMirrorNode): string {
@@ -1285,7 +1286,20 @@ export function serializeSelection(doc: ProseMirrorNode, from: number, to: numbe
   return serializeEditorDoc(doc.type.create(null, content)).value;
 }
 
+// Docs are immutable, and one keystroke serializes the same doc from the marker
+// plugin, onUpdate, and onSelectionUpdate.
+const serializedDocs = new WeakMap<ProseMirrorNode, RichDocMap>();
+
 export function serializeEditorDoc(doc: ProseMirrorNode): RichDocMap {
+  let map = serializedDocs.get(doc);
+  if (!map) {
+    map = serializeDoc(doc);
+    serializedDocs.set(doc, map);
+  }
+  return map;
+}
+
+function serializeDoc(doc: ProseMirrorNode): RichDocMap {
   const acc: RichAccumulator = { runs: [], value: "", flat: 0, md: 0 };
   const blocks: ProseMirrorNode[] = [];
   doc.content.forEach((node) => {