diff --git a/js/desktop/src/features/particles/text-reaction-input.tsx b/js/desktop/src/features/particles/text-reaction-input.tsx
index 5f2c172..9820064 100644
--- a/js/desktop/src/features/particles/text-reaction-input.tsx
+++ b/js/desktop/src/features/particles/text-reaction-input.tsx
@@ -1,6 +1,7 @@
 import { useEffect, useRef, useState } from "react";
 import { Send } from "lucide-react";
 import { useSuspendPlayback } from "@/hooks/use-suspend-playback";
+import { sanitizeReactionText } from "@/lib/firestore-particles";
 import { cn } from "@/lib/utils";
 
 const MAX_LENGTH = 40;
@@ -44,7 +45,9 @@ export function TextReactionInput({ open, onSubmit, onClose }: TextReactionInput
       <input
         ref={inputRef}
         value={value}
-        onChange={(e) => setValue(e.target.value.slice(0, MAX_LENGTH))}
+        onChange={(e) =>
+          setValue(sanitizeReactionText(e.target.value).slice(0, MAX_LENGTH))
+        }
         onBlur={onClose}
         onKeyDown={(e) => {
           if (e.key === "Enter") {
diff --git a/js/desktop/src/lib/firestore-particles.ts b/js/desktop/src/lib/firestore-particles.ts
index efa8349..2c6fe12 100644
--- a/js/desktop/src/lib/firestore-particles.ts
+++ b/js/desktop/src/lib/firestore-particles.ts
@@ -14,6 +14,7 @@ import {
   Timestamp,
   arrayUnion,
   arrayRemove,
+  FieldPath,
   type DocumentData,
   type FirestoreDataConverter,
   type QueryDocumentSnapshot,
@@ -399,17 +400,30 @@ export async function updateStreamPlaybackMarker(
   });
 }
 
+// Strip characters that Firestore's dotted field-path syntax treats specially.
+// Using FieldPath bypasses the parser, but we also forbid these in stored keys
+// so reaction maps stay portable across any future read path.
+const RESERVED_REACTION_CHARS = /[~*/[\]]/g;
+
+export function sanitizeReactionText(text: string): string {
+  return text.replace(RESERVED_REACTION_CHARS, "");
+}
+
 export async function toggleParticleReaction(
   docPath: string,
   emoji: string,
   humanId: string,
   currentReactions?: Reactions,
 ): Promise<void> {
+  const key = sanitizeReactionText(emoji);
+  if (!key) return;
   const particleRef = typedDoc(docPath);
-  const field = `reactions.${emoji}`;
-  const alreadyReacted = currentReactions?.[emoji]?.includes(humanId) ?? false;
-  await updateDoc(particleRef, {
-    [field]: alreadyReacted ? arrayRemove(humanId) : arrayUnion(humanId),
-    updated_at: serverTimestamp(),
-  });
+  const alreadyReacted = currentReactions?.[key]?.includes(humanId) ?? false;
+  await updateDoc(
+    particleRef,
+    new FieldPath("reactions", key),
+    alreadyReacted ? arrayRemove(humanId) : arrayUnion(humanId),
+    "updated_at",
+    serverTimestamp(),
+  );
 }
diff --git a/js/mobile/src/features/stream-view/ReactionSheet.tsx b/js/mobile/src/features/stream-view/ReactionSheet.tsx
index 609f29e..4cdd6b2 100644
--- a/js/mobile/src/features/stream-view/ReactionSheet.tsx
+++ b/js/mobile/src/features/stream-view/ReactionSheet.tsx
@@ -33,6 +33,7 @@ import Animated, {
 import { REACTION_EMOJIS, type Reactions } from "@/api/types";
 import type { Human } from "@/api/types";
 import { resolveHumanDisplay } from "@/lib/humans";
+import { sanitizeReactionText } from "@/lib/firestore-particles";
 import { cn } from "@/lib/utils";
 import { useSuspendPlayback } from "@/hooks/use-suspend-playback";
 
@@ -327,7 +328,9 @@ export function ReactionSheet({
                   <View className="flex-1 bg-white/10 rounded-full px-4 py-2.5">
                     <TextInput
                       value={text}
-                      onChangeText={(v) => setText(v.slice(0, TEXT_REACTION_MAX))}
+                      onChangeText={(v) =>
+                        setText(sanitizeReactionText(v).slice(0, TEXT_REACTION_MAX))
+                      }
                       placeholder="Send a quick reply..."
                       placeholderTextColor="rgba(255,255,255,0.4)"
                       maxLength={TEXT_REACTION_MAX}
diff --git a/js/mobile/src/lib/firestore-particles.ts b/js/mobile/src/lib/firestore-particles.ts
index ce1d19e..0044169 100644
--- a/js/mobile/src/lib/firestore-particles.ts
+++ b/js/mobile/src/lib/firestore-particles.ts
@@ -14,6 +14,7 @@ import {
   Timestamp,
   arrayUnion,
   arrayRemove,
+  FieldPath,
   type DocumentData,
   type FirestoreDataConverter,
   type QueryDocumentSnapshot,
@@ -419,17 +420,30 @@ export async function updateStreamPlaybackMarker(
   });
 }
 
+// Strip characters that Firestore's dotted field-path syntax treats specially.
+// Using FieldPath bypasses the parser, but we also forbid these in stored keys
+// so reaction maps stay portable across any future read path.
+const RESERVED_REACTION_CHARS = /[~*/[\]]/g;
+
+export function sanitizeReactionText(text: string): string {
+  return text.replace(RESERVED_REACTION_CHARS, "");
+}
+
 export async function toggleParticleReaction(
   docPath: string,
   emoji: string,
   humanId: string,
   currentReactions?: Reactions,
 ): Promise<void> {
+  const key = sanitizeReactionText(emoji);
+  if (!key) return;
   const particleRef = typedDoc(docPath);
-  const field = `reactions.${emoji}`;
-  const alreadyReacted = currentReactions?.[emoji]?.includes(humanId) ?? false;
-  await updateDoc(particleRef, {
-    [field]: alreadyReacted ? arrayRemove(humanId) : arrayUnion(humanId),
-    updated_at: serverTimestamp(),
-  });
+  const alreadyReacted = currentReactions?.[key]?.includes(humanId) ?? false;
+  await updateDoc(
+    particleRef,
+    new FieldPath("reactions", key),
+    alreadyReacted ? arrayRemove(humanId) : arrayUnion(humanId),
+    "updated_at",
+    serverTimestamp(),
+  );
 }