diff --git a/src/lib/json_buffer.ts b/src/lib/json_buffer.ts
index 36ec6e1..078b023 100644
--- a/src/lib/json_buffer.ts
+++ b/src/lib/json_buffer.ts
@@ -604,7 +604,15 @@ class SharedJsonBufferImpl<T extends Proxyable> extends Serializable {
         return Reflect.get(target, prop, receiver);
       }
       if (prop === "__ptr") return target.__ptr;
-      if (prop === "toJSON") return () => this.toJSON(target.__ptr);
+      if (prop === "toJSON")
+        return () => {
+          // Clear caches that may be stale from another thread's GC
+          // compacting the shared buffer and recycling addresses.
+          this.stringCache.clear();
+          this.proxyCache.clear();
+          this.propertyHints.clear();
+          return this.toJSON(target.__ptr);
+        };
 
       const ptr = target.__ptr;
       if (ptr === 0) return undefined;
@@ -1211,7 +1219,13 @@ class SharedJsonBufferImpl<T extends Proxyable> extends Serializable {
       if (prop === CONSOLE_VIEW) return () => this.toConsoleView(target.__ptr);
       if (prop === toSerialized) return () => this[toSerialized]();
       if (prop === "__ptr") return target.__ptr;
-      if (prop === "toJSON") return () => this.toJSON(target.__ptr);
+      if (prop === "toJSON")
+        return () => {
+          this.stringCache.clear();
+          this.proxyCache.clear();
+          this.propertyHints.clear();
+          return this.toJSON(target.__ptr);
+        };
       if (prop === Symbol.iterator) {
         return () => new ArrayCursor(this, target.__ptr);
       }
diff --git a/src/lib/sync/mpmc.ts b/src/lib/sync/mpmc.ts
index 302d010..75d7f35 100644
--- a/src/lib/sync/mpmc.ts
+++ b/src/lib/sync/mpmc.ts
@@ -289,9 +289,15 @@ export class Receiver<T> extends ChannelHandle<T> {
         : { ok: false, error: ERR_SPURIOUS };
     }
 
+    // Deep-copy under sendLock to prevent concurrent GC from
+    // compacting the SharedArrayBuffer while we traverse the proxy.
+    const sendToken = await this.internals.sendLock.acquire();
+    const copied = typeof (val as any)?.toJSON === "function" ? (val as any).toJSON() : val;
+    sendToken[Symbol.dispose]();
+
     // Handover: Item token consumed -> Slot token released
     this.internals.slotsAvailable[INTERNAL_SEMAPHORE_CONTROLLER].release(1);
-    return { ok: true, value: val };
+    return { ok: true, value: copied };
   }
 
   blockingRecv(): Result<T, Error> {
@@ -320,8 +326,14 @@ export class Receiver<T> extends ChannelHandle<T> {
         : { ok: false, error: ERR_SPURIOUS };
     }
 
+    // Deep-copy under sendLock to prevent concurrent GC from
+    // compacting the SharedArrayBuffer while we traverse the proxy.
+    const sendToken = this.internals.sendLock.blockingAcquire();
+    const copied = typeof (val as any)?.toJSON === "function" ? (val as any).toJSON() : val;
+    sendToken[Symbol.dispose]();
+
     this.internals.slotsAvailable[INTERNAL_SEMAPHORE_CONTROLLER].release(1);
-    return { ok: true, value: val };
+    return { ok: true, value: copied };
   }
 
   async *[Symbol.asyncIterator](): AsyncGenerator<T, void, void> {