diff --git a/bindings/otel-thread-ctx.cc b/bindings/otel-thread-ctx.cc
index 69d7aa43..15477a11 100644
--- a/bindings/otel-thread-ctx.cc
+++ b/bindings/otel-thread-ctx.cc
@@ -614,14 +614,21 @@ void CtxWrap::Append(const FunctionCallbackInfo<Value>& args) {
     isolate->ThrowError("allocation failed");
     return;
   }
+  // Capture before the copy: the point of the assert below is that the memcpy
+  // carried the header across intact, not that the record is valid. It used to
+  // assert `valid == 1`, which invalidate() legitimately makes false — and
+  // since NDEBUG is not defined for this addon, that aborted release builds
+  // too, not just debug ones.
+  const uint8_t src_valid = self->record_->valid;
   // Copy the existing record (header + already-written attrs_data).
   memcpy(
       new_rec.get(), self->record_, sizeof(OtelThreadCtxRecord) + current_used);
   // Append the new entries and update attrs_data_size.
   memcpy(&new_rec->attrs_data[current_used], appended.data(), appended.size());
   new_rec->attrs_data_size = static_cast<uint16_t>(new_used);
-  // The copy should've preserved valid=1 from the source record.
-  assert(new_rec->valid == 1);
+  // The copy should've carried the source record's header across verbatim,
+  // whatever its validity was.
+  assert(new_rec->valid == src_valid);
 
   // Publish: the pointer swap is the atomic boundary the reader sees. The
   // first fence keeps the new_rec content writes ordered before the pointer
diff --git a/ts/test/otel-invalidate-append.ts b/ts/test/otel-invalidate-append.ts
new file mode 100644
index 00000000..9d12dc16
--- /dev/null
+++ b/ts/test/otel-invalidate-append.ts
@@ -0,0 +1,57 @@
+/*
+ * Copyright 2026 Datadog, Inc
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+// Runs in a forked process because the failure mode is an abort, which would
+// take the whole mocha run down with it.
+//
+// Append's reallocate path used to assert that the copied record had
+// `valid == 1`. invalidate() sets that byte to 0 and appending afterwards is
+// supported, so an append too large to fit in place aborted the process:
+//
+//     Assertion `new_rec->valid == 1' failed.
+//
+// Only the reallocate path is affected — an append that fits the current
+// capacity is written in place and never copies the header. A fresh record has
+// 36 bytes of attrs_data capacity (64 - sizeof(header)), so the value below is
+// comfortably past it.
+
+import assert from 'assert';
+
+import {otelThreadCtx} from '../src/index';
+
+const VALUE = 'x'.repeat(200);
+
+const ctx = new otelThreadCtx.ThreadContext(
+  Buffer.alloc(16, 1),
+  Buffer.alloc(8, 2),
+);
+
+ctx.run(() => {
+  ctx.invalidate();
+  ctx.appendAttributes([VALUE]);
+
+  const bytes = ctx.debugBytes();
+  const attrsDataSize = bytes[26] | (bytes[27] << 8);
+
+  // invalidate() must stick: growing the record does not resurrect it.
+  assert.strictEqual(bytes[24], 0, 'valid byte should still be 0');
+  // key index (1) + length (1) + the value itself.
+  assert.strictEqual(attrsDataSize, VALUE.length + 2, 'attrs_data_size');
+});
+
+console.log('ok');
diff --git a/ts/test/test-otel-thread-ctx.ts b/ts/test/test-otel-thread-ctx.ts
index f26af9db..f4d6683b 100644
--- a/ts/test/test-otel-thread-ctx.ts
+++ b/ts/test/test-otel-thread-ctx.ts
@@ -770,6 +770,45 @@ function captureBytes(opts: {
       });
     });
 
+    describe('invalidate then grow', () => {
+      // Regression test: Append's reallocate path asserted that the copied
+      // record had valid == 1, which invalidate() legitimately makes false, so
+      // an append too large to fit in place aborted the process. NDEBUG is not
+      // defined for this addon, so that hit release builds too. The sibling
+      // test above appends 6 bytes, which fits the initial 36-byte capacity and
+      // is written in place, so it never reached the copy. Forked, because the
+      // failure is an abort rather than a test failure.
+      it('should survive an append that reallocates after invalidate', async function () {
+        this.timeout(30000);
+
+        const proc = fork(join(__dirname, 'otel-invalidate-append.js'), {
+          silent: true,
+        });
+        let output = '';
+        proc.stdout?.on('data', chunk => {
+          output += chunk;
+        });
+        proc.stderr?.on('data', chunk => {
+          output += chunk;
+        });
+
+        await new Promise<void>((resolve, reject) => {
+          proc.on('error', reject);
+          proc.on('close', (code, signal) => {
+            if (code === 0) {
+              resolve();
+            } else {
+              reject(
+                new Error(
+                  `otel-invalidate-append exited with code=${code} signal=${signal}\n${output}`,
+                ),
+              );
+            }
+          });
+        });
+      });
+    });
+
     describe('getProcessContextAttributes', () => {
       it('rejects non-array keys', () => {
         strictAssert.throws(