diff --git a/core/packages/gax/src/observability/errorUtil.ts b/core/packages/gax/src/observability/errorUtil.ts
index ce07114783ce..b54bc3a7276c 100644
--- a/core/packages/gax/src/observability/errorUtil.ts
+++ b/core/packages/gax/src/observability/errorUtil.ts
@@ -15,6 +15,7 @@
  */
 
 import {Attributes, Span} from '@opentelemetry/api';
+import * as protobuf from 'protobufjs';
 import {GoogleError} from '../googleError';
 import {Status} from '../status';
 /**
@@ -798,9 +799,46 @@ export function isServerSideError(
   return resolveServerErrorCode(e, rpcType) !== undefined;
 }
 
+/**
+ * Returns the bytes of binary data: an ArrayBuffer, or a view of one such as a Buffer
+ * or Uint8Array.
+ *
+ * @param {unknown} value - The value to check.
+ * @returns {Uint8Array | undefined} The bytes of `value`, or undefined if it is not binary data.
+ */
+function toBytes(value: unknown): Uint8Array | undefined {
+  if (ArrayBuffer.isView(value)) {
+    return new Uint8Array(value.buffer, value.byteOffset, value.byteLength);
+  }
+  if (value instanceof ArrayBuffer) {
+    return new Uint8Array(value);
+  }
+  return undefined;
+}
+
+/**
+ * Encodes bytes as base64, using Buffer when it is available and a pure JavaScript
+ * encoder otherwise (e.g. in browsers).
+ *
+ * @param {Uint8Array} bytes - The bytes to encode.
+ * @returns {string} The base64 encoding of `bytes`.
+ */
+function encodeBase64(bytes: Uint8Array): string {
+  if (typeof Buffer !== 'undefined') {
+    return Buffer.from(
+      bytes.buffer,
+      bytes.byteOffset,
+      bytes.byteLength,
+    ).toString('base64');
+  }
+  return protobuf.util.base64.encode(bytes, 0, bytes.length);
+}
+
 /**
  * Safely converts a value to a JSON string without throwing exceptions on
- * circular references, BigInt values, or non-serializable properties.
+ * circular references, BigInt values, or non-serializable properties. Binary data
+ * (e.g. a Buffer or Uint8Array) is serialized as a base64 string rather than one
+ * JSON entry per byte.
  *
  * @param {unknown} value - The value to serialize.
  * @returns {string | undefined} The serialized JSON string, or undefined if `value` is undefined or unserializable.
@@ -813,13 +851,24 @@ export function safeJsonStringify(value: unknown): string | undefined {
     const ancestors: unknown[] = [];
     return JSON.stringify(
       value,
-      function (this: unknown, _key: string, val: unknown) {
+      function (this: unknown, key: string, val: unknown) {
         if (typeof val === 'bigint') {
           return val.toString();
         }
         if (typeof val !== 'object' || val === null) {
           return val;
         }
+        // Encode binary data as base64. Buffer#toJSON runs before this replacer and
+        // turns a Buffer into {type: 'Buffer', data: [...]}, so in that case the
+        // original Buffer is read back from the holder.
+        const bytes = toBytes(
+          (val as {type?: unknown}).type === 'Buffer'
+            ? (this as Record<string, unknown>)[key]
+            : val,
+        );
+        if (bytes) {
+          return encodeBase64(bytes);
+        }
         // Track object ancestry to replace circular references.
         if (ancestors.includes(this)) {
           while (
@@ -845,6 +894,38 @@ export function safeJsonStringify(value: unknown): string | undefined {
   }
 }
 
+/**
+ * Resolves the `response.payload` attribute: the full response payload of a call,
+ * serialized as JSON without truncation or redaction. String payloads are returned
+ * as-is and binary payloads (e.g. a Buffer or Uint8Array) as base64, as are binary
+ * values nested in the response (e.g. `bytes` fields). Prefers `rawResponse`, the
+ * unprocessed response gax provides for paginated and long-running calls (the full
+ * page response or the raw Operation), over the processed `response`.
+ *
+ * @param {unknown} response - The response passed to the callback or resolved by the promise.
+ * @param {unknown} [rawResponse] - The raw (unprocessed) response, when gax provides one.
+ * @returns {string | undefined} The serialized payload, or undefined if there is no response.
+ */
+export function resolveResponsePayload(
+  response: unknown,
+  rawResponse?: unknown,
+): string | undefined {
+  const payload = rawResponse ?? response;
+  if (payload === undefined || payload === null) {
+    return undefined;
+  }
+  // TODO(security): The payload is returned in full because `response.payload` is defined
+  // as the raw response, so it can contain sensitive data (e.g. secret values or access
+  // tokens). Only emit it when logging is explicitly enabled (see `checkLoggingEnabled`),
+  // and consider redacting known sensitive fields.
+  if (typeof payload === 'string') {
+    return payload;
+  }
+  // Binary payloads are base64 encoded and, like strings, not wrapped in JSON quotes.
+  const bytes = toBytes(payload);
+  return bytes ? encodeBase64(bytes) : safeJsonStringify(payload);
+}
+
 /**
  * Extracts the server-side error details message attached by GFE/backend on
  * server-side errors, checking the error and its `.cause` chain and falling
diff --git a/core/packages/gax/test/unit/errorUtil.ts b/core/packages/gax/test/unit/errorUtil.ts
index e390e2223034..205d485b87d2 100644
--- a/core/packages/gax/test/unit/errorUtil.ts
+++ b/core/packages/gax/test/unit/errorUtil.ts
@@ -51,6 +51,7 @@ import {
   resolveExceptionMessage,
   resolveExceptionStacktrace,
   resolveResponseStatusCode,
+  resolveResponsePayload,
   resolveServerExceptionDetails,
   recordExceptionEvent,
   isPreConnectionFailure,
@@ -1121,6 +1122,109 @@ describe('errorUtil', () => {
       };
       assert.strictEqual(safeJsonStringify(badObj), undefined);
     });
+
+    it('serializes binary data as base64 instead of one entry per byte', () => {
+      // Buffers reach the replacer already converted by Buffer#toJSON, unlike other
+      // binary data such as Uint8Arrays, so both are covered.
+      assert.strictEqual(
+        safeJsonStringify({
+          buffer: Buffer.from('hello'),
+          list: [new Uint8Array([1, 2, 3])],
+        }),
+        '{"buffer":"aGVsbG8=","list":["AQID"]}',
+      );
+      // Objects that only look like a serialized Buffer are left as-is.
+      assert.strictEqual(
+        safeJsonStringify({type: 'Buffer', data: [1, 2]}),
+        '{"type":"Buffer","data":[1,2]}',
+      );
+    });
+  });
+
+  describe('resolveResponsePayload', () => {
+    it('serializes the full response as JSON and returns string payloads as-is', () => {
+      assert.strictEqual(
+        resolveResponsePayload({
+          name: 'projects/p/secrets/s',
+          labels: {env: 'prod'},
+          versions: [1, 2],
+        }),
+        '{"name":"projects/p/secrets/s","labels":{"env":"prod"},"versions":[1,2]}',
+      );
+      assert.strictEqual(resolveResponsePayload('raw body'), 'raw body');
+      // Empty responses (e.g. google.protobuf.Empty) still have a payload.
+      assert.strictEqual(resolveResponsePayload({}), '{}');
+    });
+
+    it('prefers the raw response provided for paginated and long-running calls', () => {
+      const resources = [{name: 'a'}, {name: 'b'}];
+      const rawResponse = {items: resources, nextPageToken: 'token'};
+      assert.strictEqual(
+        resolveResponsePayload(resources, rawResponse),
+        '{"items":[{"name":"a"},{"name":"b"}],"nextPageToken":"token"}',
+      );
+      // A null raw response falls back to the response.
+      assert.strictEqual(
+        resolveResponsePayload({name: 'a'}, null),
+        '{"name":"a"}',
+      );
+    });
+
+    it('returns undefined when there is no response', () => {
+      assert.strictEqual(resolveResponsePayload(undefined), undefined);
+      assert.strictEqual(resolveResponsePayload(null), undefined);
+    });
+
+    it('serializes protobuf messages through their JSON representation', () => {
+      const message = new GoogleErrorDecoder().root
+        .lookupType('google.rpc.ErrorInfo')
+        .fromObject(SERVICE_DISABLED_ERROR_INFO);
+      assert.strictEqual(
+        resolveResponsePayload(message),
+        '{"reason":"SERVICE_DISABLED","domain":"googleapis.com",' +
+          '"metadata":{"service":"translate.googleapis.com","consumer":"projects/123"}}',
+      );
+    });
+
+    it('encodes binary payloads as base64', () => {
+      assert.strictEqual(
+        resolveResponsePayload(Buffer.from('hello')),
+        'aGVsbG8=',
+      );
+      assert.strictEqual(
+        resolveResponsePayload(new Uint8Array([1, 2, 3]).buffer),
+        'AQID',
+      );
+      // Only the viewed bytes are encoded, not the rest of the underlying ArrayBuffer.
+      assert.strictEqual(
+        resolveResponsePayload(new Uint8Array([0, 1, 2, 3, 4]).subarray(1, 4)),
+        'AQID',
+      );
+      // Typed arrays from another realm are recognized too.
+      assert.strictEqual(
+        resolveResponsePayload(vm.runInNewContext('new Uint8Array([1, 2, 3])')),
+        'AQID',
+      );
+    });
+
+    it('encodes binary payloads as base64 when Buffer is not defined', () => {
+      const originalBuffer = (globalThis as Record<string, unknown>).Buffer;
+      try {
+        delete (globalThis as Record<string, unknown>).Buffer;
+        assert.strictEqual(
+          resolveResponsePayload(
+            new Uint8Array([0, 1, 2, 3, 4]).subarray(1, 4),
+          ),
+          'AQID',
+        );
+        assert.strictEqual(
+          resolveResponsePayload(new Uint8Array([255])),
+          '/w==',
+        );
+      } finally {
+        (globalThis as Record<string, unknown>).Buffer = originalBuffer;
+      }
+    });
   });
 
   describe('isPreConnectionFailure', () => {