fix(workflow): decode base64 bytes after JSON session resume

src/google/adk/workflow/_function_node.py

@@ -14,6 +14,9 @@
 
 from __future__ import annotations
 
+import base64
+import binascii
+
 import asyncio
 import collections.abc
 from collections.abc import AsyncGenerator
@@ -59,6 +62,11 @@
 from .utils._workflow_hitl_utils import process_auth_resume
 
 logger = logging.getLogger("google_adk." + __name__)
+
+def _annotation_is_bytes(annotated_type: Any) -> bool:
+  """True when the annotation is exactly ``bytes`` or ``bytearray``."""
+  return annotated_type is bytes or annotated_type is bytearray
+
 _ANY_TYPE_ADAPTER: TypeAdapter[Any] = TypeAdapter(Any)
 
 
@@ -696,6 +704,15 @@ def _coerce_param(
       value = _unwrap_content_payload(value, annotated_type)
     if param_name in self._failed_type_adapters:
       return value
+    # JSON session stores (DatabaseSessionService / SqliteSessionService) dump
+    # bytes inside Event.output (typed Any) as base64 strings. TypeAdapter(bytes)
+    # then utf-8-encodes the string (so b"\x00\x01" becomes b"AAE="). Prefer
+    # base64 decode when the annotation is bytes and the value is still a str.
+    if isinstance(value, str) and _annotation_is_bytes(annotated_type):
+      try:
+        return base64.b64decode(value, validate=True)
+      except binascii.Error:
+        pass
     adapter = self._type_adapters.get(param_name)
     if adapter is None:
       adapter = self._build_type_adapter(param_name, annotated_type)

tests/unittests/workflow/test_function_node_bytes_json_resume.py

@@ -0,0 +1,54 @@
+# Copyright 2026 Google LLC
+#
+# 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.
+
+"""Regression: bytes node_input after JSON Event round-trip (Fixes #7852)."""
+
+from __future__ import annotations
+
+import base64
+
+from google.adk.events.event import Event
+from google.adk.workflow._function_node import FunctionNode
+
+
+def test_function_node_coerces_base64_str_to_bytes_after_event_json_roundtrip():
+  """Database/Sqlite session stores Event.output as JSON; bytes become base64.
+
+  Event.output is typed Any, so val_json_bytes does not restore bytes on load.
+  FunctionNode must base64-decode when the parameter annotation is bytes.
+  """
+
+  def consume(node_input: bytes):
+    return node_input
+
+  node = FunctionNode(func=consume, name="consume")
+  original = b"\x00\x01"
+  event = Event(author="produce", invocation_id="inv", output=original)
+  restored = Event.model_validate(event.model_dump(mode="json", exclude_none=True))
+  assert isinstance(restored.output, str)
+  assert restored.output == base64.b64encode(original).decode("ascii")
+
+  coerced = node._coerce_param("node_input", restored.output, bytes)
+  assert coerced == original
+  assert isinstance(coerced, bytes)
+
+
+def test_function_node_bytes_coerce_falls_back_for_non_base64_str():
+  def consume(node_input: bytes):
+    return node_input
+
+  node = FunctionNode(func=consume, name="consume")
+  # Not valid base64 (wrong padding / alphabet) → TypeAdapter encodes utf-8
+  out = node._coerce_param("node_input", "hello", bytes)
+  assert out == b"hello"