diff --git a/src/google/adk/runners.py b/src/google/adk/runners.py
index 17737297..8723ea2e 100644
--- a/src/google/adk/runners.py
+++ b/src/google/adk/runners.py
@@ -84,6 +84,19 @@ def _has_non_empty_transcription_text(transcription) -> bool:
   )
 
 
+def _apply_run_config_custom_metadata(
+    event: Event, run_config: RunConfig | None
+) -> None:
+  """Merges run-level custom metadata into the event, if present."""
+  if not run_config or not run_config.custom_metadata:
+    return
+
+  event.custom_metadata = {
+      **run_config.custom_metadata,
+      **(event.custom_metadata or {}),
+  }
+
+
 class Runner:
   """The Runner class is used to run agents.
 
@@ -695,6 +708,9 @@ class Runner:
           author='model',
           content=early_exit_result,
       )
+      _apply_run_config_custom_metadata(
+          early_exit_event, invocation_context.run_config
+      )
       if self._should_append_event(early_exit_event, is_live_call):
         await self.session_service.append_event(
             session=session,
@@ -721,6 +737,9 @@ class Runner:
 
       async with Aclosing(execute_fn(invocation_context)) as agen:
         async for event in agen:
+          _apply_run_config_custom_metadata(
+              event, invocation_context.run_config
+          )
           if is_live_call:
             if event.partial and _is_transcription(event):
               is_transcribing = True
@@ -775,7 +794,13 @@ class Runner:
           modified_event = await plugin_manager.run_on_event_callback(
               invocation_context=invocation_context, event=event
           )
-          yield (modified_event if modified_event else event)
+          if modified_event:
+            _apply_run_config_custom_metadata(
+                modified_event, invocation_context.run_config
+            )
+            yield modified_event
+          else:
+            yield event
 
     # Step 4: Run the after_run callbacks to perform global cleanup tasks or
     # finalizing logs and metrics data.
@@ -846,6 +871,7 @@ class Runner:
           author='user',
           content=new_message,
       )
+    _apply_run_config_custom_metadata(event, invocation_context.run_config)
     # If new_message is a function response, find the matching function call
     # and use its branch as the new event's branch.
     if function_call := invocation_context._find_matching_function_call(event):
diff --git a/tests/unittests/test_runners.py b/tests/unittests/test_runners.py
index d692f7e3..c347a789 100644
--- a/tests/unittests/test_runners.py
+++ b/tests/unittests/test_runners.py
@@ -16,6 +16,7 @@ import importlib
 from pathlib import Path
 import sys
 import textwrap
+from typing import AsyncGenerator
 from typing import Optional
 from unittest.mock import AsyncMock
 
@@ -23,6 +24,7 @@ from google.adk.agents.base_agent import BaseAgent
 from google.adk.agents.context_cache_config import ContextCacheConfig
 from google.adk.agents.invocation_context import InvocationContext
 from google.adk.agents.llm_agent import LlmAgent
+from google.adk.agents.run_config import RunConfig
 from google.adk.apps.app import App
 from google.adk.apps.app import ResumabilityConfig
 from google.adk.artifacts.in_memory_artifact_service import InMemoryArtifactService
@@ -54,7 +56,9 @@ class MockAgent(BaseAgent):
     if parent_agent:
       self.parent_agent = parent_agent
 
-  async def _run_async_impl(self, invocation_context):
+  async def _run_async_impl(
+      self, invocation_context: InvocationContext
+  ) -> AsyncGenerator[Event, None]:
     yield Event(
         invocation_id=invocation_context.invocation_id,
         author=self.name,
@@ -78,7 +82,9 @@ class MockLlmAgent(LlmAgent):
     self.disallow_transfer_to_parent = disallow_transfer_to_parent
     self.parent_agent = parent_agent
 
-  async def _run_async_impl(self, invocation_context):
+  async def _run_async_impl(
+      self, invocation_context: InvocationContext
+  ) -> AsyncGenerator[Event, None]:
     yield Event(
         invocation_id=invocation_context.invocation_id,
         author=self.name,
@@ -88,6 +94,25 @@ class MockLlmAgent(LlmAgent):
     )
 
 
+class MockAgentWithMetadata(BaseAgent):
+  """Mock agent that returns event-level custom metadata."""
+
+  def __init__(self, name: str):
+    super().__init__(name=name, sub_agents=[])
+
+  async def _run_async_impl(
+      self, invocation_context: InvocationContext
+  ) -> AsyncGenerator[Event, None]:
+    yield Event(
+        invocation_id=invocation_context.invocation_id,
+        author=self.name,
+        content=types.Content(
+            role="model", parts=[types.Part(text="Test response")]
+        ),
+        custom_metadata={"event_key": "event_value"},
+    )
+
+
 class MockPlugin(BasePlugin):
   """Mock plugin for unit testing."""
 
@@ -495,6 +520,41 @@ async def test_runner_allows_nested_agent_directories(tmp_path, monkeypatch):
     assert result is False
 
 
+@pytest.mark.asyncio
+async def test_run_config_custom_metadata_propagates_to_events():
+  session_service = InMemorySessionService()
+  runner = Runner(
+      app_name=TEST_APP_ID,
+      agent=MockAgentWithMetadata("metadata_agent"),
+      session_service=session_service,
+      artifact_service=InMemoryArtifactService(),
+  )
+  await session_service.create_session(
+      app_name=TEST_APP_ID, user_id=TEST_USER_ID, session_id=TEST_SESSION_ID
+  )
+
+  run_config = RunConfig(custom_metadata={"request_id": "req-1"})
+  events = [
+      event
+      async for event in runner.run_async(
+          user_id=TEST_USER_ID,
+          session_id=TEST_SESSION_ID,
+          new_message=types.Content(role="user", parts=[types.Part(text="hi")]),
+          run_config=run_config,
+      )
+  ]
+
+  assert events[0].custom_metadata is not None
+  assert events[0].custom_metadata["request_id"] == "req-1"
+  assert events[0].custom_metadata["event_key"] == "event_value"
+
+  session = await session_service.get_session(
+      app_name=TEST_APP_ID, user_id=TEST_USER_ID, session_id=TEST_SESSION_ID
+  )
+  user_event = next(event for event in session.events if event.author == "user")
+  assert user_event.custom_metadata == {"request_id": "req-1"}
+
+
 class TestRunnerWithPlugins:
   """Tests for Runner with plugins."""