diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/tool_auth_handler.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/tool_auth_handler.py
index 21337cfa51..0d78a5759b 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/tool_auth_handler.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/tool_auth_handler.py
@@ -242,6 +242,12 @@ async def _get_existing_credential(
             existing_credential = await refresher.refresh(
                 existing_credential, self.auth_scheme
             )
+            # Persist the refreshed credential so the next invocation
+            # reads the new tokens instead of the stale pre-refresh ones.
+            # Without this, providers that rotate refresh_tokens on each
+            # refresh (e.g. Salesforce, many OIDC providers) will fail
+            # because the old refresh_token has already been invalidated.
+            self._store_credential(existing_credential)
         return existing_credential
     return None
 
diff --git a/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_tool_auth_handler.py b/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_tool_auth_handler.py
index a6babce651..1c9ace5c18 100644
--- a/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_tool_auth_handler.py
+++ b/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_tool_auth_handler.py
@@ -292,3 +292,75 @@ async def test_openid_connect_existing_oauth2_token_refresh(
   assert result.state == 'done'
   # The result should contain the refreshed credential after exchange
   assert result.auth_credential is not None
+
+
+@patch(
+    'google.adk.tools.openapi_tool.openapi_spec_parser.tool_auth_handler.OAuth2CredentialRefresher'
+)
+@pytest.mark.asyncio
+async def test_refreshed_credential_is_persisted_to_store(
+    mock_oauth2_refresher, openid_connect_scheme, openid_connect_credential
+):
+  """Test that refreshed OAuth2 credentials are persisted back to the store.
+
+  Regression test for https://github.com/google/adk-python/issues/5329.
+  Without persisting, the next invocation reads stale pre-refresh tokens from
+  state.  Providers that rotate refresh_tokens on each refresh (e.g.
+  Salesforce, many OIDC providers) will then fail because the old
+  refresh_token has already been invalidated.
+  """
+  # Create existing OAuth2 credential with an "old" refresh token.
+  existing_credential = AuthCredential(
+      auth_type=AuthCredentialTypes.OPEN_ID_CONNECT,
+      oauth2=OAuth2Auth(
+          client_id='test_client_id',
+          client_secret='test_client_secret',
+          access_token='old_access_token',
+          refresh_token='old_refresh_token',
+      ),
+  )
+
+  # The refresher will return a credential with rotated tokens.
+  refreshed_credential = AuthCredential(
+      auth_type=AuthCredentialTypes.OPEN_ID_CONNECT,
+      oauth2=OAuth2Auth(
+          client_id='test_client_id',
+          client_secret='test_client_secret',
+          access_token='new_access_token',
+          refresh_token='new_refresh_token',
+      ),
+  )
+
+  from unittest.mock import AsyncMock
+
+  mock_refresher_instance = MagicMock()
+  mock_refresher_instance.is_refresh_needed = AsyncMock(return_value=True)
+  mock_refresher_instance.refresh = AsyncMock(return_value=refreshed_credential)
+  mock_oauth2_refresher.return_value = mock_refresher_instance
+
+  tool_context = create_mock_tool_context()
+  credential_store = ToolContextCredentialStore(tool_context=tool_context)
+
+  # Store the existing (stale) credential.
+  key = credential_store.get_credential_key(
+      openid_connect_scheme, openid_connect_credential
+  )
+  credential_store.store_credential(key, existing_credential)
+
+  handler = ToolAuthHandler(
+      tool_context,
+      openid_connect_scheme,
+      openid_connect_credential,
+      credential_store=credential_store,
+  )
+
+  await handler.prepare_auth_credentials()
+
+  # The critical assertion: the *refreshed* credential must now be in the
+  # store so that the next invocation reads the new tokens, not the old ones.
+  persisted = credential_store.get_credential(
+      openid_connect_scheme, openid_connect_credential
+  )
+  assert persisted is not None
+  assert persisted.oauth2.access_token == 'new_access_token'
+  assert persisted.oauth2.refresh_token == 'new_refresh_token'