feat: Support dynamic per-request headers in MCPToolset

Add a header_provider param which is a callable[ReadonlyContext, Dict[str, Any]] for users to build headers in MCPToolset
fix: https://github.com/google/adk-python/issues/3156
PiperOrigin-RevId: 820412372

contributing/samples/mcp_dynamic_header_agent/header_server.py

@@ -0,0 +1,50 @@
+# Copyright 2025 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.
+
+from __future__ import annotations
+
+from fastapi import Request
+from mcp.server.fastmcp import Context
+from mcp.server.fastmcp import FastMCP
+
+mcp = FastMCP('Header Check Server', host='localhost', port=3000)
+
+TENANT_DATA = {
+    'tenant1': {'name': 'Tenant 1', 'data': 'Data for tenant 1'},
+    'tenant2': {'name': 'Tenant 2', 'data': 'Data for tenant 2'},
+}
+
+
+@mcp.tool(
+    description='Returns tenant specific data based on X-Tenant-ID header.'
+)
+def get_tenant_data(context: Context) -> dict:
+  """Return tenant specific data."""
+  if context.request_context and context.request_context.request:
+    headers = context.request_context.request.headers
+    tenant_id = headers.get('x-tenant-id')
+    if tenant_id in TENANT_DATA:
+      return TENANT_DATA[tenant_id]
+    else:
+      return {'error': f'Tenant {tenant_id} not found'}
+  else:
+    return {'error': 'Could not get request context'}
+
+
+if __name__ == '__main__':
+  try:
+    print('Starting Header Check MCP server on http://localhost:3000')
+    mcp.run(transport='streamable-http')
+  except KeyboardInterrupt:
+    print('\nServer stopped.')