Add default Origin for streamable HTTP client

Author: tarunag10

src/mcp/client/streamable_http.py

@@ -7,6 +7,7 @@
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from contextlib import asynccontextmanager
 from dataclasses import dataclass
+from urllib.parse import urlsplit
 
 import anyio
 import httpx
@@ -50,6 +51,15 @@
 MAX_RECONNECTION_ATTEMPTS = 2  # Max retry attempts before giving up
 
 
+def _get_default_origin(url: str) -> str | None:
+    parsed_url = urlsplit(url)
+    if parsed_url.scheme not in {"http", "https"} or not parsed_url.netloc:
+        return None
+
+    authority = parsed_url.netloc.rsplit("@", 1)[-1]
+    return f"{parsed_url.scheme}://{authority}"
+
+
 class StreamableHTTPError(Exception):
     """Base exception for StreamableHTTP transport errors."""
 
@@ -72,13 +82,16 @@ class RequestContext:
 class StreamableHTTPTransport:
     """StreamableHTTP client transport implementation."""
 
-    def __init__(self, url: str) -> None:
+    def __init__(self, url: str, default_origin: str | None = None) -> None:
         """Initialize the StreamableHTTP transport.
 
         Args:
             url: The endpoint URL.
+            default_origin: Origin header to include when the caller has not
+                configured one on the HTTP client.
         """
         self.url = url
+        self.default_origin = default_origin
         self.session_id: str | None = None
         self.protocol_version: str | None = None
 
@@ -92,6 +105,8 @@ def _prepare_headers(self) -> dict[str, str]:
             "accept": "application/json, text/event-stream",
             "content-type": "application/json",
         }
+        if self.default_origin:
+            headers["Origin"] = self.default_origin
         # Add session headers if available
         if self.session_id:
             headers[MCP_SESSION_ID] = self.session_id
@@ -547,7 +562,8 @@ async def streamable_http_client(
         # Create default client with recommended MCP timeouts
         client = create_mcp_http_client()
 
-    transport = StreamableHTTPTransport(url)
+    default_origin = None if "origin" in client.headers else _get_default_origin(url)
+    transport = StreamableHTTPTransport(url, default_origin=default_origin)
 
     logger.debug(f"Connecting to StreamableHTTP endpoint: {url}")
 

tests/shared/test_streamable_http.py

@@ -29,7 +29,7 @@
 
 from mcp import MCPError, types
 from mcp.client.session import ClientSession
-from mcp.client.streamable_http import StreamableHTTPTransport, streamable_http_client
+from mcp.client.streamable_http import StreamableHTTPTransport, _get_default_origin, streamable_http_client
 from mcp.server import Server, ServerRequestContext
 from mcp.server.streamable_http import (
     MCP_PROTOCOL_VERSION_HEADER,
@@ -767,6 +767,58 @@ def test_streamable_http_transport_init_validation():
         StreamableHTTPServerTransport(mcp_session_id="test\n")
 
 
+def test_get_default_origin_derives_origin_from_url():
+    assert _get_default_origin("https://example.com:8443/mcp?token=abc") == "https://example.com:8443"
+    assert _get_default_origin("http://user:pass@[::1]:8080/mcp") == "http://[::1]:8080"
+
+
+@pytest.mark.anyio
+async def test_streamable_http_client_sets_default_origin_on_http_client():
+    recorded_headers: list[httpx.Headers] = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        recorded_headers.append(request.headers)
+        return httpx.Response(202, request=request)
+
+    async with httpx.AsyncClient(transport=httpx.MockTransport(handler)) as client:
+        async with streamable_http_client("https://mcp.example.com:8443/mcp", http_client=client) as (
+            _read_stream,
+            write_stream,
+        ):
+            await write_stream.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")))
+            with anyio.fail_after(1):
+                while not recorded_headers:
+                    await anyio.sleep(0.01)
+
+        assert recorded_headers[0]["origin"] == "https://mcp.example.com:8443"
+        assert "origin" not in client.headers
+
+
+@pytest.mark.anyio
+async def test_streamable_http_client_preserves_custom_origin_header():
+    recorded_headers: list[httpx.Headers] = []
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        recorded_headers.append(request.headers)
+        return httpx.Response(202, request=request)
+
+    async with httpx.AsyncClient(
+        headers={"Origin": "https://proxy.example"},
+        transport=httpx.MockTransport(handler),
+    ) as client:
+        async with streamable_http_client("https://mcp.example.com/mcp", http_client=client) as (
+            _read_stream,
+            write_stream,
+        ):
+            await write_stream.send(SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")))
+            with anyio.fail_after(1):
+                while not recorded_headers:
+                    await anyio.sleep(0.01)
+
+        assert recorded_headers[0]["origin"] == "https://proxy.example"
+        assert client.headers["origin"] == "https://proxy.example"
+
+
 def test_session_termination(basic_server: None, basic_server_url: str):
     """Test session termination via DELETE and subsequent request handling."""
     response = requests.post(