modelcontextprotocol
diff --git a/docs/troubleshooting.md b/docs/troubleshooting.md
@@ -202,7 +202,7 @@ Invalid Host header
 The fix is the same `transport_security=TransportSecuritySettings(allowed_hosts=[...], allowed_origins=[...])` shown under `Server returned an error response`. Two of its edges are worth naming:
 
 * An `allowed_hosts` entry is an exact string. `"mcp.example.com"` matches a bare `Host` header and `"mcp.example.com:*"` matches any explicit port. List both.
-* A `403` with the body `Invalid Origin header` is the sibling check on the `Origin` header. It only fires for browsers (nothing else sends `Origin`), and `allowed_origins=` is its allowlist.
+* A `403` with the body `Invalid Origin header` is the sibling check on the `Origin` header, and `allowed_origins=` is its allowlist. Browsers send `Origin`, and so does the python `Client`: it stamps a same-origin value (`scheme://host[:port]`) derived from the URL you connect to, so that spec-compliant servers enforcing CSRF / DNS-rebinding protection accept the handshake. That is why the allowlist above names `http://mcp.example.com` alongside the host — the client's own `Origin` has to be on it.
 
 **[Deploy & scale](run/deploy.md)** has the full treatment, including when switching the check off is the honest configuration.
 

diff --git a/docs_src/troubleshooting/tutorial004.py b/docs_src/troubleshooting/tutorial004.py
@@ -13,6 +13,6 @@ def forecast(city: str) -> str:
 app = mcp.streamable_http_app(
     transport_security=TransportSecuritySettings(
         allowed_hosts=["mcp.example.com", "mcp.example.com:*"],
-        allowed_origins=["https://app.example.com"],
+        allowed_origins=["http://mcp.example.com", "http://mcp.example.com:*"],
     )
 )
diff --git a/src/mcp/client/streamable_http.py b/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
@@ -108,6 +109,19 @@ def __init__(self, url: str) -> None:
         # `notifications/cancelled` at 2026 can abort it; see
         # `_consume_modern_cancellation`. Keys are verbatim-typed ("1" is not 1).
         self._in_flight_posts: dict[RequestId, _InFlightPost] = {}
+        self._default_origin = self._derive_origin(url)
+
+    @staticmethod
+    def _derive_origin(url: str) -> str | None:
+        """Derive a same-origin ``Origin`` value (scheme://host[:port]) from a URL.
+
+        Returns ``None`` when the URL has no scheme or host, in which case no
+        ``Origin`` header is added.
+        """
+        parsed = urlsplit(url)
+        if not parsed.scheme or not parsed.netloc:
+            return None
+        return f"{parsed.scheme}://{parsed.netloc}"
 
     def _prepare_headers(self) -> dict[str, str]:
         """Build MCP-specific request headers for any outbound HTTP request.
@@ -123,6 +137,13 @@ def _prepare_headers(self) -> dict[str, str]:
             "accept": "application/json, text/event-stream",
             "content-type": "application/json",
         }
+        # Send a same-origin Origin header by default so spec-compliant servers
+        # that enforce anti-DNS-rebinding / CSRF protection (e.g. the Go SDK's
+        # http.CrossOriginProtection) accept the handshake instead of returning
+        # 403. Callers needing a different Origin can set one on the underlying
+        # httpx client's default headers.
+        if self._default_origin is not None:
+            headers["origin"] = self._default_origin
         if self.session_id:
             headers[MCP_SESSION_ID] = self.session_id
         if self._protocol_version_header:

diff --git a/tests/shared/test_streamable_http.py b/tests/shared/test_streamable_http.py
@@ -1608,6 +1608,26 @@ async def bad_client():
         assert tools.tools
 
 
+def test_prepare_headers_includes_same_origin():
+    """Default Origin header is derived from the target URL (scheme://host[:port]).
+
+    Regression test for #2727: spec-compliant servers enforcing
+    anti-DNS-rebinding / CSRF protection reject requests with no Origin.
+    """
+    transport = StreamableHTTPTransport(url="http://my-go-server:8081/mcp")
+    headers = transport._prepare_headers()
+    assert headers["origin"] == "http://my-go-server:8081"
+
+    https_transport = StreamableHTTPTransport(url="https://example.com/mcp/path?x=1")
+    assert https_transport._prepare_headers()["origin"] == "https://example.com"
+
+
+def test_prepare_headers_omits_origin_for_invalid_url():
+    """No Origin header is added when the URL lacks a scheme or host."""
+    transport = StreamableHTTPTransport(url="not-a-url")
+    assert "origin" not in transport._prepare_headers()
+
+
 @pytest.mark.anyio
 async def test_handle_sse_event_skips_empty_data() -> None:
     """_handle_sse_event skips empty SSE data (keep-alive pings) without writing to the stream."""