René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Support for API Gateway Path Prefixes in SSE Client URL Handling","articleBody":"**Is your feature request related to a problem? Please describe.**\n\nWhen using the client SDK to connect MCP server behind an `API gateway` that adds a deployment-specific prefix (e.g., `/gateway_prefix/\u003cdeployment_name\u003e`), the backend mcp server is unaware of this prefix and only exposes routes like `/v1/sse` and `/v1/messages`. The client SDK currently only supports [url](https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/sse.py#L25) and uses [urljoin](https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/sse.py#L75) to resolve endpoint URLs, which causes the gateway prefix to be dropped when joining paths that start with /. This results in incorrect routing through the gateway.\n\ndebug logs:\n\n```\n❯ python3 client.py\nDEBUG:asyncio:Using selector: KqueueSelector\nINFO:mcp.client.sse:Connecting to SSE endpoint: https://example.com/gateway_prefix/\u003cdeployment_name\u003e/v1/sse\nDEBUG:httpcore.connection:connect_tcp.started host='https://example.com' port=443 local_address=None timeout=5 socket_options=None\nDEBUG:httpcore.connection:connect_tcp.complete return_value=\u003chttpcore._backends.anyio.AnyIOStream object at 0x10889fef0\u003e\nDEBUG:httpcore.connection:start_tls.started ssl_context=\u003cssl.SSLContext object at 0x10895ca50\u003e server_hostname='example.com' timeout=5\nDEBUG:httpcore.connection:start_tls.complete return_value=\u003chttpcore._backends.anyio.AnyIOStream object at 0x107fb45c0\u003e\nDEBUG:httpcore.http11:send_request_headers.started request=\u003cRequest [b'GET']\u003e\nDEBUG:httpcore.http11:send_request_headers.complete\nDEBUG:httpcore.http11:send_request_body.started request=\u003cRequest [b'GET']\u003e\nDEBUG:httpcore.http11:send_request_body.complete\nDEBUG:httpcore.http11:receive_response_headers.started request=\u003cRequest [b'GET']\u003e\nDEBUG:httpcore.http11:receive_response_headers.complete return_value=(b'HTTP/1.1', 200, b'OK', [(b'cache-control', b'no-store'), (b'content-type', b'text/event-stream; charset=utf-8'), (b'date', b'Fri, 23 May 2025 15:45:20 GMT'), (b'x-accel-buffering', b'no'), (b'x-upstream-service-time', b'7'), (b'transfer-encoding', b'chunked')])\nINFO:httpx:HTTP Request: GET https://example.com/gateway_prefix/\u003cdeployment_name\u003e/v1/sse \"HTTP/1.1 200 OK\" \u003c== correct sse endpoint\nDEBUG:mcp.client.sse:SSE connection established\nDEBUG:httpcore.http11:receive_response_body.started request=\u003cRequest [b'GET']\u003e\nDEBUG:mcp.client.sse:Received SSE event: endpoint\nDEBUG:mcp.client.sse:urljoin: https://example.com/gateway_prefix/\u003cdeployment_name\u003e/v1/sse + /v1/messages/?session_id=80b1237100a34eaa9795a5993661c76a \u003c== wrong messages endpoint\nINFO:mcp.client.sse:Received endpoint URL: https://example.com/v1/messages/?session_id=80b1237100a34eaa9795a5993661c76a\nINFO:mcp.client.sse:Starting post writer with endpoint URL: https://example.com/v1/messages/?session_id=80b1237100a34eaa9795a5993661c76a\n```\n\n**Describe the solution you'd like**\nThe client SDK should preserve the gateway prefix when joining endpoint URLs, so that requests are routed correctly through the gateway. Ideally, the SDK should provide a way to configure or detect the gateway prefix and ensure all endpoint URLs are constructed with it.\n\n**Describe alternatives you've considered**\n\n**Additional context**\n\nExample:\n```python\nurl = \"https://example.com/gateway_prefix/\u003cdeployment_name\u003e/v1/sse\"\npath = \"/v1/messages\"\nurljoin(url, path)\n# Actual:   https://example.com/v1/messages\n# Expected: https://example.com/gateway_prefix/\u003cdeployment_name\u003e/v1/messages\n```\nThis is a common pattern for API gateways and reverse proxies, so supporting this would improve compatibility.\n\nsimilar request: \nhttps://github.com/modelcontextprotocol/python-sdk/issues/412#issuecomment-2820077952\nhttps://github.com/modelcontextprotocol/python-sdk/issues/733\nhttps://github.com/modelcontextprotocol/python-sdk/pull/386\n","author":{"url":"https://github.com/lizzzcai","@type":"Person","name":"lizzzcai"},"datePublished":"2025-05-23T16:14:35.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/795/python-sdk/issues/795"}