Fix SSE client handling of nested path URLs

[danilpavlov]

Fix SSE client failing when connecting to servers with nested paths (e.g., http://domain/path1/path2/sse). Previously, the client worked only with simple URLs like http://domain/sse.

The fix extracts the base path segment between domain and /sse endpoint using regex, then correctly constructs the response URL by preserving the path structure. This ensures proper communication with MCP servers hosted under non-root paths.

Without this fix, clients couldn't connect to SSE servers deployed under nested paths, as the endpoint URLs for posting messages were constructed incorrectly.

Motivation and Context

I got problem while deploying my MCP server inside a Kubernetes cluster. The SSE client was only working with simple URLs like http://example-domain/sse, but failed to properly handle more complex paths such as http://example-domain/path1/path2/sse.

When the server was deployed under nested paths, the client couldn't establish a proper connection because it was incorrectly constructing the endpoint URLs for posting messages. This made it impossible to use MCP servers hosted under non-root paths, which is a common scenario in Kubernetes environments where services often run behind path-based routing.

How Has This Been Tested?

I ve tested on version for local mcp server (http://localhost:8000/sse) and with special mask (http://domain/dummy/sse)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[ylassoued]

Thank you very much @danilpavlov!!! You saved my day with your PR. I hope it will be merged soon.

[jonathanglima]

Nice!

Just wanted to add that using this alongside langchain's adapters that use the /mcp/sse prefix made the /mcp to get repeated. I had to change it a bit to fix it. Worked on:

• http://localhost:8000/sse
• http://localhost:8000/mcp/sse
• http://localhost:8000/myversion/mcp/sse
• http://localhost:8000/myversion/sse

Please note that I also had to change the urljoin

url_parsed = urlparse(url)

base_path = re.search(
    r"https?://[^/]+/(.+?)(?:/mcp)?/sse$", url
)
base_path = (
    base_path.group(1) if base_path else ""
)
endpoint_url = urljoin(
    url_parsed.scheme + "://" + url_parsed.netloc,
    base_path + sse.data
)
logger.info(
    f"Received endpoint URL: {endpoint_url}"
)

[jonathanglima]

Please see danilpavlov#1 @danilpavlov

[danilpavlov]

Nice!

Just wanted to add that using this alongside langchain's adapters that use the /mcp/sse prefix made the /mcp to get repeated. I had to change it a bit to fix it. Worked on:

• http://localhost:8000/sse
• http://localhost:8000/mcp/sse
• http://localhost:8000/myversion/mcp/sse
• http://localhost:8000/myversion/sse

Please note that I also had to change the urljoin

url_parsed = urlparse(url)

base_path = re.search(
    r"https?://[^/]+/(.+?)(?:/mcp)?/sse$", url
)
base_path = (
    base_path.group(1) if base_path else ""
)
endpoint_url = urljoin(
    url_parsed.scheme + "://" + url_parsed.netloc,
    base_path + sse.data
)
logger.info(
    f"Received endpoint URL: {endpoint_url}"
)

Thank you @jonathanglima so much for that feature!

[danilpavlov]

@dsp-ant Can you please check this PR. Forget to mark as reviewer :/