Use interceptors for session/stateful tool calls

[jplattel]

With #297 (comment) having an interceptor works for stateless tool requests. That's nice!

Due to optimizations and our stateful backend it would be very nice that a session would also use the client.tool_interceptors when making a tool call. So using:

with client.session() as session: 
    session.call_tool(toolname, **kwargs)

Would also call the interceptor, allowing us to override the headers for example. This currently does not seem to happen.

[eyurtsev]

Could you explain what you want to do with an interceptor?

Given that you're using the MCP sdk directly and are calling a call_tool method rather than invoking via a lnagchain tool, so there shouldn't be any need for abstraction to intercept the logic.

you can directly add some code before and after the tool call

with client.session() as session: 
    do stuff here
    session.call_tool(toolname, **kwargs)
    do other stuff here

[jplattel]

Hi @eyurtsev thank you for replying. Reading it back it seems that I didn't supply some needed context.

Right know we use an agent with a prebuilt langgraph workflow with a MultiServerMCPClient. Using interceptors would be a solution to add a access token header to each tool-call during runtime, allowing us to authorise a user for the mcp-server.

I've tested it without a session and then the interceptors seem to work. With a session it doesn't seem to add the header to the tool-call. I could setup a new session for each tool-call but that is way to much overhead imho.

[ankwok]

I'm looking for a similar functionality.

Currently, we're using langgraph and assigning a trace_id to incoming requests. Ideally, we would be able to forward this trace_id as a header to the MCP server. It's not clear to me how the current implementation of interceptors could support this.

Having access to the LG state from within an interceptor would solve this issue.

[eyurtsev]

@jplattel
Does providing https://www.python-httpx.org/advanced/authentication/#custom-authentication-schemes not solve for the access token?

it's possible to create custom auth and set it on the session (supported by the underlying MCP-sdk).

Sorry I haven't dug deeply into it -- so may be unaware of some basic limitations

[eyurtsev]

@ankwok we'll working to expose langgraph state.

You may also be able to just use the thread_id or run_id via the langsmith sdk. the sdk relying on python's context var API, so you should be able to set / access information like the run_id (i'd think of this as more of a work-around since it's not the most intuitive solution probably)

[sydney-runkle]

Having access to the LG state from within an interceptor would solve this issue.

We've now exposed request.runtime, which has the langgraph state! Does this resolve your issue?

[jplattel]

@jplattel Does providing https://www.python-httpx.org/advanced/authentication/#custom-authentication-schemes not solve for the access token?

it's possible to create custom auth and set it on the session (supported by the underlying MCP-sdk).

Sorry I haven't dug deeply into it -- so may be unaware of some basic limitations

Thank you, I'll have a look if that is possible. Will report back :)