Resource template parameters do not support values with slashes

[phernandez]

Describe the bug

Overview

Resource template values containing path separators (slashes) do not work.

Working Examples

Simple Static Resources

@mcp.resource("resource://my-resource")
def get_data() -> dict[str, Any]:
    return {
        "contents": [{
            "uri": "resource://my-resource",
            "mimeType": "text/plain",
            "text": "Hello, world!"
        }]
    }

This works reliably and shows up in the Inspector UI.

Parameter

@mcp.resource("resource://{value}")
def get_content(value: str) -> dict[str, Any]:
    # This works for: resource://
    # But fails for: resourse://value/with/slashes
    ...

To Reproduce

1. Template Pattern with Regex

@mcp.resource("resource://{value:.*}")
def get_content(value: str) -> dict[str, Any]:
    # Error: Mismatch between URI parameters set() and function parameters {'permalink'}

2. Generic Catch-all Pattern

@mcp.resource("memory://*")
def get_content(uri: str) -> dict[str, Any]:
    # Error: Mismatch between URI parameters set() and function parameters {'uri'}

3. Path Parameter Approach

@mcp.resource("resource://content/{path}")
def get_content(path: str) -> dict[str, Any]:
    # Works for simple paths but fails with slashes in {path}

4. Base Path with Query Parameters

@mcp.resource("resource://content")
def get_content(path: str = None) -> dict[str, Any]:
    # Error: Mismatch between URI parameters set() and function parameters {'path'}

Expected behavior

Resource Templates with slashes in the value should be passed to the decorated function.

[dsp-ant]

Thank you for reporting. I am curious if you mind maybe sending a test case for this or even attempt a fix? I would appreciate a PR. Of course if you cannot, I will see if i can make time to fix this.

[tahseenadit]

Hi, if no one has started working on this, then I can try to fix this. I am interested. Thanks!

[g-votte]

I created a PR to support slashes in resource template parameters: #197

While implementing the fix, I noticed there's an existing test case that explicitly expects an error for this case:

python-sdk/tests/issues/test_141_resource_templates.py

Lines 61 to 64 in f10665d

	with pytest.raises(ValueError, match="Unknown resource"):
	await mcp.read_resource(
	"resource://users/123/posts/456/extra"
	) # Extra path component

@dsp-ant
Could you please provide guidance on the intended behavior here? If rejecting values with slashes is the expected design, we could resolve this issue as intended behavior. Otherwise, I'll proceed with the implementation in #197.

[dsp-ant]

After looking into more detail and particularly thanks go @g-votte PR, I have thought more about this and looked into how FastAPI does this. I think the way we do this in FastMCP is currently correct. We should not by default match paths. As an example FastAPI also matches only non-paths by default but then allows for matching paths with a path convertor, e.g {value:path} [1]. I will close this bug out in favour of a #220. In the meantime, if you do need this behaviour, you can use the lowlevel Server framework to parse resources as you wish and can built a custom decorator framework on top.

[1] https://fastapi.tiangolo.com/tutorial/path-params/#path-convertor