Question: How to get a resource argument containing "/" ?

I want to make the resource like

@mcp.resource("folder://{path}") def folder_contents(path: str) -> list[str]:

The path should be something like xxx/. or xxx/abc/zzz/

With the @mcp.resource("folder://{path}") it doesn't work. Is there a way to implement it?

Hi is this still a problem? If yes could you provide a code snippet that demonstrates the issue, your expectations and where it fails?

@Kludex

Seems this could still be relevant, maybe we could consider looking into doing something similar to FastAPI/Starlette with {file_path:path} where path specifies this can have slashes in it.

@Kludex do you think this makes sense? Did it work well for Starlette?

Claude's repro:

"""Test resource templates with path parameters containing forward slashes."""

import json
from typing import Any

import pytest

from mcp.server.fastmcp.resources import FunctionResource, ResourceTemplate


def test_template_with_path_parameter_single_level():
    """Test matching URIs with single-level path parameters."""

    def folder_contents(path: str) -> list[str]:
        return ["file1.txt", "file2.txt"]

    template = ResourceTemplate.from_function(
        fn=folder_contents,
        uri_template="folder://{path}",
        name="folder",
    )

    # Single level path should work
    params = template.matches("folder://root")
    assert params == {"path": "root"}


def test_template_with_path_parameter_nested_path():
    """Test matching URIs with nested path parameters containing forward slashes.

    This is the issue reported in #436.
    A user wants to define a resource like:
      @mcp.resource("folder://{path}")
      def folder_contents(path: str) -> list[str]:

    And then be able to match URIs like "folder://xxx/abc/zzz/" where the path
    parameter contains forward slashes.

    Currently, this fails because the regex pattern uses [^/]+ which excludes
    forward slashes.
    """

    def folder_contents(path: str) -> list[str]:
        # This would return the contents of the nested directory
        return ["file1.txt", "file2.txt"]

    template = ResourceTemplate.from_function(
        fn=folder_contents,
        uri_template="folder://{path}",
        name="folder",
    )

    # Nested paths with slashes should work but currently fail
    params = template.matches("folder://xxx/abc/zzz/")

    # This should match and extract the full path
    assert params is not None, "Failed to match nested path with slashes"
    assert params == {"path": "xxx/abc/zzz/"}


def test_template_with_path_parameter_various_paths():
    """Test matching various nested paths."""

    def get_file(path: str) -> str:
        return f"Content of {path}"

    template = ResourceTemplate.from_function(
        fn=get_file,
        uri_template="file://{path}",
        name="file",
    )

    # Test various nested paths
    test_cases = [
        ("file://simple", {"path": "simple"}),
        ("file://one/two", {"path": "one/two"}),
        ("file://one/two/three", {"path": "one/two/three"}),
        ("file://very/deep/nested/path/to/file.txt",
         {"path": "very/deep/nested/path/to/file.txt"}),
        ("file://path/with/trailing/slash/",
         {"path": "path/with/trailing/slash/"}),
        ("file://path.with.dots/file.txt",
         {"path": "path.with.dots/file.txt"}),
    ]

    for uri, expected_params in test_cases:
        params = template.matches(uri)
        assert params is not None, f"Failed to match URI: {uri}"
        assert params == expected_params, (
            f"For URI {uri}, expected {expected_params} but got {params}"
        )


def test_template_with_multiple_params_and_path():
    """Test template with multiple parameters where one is a path."""

    def get_resource(bucket: str, path: str) -> dict[str, Any]:
        return {"bucket": bucket, "path": path}

    template = ResourceTemplate.from_function(
        fn=get_resource,
        uri_template="s3://{bucket}/{path}",
        name="s3",
    )

    # This should extract bucket and path separately,
    # with path potentially containing slashes
    params = template.matches("s3://mybucket/folder/subfolder/file.txt")

    assert params is not None

    # If the regex allowed slashes in the last parameter,
    # this would work correctly
    assert params["bucket"] == "mybucket"
    # The path might contain slashes
    assert "file.txt" in params["path"]


@pytest.mark.anyio
async def test_create_resource_with_nested_path():
    """Test creating a resource from template with nested path parameter."""

    def read_folder(path: str) -> list[str]:
        # Simulate reading a folder at the given path
        if path == "project/src/components":
            return ["Button.tsx", "Card.tsx", "Header.tsx"]
        return []

    template = ResourceTemplate.from_function(
        fn=read_folder,
        uri_template="folder://{path}",
        name="folder",
    )

    # Try to match and create a resource with a nested path
    uri = "folder://project/src/components"
    params = template.matches(uri)

    assert params is not None
    resource = await template.create_resource(uri, params)
    assert isinstance(resource, FunctionResource)
    content = await resource.read()
    assert isinstance(content, str)
    data = json.loads(content)
    assert "Button.tsx" in data
    assert "Card.tsx" in data
    assert "Header.tsx" in data

Seems this could still be relevant, maybe we could consider looking into doing something similar to FastAPI/Starlette with {file_path:path} where path specifies this can have slashes in it.

@Kludex do you think this makes sense? Did it work well for Starlette?

It does work well, there's no way to do that without the converters.

Seems this could still be relevant, maybe we could consider looking into doing something similar to FastAPI/Starlette with {file_path:path} where path specifies this can have slashes in it.
@Kludex do you think this makes sense? Did it work well for Starlette?

It does work well, there's no way to do that without the converters.

Can we just import from starlette or do we need to implement something custom

Seems this could still be relevant, maybe we could consider looking into doing something similar to FastAPI/Starlette with {file_path:path} where path specifies this can have slashes in it.
@Kludex do you think this makes sense? Did it work well for Starlette?

It does work well, there's no way to do that without the converters.

Can we just import from starlette or do we need to implement something custom

I think we can copy that logic in here.

related: #220

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Question: How to get a resource argument containing "/" ? #436

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felixweinberger commented on Oct 7, 2025

maxisbey commented on Oct 8, 2025

maxisbey commented on Oct 8, 2025

Kludex commented on Oct 8, 2025

maxisbey commented on Oct 8, 2025

Kludex commented on Dec 4, 2025

maxisbey commented on Dec 8, 2025

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