ResourceTemplate fails when user selects <Empty> for optional parameter

[dudo]

Description

When using ResourceTemplate with optional parameters (e.g., {param?}), if a user selects <Empty> from the completion dropdown, the resulting URI doesn't match the template pattern, causing the resource to fail to load.

The resource kubernetes://resources/nodes/ was not found.

Steps to Reproduce

1. Create a ResourceTemplate with an optional parameter: "scheme://path/{required}/{optional?}"
2. User selects a value for required parameter
3. User selects <Empty> for the optional parameter from the completion dropdown
4. The resulting URI becomes something like scheme://path/required/<Empty>
5. This URI doesn't match the original template pattern and fails validation

Expected Behavior

When a user selects <Empty> for an optional parameter, the URI should either:

• Omit that path segment entirely (e.g., scheme://path/required)
• Map <Empty> to an empty string and handle it gracefully in the URI parsing

Current Workaround

We have to manually handle both cases in our readCallback:

• scheme://path/required (when optional param is truly omitted)
• scheme://path/required/<Empty> (when user explicitly selects <Empty>)

Impact

This makes ResourceTemplates with optional parameters brittle and requires additional error-prone parsing logic in every implementation.

Environment

• MCP TypeScript SDK version: latest
• Context: ResourceTemplate with pattern like kubernetes://resources/{resourceType}/{namespace?}

[SVLaursen]

Experiencing the same issue when supplying the resource with multiple optional parameters, e.g.:

scheme://path/to/{param1,param2?}

EDIT:
Also for context, this breaks the expected RFC standard and seems to involve all optional parameter types from what I've been able to test.

[felixweinberger]

Confirmed. The {param?} optional path syntax isn't properly implemented in UriTemplate - the ? suffix is kept as part of the variable name rather than being parsed as optional. Related to #1079 (both are URI template issues).