Resource template UI does not support RFC 6570 expansion

[delaneyj]

Description

MCP Inspector v2 expands resource templates using plain string replacement instead of RFC 6570 expansion.

This causes two related problems:

1. Query expressions such as {?topic} do not produce an input control.
2. Reserved characters in simple variables are inserted without percent encoding.

Reproduction

Advertise this resource template:

{
  "name": "events",
  "uriTemplate": "foobar://events/{topic}"
}

In Inspector:

1. Open the resource template.
2. Enter foo/bar for topic.
3. Click Read Resource.
4. Inspect the generated resources/read request.

Actual result

Inspector sends:

{
  "uri": "foobar://events/foo/bar"
}

The inserted slash creates another path segment. A standards-compliant resource-template matcher rejects the URI:

{
  "code": -32602,
  "message": "Resource not found",
  "data": {
    "uri": "foobar://events/foo/bar"
  }
}

When the server advertises:

foobar://events{?topic}

Inspector does not display a topic input.

Expected result

Inspector should expand templates according to RFC 6570.

For {topic}, the value foo/bar should produce:

foobar://events/foo%2Fbar

For {?topic}, Inspector should display a topic input and generate an encoded query expression.

Suspected cause

The web client appears to extract and replace only plain variables:

const variables = /\{(\w+)\}/g
template.replace(/\{(\w+)\}/g, (_, name) => values[name])

This does not support RFC 6570 operators or encode values.

Suggested correction

Use an RFC 6570 implementation for:

• Variable discovery.
• Form generation.
• URI expansion.
• Reserved-character encoding.
• Query-expression expansion.

Add coverage for:

foobar://events/{topic}
foobar://events{?topic}

Test values containing /, ?, #, %, spaces, and Unicode text.

Environment

• MCP Inspector: v2.0.0
• Protocol revision: 2026-07-28
• Transport: Streamable HTTP
• Client: Inspector web UI