Claude Code: Support MCP completion/complete for resource template parameters in @ mention UI

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When typing @server:db://object/asp in the @ mention autocomplete, Claude Code should send completion/complete requests to the MCP server to get suggestions for resource template URI parameters. Currently, the @ mention UI only performs fuzzy search over static resources from resources/list — it does not invoke the MCP completions protocol for resource templates.

MCP servers that expose large datasets (databases, APIs with millions of entities) define resource templates like db://object/{object_name} with @server.completion() handlers that return matching names as the user types. This is the MCP-standard way to enable discovery of valid identifiers without enumerating all possible resources upfront.

My server-side implementation works correctly — ReadMcpResourceTool resolves template URIs. But the @ mention in claude code UI never sends completion/complete requests, so users get no suggestions while typing.

Proposed Solution

Expected Behavior

1. User types @server:db://object/ — Claude Code recognizes a registered resource template (defined with that resource pattern)
2. As the user types parameter characters (e.g., asp), Claude Code sends completion/complete requests
3. Returned Completion.values appear as autocomplete suggestions
4. Selecting a suggestion completes the URI (e.g., @server:db://object/aspirin)

MCP Protocol Reference

{
"method": "completion/complete",
"params": {
"ref": { "type": "ref/resource", "uri": "db://object/{object_name}" },
"argument": { "name": "object_name", "value": "asp" }
}
}

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

1. User types @server:db://object/ — Claude Code recognizes a registered resource template (defined with that resource pattern)
2. As the user types parameter characters (e.g., asp), Claude Code sends completion/complete requests
3. Returned Completion.values appear as autocomplete suggestions
4. Selecting a suggestion completes the URI (e.g., @server:db://object/aspirin)

MCP Protocol Reference

{
"method": "completion/complete",
"params": {
"ref": { "type": "ref/resource", "uri": "db://object/{object_name}" },
"argument": { "name": "object_name", "value": "asp" }
}
}

Additional Context

issue 3326 may be related