Sentry MCP server fails to initialize: cannot unmarshal JSON Schema array union type into string

[ejaramillompg]

Summary

mcpx v1.5.0 fails to initialize the @sentry/mcp-server with the following error:

error: server "sentry": initialize: mcp: initialization failed: mcp: transport closed

Running with verbose output reveals the root cause:

json: cannot unmarshal array into Go struct field PropertySchema.tools.inputSchema.properties.type of type string

Root Cause

The Sentry MCP server uses JSON Schema draft-07 union types where type is an array of strings for optional/nullable fields:

{
  "type": ["string", "null"],
  "description": "..."
}

mcpx's Go JSON unmarshaler maps inputSchema.properties.*.type to type string, which cannot hold an array value. The spec allows both "type": "string" and "type": ["string", "null"].

Expected behavior

mcpx should handle type as either a string or []string (union) per the JSON Schema spec. Unknown or unsupported types could fall back to "any" / untyped rather than hard-failing initialization.

Reproduction

1. Add to ~/.mcpx/config.yml:

   sentry:
     command: npx
     args: ["-y", "@sentry/mcp-server"]
     transport: stdio

2. Authenticate: npx -y @sentry/mcp-server auth login
3. Run: mcpx list sentry -v

Result: initialization error as above.

Environment

• mcpx version: 1.5.0 (brew, codestz/tap)
• @sentry/mcp-server version: latest (npx -y)
• OS: macOS 25.3.0 (Darwin)

Workaround

Use the Sentry REST API directly with the OAuth token cached at ~/.sentry/mcp.json by the MCP server's device code flow.