[BUG] Valid MCP tool silently dropped: API schema validation rejects property names containing `[]`

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary

A tool from a spec-compliant MCP server is silently discarded at session start because its
input schema contains a property named uids[]. Tool schemas are validated against
^[a-zA-Z0-9_.-]{1,64}$ for property keys, and the square brackets fail that pattern.

uids[] is a legal property name in JSON Schema — the spec places no constraints on the keys
of properties — and the MCP specification defines inputSchema as a JSON Schema object.
So the server is valid on both counts; the rejection comes from a stricter constraint layered
on top by the API. The result is that a working MCP server loses functionality with no
actionable error surfaced to the user.

Environment

• Claude Code: 2.1.263 (VS Code extension)
• Platform: Linux 7.0.12-linuxkit (devcontainer), Node v24.19.0
• MCP server: pushpad-mcp@0.1.1 (stdio, npx -y pushpad-mcp)
• Affected tool: subscriptions_list

Reproduction

1. Configure the MCP server in .mcp.json:

``json
{
"mcpServers": {
"pushpad": {
"type": "stdio",
"command": "npx",
"args": ["-y", "pushpad-mcp"],
"env": { "PUSHPAD_TOKEN": "<token>" }
}
}
}
``

1. Start a session and ask the model to list subscriptions for a project.

Expected

subscriptions_list is available, like the server's other 18 tools.

Actual

The tool is excluded before the model ever sees it. The only trace is an internal note:

- "subscriptions_list" (MCP server "pushpad"): "property key uids[] does not match /^[a-zA-Z0-9_.-]{1,64}$/"

Root cause

pushpad-mcp is a generic OpenAPI-to-MCP bridge; it fetches https://pushpad.xyz/openapi.yaml
at startup and derives tools from it. The spec declares:

uids:
  in: query
  name: uids[]
  style: form
  explode: true
  schema:
    type: array
    items:
      type: string

This is valid OpenAPI 3 and the standard way to express a repeated bracketed query parameter
(?uids[]=a&uids[]=b), the convention used by Rails, PHP, and many other backends. The bridge
maps the query-parameter name directly to a JSON Schema property name, yielding uids[].

subscriptions_list is the only one of the spec's 19 operations that references a bracketed
parameter, which is exactly why it is the only tool dropped — the failure is entirely attributable
to the character class, not to anything else about the server.

Impact

• Any OpenAPI-derived MCP server exposing bracket-style array query params loses those tools.

This is a widespread convention, so the blast radius is larger than one server.

• The failure is silent from the user's perspective. Nothing appears in the UI: the tool simply

does not exist, and the model can only report it as missing after the fact.

• There is no user-side remedy. Fixing it requires changing the upstream API's parameter naming

or patching the bridge — neither is available to someone who just installed the server.

Suggested resolutions

Any one of these would resolve it:

1. Relax the property-key pattern to accept the characters JSON Schema permits, or at minimum

[ and ]. (recommended)

1. Sanitize on the client side. Rewrite non-conforming keys to a safe alias when registering

the tool, and map them back when invoking it. This keeps the constraint intact while preserving
compatibility with compliant servers.

What Should Happen?

API schema validation should allow property names containing []

Error Messages/Logs

Steps to Reproduce

1. Configure the MCP server in .mcp.json:

``json
{
"mcpServers": {
"pushpad": {
"type": "stdio",
"command": "npx",
"args": ["-y", "pushpad-mcp"],
"env": { "PUSHPAD_TOKEN": "<token>" }
}
}
}
``

1. Start a session and ask the model to list subscriptions for a project.

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.263 (VS Code extension)

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_

property key "tags[]" does not match /^[a-zA-Z0-9_.-]{1,64}$/