bug(mcp): tools whose input schema carries `$schema` cannot be published as Client Capabilities

[jsiu93]

What happened

maka runtime-host capability-provider serve fails before publishing a single tool when an MCP server advertises input schemas with a root $schema annotation. The descriptor is copied into the Client Capability frame unchanged, and the protocol's strict keyword allowlist rejects it:

Runtime Host capability provider stopped: Unsupported Client Capability tool schema keyword
CapabilityProviderPublicationError: Unsupported Client Capability tool schema keyword
[exit=1]

This hits most servers built with the TypeScript SDK. @modelcontextprotocol/sdk converts Zod input schemas in server/zod-json-schema-compat.js with zod-to-json-schema (Zod v3) or z.toJSONSchema (Zod v4), and both emit a root $schema by default. A direct tools/list from XcodeBuildMCP 2.7.0 had $schema on 44 of 44 tools, and it was the only keyword outside the allowlist in any of them; an earlier probe through mcp_router saw the same on 31 of 31 tools. Python FastMCP servers generally do not emit it.

$schema and $comment are annotations with no validation meaning. Expected: the provider publishes these tools; the allowlist can stay strict for everything else.

How to reproduce

1. Start a fresh Runtime Host with a loopback WebSocket listener and issue a capability-provider credential (supply it through MAKA_RUNTIME_HOST_ACCESS_CREDENTIAL).
2. Point an MCP config at any TypeScript SDK server, for example:

{
  "version": 2,
  "mcpServers": {
    "xcodebuildmcp": {
      "command": "bunx",
      "args": ["xcodebuildmcp", "mcp"]
    }
  }
}

3. Run:

maka runtime-host capability-provider serve \
  --url ws://127.0.0.1:47633/runtime-host \
  --mcp-config ./mcp.json \
  --expected-root "$ROOT_ID" \
  --client-identity ./capability-provider-identity.json

4. The provider exits 1 with the error above and publishes nothing.

Unit reproduction: an MCP manager test against a stdio fixture whose tool schema carries a root $schema, a nested $schema under properties, a $comment, and an $id. On origin/main the descriptor still contains all three annotations.

Environment

• Maka version or commit: 0.1.11 dev build; confirmed on origin/main at 6ada6b546
• OS and version: macOS 26.6.1 (25G76), arm64
• Surface: CLI capability provider and Runtime Host
• Node.js v22.22.2, npm 10.9.7
• MCP server: XcodeBuildMCP 2.7.0 (TypeScript SDK)

Logs, screenshots, or additional context

Census of the captured XcodeBuildMCP tools/list, recursing through the allowlisted child positions:

tools=44
root_$schema=44
unsupported_keywords={"$schema":44}

I have a fix that strips $schema and $comment at MCP discovery and leaves the validator untouched; PR to follow.