MCP server: tools/list crashes on enrichment_create — z.union() schema triggers '_zod' TypeError

[LucianaCedalio]

Summary

instantly mcp registers all 171 tools successfully, but every tools/list request fails with:

Cannot read properties of undefined (reading '_zod')

As a result, no MCP client can enumerate any tools — the server is effectively unusable end-to-end, even though the stdio handshake and tool registration succeed.

Environment

• instantly-cli@0.1.17 (latest)
• Node v20.20.2, macOS
• @modelcontextprotocol/sdk@1.29.0 (bundled)
• zod@4.3.6 (bundled)

Repro

export INSTANTLY_API_KEY=...
(printf '%s\n' \
  '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"t","version":"1"}}}' \
  '{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}' \
  '{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}'; \
 sleep 2) | instantly mcp

Observed:

Instantly MCP server started. Tools registered: 171
{"jsonrpc":"2.0","id":2,"error":{"code":-32603,"message":"Cannot read properties of undefined (reading '_zod')"}}

Root cause

I patched the vendored @modelcontextprotocol/sdk ListToolsRequestSchema handler to wrap each tool's serialization in a try/catch and log the failing name. One and only one of the 171 tools triggers the crash:

[BROKEN TOOL] enrichment_create: Cannot read properties of undefined (reading '_zod')

Looking at dist/index.js around the enrichmentCreateCommand definition (≈line 3188), its inputSchema is the only schema in the whole CLI that uses z.union([...]):

inputSchema: z72.object({
  resource_id: z72.string().describe(...),
  type: z72.enum(ENRICHMENT_TYPES).describe(...),
  limit: z72.coerce.number().optional().describe(...),
  filters: z72.union([                                    // <-- unique in the codebase
    z72.string(),
    z72.array(z72.record(z72.unknown()))
  ]).optional().describe(...),
  custom_flow: z72.string().optional().describe(...)
}),

grep -c "z\.union\|z72\.union" across dist/index.js returns 1. That single z.union is the crash trigger — the SDK's toJsonSchemaCompat(..., { strictUnions: true, pipeStrategy: 'input' }) path (called from server/mcp.js ListToolsRequestSchema handler) dereferences ._zod on something undefined when walking the union branches in zod v4 Mini's compat layer.

Skipping enrichment_create via a try/catch patch returns the other 170 tools correctly and the MCP becomes fully functional.

Suggested fixes (pick one)

1. Quick: replace the union with two separate optional fields, or accept a string and JSON-parse it in the handler (the handler already does typeof input.filters === "string" ? JSON.parse(input.filters) : input.filters, so a plain z.string() plus validation at call time would work).
2. Correct: bump @modelcontextprotocol/sdk if a later version handles v4 unions + z.record(z.unknown()) inside arrays correctly, or drop strictUnions: true for the tools/list serialization.

Happy to PR the first option if useful.

Impact

Until fixed, any MCP client (Claude Desktop, Cursor, Gemini CLI) connecting to instantly mcp sees zero tools — the server is unusable without a manual patch to the vendored SDK.