Gemini Flash: one malformed MCP array enum breaks all prompts with HTTP 400 #4835

import { createInterface } from 'node:readline';

const mode = process.argv[2];
const count = Number(process.argv[3] ?? 1);
const allowedModes = ['array-enum-bad', 'array-enum-fixed', 'count'];

if (!allowedModes.includes(mode) ||
    !Number.isInteger(count) ||
    count < 1 ||
    count > 400) {
  throw new Error('Usage: node schema-repro.mjs array-enum-bad|array-enum-fixed|count [1..400]');
}

const levels = { type: ['array', 'null'], items: { type: 'integer' } };
if (mode === 'array-enum-bad') {
  levels.enum = [0, 1, 2, 3, 4, 25];
} else {
  levels.items.enum = [0, 1, 2, 3, 4, 25];
}

const tools = Array.from({ length: mode === 'count' ? count : 1 }, (_, index) => ({
  name: `probe_${String(index).padStart(3, '0')}`,
  description: 'Schema diagnostic only; performs no operation.',
  inputSchema: {
    type: 'object',
    properties: mode === 'count' ? { value: { type: 'string' } } : { levels },
  },
}));

for await (const line of createInterface({ input: process.stdin, crlfDelay: Infinity })) {
  if (!line.trim()) {
    continue;
  }
  const request = JSON.parse(line);
  if (request.id === undefined) {
    continue;
  }

  let result;
  if (request.method === 'initialize') {
    result = {
      protocolVersion: request.params.protocolVersion,
      capabilities: { tools: {} },
      serverInfo: { name: 'schema-repro', version: '1.0.0' },
    };
  } else if (request.method === 'tools/list') {
    result = { tools };
  } else if (request.method === 'tools/call') {
    result = {
      isError: true,
      content: [{ type: 'text', text: 'This diagnostic never executes an operation.' }],
    };
  } else if (request.method === 'ping') {
    result = {};
  } else {
    process.stdout.write(`${JSON.stringify({
      jsonrpc: '2.0',
      id: request.id,
      error: { code: -32601, message: 'Method not found' },
    })}\n`);
    continue;
  }

  process.stdout.write(`${JSON.stringify({ jsonrpc: '2.0', id: request.id, result })}\n`);
}