MCP: "Failed to get tools" when a tool inputSchema contains a boolean sub-schema in properties

Summary

OpenCode marks an entire MCP server as failed with "Failed to get tools" when any tool's inputSchema contains a boolean sub-schema inside properties (e.g. "properties": { "raw_input": true }). Boolean sub-schemas are valid JSON Schema (2020-12 permits true/false where a schema is expected), and the same server works in clients that tolerate them. A single such tool takes down every tool on the server.

This is the same failure path as #21591 / #26529 (a schema-processing error in listTools() collapsing to "Failed to get tools"), but a different trigger: those were outputSchema $refs and were fixed by the tolerant retry keyed off isOutputSchemaValidationError(). Boolean inputSchema sub-schemas are not covered by that retry.

Related: #47644 (the underlying error is discarded), #48196 (silent failure), and the same class in the MCP Python SDK surface types — modelcontextprotocol/python-sdk#3353 / PR #3354.

Steps to reproduce

repro-mcp.mjs — dependency-free stdio MCP server with one offending tool:

let buffer = "";
const send = (message) => process.stdout.write(JSON.stringify(message) + "\n");

process.stdin.on("data", (chunk) => {
  buffer += chunk;
  let newline;
  while ((newline = buffer.indexOf("\n")) >= 0) {
    const line = buffer.slice(0, newline).trim();
    buffer = buffer.slice(newline + 1);
    if (!line) continue;
    const message = JSON.parse(line);
    if (message.method === "initialize") {
      send({ jsonrpc: "2.0", id: message.id, result: {
        protocolVersion: "2024-11-05",
        capabilities: { tools: {} },
        serverInfo: { name: "repro", version: "1.0.0" },
      }});
    } else if (message.method === "tools/list") {
      send({ jsonrpc: "2.0", id: message.id, result: { tools: [{
        name: "read_everything",
        description: "Example tool.",
        inputSchema: {
          type: "object",
          additionalProperties: false,
          properties: {
            reason: { type: "string" },
            raw_input: true,               // <-- boolean sub-schema
            input: {
              type: "object",
              properties: { input: true }, // <-- boolean sub-schema (nested)
            },
          },
        },
      }]}});
    } else if (message.id !== undefined) {
      send({ jsonrpc: "2.0", id: message.id, result: {} });
    }
  }
});

opencode.json beside it:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "repro": { "type": "local", "command": ["node", "repro-mcp.mjs"], "enabled": true }
  }
}

Then:

opencode mcp list --print-logs --log-level DEBUG

Observed

• The server process starts, the initialize handshake completes, and tools/list returns a valid response over stdio (verifiable with any raw MCP client).
• OpenCode closes the connection and reports the whole server as failed:

●  ✗ repro  failed
│      Failed to get tools
│      node repro-mcp.mjs

• No underlying schema error is surfaced, even with --print-logs --log-level DEBUG — only the constant "Failed to get tools" (#47644).

Expected

One of:

1. Boolean sub-schemas in inputSchema/outputSchema properties are accepted (they are valid JSON Schema 2020-12), so the tools load; or
2. A schema-processing failure on a single tool is skipped/downgraded and the remaining tools still load, with a visible warning; and
3. The real underlying error is surfaced instead of the constant string.

Workaround

Because MCP tool definitions skip the tool.definition hook (#46628), a plugin cannot fix this. The only user-space workaround is to proxy the server and rewrite the tools/list response — rewriting boolean sub-schemas (true -> {}, false -> {"not": {}}) and dropping outputSchema. I verified this against a real server with 45 tools carrying "raw_input": true: after sanitizing, opencode mcp list reports connected and all 45 tools are preserved.

Environment

• OpenCode version: 1.18.31
• OS: macOS (darwin) x86_64
• MCP transport: stdio (local), server protocol 2024-11-05
• Also reproduced against a Go-based stdio MCP server; the same class is reported for third-party servers against Claude Code (anthropics/claude-code#25081) and the MCP Python SDK (modelcontextprotocol/python-sdk#3353).

Plugins

None — reproduced with the server alone; MCP tools do not pass through tool.definition.