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claude-code - 💡(How to fix) Fix MCP servers with large tool schemas silently fail to register tools [1 comments, 2 participants]

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anthropics/claude-code#51836•Fetched 2026-04-23 07:43:39
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Error Message

MCP servers that pass the health check (claude mcp list shows "Connected") but their tools never appear in sessions. No error, no warning — completely silent failure. All MCP server tools should register regardless of schema size. At minimum, a warning or error should be logged when tools fail to register so users can diagnose the issue.

Root Cause

Total tool schema payload size. There appears to be an undocumented size threshold above which tool registration silently fails.

Fix Action

Workaround

Using direct curl API calls instead of MCP for the affected services.

Code Example

claude mcp add calcom -- npx -y @calcom/cal-mcp
   claude mcp add notion -- npx -y @notionhq/notion-mcp-server

---

claude mcp list

---

claude -p "list all MCP tools"
RAW_BUFFERClick to expand / collapse

Environment

  • Claude Code version: 2.1.116
  • OS: Ubuntu 24.04, headless VPS, running as non-root user via su - aios

Problem

MCP servers that pass the health check (claude mcp list shows "Connected") but their tools never appear in sessions. No error, no warning — completely silent failure.

Details

Working servers register tools fine:

  • n8n: 1,396 chars total tool schema — works
  • Asana: 4,907 chars total tool schema — works

Broken servers connect but tools never register:

  • Cal.com (@calcom/cal-mcp): 53,044 chars across 9 tools — connects, tools missing
  • Notion (@notionhq/notion-mcp-server): 73,396 chars across 22 tools — connects, tools missing

All 4 servers return identical protocolVersion (2024-11-05), same initialize handshake, same tools/list response format. The only difference is schema size.

Notion also uses $defs/$ref in its tool schemas, which may compound the issue.

Root Cause

Total tool schema payload size. There appears to be an undocumented size threshold above which tool registration silently fails.

Reproduction

  1. Add both servers: 
    claude mcp add calcom -- npx -y @calcom/cal-mcp
claude mcp add notion -- npx -y @notionhq/notion-mcp-server
  2. Verify connection: 
    claude mcp list
    Both show Connected.
  3. Check available tools: 
    claude -p "list all MCP tools"
    Cal.com and Notion tools are missing from the tool list.

Expected Behavior

All MCP server tools should register regardless of schema size. At minimum, a warning or error should be logged when tools fail to register so users can diagnose the issue.

Workaround

Using direct curl API calls instead of MCP for the affected services.

extent analysis

TL;DR

The issue can be mitigated by implementing a workaround using direct curl API calls for the affected services, as the total tool schema payload size appears to be causing a silent failure in tool registration.

Guidance

  • Investigate the Claude Code version 2.1.116 to see if there are any known issues or limitations related to tool schema size.
  • Verify that the issue is indeed caused by the schema size by testing with a smaller schema for the Cal.com and Notion servers.
  • Consider reaching out to the developers of the affected services (Cal.com and Notion) to see if they have any recommendations or workarounds for this issue.
  • Use the provided workaround of using direct curl API calls for the affected services as a temporary solution.

Example

No code example is provided as the issue is more related to the configuration and setup of the MCP servers rather than a specific code snippet.

Notes

The root cause of the issue appears to be the total tool schema payload size, but it is unclear what the exact threshold is or how to increase it. The workaround provided may not be a long-term solution and may require additional maintenance and upkeep.

Recommendation

Apply workaround: Using direct curl API calls for the affected services is the recommended solution at this time, as it allows for continued functionality while the underlying issue is investigated and resolved.

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