[Claude Desktop Windows] Schema compilation fails with complex MCP outputSchemas containing $defs

[Maladjez]

Bug Description

Claude Desktop (Windows) v1.0.1768 fails to compile MCP tool schemas that contain complex outputSchema definitions with nested $defs and $ref references. This prevents MCP tools from being usable despite successful server startup and tool registration.

Environment

• OS: Windows Server 2022 Enterprise (VMware VM)
• Claude Desktop Version: 1.0.1768
• Architecture: x64
• Node Version: 22.21.1 (bundled)

Affected MCP Servers

• @modelcontextprotocol/server-postgres (v1.22.0)
• @modelcontextprotocol/server-filesystem (Extension v0.2.0)
• mcp-server-jetbrains

All servers use complex outputSchema with $defs sections.

Symptoms

1. MCP servers start successfully ✅
2. MCP protocol handshake completes ✅
3. Tools are registered successfully ✅
4. BUT: Frontend shows 50+ schema compilation errors ❌
5. Tool calls timeout after ~5 minutes ❌
6. No tools/call requests ever reach MCP servers ❌

Error Logs

From claude_ai-web.log:

[error] Error compiling schema, function code: var refVal1 = refVal[1]; var validate = function(data, dataPath, parentData, parentDataProperty, rootData) { 'use strict';  var vErrors = null;  var errors = 0; if (rootData === undefined) rootData = data; var errs_1 = errors; var errs_2 = errors; if ((typeof data !== "number" || (data % 1) || data !== data)) { validate.errors = [ { keyword: 'type' , dataPath: (dataPath || '') + "" , schemaPath: '#/definitions/nonNegativeInteger/type' , params: { type: 'integer' } , message: 'should be integer' } ]; return false; }...

Repeated 50+ times for different schema definitions

MCP Communication Flow

✅ 1. Server starts: postgres-mcp v1.22.0
✅ 2. Handshake: {"method":"initialize",...}
✅ 3. Server responds: {"result":{"protocolVersion":"2025-06-18",...}}
✅ 4. tools/list request sent
✅ 5. Server returns tools with complex outputSchema containing $defs
❌ 6. Frontend attempts schema compilation → FAILS
❌ 7. User initiates tool call → Frontend sends COMPLETION request to claude.ai
❌ 8. Request times out after ~5 minutes: "TypeError: Failed to fetch"
❌ 9. No tools/call request ever sent to MCP server

Example Problematic Schema

From PostgreSQL MCP server tools/list response:

{
  "name": "list_schemas",
  "outputSchema": {
    "$defs": {
      "Annotations": {
        "properties": {
          "priority": {
            "anyOf": [
              {
                "maximum": 1,
                "minimum": 0,
                "type": "number"
              },
              {"type": "null"}
            ]
          }
        }
      },
      "TextContent": {...},
      "EmbeddedResource": {
        "properties": {
          "resource": {
            "anyOf": [
              {"$ref": "#/$defs/TextResourceContents"},
              {"$ref": "#/$defs/BlobResourceContents"}
            ]
          }
        }
      }
    },
    "properties": {
      "result": {
        "items": {
          "anyOf": [
            {"$ref": "#/$defs/TextContent"},
            {"$ref": "#/$defs/ImageContent"}
          ]
        }
      }
    }
  }
}

Reproduction Steps

1. Install Claude Desktop v1.0.1768 on Windows
2. Configure claude_desktop_config.json:

{
  "mcpServers": {
    "postgres": {
      "command": "uvx",
      "args": ["mcp-server-postgres", "postgresql://user:pass@host:5432/db"]
    }
  }
}

3. Start Claude Desktop
4. Observe in logs:
   • main.log: Servers start successfully
   • mcp.log: tools/list returns schemas with $defs
   • claude_ai-web.log: 50+ "Error compiling schema" errors
5. Attempt to use any tool → Timeout after 5 minutes

Expected Behavior

• Schema validator should handle JSON Schema Draft-07 with $defs
• Tool calls should be sent to MCP servers
• Tools should execute successfully

Actual Behavior

• Schema compilation fails silently
• Frontend never sends tools/call requests to MCP servers
• User sees "Claude is not available" after 5-minute timeout

Workaround

None found. MCP tools are completely unusable in Claude Desktop v1.0.1768 on Windows when servers use complex outputSchemas.

Additional Context

• Problem occurs with all MCP servers that use $defs in outputSchema
• Problem persists even without Filesystem Extension
• Network connectivity confirmed working (corporate proxy configured correctly)
• MCP servers themselves work correctly (verified by examining stdio communication)
• Schema validator appears to choke specifically on nonNegativeInteger type definitions and nested $ref structures

Logs Available

Can provide full logs if needed:

• main.log - Startup sequence
• mcp.log - MCP protocol communication
• claude_ai-web.log - Frontend errors
• mcp-server-*.log - Individual server logs

Request

Please investigate the JSON Schema validator used in Claude Desktop Windows build. The validator appears incompatible with the schema format used by official MCP servers from @modelcontextprotocol.

[Maladjez]

Additional Repro Details and Logs

Below is a more precise description of the minimal scenario and the attached log snippets.

Minimal Reproduction Scenario

1. Start Claude Desktop on Windows Server 2022 with the following MCP servers configured:
   • @modelcontextprotocol/server-postgres (v1.22.0)
   • @modelcontextprotocol/server-filesystem (Extension v0.2.0)
   • mcp-server-jetbrains (PyCharm MCP server)
2. Open a new chat, select a Claude model, and ensure only one MCP server is enabled (I reproduced this with each server individually: filesystem-only, postgres-only, jetbrains-only).
3. Use a simple prompt that should obviously require the corresponding MCP tool, for example:
   • For Postgres: “List all schemas in the database.”
   • For Filesystem: “List the files in directory X.”
   • For JetBrains: “Show me the run configurations in this project.”
4. Wait ~5–8 minutes.

Observed Behaviour in the UI

• The chat appears to “hang” for several minutes, then:
  • Either the session is silently reset and the original prompt reappears in the input field, or
  • The UI shows a message like “No MCP server could be addressed” and no tool output is produced, or
  • The UI reports that “Claude is not available” after a long delay.
• In all cases, the MCP tools are never actually invoked from the user’s perspective.

Observed Behaviour in the Logs

From the attached, time-snipped logs:

• main.log:
  • Confirms app version, platform and MCP startup:
    • appVersion 1.0.1768, platform win32, arch x64, nodeVersion 22.21.1
    • MCP servers for Filesystem, jetbrains, and postgres are launched successfully.
• mcp.log:
  • Shows that all MCP servers:
    • Receive the initialize request from the client.
    • Respond with protocolVersion "2025-06-18" and their capabilities.
    • Successfully answer tools/list, prompts/list, resources/list.
  • For the time window where the prompt is sent, there are no tools/call requests at all for any of the configured MCP servers.
• claude_ai-web.log:
  • Contains dozens of repeated Error compiling schema entries when the client tries to compile MCP schemas that include complex outputSchema definitions with $defs/$ref (see example in the main issue body).
  • After the user prompt is issued, the frontend sends a completion request to claude.ai, then eventually hits a TypeError: Failed to fetch / timeout after several minutes.
  • The UI then resets the conversation or shows one of the error states described above.
• mcp-server-*.log (filesystem/postgres/jetbrains):
  • Show that each MCP server:
    • Starts correctly.
    • Handles initialize and tools/list normally.
    • Does not receive any tools/call requests in the failing scenario, which confirms that the problem is on the Claude Desktop client side, not in the MCP servers.

Why This Looks Like a Claude Desktop Schema/Validation Bug

• The MCP servers clearly work as expected: they start, handshake, and advertise tools without errors.
• The problem occurs even when only one MCP server is enabled, and it affects:
  • server-postgres (complex outputSchema with $defs and $ref),
  • server-filesystem (Draft-07 schemas),
  • mcp-server-jetbrains (complex input/output schema structures).
• The frontend’s schema compiler throws many errors as soon as it encounters these complex schemas, and from that point on it never sends any tools/call requests, but still lets the user initiate prompts that end in a long timeout.

This strongly suggests that the JSON Schema validator / compiler used in the Windows build of Claude Desktop 1.0.1768 cannot correctly handle these MCP schemas (especially $defs/$ref patterns and Draft-07), and fails in a way that prevents any MCP tool invocation.

Attached Files

I have attached time-snipped versions of:

• main.log
• mcp.log
• claude_ai-web.log
• mcp-server-Filesystem.log
• mcp-server-postgres.log
• mcp-server-jetbrains.log

These cover the window from app startup through MCP initialization and the failing tool invocation attempt.

claude.ai-web.log
main.log
mcp.log
mcp-server-Filesystem.log
mcp-server-jetbrains.log
mcp-server-postgres.log