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Claude models fail on MCP tools with no required parameters (undefined vs {}) #6559

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opened 2026-02-16 18:04:35 -05:00 by yindo · 3 comments
yindo commented 2026-02-16 18:04:35 -05:00
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Originally created by @belovedEmperor on GitHub (Jan 16, 2026).

Originally assigned to: @thdxr on GitHub.

Description

Claude models send undefined for MCP tool arguments when a tool has no required parameters. OpenCode passes this through unchanged, causing Zod validation to fail on the MCP server side.

Gemini models send {} for the same tools and work fine.

Steps to reproduce

  1. Configure @modelcontextprotocol/server-memory MCP server
  2. Use a Claude model (e.g., claude-sonnet-4-20250514)
  3. Trigger memory_read_graph tool (no required params)
  4. Tool execution aborted with invalid_union error

Error output

[
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "expected": "record",
          "code": "invalid_type",
          "path": [
            "state",
            "input"
          ],
          "message": "Invalid input: expected record, received undefined"
        }
      ],
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": [
            "part"
          ],
          "message": "Invalid input: expected object, received undefined"
        },
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [
            "delta"
          ],
          "message": "Invalid input: expected string, received undefined"
        }
      ],
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": [
            "part"
          ],
          "message": "Invalid input: expected object, received undefined"
        },
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [
            "delta"
          ],
          "message": "Invalid input: expected string, received undefined"
        }
      ]
    ],
    "path": [],
    "message": "Invalid input"
  }
]

Root cause

In packages/opencode/src/mcp/index.ts, convertMcpTool's execute function passes args directly:

arguments: args as Record<string, unknown>

When Claude sends undefined, this becomes arguments: undefined to the MCP server.

Suggested fix

arguments: args ?? {}

Environment

  • OS: NixOS
  • Models affected: Claude (all versions tested)
  • Models working: Gemini
Originally created by @belovedEmperor on GitHub (Jan 16, 2026). Originally assigned to: @thdxr on GitHub. ## Description Claude models send `undefined` for MCP tool arguments when a tool has no required parameters. OpenCode passes this through unchanged, causing Zod validation to fail on the MCP server side. Gemini models send `{}` for the same tools and work fine. ## Steps to reproduce 1. Configure `@modelcontextprotocol/server-memory` MCP server 2. Use a Claude model (e.g., `claude-sonnet-4-20250514`) 3. Trigger `memory_read_graph` tool (no required params) 4. Tool execution aborted with `invalid_union` error ## Error output ```json [ { "code": "invalid_union", "errors": [ [ { "expected": "record", "code": "invalid_type", "path": [ "state", "input" ], "message": "Invalid input: expected record, received undefined" } ], [ { "expected": "object", "code": "invalid_type", "path": [ "part" ], "message": "Invalid input: expected object, received undefined" }, { "expected": "string", "code": "invalid_type", "path": [ "delta" ], "message": "Invalid input: expected string, received undefined" } ], [ { "expected": "object", "code": "invalid_type", "path": [ "part" ], "message": "Invalid input: expected object, received undefined" }, { "expected": "string", "code": "invalid_type", "path": [ "delta" ], "message": "Invalid input: expected string, received undefined" } ] ], "path": [], "message": "Invalid input" } ] ``` ## Root cause In `packages/opencode/src/mcp/index.ts`, `convertMcpTool`'s execute function passes args directly: ```typescript arguments: args as Record<string, unknown> ``` When Claude sends `undefined`, this becomes `arguments: undefined` to the MCP server. ## Suggested fix ```typescript arguments: args ?? {} ``` ## Environment - OS: NixOS - Models affected: Claude (all versions tested) - Models working: Gemini
yindo added the nix label 2026-02-16 18:04:35 -05:00
yindo closed this issue 2026-02-16 18:04:35 -05:00
yindo commented 2026-02-16 18:04:35 -05:00
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@github-actions[bot] commented on GitHub (Jan 16, 2026):

This issue might be a duplicate of existing issues. Please check:

  • #8184: Strict JSON Schema validation fail on backends like SGLang (Missing required field in empty tool parameters)

Feel free to ignore if none of these address your specific case.

@github-actions[bot] commented on GitHub (Jan 16, 2026): This issue might be a duplicate of existing issues. Please check: - #8184: Strict JSON Schema validation fail on backends like SGLang (Missing required field in empty tool parameters) Feel free to ignore if none of these address your specific case.
yindo commented 2026-02-16 18:04:35 -05:00
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@belovedEmperor commented on GitHub (Jan 16, 2026):

Also confirmed this affects the Playwright MCP server - not specific to server-memory. Any MCP tool with no required parameters will fail when called by Claude.

@belovedEmperor commented on GitHub (Jan 16, 2026): Also confirmed this affects the Playwright MCP server - not specific to server-memory. Any MCP tool with no required parameters will fail when called by Claude.
yindo commented 2026-02-16 18:04:36 -05:00
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@BingLi37 commented on GitHub (Jan 20, 2026):

me too

@BingLi37 commented on GitHub (Jan 20, 2026): me too
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Reference: anomalyco/opencode#6559
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