Adding AWS MSK MCP server to mcp.json breaks Amazon Q CLI with ValidationException

4 answers

• Best answerAgentArmorHQ teamTeam4d ago

  Unresolved: the issue was closed as not planned in January 2026 after being marked stale, and no maintainer confirmed a fix. A contributor said they opened a pull request to address it, and a later comment linked PR #1118, but the thread does not confirm the fix landed.

  The failure reproduces on macOS when awslabs.aws-msk-mcp-server is added to ~/.aws/amazonq/mcp.json. Every prompt — even a first "hello" — fails with:

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  Amazon Q is having trouble responding right now:
     0: unhandled error (ValidationException)
     3: Error { code: "ValidationException", message: "Improperly formed request.", aws_request_id: "..." }
  

  The reported config was:

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  {
    "mcpServers": {
      "awslabs.core-mcp-server": {
        "command": "uvx",
        "args": ["awslabs.core-mcp-server@latest"]
      },
      "awslabs.aws-msk-mcp-server": {
        "command": "uvx",
        "args": ["awslabs.aws-msk-mcp-server@latest", "--allow-writes"],
        "env": { "FASTMCP_LOG_LEVEL": "ERROR" },
        "disabled": false,
        "autoApprove": []
      }
    }
  }
  

  The only workaround reported in the thread is to disable or delete the MSK server entry; Q CLI then works again, at the cost of not being able to use that server. Switching Q CLI models between Sonnet 3.5, 3.7, and 4 did not help, and no root cause was given in the thread. General advice: when testing MCP servers, add them one at a time so a single failing server doesn't break the whole client.

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• AgentArmorHQ teamTeam4d ago

  What a GitHub user tried on GitHub (Didn't work)

  Configured awslabs.aws-msk-mcp-server (uvx awslabs.aws-msk-mcp-server@latest --allow-writes, FASTMCP_LOG_LEVEL=ERROR) alongside awslabs.core-mcp-server in ~/.aws/amazonq/mcp.json. Q CLI then failed on every prompt with ValidationException "Improperly formed request."

  Source: https://github.com/awslabs/mcp/issues/912 (opens external site) ↗

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• AgentArmorHQ teamTeam4d ago

  What @aarjvi17 tried on GitHub (Didn't work)

  Switched Q CLI models between Sonnet 3.5, 3.7, and 4. The ValidationException persisted.

  Source: https://github.com/awslabs/mcp/issues/912#issuecomment-3105197433 (opens external site) ↗

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• AgentArmorHQ teamTeam4d ago

  What @rajatechai tried on GitHub (Worked)

  Disabled or deleted the awslabs.aws-msk-mcp-server entry from mcp.json. Q CLI worked again, but the MSK MCP server could no longer be used.

  Source: https://github.com/awslabs/mcp/issues/912#issuecomment-3150890162 (opens external site) ↗

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