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Validate an MCP message

Start with one copied message. Identify its role, fix the core JSON-RPC or tool-definition shape, and write down what still needs an SDK test.

  1. 1Identify the message
  2. 2Read findings
  3. 3Inspect correction
  4. 4Compare the pair
Remove secrets first.Validation stays in this browser tab and does not contact an MCP server.
MCP payload labClassify, validate, correct, compare, save fixture
1

Identify the source and message

The source does not change validation. It records where the fixture came from.

2

Read the structural findings

Errors block the core shape. Warnings identify a likely interoperability problem.

Detected type—
    3

    Inspect the corrected fixture

    The correction fills only required core structure. It cannot infer server-specific arguments or permissions.

    4

    Compare a request and response

    Paste the related message to check direction and matching IDs. Leave it empty when you only have one message.

    • Paste a related message to compare the pair.

    Ready

    Use this workflow when…

    • MCP Inspector shows a message but the failure is not obvious.
    • An SDK test or debug log contains a tools/list or tools/call payload.
    • You need to separate JSON-RPC envelope errors from runtime, transport, auth, or tool errors.

    Identify the message before fixing it

    A request, response, tools/list result, and single tool definition have different required fields.

    Requestjsonrpc + method + id?

    Check the method and whether a response is expected.

    Responsejsonrpc + id + result/error

    A response cannot contain both result and error.

    Tool definitionname + inputSchema

    The schema describes the arguments accepted by that tool.

    A structurally valid message can still fail because of protocol-version differences, transport framing, authorization, server state, invalid arguments, or tool execution. Run the corrected message through the exact client and server versions you use.

    Check these failure boundaries

    The local validator deliberately stops before transport and execution.

    Observed resultThis workflow checksTest elsewhere
    JSON parse errorJSON syntaxNothing until syntax is fixed
    Missing methodRequest envelopeClient serialization
    Tool has no inputSchemaTool definitionSDK schema compatibility
    Valid message, runtime failureCore structure onlyTransport, auth, server, and tool code

    Where the result can go

    Download the result first. It stays useful without another product.

    1Keep the file

    Copy or download the cleaned output and use it in any compatible workflow.

    2Read the versioned SEO MCP reference

    seo-mcp.de covers capability design, permission boundaries, integration choices, and version-specific implementation details.

    Open the SEO MCP referenceDevAwesome and seo-mcp.de are operated by Matthias Ramahi. This is a related workflow handoff, not an independent recommendation.

    Fix syntax first, then preserve the expected behavior as a test.

    JSON formatterFind and repair JSON syntax errors.Simple test case builderRecord one expected MCP response or failure.Reproducible tool testsPublish a bounded result with versions and failure cases.