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Validate an MCP message

Choose the protocol revision, load or paste one message, inspect the corrected structure, and compare a related response or expected JSON shape.

  1. 1Choose the version
  2. 2Read findings
  3. 3Inspect correction
  4. 4Compare output
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 the related message or expected output

    Check request and response IDs, or compare an expected JSON shape with the actual message.

    • Paste a related message to compare the pair.

    Ready

    MCP validation recipeSave or reload the rules. Input and output are never included.
    Recipe preflight

    Review the saved mappings

      MCP validation run manifestDownload settings, counts, and SHA-256 receipts for this run. Raw input and output stay out of the file.
      Session-level evidence.Pair every supplied request and response without contacting the client or server.
      Session mode

      Analyze a complete MCP session

      Load JSONL or a JSON array. Requests and responses are paired by ID; gaps remain visible.

      Ready to pair the supplied messages.

      Negotiation
      Unanswered or orphaned
        Errors

          The analyzer reads supplied messages only. It does not replay transport, authentication, cancellation, timeouts, or tool execution.

          Session fixture

          One pair, one unanswered request, one orphan error.

          The JSONL file makes the pairing rules visible without requiring an MCP server, secret, or network request.

          Download JSONL sessionDownload expected summary

          The expected result is tied to the local session analyzer contract, not a complete normative MCP conformance suite.

          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, request ID, and revision-specific metadata.

          Responsejsonrpc + id + result/error

          A response cannot contain both result and error.

          Tool definitionname + inputSchema

          The schema describes the arguments accepted by that tool.

          The 2026-07-28 revision uses a stateless core and requires version and client-capability metadata on every request. The older revisions use different lifecycle rules. A structurally valid message can still fail because of transport, authorization, invalid arguments, or tool execution. Run the corrected message through the exact client and server versions you use.

          Worked fixture

          Expected shape and actual response stay separate.

          The synthetic fixture expects a successful JSON-RPC response with one text content block. The actual response adds the text value. Expected-shape comparison accepts the extra field while still requiring every declared path and value.

          Protocol revision
          2026-07-28
          Comparison
          Expected JSON is treated as the required subset of the actual response.
          Boundary
          No transport, authorization, SDK, or server execution is tested.
          Download expected JSONDownload actual JSONDownload recipe

          Version rules follow the official MCP 2026-07-28 base protocol.

          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.