Spot the bug revealed by JSON-RPC tracing in this MCP exchange · Zenaique

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Quick reference

TL;DR

The response carries id 2 but the request was id 1. JSON-RPC correlates by echoing the request id, so the client can never match this reply.

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Easy to grasp

Picture a busy coffee counter where everyone orders at once. The barista hands each order a numbered ticket and calls that number when the drink is ready. You hold ticket 1, but the barista shouts 'order 2!' and sets down your latte. You have no way to know it is yours, so you keep waiting while the coffee goes cold on the counter. JSON-RPC works the same way. The client tags each request with an id and keeps a list of tickets it is still waiting on. The server must reply with the exact same id so the client can match the answer to the right question. Here the request used id 1 and the reply came back as id 2, so the client never finds a match and the call stalls.

Key concepts

JSON-RPC 2.0 Request-response correlation MCP tracing and debugging

Detailed answer & concept explanation

~7 min read

Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.

Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example.

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Quick scan · 2–4 min

Everything important, quickly.

3 min: identify the id mismatch, explain the pending-request map and correlation, give the two server bugs that cause it, and note why tracing is the tool that catches it.

Side-by-side

Aspect	Correct response	Buggy response shown
id value	Echoes request id (1)	New value (2)
Client lookup	Finds pending entry for id 1	Finds nothing for id 2
Outcome	Call resolves with the result	Original call hangs and times out
Data payload	Alice's row, correct	Alice's row, still correct

Real-world examples

Real products, models, and research that use this idea.

The mcp-inspector tool surfaces this by printing paired request and response frames so a mismatched id is visible at a glance.
Claude Code and Cursor multiplex many concurrent tools/call requests over one stdio connection, so id correlation is what keeps replies from crossing wires.
An MCP server built on a naive JSON-RPC library that auto-numbers outgoing messages will stamp wrong ids and cause intermittent client hangs.

Interviewer follow-ups

What an interviewer would ask next. Try answering before peeking at the approach.

QHow would you design the client's pending-request map to detect a mismatched or duplicate id early?

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QHow do JSON-RPC notifications change the correlation story, and how should a client handle them?

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QUnder concurrent load, what observability would you add to catch id correlation bugs before users do?

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Don't say this

Red flags and common mistakes that signal junior thinking. Click to expand.

Most common mistake

Looking at the SQL or the result payload for the bug. The transport layer is broken first: the response id does not match the request id, so correlation fails.

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60-second cheat sheet

The night-before-the-interview bullets. Scan these on the way to the call.

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References

Primary sources. Skim if you want the original framing.