So the reason that Claude code is capable of outputting valid json is because if the prompt text suggests it should be JSON then it enters a special loop in the main query engine that just validates it against JSON schema for JSON and then feeds the data with the error message back into itself until it is valid JSON or a retry limit is reached.
This code is so eye wateringly spaghetti so I am still trying to see if this is true, but this seems to be how it not only returns json to the user, but how it handles *all* LLM-to-JSON, including internal output from its tools. There appears to be an unconditional hook where if the JSON output tool is present in the session config at all, then all tool calls must be followed by the "force into JSON" loop.
If that's true, that's just *mind blowingly expensive*
jonny (nonvenomous)
npub1lj…pw8tm
2026-03-31 20:08:36 CEST
in reply to nevent1q…t0g8
Author Public Key
npub1ljmfkwmllavdpnf5tgmrfay6mj4t78c0xryugfw4qka0c4exas0q2pw8tmPublished at
2026-03-31 20:08:36 CESTEvent JSON
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"content": "So the reason that Claude code is capable of outputting valid json is because if the prompt text suggests it should be JSON then it enters a special loop in the main query engine that just validates it against JSON schema for JSON and then feeds the data with the error message back into itself until it is valid JSON or a retry limit is reached.\n\nThis code is so eye wateringly spaghetti so I am still trying to see if this is true, but this seems to be how it not only returns json to the user, but how it handles *all* LLM-to-JSON, including internal output from its tools. There appears to be an unconditional hook where if the JSON output tool is present in the session config at all, then all tool calls must be followed by the \"force into JSON\" loop.\n\nIf that's true, that's just *mind blowingly expensive*",
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