parsers.mdx

title	Parsers
description	Output-fixing and retry parsers for when an LLM emits dirty JSON.
sidebarTitle	Parsers

Parser examples enact what happens when the LLM gets the JSON contract slightly wrong. Sources under examples/parsers/.

Name	Scenario	Source
parsers_fixing	LLM emits malformed JSON — OutputFixingParser re-prompts with the parse error, the model usually fixes it on the second try.	src
parsers_retry	When one fix isn't enough — RetryParser loops the fixer + parse cycle up to N times until parsing succeeds, surfacing the last error if all attempts fail.	src

How to run

cargo run -p cognis-examples --example parsers_fixing
cargo run -p cognis-examples --example parsers_retry

When to use which

• OutputFixingParser — model is capable of valid JSON but slipped this time. Cheaper: one repair attempt with the parse error in the prompt.
• RetryParser — one repair pass isn't enough; loop fixer + parse up to N times. More expensive but more robust against models that produce different-but-still-broken JSON each attempt.

In production, layer them: RetryParser::with_retries(OutputFixingParser::new(inner, fixer), 3).

See also

The user guide for parsers and recovery strategies. `chains_structured_parsing` shows the parsers in a chain.