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JSON repair can turn an interrupted response into valid-looking JSON—and hide the fact that content went missing. In ToNBi’s October 6, 2026 experiment, the json-repair library returned plausible-looking JSON for 840 of 853 truncated samples. A required-field schema check still accepted 137 damaged results. Check why generation stopped before repairing its output, and use a completion marker as an additional safeguard.
Why repaired JSON can conceal truncation
A response cut off before its closing brackets is malformed JSON. A repair library may close those structures so a parser can read the result. That fixes the syntax, not the missing content: the output can look complete even though generation stopped early.
ToNBi tested five JSON samples, truncating each at every character boundary for 853 truncated strings. With json-repair version 0.63.5, 840 of 853 strings (98.5%) produced plausible-looking JSON. The experiment also found 107 cases where arrays lost elements and 91 where strings were cut short; some cases involved both kinds of damage. These are results from the author’s constructed demonstration, not estimates of how often truncation occurs in production.
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Why a schema may accept incomplete data
A required-field JSON Schema can check that expected fields exist and have the expected types. It cannot necessarily establish that a value contains all the information originally generated. In ToNBi’s experiment, 137 of 853 truncated outputs (16.1%) passed the described required-field validation while differing from the original data.
- An array can retain a valid structure after later elements have disappeared.
- A string can end early and still be a valid string.
- A number cut off at a digit boundary can remain a valid integer while no longer matching the intended value.
Syntax validity and schema conformity are therefore not proof of completeness.
Use safeguards in the order they catch the problem
| Safeguard | What it can detect | What it does not establish |
|---|---|---|
| API termination status | Whether generation stopped because it reached an output limit or another reported stop condition. | That the returned content is semantically correct. |
| JSON parsing | Whether the text is syntactically valid JSON. | That valid JSON contains every intended element or character. |
| Schema validation | Whether the parsed value meets the schema’s required shape and type constraints. | That arrays, strings, or numbers preserve all intended content. |
| Required completion marker | Whether the expected end-of-generation signal is present. | That every value is complete or correct. |
1. Check why generation stopped
Inspect the API’s finish or stop status before passing output to a repair library. If the provider reports that generation ended at an output limit, treat the response as incomplete and retry or otherwise handle it explicitly. An HTTP 200 status alone does not tell you, from the evidence in this experiment, whether the model completed the intended output; check the generation-stop metadata your API provides.
2. Parse and validate without treating either as proof of completion
After checking the stop status, parse the JSON and validate it against the schema. If you use a repair library for malformed input, preserve the original response and record that repair occurred. Do not let a successful parse or schema check override a signal that generation stopped at a limit.
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ToNBi’s proposed fallback is a required marker emitted last and constrained to the value true. Reject output if the marker is absent or invalid. In the author’s experiment, this check allowed 0 of 943 tested truncations to pass validation. The result depended on generating fields in schema order; verify that ordering and behavior in your own structured-output setup. A marker is an additional completion signal, not a guarantee that the data is semantically accurate.
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How much weight to put on the experiment
The figures show a concrete failure mode, but they are not general failure rates. ToNBi used five samples and assumed truncation was equally likely at every character position. Real responses and real cutoff locations can differ, so the observed rates may not predict a particular application. The post describes the experiment and its limits here: ToNBi’s demonstration of repairing truncated JSON.
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