JSON.parse requires complete, valid JSON, but an LLM stream arrives in fragments that may end halfway through a string, value, object, or array. For a live interface, use a partial-JSON parser to produce provisional snapshots; when the stream ends, strictly parse and validate the complete raw response. One JavaScript option is SoFar, whose author describes a 425-byte-gzipped parser, but that size and its reported benchmark are the author’s claims, not independently verified measurements.
Why JSON.parse fails during streaming
A stream delivers pieces of text, not necessarily complete JSON documents. Chunk boundaries can fall anywhere, including inside a quoted string or between the colon and value of an object property. Parsing a chunk on its own will therefore fail unless that chunk happens to be complete JSON. Parsing the accumulated buffer can fail for the same reason while the response is still unfinished. Chrome for Developers’ guide to LLM streaming describes the fragmented nature of streamed responses.
For example, after receiving {"status":"rea, the consumer has not yet received a complete JSON string or object. A thrown parse error at that point is expected; it does not by itself mean the model produced invalid final JSON.
Use partial parsing for provisional UI updates
A partial parser attempts to recover a useful value from unfinished input. Treat the result as a snapshot of what can be safely understood so far—not as a confirmed, complete response. It can help update a streaming interface, but should not trigger irreversible actions or stand in for final validation.
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Symfony’s PartialJsonParser documentation describes recovering the largest valid structure from incomplete payloads for incremental UI updates. It also says validation runs on the final object, not on partial snapshots; unchanged snapshots may be skipped.
SoFar: a JavaScript option for partial JSON
Manthan Kansagra’s DEV Community article describes SoFar, a small JavaScript library with parsePartialJSON for a partial buffer and createJSONStream for a stateful feed interface. The article reports a size of 425 bytes gzipped and a benchmark of about 1.3× the cost of bare JSON.parse on a 1.6 MB buffer. These are the author’s claims; they were not independently measured here. The article’s displayed publication date is September 18, without a year in the available date information. Read the author’s SoFar description.
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According to the article, SoFar scans for open containers, string and escape state, and safe cut points. It attempts to parse a best-effort completion, then tries earlier safe cut points if needed. Its conservative behavior matters: an incomplete prefix such as tru does not establish the literal true, and the parser does not guess incomplete numbers or literals. The article says it will not repair JSON that was never going to be valid.
The “425-byte fix” is a concise way to describe the author’s reported gzipped package size, not a guarantee that partial parsing fixes every stream error. Choose a parser based on its recovery behavior and your application’s requirements.
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Wire partial parsing into a stream safely
- Collect the right text. Follow the streaming API’s event format and concatenate the response content fragments in order. Do not assume a network chunk or API fragment ends at a JSON token or syntax boundary.
- Parse the accumulated buffer for display. Feed the growing text to a partial parser, or use its stateful streaming interface. Render any recovered value as provisional UI state, and handle cases where no useful snapshot is available yet.
- Wait for the stream to finish. Keep the raw response text. Once generation is complete, call
JSON.parseon that complete text and validate the resulting value against the application’s expected schema. - Handle final failure explicitly. If strict parsing or schema validation fails, report the error or provide a deliberate recovery path. Do not silently promote a partial parser’s fabricated or incomplete value to authoritative data.
Keep persistence, business decisions, and other consequential operations behind final parsing and validation. A snapshot that is useful for rendering is not necessarily safe for those tasks.
Partial parsing is not general-purpose JSON repair
Incomplete streams and malformed completed input are different problems. A partial-stream parser tries to extract a useful value before all expected text has arrived; a repair tool attempts to fix malformed JSON-like text, such as input containing comments or unquoted keys. The SoFar article points to jsonrepair for that separate repair use case.
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Recovery policies vary across partial parsers. Symfony AI documents recovery cases including trailing commas, unclosed strings, dangling colons, partial literals, and open containers. SoFar, by contrast, describes conservative handling that does not guess incomplete literals or numbers. Neither behavior should be assumed from the label “partial JSON parser”; check the implementation and test it against the inputs your application can receive.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Account for resource limits in production
Parsing a growing buffer repeatedly can consume more work as a response grows. Limits on input bytes, fragment counts, nesting depth, and parsing effort help prevent unexpectedly large or deeply nested streams from consuming unbounded resources. The OpenAI Node SDK’s ChatCompletionStream source includes bounds for bytes, fragments, nesting depth, and parse work. That implementation is evidence that production stream consumers need safeguards; it is not, by itself, a general-purpose partial-parser recommendation.
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There is no independently established adoption, reliability, or industry-wide performance statistic for partial JSON parsers in the sources cited here. Evaluate candidate libraries against your own stream sizes, malformed-input cases, output schema, and latency needs.
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