Show HN: Fastest non lossy JSON parser

Best JSON Formatter 0.2.0, a non-lossy JSON parser and formatter that preserves duplicate keys, integer precision, and exact token spelling, is the fastest fully lossless formatter in benchmarks, outperforming Biome 2.5.6, Prettier 3.9.6, and jsonc-parser 3.3.1 on files up to 5.35 MB. The tool runs locally via npx or the bjf CLI, requires Node.js 20+, and includes commands for validation, formatting, repair, conversion, and schema operations.

Fast, local JSON formatting and correctness checks without lossy JSON.parse / JSON.stringify round trips. npx -y bestjsonformatter@latest check response.json npx -y bestjsonformatter@latest format response.json --write Best JSON Formatter preserves duplicate object properties, integers beyond JavaScript's safe range, decimal and exponent spelling, string escape spelling, and property order. It performs no network requests, collects no analytics, and requires no MCP server or account. Use it without installing: npx -y bestjsonformatter@latest check package.json Or install the short bjf command: npm install --global bestjsonformatter bjf check package.json Node.js 20 or newer is required. check validates syntax and finds duplicate keys and numbers that ordinary JavaScript parsing may round, overflow, underflow, or rewrite: bjf check response.json bjf check response.json --json bjf check response.json --schema response.schema.json Exit codes are stable for CI and coding agents: | Code | Meaning | |---|---| 0 | Valid JSON with no selected correctness findings | 1 | Invalid JSON, correctness findings, schema errors, or formatting differences | 2 | Invalid command, inaccessible file, or internal failure | bjf format response.json bjf format response.json --check bjf format response.json --write bjf format response.json --indent minified bjf format response.json --indent tab bjf format response.json --sort Ordinary formatting changes whitespace only. --sort explicitly reorders object properties while preserving array order and exact scalar tokens. --write uses a same-directory temporary file and atomic rename. Multiple files are supported when checking or writing: bjf format package.json fixtures/ .json --check bjf format package.json fixtures/ .json --write bjf check fixtures/ .json --json bjf repair broken.json bjf repair broken.json --write bjf duplicates response.json bjf precision response.json bjf compare before.json after.json bjf query '$.items .id' response.json bjf to-yaml response.json bjf to-csv records.json bjf to-xml response.json bjf schema-infer response.json bjf schema-validate response.json schema.json bjf schema-sample schema.json Repair output is a proposal unless --write is explicitly supplied. Use -o output.ext to write conversion or repair output to a separate file. Agents can call the CLI directly through their normal shell, so JSON never has to be copied into an MCP tool argument or model context. Install the agent skill: npx skills add skrcode/bestjsonformatter-cli The skill teaches Codex, Claude Code, Cursor, GitHub Copilot, and other compatible agents to run compact checks first and request explicit authorization before --write . The same engine is available as an ESM library: js import { checkJson, formatLosslessJson } from "bestjsonformatter"; const checked = checkJson source ; const formatted = formatLosslessJson source, 2 .formatted; Heavy repair, diff, query, conversion, and schema dependencies are loaded only when their operations are requested. Formatting, validation, and correctness checks stay on the small local hot path. In the repository's reproducible five-run benchmark, Best JSON Formatter is the fastest formatter that passes every exact-token check. | Fresh-process median | 155 B | 105 KB | 1.07 MB | 5.35 MB | Fully lossless | |---|---|---|---|---|---| | Best JSON Formatter 0.2.0 | 54 ms | 76 ms | 143 ms | 473 ms | Yes | | Biome 2.5.6 | 58 ms | 88 ms | 280 ms | 1,150 ms | No | | Prettier 3.9.6 | 120 ms | 234 ms | 955 ms | 3,926 ms | No | | jsonc-parser 3.3.1 | 63 ms | 825 ms | 15,000 ms | 15,000 ms | Yes | | jq 1.8.2 | 5 ms | 14 ms | 105 ms | 500 ms | No | The comparison uses identical stdin payloads and complete fresh CLI processes. jq is faster on small and one-megabyte inputs but discards duplicate keys and rewrites number and escape spelling. Results are bounded to the recorded machine, versions, inputs, and command boundaries—not a universal speed claim. See the complete method, raw samples, Python and native Node results /skrcode/bestjsonformatter-cli/blob/main/docs/BENCHMARKS.md . Documents are read and processed inside the local Node.js process. The package contains no telemetry client, post-install script, network request, account system, or hosted document API. npm install npm run check npm run benchmark Every formatting change must preserve the exact-token fixtures. Performance changes must record the complete journey, competitor versions, raw samples, median, p95, correctness, and timeouts. MIT licensed. The browser tool is available at bestjsonformatter.org https://bestjsonformatter.org .