Pi releases pi-schema, a JSON Schema validator for runtimes that block eval

• Pi-schema compiles JSON Schema into closures for JavaScript runtimes that block eval, as described in the launch thread.
• Strict-CSP performance is the standout: pi-schema keeps sub-microsecond checks across the three scenarios in the launch chart.
• Pi-ai and pi-durable face an import break: the migration announcement says their TypeBox dependency and re-exports are going away.

The error reporter is built only after a validation failure, so always-valid inputs never pay to construct it. There is a naming gotcha in the builder API: oneOf() emits enum, while union() emits anyOf.

@earendil-works/pi-schema is a dependency-free builder and validator, installed with npm install @earendil-works/pi-schema. Schemas compile once into ordinary JavaScript closures, without generating source code through eval or new Function.

Mario Zechner, pi-schema’s author, credited SICP and Graal/Truffle as inspirations for the closure approach.

Three API details:

• Builders return plain, serializable JSON Schema.
• check() acts as a TypeScript type guard.
• obj() rejects additional properties by default.

Timings cover boolean checks of valid values, with code generation disabled via --disallow-code-generation-from-strings for the CSP comparison. The suite ran on Node 24.4.1, using one i9-13900 core and the median of 15 rounds after warm-up.

Published validation timings, including TypeBox with runtime code generation blocked

TypeBox retains an advantage on simpler schemas when code generation is available: the README reports 9 ns for a read-tool schema against pi-schema’s 31 ns. Compiling a new schema takes 21 µs for pi-schema versus 203 µs for TypeBox in the same benchmark suite.

Official JSON Schema core-suite results reported in the launch

The core comparison excludes format tests. With its optional format map, pi-schema separately reports 895 passes from 895 draft 2020-12 format tests.

Pi-schema leaves these features unsupported:

• Meta-schemas that switch vocabularies on or off through $vocabulary.
• contentMediaType and contentEncoding assertions, which are optional under the specification.
• Draft 2019-09’s $recursiveRef.

The published measurements compare retained heap after compiling and checking 3,200 distinct tool-like schemas, plus validator bundle sizes:

| Measurement | pi-schema | TypeBox 1.3.27 | cfworker 4.1.1 | omptype 18.13.3 |
| --- | ---: | ---: | ---: | ---: |
| Retained heap | 7.1 MB | 54.7 MB | 7.6 MB | 20.0 MB |
| Minified, gzipped bundle | 8.5 KB | 35.2 KB | 5.8 KB | 38.8 KB |

Heap measurements used separate processes after garbage collection and excluded the schemas themselves, which occupied 11 MB. Bundle measurements used esbuild 0.28 and gzip -9; pi-schema’s optional format checks were excluded.

Pi-ai and pi-durable will stop depending on TypeBox and re-exporting it, according to the migration announcement. Internal schemas are being switched to pi-schema, while existing TypeBox-authored schemas remain compatible.

That compatibility covers TypeBox 1.x JSON Schemas, with two documented exclusions:

• JavaScript-only types such as BigInt, Undefined, and Function.
• Refinements and codecs.

Zechner said he had not inspected the code, relying on tests, benchmarks and profiling to drive the implementation. He described his contribution as a single informed prompt, then wrote that the result “does not feel like an accomplishment at all.”