UltraJSON (ujson)
Ultra-fast JSON encoder and decoder for Python written as a C extension. Drop-in replacement for Python's standard json module offering 2-5x faster encoding and decoding. Used by pandas, PyPI, and many high-throughput Python services where JSON serialization is a bottleneck. Maintains the same API as the standard json module — json.loads/json.dumps compatible.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Pure serialization library. C extension with active security maintenance. No network calls. Historical CVEs patched — keep updated.
⚡ Reliability
Best When
You need a simple drop-in json module replacement that's faster than stdlib with no API changes — ujson is the path of least resistance for JSON speedup.
Avoid When
You need the absolute fastest JSON performance or Pydantic/dataclass support — orjson is faster and more feature-complete. Also avoid for financial data with float precision requirements.
Use Cases
- • Replace Python's standard json module with a drop-in faster alternative in API services processing large JSON payloads
- • Speed up agent pipelines that serialize/deserialize large JSON responses from LLM APIs or data services
- • Reduce JSON serialization overhead in high-throughput Python web services (Flask, FastAPI, Django)
- • Process large JSON datasets in data pipelines faster than standard json without changing existing code
- • Benchmark JSON performance in Python services to identify if serialization is a bottleneck before adopting orjson
Not For
- • Maximum performance requirements — orjson is faster than ujson and handles more edge cases correctly
- • Dataclass/Pydantic serialization — orjson has native support for Python dataclasses, datetime, UUID; ujson requires manual handling
- • Applications requiring exact decimal precision — ujson can lose precision on large floats; use standard json or orjson for financial data
Interface
Authentication
Python library. No authentication required.
Pricing
BSD license. Originally from ESN Social Software, now community maintained.
Agent Metadata
Known Gotchas
- ⚠ ujson does not support Python's json module 'default' serializer hook for custom types — custom objects must be converted to dicts/lists before serialization
- ⚠ Float precision can differ from stdlib json — large floats may lose precision; validate with decimal-sensitive data before adopting
- ⚠ ujson returns str (not bytes) from dumps, unlike orjson which returns bytes — this can cause encoding issues if bytes are expected downstream
- ⚠ sort_keys parameter behavior may differ subtly from stdlib json in edge cases involving non-string keys
- ⚠ C extension compilation requires build tools at install time — may fail in minimal Docker images without gcc; use pre-built wheels from PyPI where available
- ⚠ ujson is faster for most workloads but orjson typically beats it in benchmarks — benchmark your specific workload before choosing
Alternatives
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Scores are editorial opinions as of 2026-03-06.