AmberPen vs Harper
Harper is a free, open-source grammar checker that runs entirely on your device. AmberPen is a hosted, model-based proofreading API. Harper is the better fit when local execution, zero cost, and very low latency matter most; AmberPen is built for applications that need the best correction quality and multilingual proofreading.
At a glance
| Capability | AmberPen | Harper 2.4.0 |
|---|---|---|
| Best for | High-quality, contextual proofreading | Simple local English checking |
| Runs locally | No — hosted API | Yes |
| Price | Usage-based, with a free allowance | Free and open source |
| Languages | Multilingual | English dialects |
| Structured edits | Yes, with offsets and optional explanations | Yes, through local integrations |
| Submitted text used for training | No | No text leaves your device |
Correction quality
We ran both correctors over the same 754 JFLEG development sentences and 6,845 CWEB test sentences, then scored both with the same local evaluation tools. JFLEG rewards fluent corrections; CWEB rewards precise, restrained edits on web text where most words are already correct.
| Metric | AmberPen | Harper |
|---|---|---|
| JFLEG GLEU | 0.5610 | 0.3437 |
| CWEB precision | 37.35% | 5.80% |
| CWEB recall | 65.54% | 13.79% |
| CWEB ERRANT F0.5 | 40.87 | 6.56 |
AmberPen leads JFLEG by 0.2173 GLEU. On CWEB it finds nearly five times as many annotated errors and its precision is more than six times Harper's. Harper's very low CWEB precision reflects a recurring failure mode: unfamiliar names, abbreviations, and specialist terms are sometimes changed into common dictionary words.
Examples from CWEB
In the first three examples, both human references left the highlighted source terms unchanged. The final two show corrections both references required but Harper missed or only partly fixed.
| Source | Harper output | What went wrong |
|---|---|---|
| outlined in the CTA | outlined in the Cat | An acronym was treated as a misspelling. |
| oral rehydration solution…temporary ileostomy | oral dehydration solution…temporary blossomy | Medical terms were replaced, changing the meaning. |
| Nemanja Matic is a Serbian footballer | Newman's Magic is a Serbian footballer | A person's name was rewritten as familiar English words. |
| Even if it takes places… | No change | Both references corrected this to “takes place.” |
| i am…waiting for solution please | I am…waiting for solution please | Harper capitalized “I” but missed “a solution” and the final period. |
Speed
| CWEB latency per sentence | AmberPen | Harper |
|---|---|---|
| p50 | 217 ms | 2 ms |
| p95 | 1,024 ms | 8 ms |
| p99 | 1,413 ms | 14 ms |
Harper is roughly 100 times faster at the median. That is a real advantage, although the numbers are not an infrastructure comparison: Harper executes in-process, while AmberPen's measurements include a network request to a hosted service.
Privacy and data handling
Harper offers the strongest possible locality: proofreading happens on your device, so the text never needs to leave it. AmberPen sends text to its hosted service for correction, but does not use customer text to train models, and does not retain the text submitted for proofreading once the response has been returned.
Incremental proofreading does not change that: it runs inside the SDK, in your own process, which diffs the text locally and sends only the chunks that changed. Account, billing, usage, and security records are handled separately from proofreading text. See the privacy policy for the complete details.
Language support
Harper is designed around English and its regional dialects. AmberPen preserves the input language and is the better choice for multilingual applications. The benchmark numbers above are English-only, however, so they should not be presented as a measured multilingual quality score.
Which should you choose?
- Choose AmberPen when correction quality matters most, when errors depend on context and meaning, or when your product needs multilingual proofreading.
- Choose Harper when you want a simple, free, extremely fast, fully local English grammar checker and can accept substantially lower benchmark quality.
Bottom line: between these two, AmberPen is better for the best correction quality and multilingual work. Harper is best for a simple and free local grammar checker.
See Benchmarks & performance for the full comparison and methodology.
This page covers AmberPen and Harper head to head. If you are still shortlisting, the grammar checker API comparison reviews seven options side by side, and how grammar correction is measured explains what the scores above do and do not prove.