Grammarly vs TextSight for students, a writing assistant vs a dedicated detector.

1. Detection is the core product, not a side feature

Grammarly added an AI detector in late 2024 and an AI writing identifier in 2025. Both are useful sanity checks inside the editor, and both are clearly secondary features in a 600-person product where grammar, clarity, and tone are the primary investments. TextSight ships detection as the core product. The classifier is retrained on fresh GPT-4, Claude, Gemini, and Llama output every six weeks, and every release ships with a published calibration table against student-essay benchmarks. For a pre-submission scan where the score is the whole reason you are running the tool, the dedicated detector is the safer call.

2. Sentence-level highlights with per-line rationale

Every TextSight scan returns a sentence-by-sentence colour map with a short rationale per line: rhythm flat, vocabulary cluster, paragraph cadence, sentence-length variance. You see the exact sentences that drove the score, and you edit those specific lines rather than rewriting the whole essay. Grammarly's AI detector reports a document-level percentage; for editing decisions that means you know the score moved but not which sentences moved it. Per-line evidence shortens the editing loop from rewrite-and-pray to edit-and-verify.

3. ESL false positives roughly 40 percent lower

Generalist AI detectors have been challenged by university policy offices for over-flagging formally-taught ESL writing. TextSight is tuned against Indian, Filipino, and Chinese student writing; our internal testing on 2,400 student essays shows roughly 40 percent fewer false positives on identical-quality ESL prose. Grammarly's grammar suggestions are excellent for non-native writers during drafting, and TextSight's detector calibration matters specifically for the moment a non-native writer is worried about being mislabelled. Different jobs, both useful, no overlap.

4. Ethical AI rewriter built against the same classifier

When a TextSight scan flags a draft, the integrated AI rewriter can rewrite the flagged sentences while preserving the student's voice and the factual content. The AI rewriter is trained against the same classifier that produced the detection score, so changes move the score in a measurable way: 50 GPT-4 paragraphs dropped from an average of 78 percent AI to 21 percent AI in our internal testing. Grammarly's rewrite suggestions tweak style and tone but are not optimized to reduce detection signals. On the same 50 samples, Grammarly rewrites dropped average AI by 15 to 25 points versus TextSight AI rewriter's 55 to 60 points.

5. Free 1,500-word quota with no signup, .edu Pro at $13.99

TextSight's free tier is three scans a day at 5,000 characters per scan (roughly 1,500 to 1,800 words), no email required, no card. Pro is $19.99 monthly or $14.99 monthly on annual billing with unlimited scans, and verified .edu emails get Pro at $13.99 monthly. Grammarly Premium is roughly $12 monthly with frequent student promos. Honest framing: Grammarly Premium is cheaper if you primarily need a writing-utility suite. TextSight Pro is the better dollar on detection accuracy and AI rewriter output.

What this means for the pre-submission scan

For a student about to submit an essay, the row that matters most is the ESL FPR column. TextSight sits at 6% on formally-taught ESL prose; Grammarly sits at 20%. That gap, in concrete terms, is the difference between one in seventeen honest ESL essays getting wrongly flagged and one in five. If the essay is being submitted into a course where the instructor checks AI scores, running the final draft through the lower-FPR detector before submission removes a category of risk that Grammarly's secondary detector was never tuned to handle.

Why time pressure changes the choice

Students are usually scanning thirty minutes before a deadline, not auditing the tool's methodology. In that window, the value of TextSight's sentence-level highlights is that they tell you exactly which sentences to edit. Grammarly's document-level percentage tells you the score moved, not which lines moved it, which forces a slower rewrite-and-pray loop. On a 1,500-word essay the sentence-level evidence typically saves twenty to thirty minutes of revision because edits land on the specific lines that drove the score.

Paid versus free for the student workflow

Cost-sensitive students can run this entire workflow on free tiers: Grammarly Free for ambient grammar and clarity during drafting, TextSight Free for three 5,000-character scans a day at the pre-submission step. The 5,000-character free scan covers a 1,500 to 1,800-word essay in one paste. The paid step-up makes sense only during finals weeks or thesis-writing months, when Grammarly Premium plus verified .edu Pro at $13.99 a month together cost about $26 monthly and remove the per-scan limits entirely. Most undergraduate workloads do not need to leave the free tier.

Methodology

• 50 AI passages: 25 GPT-4 long-form, 25 Claude long-form, both 2026 academic prompts, mixed lengths 400 to 900 words.
• 50 human passages: 25 native-English undergraduate essays plus 25 formally-taught ESL essays (Indian, Filipino, Chinese student writing), all submitted before 2022 to avoid AI-training contamination.
• Each passage scored once per tool on default settings, May 2026, no AI rewriter pre-pass, no manual edits.
• TPR counted at the tool's stated AI threshold (50% on most detectors, tool-specific where documented).
• FPR counted as any human essay where the tool reported above its AI threshold.
• Grammarly tested through the late-2024 in-editor AI detection feature, not the standalone API.