AI red-pen grader for essays and resumes

What to build

A free, single-purpose web app that takes a block of submitted writing (essay, resume, cover letter) and returns it marked up with simulated handwritten red-pen corrections, margin notes, and a letter grade, built on top of an LLM API for the grading logic and a canvas/SVG layer for the handwritten-markup rendering.

A thin, highly differentiated UX wrapper around commoditized LLM writing feedback — the 'graded like a teacher' visual is the product, not the underlying AI.

Demand

Students, ESL learners, and job seekers want fast, low-friction writing feedback without signing up for a bloated AI assistant; the teacher-red-pen framing makes the feedback feel more authoritative and legible than a wall of AI text.

• r/SideProject (Reddit)Community

  Solo builder posted gradethis.app to r/SideProject: 'a free app that returns your writing marked up in red pen, like a teacher would.'

• gradethis.appProduct

  Live product demonstrating the red-pen/margin-note/letter-grade markup format on top of an LLM backend, confirming the concept is already built and shippable by one person.

Stack

• OpenAI or Anthropic API (grading + feedback generation)
• HTML5 Canvas or SVG for handwritten-style annotation rendering
• Custom/web font for handwritten red-pen look (e.g. a script font)
• Next.js
• Supabase (auth + saved history for premium tier)
• Stripe (premium tiers: history, batch grading, API access)

Solo + AI difficulty

The LLM prompting and grading logic is easy (a well-tuned prompt plus structured output parsing); the hard part is making the red-pen markup rendering look convincingly handwritten and correctly positioned over arbitrary text layouts. A rough MVP (text in, graded markup out, no accounts) is a weekend-to-one-week build; a polished version with accurate margin-note placement and saved history is closer to 3-4 weeks solo.

Entry threshold

Low barrier: it is essentially a prompt-engineered LLM wrapper with custom CSS for the red-pen markup, already built solo in about a week; the real challenge is differentiating and monetizing in a crowded market dominated by Grammarly, QuillBot, and other AI writing/grading tools.

Window

ongoing, evergreen niche

Where to find first users

• r/SideProject and r/InternetIsBeautiful launch posts
• Product Hunt launch
• TikTok/Instagram Reels showing before/after markup (visually demo-able product)
• ESL and student-focused subreddits (r/EnglishLearning, r/ApplyingToCollege) and resume-help communities (r/resumes)

Competitors

• Grammarly
• QuillBot

Counter-signals & risks

• The core value (LLM-based writing feedback) is easily replicated by competitors like Grammarly, ChatGPT, and QuillBot, and the red-pen visual gimmick may not be a durable moat once copied.

• Reliance on a third-party LLM API creates thin margins and dependency risk; cost-per-grading could erode profitability at scale given a free-tier-heavy student user base.

• Schools, districts, and HR tools often require procurement, FERPA/COPPA compliance, and data-privacy guarantees that a lightweight micro-SaaS may struggle to meet, limiting the B2B/API/white-label upsell path.