Short answer: build it if grading is your product and your differentiator; buy it if grading is a feature your product needs but is not the thing you sell. The trap is assuming an AI grader is "just a prompt". The prompt is the easy 20%. The rubric handling, evidence grounding, review workflow, audit trail, and compliance are the 80% that quietly consumes a roadmap.
What looks easy and is
Getting a language model to read an answer and return a score is a weekend. You write a prompt, paste in the rubric and the response, and it produces something plausible. This is why build-it feels tempting. The demo works on the first try.
What looks easy and is not
The demo is not the product. The hard parts show up the moment real learners hit it:
- Rubric handling that holds. Arbitrary criteria, performance levels, weighting, partial credit. Marking has to map to the rubric reliably, not approximately, across thousands of varied answers.
- Evidence grounding. Stopping the model inventing strengths the answer does not contain. This is a real, recurring failure mode that needs deliberate design, not a hopeful prompt. See grounded AI feedback.
- Human-in-the-loop review. A reviewer needs to see the proposed grade, the cited evidence, override it, and have that captured. For consequential decisions this is not optional.
- Audit trail. Who submitted what, what the AI proposed, what evidence it cited, who reviewed it, what changed. Required the moment a grade affects someone's qualification or job.
- Compliance and isolation. GDPR, data residency, tenant isolation, the EU AI Act treatment of assessment as high-risk. Customers in education and training will ask, and "we will get to it" loses the deal.
- Cost and reliability at scale. Per-report cost, retries, queueing, handling the model's bad days.
Build if
- Grading is the core of what you sell and your edge is in how you grade.
- You have the engineering and the appetite to own the failure modes above for the long term.
- Your assessment logic is so specific that no external layer could express it.
Buy if
- Grading is a capability your product needs, but your differentiator is elsewhere - the tutor, the curriculum, the experience.
- You would rather ship the rest of your roadmap than spend two quarters on rubric edge cases and audit logging.
- You need compliance answers now, not after a future hardening sprint.
The middle path
You do not have to choose all-build or all-buy. Scorafy is a grading-and-feedback layer you can sit behind your own product: bring your rubric and learner responses, get back evidence-based, per-learner reports, with review sign-off, organisation isolation via row-level security, and EU data residency. You keep your product and your learner experience; you skip building the assessment engine. If you are building an AI learning product, see Scorafy for AI learning product builders or try the demo.