How does an AI writing assistant work under the hood?

It is an LLM call with a carefully written system prompt that defines the task — proofread, rewrite, or critique — and the user's text as input. The model returns the edited text or suggestions. The quality lives almost entirely in the system prompt, which is why a builder that gets the prompt right is the core of the assistant.

Should I use streaming?

Yes, for anything user-facing. Streaming sends the response token by token so the edited text appears as it is generated, which feels far more responsive than waiting for the whole reply. Both the OpenAI and Anthropic APIs support streaming, and most editor UIs render the partial output into a preview pane as it arrives.

How do I make grammar fixes precise rather than rewrites?

Instruct the model explicitly to fix only grammar, spelling, and punctuation while preserving the author's wording, voice, and meaning, and to change nothing else. Without that constraint models tend to "improve" the text and rewrite sentences the user did not want touched. A tight, scoped prompt is what keeps a grammar mode from becoming a rewrite mode.

Can it explain its suggestions?

Yes — add a mode that returns each change as a short list of edit and reason pairs rather than just the corrected text. This is more useful for learning and for letting users accept or reject individual changes, at the cost of a slightly more complex UI. Ask the model to return structured output so your app can render each suggestion separately.

How do I detect plagiarism-style or AI-overlap issues?

A true plagiarism check needs a corpus comparison, which an LLM alone cannot do reliably. What a model can do is flag passages that read as unoriginal, clichéd, or templated and suggest more distinctive phrasing. For genuine originality checking, pair the assistant with a dedicated similarity-search service rather than trusting the model to know what exists on the web.

How to Build an AI Writing Assistant

What an AI writing assistant is

An AI writing assistant is, at its core, an LLM call with a well-crafted prompt wrapped in a responsive editor. The user types or pastes text, picks what they want — fix grammar, rewrite the tone, tighten for clarity, or get suggestions — and the model returns the result, ideally streamed into the editor in real time. Tools like Grammarly and Notion AI are elaborate versions of exactly this. The hard part is not the API call; it is writing prompts precise enough that each mode does only what it should. The builder below generates those prompts for you, scoped per mode.

How it works

The architecture is small. A mode maps to a system prompt that defines the task and its constraints. The user’s text becomes the user message. The model’s reply is streamed back token by token and rendered into a preview pane, so edits appear as they generate rather than after a long pause. The key engineering insight is that the constraints matter as much as the task: a grammar mode must be told to change only grammar, spelling, and punctuation and to preserve the author’s voice — otherwise the model “helpfully” rewrites sentences nobody asked it to touch. A rewrite mode, by contrast, needs a target tone and a length instruction. The builder above encodes these scoped prompts for each mode.

Tips and modes

Use the builder to produce a system-and-user prompt pair, then drop it into a chat completions call with streaming enabled. A few patterns worth shipping: a grammar mode locked to mechanical corrections only; a tone mode with a selectable target voice and a preserve-length option; a clarity mode that tightens wordy sentences; and a suggestions mode that returns each change as an edit-and-reason pair so users can accept or reject individually — ask the model for structured output so your UI can render each suggestion separately. Note the limits: a model cannot reliably detect true plagiarism, which needs a corpus comparison, though it can flag clichéd or templated phrasing. To make your first underlying API call, start with the ChatGPT API beginner’s guide.