Fan Fiction AI: Write the Fic, or Step Inside the Book
Fan fiction AI comes in two shapes, and picking the right one saves you a month of frustration. If you want a finished fic to post, use a writing tool: Sudowrite has a dedicated fanfic generator, and NovelAI is a strong co-writer for long stories. If what you actually want is to be there, to walk into the world of a book you just finished and see what happens when you make the choices, you want an interactive tool instead: AI Dungeon for open sandboxes, or LLM RPG if you want the game master to have read the book itself.
This guide sorts the options by the job you are doing, says who should stay with each, and is honest about where each one runs out.
What people mean by “fan fiction AI”
Look at what ranks for the phrase and nearly every page is a generator: pick a fandom, pick characters, pick tropes, press a button, get prose. That covers one real need. It misses another one that fan fiction has always served.
A lot of fic is written because the reader was not ready to leave. The book ended, the world did not, and writing a story in it was the only way back in. Reader-insert fic exists for the same reason. So there are two different people typing the same search:
- The writer. Wants help drafting a story they will edit, publish and share on an archive. Cares about prose, voice and control.
- The visitor. Wants to spend time inside the world. Cares about the world reacting, the characters behaving like themselves, and not having to do all the remembering.
Both are fan fiction. They need different tools.
The options at a glance
| Sudowrite | NovelAI | AI Dungeon | LLM RPG | |
|---|---|---|---|---|
| Shape | Writing app | Writing partner | Text adventure | Text RPG from a book |
| You are | The author | The author | The player | The player |
| Fandom knowledge | Presets plus your notes | Your lorebook | Your scenario | Read from the book you upload |
| Output | Prose you edit | Prose you edit | A played story | A played story |
| Entry price | Free trial, then from $10/mo | Subscription from $10/mo | Free to start | Free trial, no account |
Prices and descriptions are each vendor’s own, as of September 2026.
Sudowrite: the fanfic generator for writers
Sudowrite is the most direct answer to the search. It runs a dedicated page titled “Fan Fiction Story Generator” that walks you through five steps:
Fandom: Choose a universe. Characters: Pick favorites. Setting: Timeline & Tone. Tropes: Choose dynamics. Generate: Create story.
It ships popular fandoms as presets (Harry Potter, BTS, the Marvel Cinematic Universe, Sherlock Holmes, My Hero Academia and more) with their common tropes attached, and a “Create Custom Fandom” option for anything missing. The generator hands you an outline and ideas, and the full Sudowrite app then helps you expand scenes, rework dialogue and keep voices steady.
What it costs, as of September 2026, from Sudowrite’s pricing page:
| Plan | Monthly price | Credits per month |
|---|---|---|
| Hobby & Student | $10 | 225,000 |
| Professional | $22 | 450,000 |
| Max | $44 | 2,000,000 |
Each plan starts with a free trial, and yearly billing is advertised at “up to 50% off”.
Stay with Sudowrite if you are writing a fic to publish. It is a writing tool built by writers, and it treats you as the author at every step.
NovelAI: a co-writer that keeps notes
NovelAI puts image generation first on its home page these days, but its story side is still a serious writing environment. Its own line for it: “A story generator built to write with you, not for you.” You write, it continues, you steer, and a lorebook holds the facts about characters and places so the model can be reminded of them.
For fan fiction that lorebook is the key feature and also the main chore. You are the one who fills it in. A good lorebook for a big series is a lot of typing, but once it exists NovelAI stays close to the world you described.
NovelAI’s FAQ says subscriptions “start at $10/month” and that new users can try it through a trial period (as of September 2026). Our NovelAI pricing breakdown walks through every tier, and the NovelAI review covers who it suits.
Stay with NovelAI if you want fine control over every sentence and you do not mind building the reference notes yourself.
AI Dungeon: play a story instead of writing one
AI Dungeon moves you from author to player. In its own words it is “a free AI text adventure game where players create open-ended stories, explore interactive worlds, and guide every choice online or in the app.” You type what your character does, and the story answers.
For fandom play you build a custom scenario or find one somebody in the community already made. That is flexible and quick to start. The model knows only what the scenario and your notes tell it, so in a deep series the details you did not write down tend to drift.
Stay with AI Dungeon if you want a big sandbox, community scenarios and total freedom of premise. Our AI Dungeon review and AI Dungeon vs NovelAI go deeper.
LLM RPG: step inside the book you just finished
LLM RPG is built for the visitor. You upload the novel itself, as a .txt or
.epub file, and it reads the book in advance, building an atlas of its
characters, factions, locations, items and events. Then you create your own
character and play turn by turn with a game master that grounds every scene in
what the book actually says.
What that changes for fan fiction:
- You do not write the lorebook. The book is the reference, so the minor character from chapter twelve is there without you remembering to add them.
- The world keeps its rules. Outcomes are settled before the scene is written, so the story does not simply bend to whatever you type.
- It remembers your playthrough. Your run’s state carries across turns and reloads, and facts that change in your story replace the old ones.
What it costs. You can try a ready-made world free for five turns without an account. Signing up adds 50 free credits. A standard turn costs one credit, and reading a book costs one credit per 50,000 characters. Pro is $9 a month for 1,000 credits, and there is a one-time pack of 400 credits for $5. The pricing page has the details.
It is the wrong tool if you want polished prose to post on an archive. What you get is a played story in turns, which reads like a campaign log rather than a finished chapter.
Which fan fiction AI should you pick?
- You want a fic to publish: Sudowrite, or NovelAI if you prefer to write most of it yourself with a model continuing your lines.
- You want to play in a world you invent, or one the community built: AI Dungeon.
- You want to be a character inside a specific novel, and have the world react the way that book would: LLM RPG.
The honest test is one question: do you want to read the result, or live it? Writers should pick a writing tool. Readers who were not ready for the book to end should pick something that plays.
How to start playing inside a book
- Open LLM RPG and try one of the ready-made worlds. No account is needed for the first five turns.
- If it feels right, sign up with your email and upload the novel you want to return to.
- Create your character: who they are, where they start, what they want.
- Take your first turn and see how the world answers.
If you are still weighing the wider field, our guides to AI roleplaying and games like AI Dungeon cover the other shapes this can take.