Another day, another GM tool. Mine's only real trick: it doesn't assume you play 5e
I know. The "I built a DM tool" posts come through here weekly, and most of them are a combat tracker with a 5e monster list stapled on. So let me lead with the one thing OSR.RUN actually does differently, and you can decide from there.
It ships with zero game content.
No bundled stat blocks, no baked-in system. You hand it the rules you actually play and the app rebuilds itself around them.
The way you do that: upload a rulebook PDF. A structural parser (no LLM in the pipeline) reads the printed page into a playable ruleset: classes, spells, monsters, gear, ancestries, conditions, XP tables. Small book in about 10 seconds, a 220-page book in about 30. You get an ingest report showing exactly what it read and where each field landed, with page references so you can check its work against the book. Nothing is invented. If the book didn't print it, it isn't there.
Then it's yours to homebrew and edit.
Remap a column the reader carried but didn't recognize ("this column IS morale" and every monster updates at once, with one-click undo).
Design your own pills for a book's homebrew stats and decide where they show on the monster card.
Fix anything in a full builder, add your own monsters and gear mid-session, then export the whole ruleset as a file you own and can hand to anyone.
The interface rekeys to whatever you loaded. A percentile game shows four clean numbers instead of empty d20 modifier boxes. Classless systems drop the class field entirely. You can rename the coin slots and set their exchange rate, set the check die per stat and the dice roller follows. Books that ship in two volumes (player's book + bestiary) stack into one campaign through expansion slots.
It's a 5e SRD tool too, for what it's worth. D&D 5.1 SRD, Black Flag, and Basic Fantasy are bundled one-click.
Books I've validated it against so far (the parser is held to a field-exact golden for each one, monster AC/HP/morale/XP and all):
5e: D&D 5.1 SRD, Black Flag (Tales of the Valia)
B/X & retroclones: Old-School Essentials (Basic + Advanced Players' Tome + Referee's Tome), Basic Fantasy, Swords & Wizardry
NSR / Bastionland: Cairn 2e (Player's + Warden'tric Bastionland, Mausritter
Also... Knave 2e, Shadowdark, Mörk Borg, CY_BORG, Mothership (Warden's Operations Manual + Player's Survival Guide), Worlds Without Number, Dolmenwood (Player's Book + Monster Book), and The Monster Overhaul
At the table: GM panel on a laptop or phone, player sheets in any mobile browser (no app install), and a TV view showing party HP, conditions, initiative, and dice rolls live, with a roll slam that throws nat 20s up in gold. Run Philips Hue or WLED and the room dims as torches burn down in real time. All of that is optional. It works fine with nothing connected.
Players are free, forever, with unlimited characters that travel between campaigns carrying their gear. Running a table is $8.99/mo for unlimited campaigns and the full builder with export. First week free, no card required.
Same ask as always: tell me which book breaks is how the list above got to 22.