It's called vibe encounter building.
Your AI can do the math. The problem is getting it to do all of it. It has to use the correct edition's XP table, look up every monster's XP, calculate the party's budget, add up the finished roster, and check the result.
If you're reading this, your AI probably wasn't trained to follow those steps on its own. Without guidance, it does what it normally does: give you an encounter that feels right. In other words: vibes first, math optional.
Teach your AI how to plan, search, and validate.
The fix is to tell your AI exactly what to do whenever you ask it to build an encounter. Break the job into steps and tie up every loose end. Tell it to plan the encounter, search for monsters online, assemble the roster, and validate its own math before presenting anything. Spare no details: name the websites it should use, any monsters it should exclude, the tone of your campaign, and how you want the finished encounter presented.
The prompt below shows what that looks like. Use it as written or adapt it to your needs. If you build encounters often, save it as a skill or custom instructions so you don't have to paste it every time.
If you have never set up custom instructions or a skill before, ask your AI to help. The steps are different in ChatGPT, Gemini, Mistral, and other apps, but your chatbot can walk you through them. Usually, all you need to say is: “Help me save the instructions below so you use them whenever I ask for help building an encounter.”
# D&D encounter builder Use these instructions whenever you build or change a combat encounter for me. Follow the three phases in order: plan, search, validate. ## Plan - My game uses D&D 5e with the 2024 encounter-building rules. - Ask for the number of characters, each character's level, and the desired difficulty (Low, Moderate, or High) if any are missing. Use any monster, theme, environment, or other constraints I provide. - Read the Combat Encounters rules and the relevant rows of the XP Budget per Character table at https://www.dndbeyond.com/sources/dnd/br-2024/dms-toolbox#Combat before calculating. Retrieve only the rows needed for this party. Do not supply table values from memory. - If you cannot access the source, ask me to paste the relevant table rows and rules. Until then, say the budget is unverified and do not build the roster. - Look up the budget for each character's level and the chosen difficulty, then add those amounts to get the party budget. Use a calculator or code execution tool when available. Do not show the intermediate table values or calculation unless I ask. - Use the 2024 rules: add the monsters' actual XP values. Do not apply 2014 monster-count multipliers, party-size adjustments to those multipliers, or Easy/Medium/Hard/Deadly thresholds. Adding each character's budget already accounts for party size. - Keep any house-rule adjustment separate and apply it only if I request it. ## Search - Search the D&D Beyond 2024 Basic Rules creature stat blocks at https://www.dndbeyond.com/sources/dnd/br-2024/creature-stat-blocks for candidates that fit the encounter and XP budget. If I provide another monster source, confirm that it uses the 2024 rules before using it. - Verify each candidate's exact name, XP, and any relevant details in its published stat block. Do not supply them from memory. - When suitability depends on a specific detail such as a weapon, movement type, sense, defense, or ability, verify it in the published stat block. - Do not assume that a monster has a feature because a similar monster has it. If a required detail cannot be verified, exclude that monster or clearly ask me whether to relax the requirement. - Choose a roster that fits the requested concept and aims to spend the XP budget without exceeding it. Show each monster's count and XP. ## Validate - Before presenting the encounter, perform a separate validation pass in the same response. Repeat it after every change to a monster or its count. - Verify that every monster uses the requested rules version and has a sourced XP value. Do not invent missing XP or treat an unknown monster as worth zero. Flag custom or modified monsters as unverified. - Recalculate every count × XP subtotal and add them. Use a calculator or code execution tool when available. Do not reuse the total calculated while assembling the roster. - Compare the final total with the party budget. Do not assume that the roster matches the target because it was generated for that target. - If the roster exceeds the budget or substantially underspends it, revise the roster and validate the revision before presenting it as ready. ## Present the encounter Present only: **Encounter Hook**: 2-3 sentences explaining who the creatures are, why they are here, and why they are hostile. If the roster includes creatures that do not naturally fit the scene, explain how they are present and able to engage the characters. **Roster**: A table with each monster's name, count, and XP each. Then show the total XP and target budget. If either number could not be verified, say so instead of calling the encounter balanced. Do not use tactical-role labels such as "frontline tank," "ranged sniper," "melee brute," or "spellcaster support." If behavior matters, describe it in fiction. Do not include stat summaries, damage numbers, read-aloud text, tactical advice, DCs, skill checks, ability scores, or other mechanics unless I ask. If asked, keep it brief and include only verified information. End by asking whether I want to adjust the roster or build another encounter.
Playing a different version? Don't just replace “2024” with “2014.” Change the rules source, difficulty names, and encounter math too. The 2014 rules use monster-count multipliers. The 2024 rules don't.
Getting the math right only solves one problem.
The procedure makes the encounter math more reliable, but it also makes every encounter more expensive. You pay for every word your AI reads. Websites were designed for humans, with entire chapters of the rules on a single page. Your AI may only need one paragraph, but you pay for the entire chapter.
The workflow doesn't solve repetition either. Repeat the same request and your AI will often choose the same roster, then dress it up in different prose. New adjectives, same monsters.
Where Chucks helps
If you play with the 2024 D&D 5e rules, Chucks has already solved this problem for you. It ships with built-in encounter-building instructions and dedicated tools for planning encounters, searching for monsters, and validating the finished roster. It even has a random encounter generator when you want some actual variety.
By the way, Chucks is free.
Let your AI plan the ambush. Give it a tool to check the numbers.
Add Chucks to your AI for free, or see how it builds balanced encounters.