The most common question on the consulting recruiting forums is not “which framework” or “which casebook.” It is some version of “how do I practice cases when I don’t have a partner?” One recent thread on r/MBBConsulting put it plainly: paid $200 for an hour with a case partner, felt like an idiot afterwards, and “kind of surprised there still isn’t a really solid AI tool for this. ChatGPT voice is already pretty good.”
I work on Tough Tongue AI, where we build AI interviewers, so read the rest of this with that in mind. But the question deserves a real answer rather than a pitch, because the honest one is more useful: you can do most of your case prep alone, a prompted ChatGPT or Claude is better than people think, and it breaks in exactly three places. Below is the prompt we use, a transcript of it breaking on a real Kellogg casebook case, the fix for each break, and a five week plan that splits the reps between AI and a human.
What solo practice can and cannot do
A first-round case at McKinsey, Bain or BCG runs 25 to 40 minutes. You get a prompt, ask clarifying questions, lay out a structure, and then the interviewer starts handing you data as exhibits, charts and tables, and asks what you see. You read, do the arithmetic aloud, get probed. There is a brainstorm, and then a recommendation the interviewer pushes back on.
Most of that is learnable alone. Structuring a problem in ninety seconds, mental math with units, market sizing, generating eight ideas in three buckets, giving a recommendation in thirty seconds that leads with the answer: these are drills, and drills reward repetition more than company.
What you cannot do alone with a casebook is be surprised. You open the case and the exhibit is right there, next to the question, above the answer. You cannot un-see the answer key. So solo casebook practice trains the parts of the case where you already know what is coming, and never trains the part where you do not.
The fix people reach for is an AI interviewer. The interesting question is how far it gets you.
The prompt
This is the prompt we tested. It works in ChatGPT, Claude or Gemini. Paste it, then paste the entire case underneath it: prompt, clarifying information, exhibits, and the interviewer guide with the answer key. The interviewer guide is what makes it work; a prompt on its own makes the model invent a case, and an invented case has no key to grade against.
You are a McKinsey engagement manager running a first-round, interviewer-led case interview. I am the candidate. Run the case below exactly as written and follow these rules for the whole session.
Rules
1. Play only the interviewer. Speak in short turns, one question at a time, and wait for my answer. Never answer for me and never move on until I have responded.
2. Follow the case's problem-solving steps in order. Do not skip a step and do not reveal what is coming.
3. Give clarifying information only when I ask for it. If I ask something the case does not cover, say the team does not have that data.
4. Provide an exhibit only at the point the interviewer guide says to, and only after I have asked for that kind of data.
5. Do not correct my arithmetic. If a number is wrong, ask me to walk through my calculation out loud. Give me the correct figure only if I fail to find the error after two attempts, and remember that you had to.
6. When I get a number right, still ask one follow-up on what it means or what would change my view.
7. At the recommendation, push back once with an objection built from something I said earlier in the case. Do not accept the first version of my answer.
8. Keep a rough clock: the case should take 25 to 30 minutes of conversation. If I am slow, steer me to the recommendation.
9. When the case is finished, and only then, grade me against the interviewer guide's answer key using these categories: structure, quantitative accuracy, business judgment, communication, recommendation. For each category give a score out of 10, quote what I said as evidence, and place my numbers next to the key's numbers. End with a hire band (strong hire, hire, borderline, no hire) and my three most important improvement areas.
Case (pasted from the casebook, including the interviewer guide, which you must not reveal until grading)
[paste the full case here]
Begin with a brief greeting and the fit question from the case, then read me the case prompt.Rules 5, 6 and 7 are the ones most prompts leave out, and they are the difference between a chat and an interview. Rule 9 is what turns “that went well” into feedback.
Where it breaks: a transcript
I ran this prompt with Claude on Health Coaches, case 10 in the Kellogg Consulting Club Casebook 2024. A health insurer with five million members is deciding whether to hire nurses as “health coaches” to call sick members once a month, on the evidence that monthly contact cuts health spending by 5%. I copied the case out of the PDF the way a candidate would, played the candidate, and made one deliberate mistake along the way.
To be fair to the model: it did a lot right. It asked the fit question, read the prompt, withheld the coach cost until I asked, withheld the exhibits until I asked for that kind of data, followed my correct 400,000 member sizing with “what would have to be true for that to be the wrong place to start?”, and at the end built a push-back out of a risk I had raised myself twenty minutes earlier. The grading quoted my words against the key’s numbers and put me at “hire.” If your alternative is reading the casebook alone, this is a large upgrade.
It broke in three places.
Break 1: the exhibit became a paragraph
Exhibit A in this case is a stacked bar chart: three member segments, each split into healthy, lifestyle, hereditary, degenerative and other. The entire skill being tested is whether you can look at that chart and say “65+ is the sickest segment and lifestyle is the biggest sick slice, so that is where the money is.” Here is what I got when I asked for a breakdown by segment and disease:
Interviewer: We do. Here’s Exhibit A, the client’s member segmentation by disease type. The base splits into three segments: Individual is 15% of members, Group is 65%, and 65+ is 20%. Group are employer-sponsored plans; Individual are non-group members, private contractors, the unemployed and so on; and 65+ is the senior population. Within each segment, members fall into five disease categories: Healthy, Lifestyle, Hereditary, Degenerative, and Other. Reading the chart, the 65+ segment has by far the largest share of sick members, and Lifestyle is the biggest sick category in every segment, about 40% of the 65+ segment.
Two things happened. The model narrated the chart, which means it did my reading for me and handed me the insight (“65+ has by far the largest share of sick members”) that I was supposed to find. And the “about 40%” did not come from the exhibit at all. The chart’s values do not survive a copy-paste from a PDF; what the model had was the answer key, which says “20% segment x 40% lifestyle x 5m members = 400,000.” It read the key back to me as if it were the chart. A candidate would never know.
This is not a prompt problem. A text model cannot put a chart in front of you, so every exhibit becomes prose, and prose comes with the conclusion attached.
Break 2: it hinted instead of waiting
The Health Coaches case has a units trap. Costs are given per member per month, coach salary is annual, and most people drop the factor of twelve at least once. I dropped it on purpose:
Candidate: Each 65+ lifestyle member costs $1,200, and coaching saves 5% of that, so $60 per member. Times 200 members is $12,000 in savings per coach. A coach costs $72,000. So that’s a loss of $60,000 per coach, which means the program doesn’t pay even in the best segment, and I’d be inclined to recommend against launching.
Interviewer: Before you land on that, walk me through the calculation out loud once more. In particular, check your units: the $1,200 is per member per month, and your $72,000 is a cost per coach per what?
Rule 5 half worked. The model did not hand me the number. But “check your units: the $1,200 is per member per month” is not “walk me through it,” it is the answer with one step left. A McKinsey interviewer says “walk me through that” and then says nothing, and the silence is the test. The model could not stand the silence. Then, in the grading, it wrote that I “found the error on the first prompt without my giving you the answer,” which is generous to both of us.
Break 3: the key leaked
Small, but it adds up. The clarifying information in this case says a coach can reach 8 members a day, 25 days a month, and the interviewer is meant to give that only when asked. The model volunteered it alongside Exhibit B. Combined with the 40% from break 1, I was handed two of the four inputs I was supposed to ask for. Each leak makes the case a little easier than the real one, and you do not find out until the real one.
How to fix each break
You can get most of the way with a few adjustments, and the rest with a different kind of tool.
For the exhibit problem, print the exhibits before you start, or open them in a second window, and add a line to the prompt: “When the guide says to provide an exhibit, say only ‘Here is Exhibit A’ and nothing else. Do not describe it. I will read it.” That stops the narration. It does not stop the model from knowing the answer key’s reading of the chart, so keep the guide’s numbers out of the exhibit section when you paste. And use voice mode if you have it; reading a chart while talking is a different skill from reading it in silence.
For the hinting problem, tighten rule 5: “If my number is wrong, say only ‘Walk me through that calculation’ and wait. Do not name the step or the unit that is wrong. Only after my second wrong attempt may you point to the specific step.” This works better than the version above, and it still is not perfect, because language models are trained to be helpful and a silent interviewer is not helpful. Expect it to crack on the second nudge.
For the leaks, split the paste. Put the case prompt, clarifying information and exhibits in the first message, and the interviewer guide in a second message that begins “This is the interviewer guide. Use it to decide when to give information and to grade me. Never quote from it before grading.” Separating them makes the model treat the guide as reference rather than script.
When you want the actual thing, an interviewer that puts the exhibit on screen when the case reaches it, says nothing when your number is wrong, and grades against the key without leaking it, that is what we built the MBB case collection on Tough Tongue AI for: a voice interviewer that drives a slide deck, with 29 full-length cases from the Kellogg, Wharton and Ultimate Case Interview Workbook casebooks, including the Health Coaches case above. The longer post on how to prepare for MBB case interviews using AI compares it with the other tools. But if the prompt plus the fixes gets you what you need, use that.
A five week solo plan, with and without AI
The other question the forums ask is whether an AI mock plus a live case partner is redundant. It is not, but only if you give each one the job it is good at. AI is good at volume, consistency and grading against a key. A person is good at pressure, presence, interruptions and the cadence of the specific firm. Here is how I would split five weeks before a first round, which is the runway people usually post with.
Week 1: learn the shape. Read two or three cases from your casebook end to end, key included, to see what a good structure and a good recommendation look like. Do ten minutes of mental math a day with units (per month to per year, per unit to total, percentages of percentages). Run one easy case with the AI prompt so you know what the format feels like. Do not grade yourself yet.
Week 2: one case a day, AI, same case twice. Run a medium case with the prompt in the evening. Read the grading against the key. Redo the same case the next morning before the new one. This is the step everyone skips, and it produces more improvement than anything else in the plan, because it tells you whether the fix stuck or whether you just remembered the answer.
Week 3: exhibits and math under time. Switch to cases that are exhibit-heavy (Kellogg’s Healthy Foods and Health Coaches, Wharton’s Electro Chargers, the workbook’s Polystore) and use whichever exhibit fix above you can manage. Set a timer. If your structure is fine and your numbers are fine but you slow down when the chart appears, you have found the thing to work on, and it is the thing most people never find until the interview.
Week 4: first human mocks. Two sessions with a partner or a coach, no more. Bring the three improvement areas the AI grading keeps repeating and ask them to watch for those. This is where the $200 hour earns its money: you are not paying them to tell you that you dropped a factor of twelve, you are paying them to tell you that you looked at the ceiling while you recovered.
Week 5: taper. One case every other day, alternating AI and human, and no new frameworks. Redo the case you did worst on. Spend the rest of the time on the fit stories, which a text model is actually good at drilling.
Across the five weeks that is roughly twenty AI cases and four or five human ones, which is more reps than most candidates get and a better ratio than most candidates run.
Common questions
Can you practice case interviews by yourself? Yes, for most of the skills involved. The exception is the exhibit, where you need something that shows you the chart without showing you the answer.
Is ChatGPT good enough? With the prompt above and the full case pasted in, it runs a recognisable interviewer-led case and grades against the key. It narrates exhibits, hints at your math errors and leaks the key. Use it for structure, flow and recommendation; get your chart reps elsewhere.
Is an AI mock plus a live partner redundant? No. AI for volume and calibration, people for pressure and presence. Front-load the AI, back-load the humans.
Should I pay for a case partner? Late, yes, for two or three sessions. Early, no; the feedback you need early comes free from an answer key.
How many cases before the interview? Clubs say twenty to forty. With the redo loop, twenty is enough for most people.
Start here
Pick a medium case from your casebook, paste the prompt and the case into ChatGPT or Claude, put pen and paper next to you, and run it without stopping. Read the grading, then run the same case tomorrow. When you get to the exhibits and want them on a screen with an interviewer that stays quiet, the MBB collection is there, and the rest of the interview courses are on the courses page. If you try the prompt and find a fourth place it breaks, tell me in the comments; I will add it.
