By FDE Interview Prep, a SourceStrongAI company

In this post13 sections
  1. What a project deep dive is testing
  2. Where candidates report meeting it
  3. Pick the right project
  4. A five-part structure for the opening walkthrough
  5. The follow-up questions to rehearse
  6. The why ladder: a drill to run on yourself
  7. The one slide to prepare
  8. Presenting to engineers and executives at once
  9. Mistakes that sink a strong project
  10. Run the drill tonight
  11. Questions people ask
  12. Keep reading
  13. More from the blog

You know the project. You built it, you shipped it, and you can explain every box on the diagram, right up until the interviewer asks “why?” for the fifth time and lands on a decision you never made and cannot defend. A project deep dive interview is a walk through something you built, followed by questions about the decisions under it until the interviewer finds the edge of what you know. You pass it by owning that edge: prepare the decisions, the failure and your own part, then drill the whys on yourself before anyone else does. This post is one round from the FDE interview guide, and it gives you a five-part structure for the opening walkthrough, a why-ladder drill and the one slide worth preparing.

What a project deep dive is testing

The walkthrough is the easy part. What the interviewer is testing is whether you command the reasoning behind your own work, and whether you can say plainly which parts of it were yours.

The closest employer-side description comes from Ironclad. A former Ironclad leader told First Round Review that the most effective open-ended interview for Ironclad’s FDE-equivalent team asked candidates to present a problem from their career and teach how they used technology to solve it. Source 1So You Want to Hire a Forward Deployed EngineerPublisherFirst Round ReviewSource typenews report

Hiring leaders describe probing like this too. Ramp’s Director of Engineering, Leo Mehr, said on Basil Chatha’s podcast, published in July 2026, that he assesses drive and ownership by asking candidates about their careers: the decisions they made, what they were responsible for and how much responsibility they took on. Source 2Leo Mehr - Ramp's $44B Bet on Services (YouTube auto-generated English captions)PublisherBasil Chatha, YouTubeSource typerecorded talk or interview

For a , this is also the job. As of September 2026, C3 AI’s experienced-level FDE postings list leading technical deep-dives, product demonstrations and customer training sessions among the responsibilities. Source 3Forward Deployed Engineer (Redwood City) | C3 AIPublisherC3 AISource typecompany job posting An interviewer watching you explain your own system is watching a rehearsal of you explaining a system to a customer’s engineers.

So a deep dive checks three things at once:

  • Depth. Can you go two layers below any box you draw?
  • Ownership. Can you separate your calls from the team’s and the ones you inherited?
  • Candor. Can you describe a real failure without shrinking it or blaming someone else?

It often sits inside the wider hiring manager conversation, covered in the FDE hiring manager interview.

Where candidates report meeting it

Candidates in several FDE processes report being asked about their projects in depth. Source 4Cognition AI Forward Deployed Engineer Interview ExperiencePublisherAced (formerly Exponent)Source typecandidate’s personal write-upSource 5Microsoft FDE interview in a week. Anyone had any experience recently? US for AI Agent Team (comment by u/Terrible_Rooster_662)PublisherReddit r/leetcodeSource typecandidate report on RedditSource 6Microsoft FDE interview in a week. Anyone had any experience recently? US for AI Agent Team (comment by u/Terrible_Rooster_662)PublisherReddit r/leetcodeSource typecandidate report on RedditSource 7Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (post by u/ByteTrooper)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 8Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Negative_Host_4149)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 9Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Electrical_Abies_464)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 10Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Electrical_Abies_464)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 11Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Electrical_Abies_464)PublisherReddit r/developersIndiaSource typecandidate report on Reddit The formats differ:

None of this tells you how a deep dive is scored at any company, and no employer we checked publishes that. It tells you to expect it, in a slide deck, on a video call or halfway through a coding round.

Pick the right project

A deep dive can be lost before the interview, by picking the biggest project on the résumé instead of the one you can defend. Choose by these tests:

A project worth presenting

  • You made at least one design call yourself, against a reasonable alternative someone else wanted.
  • A real constraint shaped the design: a customer’s rule, a legacy system, a deadline, a budget.
  • Something broke, and you can explain the root cause in mechanical terms.
  • You can draw the whole system from memory, on a blank page, without notes.
  • You know the numbers someone could ask for: volume, latency, error rate, cost, team size.
  • You can describe it without breaking a confidentiality agreement.

A small project with a clear fork beats a flagship where you were one of many. If your strongest work is under an NDA, anonymize it up front (“a regional bank”, “a logistics customer”) and say so in your first sentence.

If you are unsure whether a project passes the ownership test, run it through the free question which decisions were yours alone first. If nothing lands in its Mine bin, pick another project.

If they pick the project, not you

Two of the candidates above reported interviewers working from the résumé, not the candidate’s chosen project. Source 7Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (post by u/ByteTrooper)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 8Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Negative_Host_4149)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 9Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Electrical_Abies_464)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 10Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Electrical_Abies_464)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 11Heizen Forward Deployed Engineer Interview – What to Expect in Next Rounds? (comment by u/Electrical_Abies_464)PublisherReddit r/developersIndiaSource typecandidate report on RedditSource 12C3 AI Senior Forward Deployed Engineer (FDE) Interview Process – My Experience (comment by u/MindlessFox6861)PublisherReddit r/OfferEngineeringSource typecandidate report on Reddit So give every project line on your résumé a one-minute version: the problem, your decision, one number, one thing that broke. Then run one rung of the why ladder on each.

A five-part structure for the opening walkthrough

This structure is ours: a way to make the first few minutes easy to follow and easy to question. Keep it short enough that the interviewer interrupts you, because the questions are where the round happens. Our rule of thumb: the whole opener in about three minutes, about half a minute per part plus the drawing.

  1. The problem and who had it. Two sentences: who was hurting, and what it cost them. “The finance team at a mid-sized distributor keyed every supplier invoice by hand, and late payments were costing them early-payment discounts.”
  2. The constraint that shaped everything. The one fact that ruled out the obvious design. “A wrong amount posted to the ledger took days to unwind, so accuracy mattered more than speed.”
  3. The design, and the decision that mattered. Draw it, then name the fork: the options you weighed, the one you chose and why. “I chose extraction with a confidence score and a human review queue over full automation, because the cost of an error was asymmetric.”
  4. Your part. One precise sentence with named roles. “I designed and built the extraction and the queue; the platform team owned the OCR vendor; the finance lead set the review rules.”
  5. What broke, and what you would change. A specific failure, how you found it, the fix, and one change with a real reason you did not make it then. “Two weeks in, a supplier switched to invoices with two tax lines and the extractor read the subtotal as the total; the daily reconciliation caught it after six postings. I added a check that line items sum to the total. I’d now build that check on day one: I skipped it because the first sample had no multi-line invoices.”

Then hand over the steering wheel: “I can go deeper into the confidence threshold or the vendor integration. Which is more useful?” You have just told them where your depth is, and given them an easy next question.

The free question take me through a project in depth has a full framework and an illustrative answer that follows the same shape.

The follow-up questions to rehearse

This is our practice list, not a script any company publishes. Each question is tagged with the kind of answer it pushes you toward, which matches the floors of the why ladder below.

  • Why this design and not the obvious one? (Trade-off.)
  • How did you know it worked? (Evidence.)
  • Which parts did you build, and which did you review? (Ownership.)
  • Who disagreed, and how was it settled? (Ownership.)
  • What broke, and why didn’t testing catch it? (Depth.)
  • What would you do with twice the traffic? (Depth.)
  • What would you cut with half the time? (Trade-off.)
  • What would you change if you rebuilt it tomorrow? (Candor.)

The why ladder: a drill to run on yourself

The deep dive is a why ladder someone else climbs for you. Climb it first.

How to run it. Take each decision in part 3 of your walkthrough. Write it at the top of a page. Ask “why?” and write the answer. Ask “why?” of that answer. Repeat until you reach one of four solid floors:

A good rung ends onExample
A constraint“The ledger could not take corrections after close.”
Evidence“We tested it on a labeled sample of past invoices.”
A trade-off“Faster to ship, but a clerk has to staff the queue.”
A person“The finance lead set that rule; I checked it held.”

If a rung ends on “it seemed right”, “that’s how it was done” or silence, you have found the gap the interviewer will find.

A worked ladder, from the fictional invoice project above:

  • Decision: low-confidence invoices go to a human review queue.
  • Why a queue? A wrong posted amount cost more to unwind than a delayed invoice. (Constraint.)
  • Why was delay acceptable? Finance paid suppliers in a weekly run, so an invoice reviewed the next morning still made it. (Constraint.)
  • Why that confidence threshold? On a labeled sample of past invoices, I found the lowest threshold where every auto-posted amount was correct, then checked that the queue it left fit one clerk’s morning. (Evidence.)
  • Why not train a better model to shrink the queue? We had a few hundred labeled invoices: enough to test a threshold, too few to train on, and the queue cost less than labeling more. (Trade-off.)
  • Why that OCR vendor? ...

The fifth rung is the dangerous one here, because the vendor was chosen before you arrived. Do not invent a reason. Write the honest answer and rehearse it:

“That was chosen before I joined; the platform team already had the contract. What I did own was checking whether it was good enough for us. It wasn’t for handwritten invoices from one supplier, so I routed that supplier straight to review.”

That answer names the owner, marks your boundary and then shows judgment inside it. A rationale made up on the spot would fall to the next why.

Two rules for the drill. Do it out loud with a friend who has permission to be annoying, because an answer that reads well on paper can still fall apart when you say it. And run it on the failure too: “Why did it break? Why didn’t testing catch it? Why was that test missing?”

The one slide to prepare

Whether or not the loop asks for a presentation, prepare one slide. It becomes your whiteboard drawing on a video call, your handout in a panel and your map when the questions scatter.

Put four things on it, and nothing else:

  • A one-line title that states the problem, in the customer’s terms, not the project’s code name.
  • The system diagram, with the boxes you built visibly marked, shaded or outlined. This answers “what was your part?” before anyone asks.
  • One decision callout beside the box it concerns: the options you weighed and the one you chose.
  • One failure marker where it broke, and the outcome in a line at the bottom.

Test it on a phone screen: if the labels are unreadable, there are too many boxes.

What to leave off: architecture you did not touch, logos, a timeline of sprints, and any customer detail you would not say aloud.

If your loop asks for a recorded walkthrough instead of a live one, the take-home walkthrough video adapts the same slide and structure.

Presenting to engineers and executives at once

One candidate reported, in April 2026, that their Cognition AI presentation was to people at different levels, which means one talk has to hold two audiences. Source 4Cognition AI Forward Deployed Engineer Interview ExperiencePublisherAced (formerly Exponent)Source typecandidate’s personal write-up Gregor Ojstersek’s Engineering Leadership newsletter reported, in August 2026, from a conversation with Colin Jarvis, that OpenAI wants FDEs who are strong engineers and good communicators who can explain a solution in a way that resonates with everyone. Source 13Inside OpenAI's Forward Deployed Engineer RolePublisherEngineering Leadership (Gregor Ojstersek)Source typenews report

Build in layers, not two talks. Open each part of the five-part structure with a sentence the executive can repeat, then go one layer down for the engineers, and signpost the change: “Here’s how that works underneath.”

In the questions, answer at the asker’s layer:

  • An executive asks “was it worth it?”: give the outcome in business terms, then offer detail.
  • An engineer asks “how did you handle retries?”: give the mechanism, and draw it if you can.
  • Anyone asks “whose idea was that?”: give the exact answer, with the role of the person who decided.

The Pro question on presenting a past project to a mixed audience covers this format in depth, and the lesson on writing for customers trains the same habit of leading with the sentence a busy reader needs.

Mistakes that sink a strong project

The component tour. Every box described, no decision defended. Fix: spend your opening time on part 3, the fork, and let the interviewer pull the rest.

The “we” fog. “We decided, we built, we shipped” leaves the interviewer nothing to evaluate. Fix: say “I” for your calls and name the owner of the rest. The post on the ownership audit has a line-by-line method.

The borrowed rationale. Defending a decision you did not make as though you did. The next why exposes it. Fix: the owner-boundary-judgment answer from the ladder.

The shrunken failure. A “failure” that was really a success, or one that was someone else’s fault. Plan to be asked about a real one: in a 2020 post on Palantir’s blog, a hiring manager said Palantir asks candidates about actual failures and is not fishing for a success story. Source 14Interviewing at Palantir: Advice from PalantiriansPublisherPalantir BlogSource typecompany blog Fix: pick a failure you caused or could have caught, and say what you changed afterward. The Pro question on your hardest technical problem is good practice for telling it plainly.

The monologue. A walkthrough so long that the interviewer never gets to ask. Fix: rehearse with a timer and cut until the opener fits in about three minutes, leaving the interviewer room to interrupt.

No numbers. “It was much faster” invites “how much faster?” Fix: know your volume, latency, error rate and cost before you walk in, and say where each number came from.

Run the drill tonight

Pick your project with the checklist. Write the five-part opener and say it aloud once. Then run the why ladder on your two biggest decisions and your failure, and write the honest answer for every rung that ends in silence.

When the answers hold, compare them with a finished one: the free project deep dive question ends with a full illustrative answer, including the failure and what the candidate changed. Say yours aloud first, then read it and mark every rung where yours is thinner. If you want to practice the customer side of the job too, the free AI practice case puts you in front of a simulated customer who only reveals what you ask for.

GlossaryForward deployed engineerA software engineer who builds and ships production systems inside a customer’s problem and environment, accountable to that customer’s outcome.More on Forward deployed engineer

Questions people ask

What is a project deep dive interview?

An interviewer asks you to walk through a project you worked on, then questions the decisions behind it: why this design, what broke, what you owned and what you would change. It tests whether you command the reasoning behind your own work, not just the architecture.

Do FDE candidates report being asked to present a project?

Some do. One candidate reported, in April 2026, that their Cognition AI forward deployed engineer process included presenting a real technical project to people at different levels. Another reported, in August 2026, being drilled on their résumé, in a reply on a Reddit thread about C3 AI’s Senior FDE process; that poster did not name the company or role.Source 4Cognition AI Forward Deployed Engineer Interview ExperiencePublisherAced (formerly Exponent)Source typecandidate’s personal write-upSource 12C3 AI Senior Forward Deployed Engineer (FDE) Interview Process – My Experience (comment by u/MindlessFox6861)PublisherReddit r/OfferEngineeringSource typecandidate report on Reddit

What questions come up in a project deep dive?

Rehearse the follow-ups that test depth, evidence, ownership and trade-offs: why you chose this design, how you knew it worked, which parts were yours, who disagreed, what broke, what you would do with twice the load, what you would cut with half the time and what you would change now. This is our list for practice, not a script any company publishes.

How do I pick which project to present?

Choose one where you made the design calls yourself, something went wrong that you can explain, and you can draw the system from memory. Avoid a project where your part is hard to separate from the team’s.

What if I cannot answer one of the why questions?

Say what you know, say who made the call if it was not you, and say what you would check. A plain answer about a decision made before you joined is stronger than a rationale invented on the spot.

Will they ask about a failure in the project?

Plan for it. In a Palantir blog post, a hiring manager said Palantir asks candidates about actual failures and is not fishing for a success story.Source 14Interviewing at Palantir: Advice from PalantiriansPublisherPalantir BlogSource typecompany blog

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