Common FDE interview mistakes, round by round, and the fix for each

By FDE Interview Prep, a SourceStrongAI company

In this post11 sections

1. The mistakes on one screen: round, mistake, fix
2. Decomposition: solving before you understand the problem
3. Coding: silent typing and no edge cases
4. Client round: yes to everything, or no to everything
5. Behavioral: the prepared script and the failure that is really a success
6. The project deep dive: a résumé line you cannot defend
7. Loving what you built more than whether anyone uses it
8. A plan to fix them this week
9. Questions people ask
10. Keep reading
11. More from the blog

Your FDE loop is coming up, or a rejection just arrived with no reason attached, and you want to know what actually sinks people. Here are six habits that sink FDE interviews, five of them warned against in public by employers, hiring leaders or former insiders: solving before you understand the problem, typing in silence, saying yes (or no) to every customer request, reciting a script, defending résumé lines you cannot explain, and falling for your own build. This post is one piece of the FDE interview guide: the mistakes round by round, and a fix for each you can practice this week.

No employer publishes why it turns candidates down, so if you were rejected, the exact reason may never come. The habits below are the next best thing: five are named somewhere public, one is our own addition, and each has a fix.

The mistakes on one screen: round, mistake, fix

Each mistake below but one carries a source where someone who hires, or someone who has worked inside the process, has named it. Every fix is our method, not a rubric any employer publishes.

The deep-dive row has no source that names it as a mistake. It is ours, and the section on it says what the one related report does and does not show. The full attributions are in each section below.

Decomposition: solving before you understand the problem

The decomposition round hands you an open, messy prompt and watches how you break it down. The failure pattern has an unusually clear description. A Blind user with a Palantir tag, who said they formerly worked there, wrote in March 2025 that a bad decomp performance means not asking questions, making large assumptions without clarifying with the interviewer, misunderstanding the problem before jumping in, giving impractical solutions, and not expanding on or optimizing the original v0. ^{[1, 2]}Source 1Palantir FDSE Interview (Blind)PublisherBlind (Teamblind)Source typecandidate report on BlindSource 2Update: interview experience - Palantir new grad FDSE interview (Blind)PublisherBlind (Teamblind)Source typecandidate report on Blind That is one person’s view, but it lines up with what Palantir publishes: its own page on open-ended questions says to articulate alternatives and trade-offs, arrive at a concrete approach, and deliver a functioning idea first, then expand it. ^[9]Source 9Palantir Careers | Navigating Open-Ended QuestionsPublisherPalantir TechnologiesSource typecompany hiring page

What the mistake sounds like. The prompt is “a hospital network wants to reduce missed appointments.” You say, “I’d build a model that predicts no-shows and sends reminders.” You have picked a solution before you know who misses appointments, what data exists, or what “reduce” means to the person asking.

The fix, in words you can say. Open with a short burst of questions, then commit:

• “Before I design anything, who feels this problem most: the clinics, the patients or the finance team?”
• “What do we have today? Is there an appointment log with outcomes, or only the schedule?”
• “I’ll assume we have a year of appointment history with a no-show flag. Tell me if that’s wrong.”
• “Here’s the smallest version that works end to end: each afternoon, pull tomorrow’s appointments, flag patients who have missed one before, and give the front desk that call list. Then we expand.”
• “Next, text reminders for everyone, because that’s cheap and we can measure it. After that, once we know whether calls change the no-show rate, rank the call list by risk instead of history.”

The last line is the one people skip. A first version that you never grow looks like you ran out of ideas. Say what the next version adds and why it comes next.

Our lesson on the open-ended round and the method on one page is free and lays out the whole sequence, and the decomposition interview guide covers the round end to end. The idea of a thin first version that runs end to end is what we call a walking skeleton. For the recovery moves once you notice you have jumped in too early, see the decomposition mistakes post.

Coding: silent typing and no edge cases

Palantir’s coding-interview guide says a core part of a good coding interview is turning a conceptual solution into working code, and it tells candidates to think about edge cases and how their code could break. ^[3]Source 3Writing Good CodePublisherPalantirSource typecompany hiring page Palantir also tells onsite candidates to talk through how they plan to approach technical problems and to ask questions if necessary. ^[10]Source 10Palantir Careers | Getting HiredPublisherPalantirSource typecompany hiring page In Palantir’s 2020 blog post on interviewing, a Forward Deployed Software Engineer described their interviews as interactive and said they talked through everything. ^[11]Source 11Interviewing at Palantir: Advice from PalantiriansPublisherPalantir BlogSource typecompany blog A long stretch of silent typing gives the interviewer nothing to work with.

A mini-scenario. You are asked to keep the newest record per customer email from a messy export. Here is a clean version:

def latest_by_email(rows):
    """Keep the newest row per customer email."""
    latest = {}
    for row in rows:
        email = (row.get("email") or "").strip().lower()
        if not email:
            continue  # skipped: say so out loud
        best = latest.get(email)
        if best is None or row["updated_at"] > best["updated_at"]:
            latest[email] = row
    return list(latest.values())

It runs, and it is where the silent candidate stops. The candidate who talks lists the ways it breaks before the interviewer has to ask:

• Case and spaces. "[email protected] " and "[email protected]" are one customer, so the key is normalized.
• Missing email. Those rows are dropped. Say it, and ask whether they should go to a review list instead.
• Ties. Two rows with the same updated_at keep the first one seen. Ask which should win.
• Timestamps as strings. Comparing strings only works when every value has the same format and offset. "2026-03-04T09:00:00+05:00" sorts after "2026-03-04T05:00:00Z" even though it is the earlier moment. Parse to timezone-aware datetimes if the export mixes offsets.
• Missing timestamp. row["updated_at"] raises KeyError if the field is absent, and a None beside a string raises TypeError. Ask whether undated rows should lose to dated ones, then read it with row.get("updated_at") and handle None first.
• Empty input. Returns an empty list, which is fine; say you checked.

The words to use. “Let me say the plan before I type: one pass, a dictionary keyed on normalized email, keep the newer row. Then I’ll walk through what breaks it.”

Our free lesson on what FDE coding rounds test covers the other formats you may meet: builds that grow in stages, bugs in someone else’s code, and rounds done with an AI assistant.

Client round: yes to everything, or no to everything

Where a loop has a customer round, the interviewer plays someone who wants something. One Blind poster reported, in July 2026, that the system architecture round in their Google FDE loop was run as a role-play with the interviewer acting as a company CTO. ^[12]Source 12FDE Interview Experience at Google (L4)PublisherBlind (teamblind.com)Source typecandidate report on Blind There are two ways to fail, and they are opposites.

The first is the eager yes. Every request gets “sure, we can do that,” and by the end you have promised a platform. Leo Mehr’s post on Ramp’s engineering blog, in August 2025, says FDEs must be able to give an enthusiastic yes to rational customer requests while pushing back against unreasonable ones. ^[4]Source 4Forward Deployed Engineering (Leo Mehr, Director, Engineering)PublisherRamp Builders (engineering blog)Source typecompany blog An all-yes answer shows only half of that.

The second is the rigid no. You decided the right design in minute three and defend it against every new fact. Shilpa Balaji, who joined Palantir as an FDE and went on to lead FDE recruiting there, told First Round Review in February 2026 that a candidate who came in overly dogmatic or set in their ways was a red flag for an FDE. ^[5]Source 5So You Want to Hire a Forward Deployed EngineerPublisherFirst Round ReviewSource typenews report Colin Jarvis, then head of forward deployed engineering at OpenAI, was quoted in The Pragmatic Engineer in August 2025 saying that what customers describe in scoping often does not match the data and system reality, so his team biases toward moving fast and then adjusting the scope. ^[13]Source 13What are Forward Deployed Engineers, and why are they so in demand? (Gergely Orosz)PublisherThe Pragmatic EngineerSource typenews report Changing your plan when the facts change is the job.

What the mistake sounds like. The customer asks for a regional view. The eager yes: “Sure, we’ll add that.” The rigid no: “That’s out of scope.” Both end the conversation instead of making a decision.

The fix: yes, with a trade-off attached. Our pattern has three parts: agree with the goal, name the cost, offer a choice.

• “Yes, adding the regional view makes sense. It pushes the first delivery back, so do you want it in this version or the next one?”
• “I’d hold off on real-time for now. Nightly covers the decision you described. If the ops team needs it within the hour, that changes my answer, and here’s what it would take.”

The second line also names what would change your mind, which is the cure for sounding dogmatic. Practice it on the scope creep question, and read our free lesson on what customer rounds look like.

Behavioral: the prepared script and the failure that is really a success

Two habits hurt here.

The script. In a Palantir hiring-manager video aimed at software engineers rather than FDEs, a hiring manager told candidates not to give a prepared statement of what they think Palantir wants to hear, and to “be real in the interview process.” ^[6]Source 6From Code to Career: Meet Palantir's Hiring Managers (YouTube uploaded English captions)PublisherPalantir, YouTubeSource typerecorded talk or interview Preparing your stories is fine. Reciting a speech tuned to the company’s values page is what that manager warned against. The fix is to prepare facts and decisions, not sentences, so that the answer comes out differently each time you tell it.

The fake failure. “My biggest weakness is that I care too much” has a cousin in the failure question: the story where the failure turns out to be a win. Palantir’s careers page says onsite interviewers ask how you have executed, challenged yourself, motivated others and made judgments, and where you have failed in the past. ^[10]Source 10Palantir Careers | Getting HiredPublisherPalantirSource typecompany hiring page In the same 2020 Palantir blog post, a hiring manager said they are not fishing for a success story and want to hear about an actual failure. ^[11]Source 11Interviewing at Palantir: Advice from PalantiriansPublisherPalantir BlogSource typecompany blog

Our fix: four beats, with the cost in the middle.

1. What you decided, in the first person.
2. What went wrong, and what it cost someone.
3. When you noticed, and what you did first.
4. What you do differently now, with one example since.

“I chose to skip the load test because the demo was two days out. The first customer batch timed out, and their ops lead lost a morning. Now I load-test anything a customer will run before I call it done” is a failure. “We shipped late but the client loved it” is not. The decision-you-reversed question is a good one to rehearse this on.

The project deep dive: a résumé line you cannot defend

Prepare as if the interviewer will pick one line of your résumé and pull on it until something gives. The mistake is a line that reads well and holds nothing up: a number you cannot explain, a system you watched someone else build, a “led” that meant “attended.”

The evidence is thin: one Reddit commenter in a C3 AI Senior FDE thread reported, in August 2026, close questioning on their résumé, then a rejection whose reason they did not give. ^[14]Source 14Comment on C3 AI Senior FDE interview threadPublisherReddit r/OfferEngineeringSource typecandidate report on Reddit

Our fix: the résumé drill. For every line, write answers to five follow-ups:

• “What was your part, and what did the team do?”
• “Where does that number come from, and what was it before?”
• “What did you choose, and what did you reject?”
• “What broke?”
• “What would you do differently?”

If a line fails two of the five, rewrite the line or cut it.

• Before: “Led the migration to a new data platform.” It fails “your part” and “what did you reject”.
• After: “Wrote the ingestion jobs and the backfill plan for our move to a new data platform, and chose a phased cutover over a single weekend switch.”

Then rehearse with the project deep dive question and which decisions were yours. The ownership audit post walks through separating your part from the team’s, sentence by sentence.

Loving what you built more than whether anyone uses it

On a South Park Commons panel in May 2026, Colin Jarvis of OpenAI named, as a failure pattern in FDE work, people who come to love the form of what they built more than its function. ^[7]Source 7The Future of Forward Deployed Engineering | OpenAI, Ramp, Nominal, DatalandPublisherSouth Park Commons (YouTube)Source typerecorded talk or interview He said the best FDEs will tear a solution up and build something different when users do not use it. ^[7]Source 7The Future of Forward Deployed Engineering | OpenAI, Ramp, Nominal, DatalandPublisherSouth Park Commons (YouTube)Source typerecorded talk or interview The same habit is easy to show in an interview: in a build round, a take-home review or a project deep dive.

In an interview, it sounds like ten minutes on your architecture and no mention of whether anyone used the thing. The fix is to lead with the user and the outcome, then the design:

• “The dispatch team used it every morning. Two weeks in, they stopped using the map view, so I cut it and put the late-truck list first.”

A close cousin is building everything. Sierra’s April 2026 post says it shares advice with candidates before its AI-native onsite, including that it’s fine to cut scope as you build and to skip boilerplate such as CRUD and auth to focus on what is unique. ^[8]Source 8The AI-native interview (Vijay Iyengar, Arya Asemanfar, Angie Wang)PublisherSierraSource typecompany blog That is one company’s advice for one format, not a universal rule. It is still a useful default in a timed build: say out loud what you are cutting and why, so the interviewer sees a decision, not an unfinished app. Our lesson on walking skeletons covers how to choose the thin version.

A plan to fix them this week

You do not need a new study plan. You need five short sessions, one habit each. If your onsite is days away, the onsite-this-week post has a shorter plan.

The free practice case puts you in front of an AI customer who holds back facts until you ask, so Friday shows you whether you still jump in early. It’s free with a sign-in. For the rest of the lessons and the full question bank, Pro starts with a 7-day free trial (pricing).

Questions people ask

What is the most common reason people fail FDE interviews?

No employer publishes why candidates are rejected or how often, so no one can name the most common reason. Employers do publish what they want to see. Palantir’s advice for open-ended problems is to articulate alternatives and trade-offs, arrive at a concrete approach and deliver a functioning idea first. Prepare against those published behaviors.^[9]Source 9Palantir Careers | Navigating Open-Ended QuestionsPublisherPalantir TechnologiesSource typecompany hiring page

What does a bad decomposition round look like, as one former Palantir employee on Blind wrote?

A Blind user who said they formerly worked at Palantir wrote, in March 2025, that a bad decomp performance means not asking questions, making large assumptions without clarifying, misunderstanding the problem before jumping in, giving impractical solutions and not expanding on the first version.^{[1, 2]}Source 1Palantir FDSE Interview (Blind)PublisherBlind (Teamblind)Source typecandidate report on BlindSource 2Update: interview experience - Palantir new grad FDSE interview (Blind)PublisherBlind (Teamblind)Source typecandidate report on Blind

Should I give the answer I think the interviewer wants to hear?

No. In a Palantir hiring-manager video aimed at software engineers, a hiring manager told candidates not to give a prepared statement of what they think Palantir wants to hear, and to be real in the interview process. Give your own view and the reason for it.^[6]Source 6From Code to Career: Meet Palantir's Hiring Managers (YouTube uploaded English captions)PublisherPalantir, YouTubeSource typerecorded talk or interview

Keep reading

Guides

• The forward deployed engineer interview, round by round
• The decomposition interview: how to break down a vague customer problem

Lessons

• The FDE decomposition interview: the method on one page
• What customer rounds look like, and the core moves
• What FDE coding rounds test, by the evidence
• The walking skeleton: a functioning first version

Questions

• Pick one project from your résumé and take me through it in depth: the problem, your design, what broke and what you would change.
• Pick a project you are proud of. Which decisions were yours alone, and which would have happened without you?
• Tell me about a decision you made that turned out to be wrong. How did you find out, and how fast did you reverse it?
• The customer’s product manager adds a new dashboard request on every weekly call, and the core pipeline is not done. Handle this week’s call.

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