Where it comes from
- Reported at PalantirSource 1Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on BlindSource 2Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on BlindSource 3Working Inside Existing SystemsPublisherPalantirSource typecompany hiring page More Palantir questions The Palantir interview guide
- Candidates report this round type at Sierra; the prompt is ours (candidates reported it for Sierra’s Agent Engineer role)Source 4Sierra Agent Engineer Interview QuestionsPublisherGlassdoor (anonymous interview review)Source typecandidate’s personal write-upSource 5Sierra Agent Engineer Interview Questions (page 2)PublisherGlassdoor (anonymous interview review)Source typecandidate’s personal write-upSource 6Sierra AI Agent Engineer Interview ExperiencePublisherAced (formerly Exponent), candidate-submitted experienceSource typecandidate’s personal write-upSource 7Sierra AI interview experiences (listing)PublisherAced (formerly Exponent)Source typecandidate’s personal write-up More Sierra questions
How to answer
Palantir names the ability to work within existing systems, codebases and infrastructure as a competency it interviews for. Source 3Working Inside Existing SystemsPublisherPalantirSource typecompany hiring page One Blind poster preparing for a Palantir onsite, who did not name the role, wrote in May 2024 that the re-engineering interview is about going through existing code, debugging and adding a new feature, and a commenter in the same poster’s earlier thread said they were given about 50 lines of Java and asked to fix a bug. Source 1Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on BlindSource 2Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on Blind The question asks how you found it, so make the route visible as you go.
- Turn the complaint into an input. Ask what customers see, on which input, and anything about when: after a deploy, later in the day, only for some accounts. A pattern in time or scope is your strongest clue.
- Read for shape, not detail. Entry point, data flow, and anything that outlives a single call: module-level names, caches, defaults, class attributes. Say the map aloud in two sentences.
- Reproduce before you touch anything. The smallest call or test that shows the wrong output. If the code can’t run, trace one input by hand and write down each value.
- Let the symptom pick the suspect. “Wrong for some customers” points at shared state or keys; “wrong at month end” at boundaries; “off by a cent” at rounding. Rule the others out aloud, with a reason each.
- Make the smallest fix in the file’s own style. Then give the cause in one sentence that ends at the customer’s symptom.
- Turn the reproduction into a regression test, and search for the same pattern elsewhere.
The trap is refactoring on sight. Unfamiliar code often looks wrong in several places; fix the one customers hit, and list the rest.
Follow-ups
What the interviewer may ask next, once your first answer is on the table.
- Some wrong invoices already went out. How do you find them, and what do you tell the account team?
- Suppose you can’t run the code. How do you convince yourself, and me, of the cause?
- The fix is in. Where else in this codebase would you look for the same mistake?
Where answers go wrong
- Rewriting the module before reproducing the bug, so nobody can say which change fixed it.
- Changing lines until the symptom goes away, then being unable to explain the cause in one sentence.
- Fixing it with no regression test, so the next tidy-up brings the bug back.
Answer this in two minutes
Write the answer you would say out loud. The clock starts with your first word.
Model answer
The code, and the report: “Customers are seeing line items for usage they never had. Support says the invoices get longer as the day goes on, and the first invoice after a deploy is always right.”