Re-engineering round

A re-engineering round is an interview in which you work inside code you did not write: you read an unfamiliar codebase or library, find and fix a bug, or add a feature, while explaining what you are doing. A learning round is the close variant in which you are first taught a concept or handed documentation, then asked to write code with it. Both sit beside algorithm problems and practical builds among the coding formats that reports describe; the lesson What FDE coding rounds test, by the evidence sorts them by company so you know which to drill.

In FDE interviews

Palantir names the ability to work within existing systems, codebases and infrastructure as a competency it interviews for. ^[1]Source 1Working 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 Palantir’s re-engineering interview is about going through existing code, debugging and adding a new feature. ^{[2, 3]}Source 2Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on BlindSource 3Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on Blind In that poster’s earlier thread, one commenter said they were given about 50 lines of Java and asked to fix a bug, and a different commenter said the interface was HackerRank and the bug was not specific to Java. ^{[2, 3]}Source 2Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on BlindSource 3Palantir re-engineering interview (Blind)PublisherBlindSource typecandidate report on Blind One Reddit commenter, who did not name the company or the role, wrote in August 2026 that in a recent learning round the interviewer first taught a technical concept and introduced a scenario, then showed them object-oriented code for querying an imaginary database and had them implement a series of small query functions; they did not have to run them, and the interviewer looked for correctness. ^[4]Source 4Palantir FDSE Intern Learning Interview (comment by u/Illustrious_Beat4472)PublisherReddit r/csMajorsSource typecandidate report on Reddit A strong candidate reads before writing and narrates the map: “The request comes in here, this class builds the query, and the date filter is applied twice. If I can run it, I’ll write a failing test for that path; if not, I’ll trace one input through by hand and show you where the rows drop. Then I’ll make the smallest fix in the style this file already uses, and leave the refactor as a note.”

The lesson Working inside existing code: learning and re-engineering rounds covers reading a codebase before you write, tracing a symptom to the line, and code review as a round.

Where it comes up

• Here is the documentation for a library you have never used. Use it to implement a small feature in the time we have.
• Here is a short piece of unfamiliar code with one bug that customers hit. Find and fix it, and explain how you found it.
• Trace one request through a large codebase you have never seen and draw its path.