Debug

Debugging sessions are an increasingly common part of the software engineering interview process. Companies use them to evaluate how you navigate unfamiliar codebases, reason about unexpected behavior, and isolate root causes under time pressure — skills that are hard to assess with traditional algorithmic questions.

Practice with 69 real-world bugs across TypeScript and Python. Each exercise drops you into a buggy codebase with a symptom description and failing tests. Find the bug, fix it, and run the tests to verify.

Debug Progress0 / 69 fixed

easy: 0/32medium: 0/27hard: 0/10

Dispatch (Webhooks)

Bug B: Retry Backoff Capped Too Lowunit-mismatch

Bug C: Event Type Filter Matches Too Broadlystring-matching

Bug D: Dead Letter Queue Off-By-Oneoff-by-one

Bug A: Signature Timestamp Mismatchunit-mismatch

Bug E: Payload Mutation by Timeline Recordermutation

Support Agent (AI)

Bug A: Session Always Expiresunit-mismatch

Bug B: Follow-up Questions Misclassifiedcomparison-logic

Bug C: Knowledge Search Returns Wrong Articlescomparison-logic

Bug D: Delivery Date Always Shows "Unavailable"type-mismatch

Course Catalog

Bug 4: Recommendations in Wrong Ordersort-logic

Bug 5: Level 1 Courses Get Zero Scoreoff-by-one

Bug 7: Status Filter Returns All Coursesdecorator

Bug 10: "NaN%" in Certification Detailsnull-safety

Bug 11: Test Expects Wrong Score for Level 1misleading-test

Bug 12: Date Parsing Test Fails in Different Timezonestimezone

Bug 1: Strategy Selection Has No Effecttype-mismatch

Bug 2: Skill-Gap Scores Are Absurdly Highinheritance

Bug 3: Error Handler Crashes with TypeErrorthis-binding

Bug 6: Deadline Penalty Applied to Wrong Coursescomparison-logic

Bug 9: Status Filter Doesn't Re-fetchreact-hooks

Bug 8: Experimental Strategy Crashes on Results Pagelsp-violation

Expense Tracker

Bug 3: CSV Import Amounts Are Not Numberstype-coercion

Bug 5: Date Range Excludes the End Dateoff-by-one

Bug 6: Average Daily Spending Loses Centsoperator

Bug 1: Tags Leak Between Expensesmutable-default

Bug 2: Duplicating an Expense Shares Metadatashallow-copy

Bug 7: Removing Expired Budgets Crashesmutate-during-iteration

Bug 9: Can't Deduplicate Expenses with set()hashability

Bug 4: Category Filters All Match the Same Categorylate-binding

Bug 8: CSV Export Silently Produces Empty Filesilent-exception

Bug 10: Monthly Report Crashes on Any Datascope

Bug 11: Monthly Total Test Passes with Wrong Valuelogic-error

Bug 12: Large Expense Total Is Always $0generator-exhaustion

React Fitness

Bug 6: Weight Unit Shows Empty Instead of "kg"optional-chaining

Bug 11: Calories Burned Returns Zero Without Body Weightdefault-parameter

Bug 1: Workout Filter Changes Are Ignoredstale-closure

Bug 2: "Add Set" Button Requires Two Clicksstale-closure

Bug 3: Workout Timer Accelerates Over Timecleanup

Bug 4: Leaderboard Sorting Recomputes Every Renderreference-equality

Bug 7: Exercise Type Filter Shows Zero Resultsenum-mismatch

Bug 8: Exercise Notes Get Jumbled After Reorderinglist-key

Bug 10: Search Bar Debounce Always Sends Empty Stringevent-pooling

Bug 5: Workout Dates Crash When Formattedtype-cast

Bug 9: Memoized ExerciseItem Re-renders on Every Parent Rendermemoization

Bug 12: Filter Shows Stale Results From Slow Previous Requestrace-condition

Issue Triage

Bug 1: Field Name Mismatch Silently Ignores Policy Selectiontype-mismatch

Bug 4: Sort Comparator Produces Ascending Instead of Descending Ordersort-logic

Bug 5: Off-by-One Error Makes Severity 1 Score Zerooff-by-one

Bug 6: SLA Penalty Applied to Non-Overdue Incidents Instead of Overduecomparison-logic

Bug 7: Decorator Delegates to Wrong Methoddecorator

Bug 10: Accessing Non-Existent Metadata Field Produces NaNnull-safety

Bug 11: Test Written to Match Buggy Behaviormisleading-test

Bug 12: Test Uses Local Time Instead of UTCtimezone

Bug 2: Missing Severity Clamp in Safety-Critical Policy Overrideinheritance

Bug 3: Lost this Context When Passing Method as Callbackthis-binding

Bug 8: Template Method Override Omits Required breakdown Arraylsp-violation

Bug 9: React useEffect Missing Dependency Causes Stale Closurereact-hooks

Music Streamer

Bug 2: Factory Registry Maps Podcast to MusicVideofactory-pattern

Bug 4: setMonth() Used Instead of setMinutes() for Durationdate-api

Bug 5: Off-by-One: >= 0 Always True for Array Lengthcomparison-logic

Bug 8: Inverted Comparison Gives Bonus to Wrong Artistscomparison-logic

Bug 10: Accessing .map() on Potentially Undefined setlistnull-safety

Bug 11: Sort Comparator a - b Sorts Ascending Instead of Descendingsort-logic

Bug 1: Constructor Calls Abstract Method Before Subclass Initinheritance

Bug 6: Missing Dependencies in useEffect Prevents Filter Updatesreact-hooks

Bug 7: .catch(this.handleError) Loses this Contextthis-binding

Bug 9: LoggingRepository Calls findAll() Instead of findByType()decorator

Bug 12: Test Asserts Months Instead of Minutes (Validates Bug 4)misleading-test

Bug 3: Polymorphic this Causes Double Discount in Subclasspolymorphism