Frontier AI Technical Interview Question Bank

Topic tags
Python
Data / SQL

Reported interview periodNot available

Earliest Completion Time in a Data Pipeline

A pipeline consists of tasks (inputs, output, duration). A dataset never produced by any task is a source available at time 0. A task may start as soon as every input is available, and independent tasks run in …

Practice focusImplement: Return the earliest time at which goal is available. A goal that is already a source returns 0 . Reject negative durations, duplicate output producers, dependency cycles, and unknown goals with ValueError . Durations are non-negative non-boolean integers; dataset names are strings. A known source is a dataset mentioned as an input but never produced; an arbitrary unmentioned name is unknown. Validate the complete pipeline, including cycles disconnected from the goal. Duplicate input names do not require multiple executions.…

Free practice

Topic tags
Python

xAICoding

Python codingWebAssembly + public tests

Reported interview periodNot available

Fixed-Window Token Limiter

Implement a per-key limiter with one shared token limit and fixed window length. Commands are:

Practice focus- ("ALLOW", key, cost, timestamp) : accept if the key's usage plus cost does not exceed the limit; a rejected request consumes nothing. - ("RESET", key, timestamp) : clear that key's usage in the fixed window containing timestamp . The window containing time t starts at (t // window) window . Costs are non-negative integers; zero is allowed. Preserve input order. Return one result per command, using booleans for ALLOW and a consistent acknowledgment for RESET .…

New public brief

Topic tags
Python
Concurrency

xAICoding

Talk-through codingNo local grader

Reported interview periodNov 2025–Apr 2026

Make a Bank Account Thread-Safe

You are given a BankAccount whose deposit method performs a read-modify-write operation and may pause between reading and writing. You cannot modify that class. Implement a subclass that makes deposits safe whe…

Practice focusCall the inherited implementation rather than copying it. Two deposits of 500 and 700 into an account starting at 0 must finish with 1200 . Ensure the lock is released if the inherited method raises. The supplied constructor creates the account and its lock. Complete deposit only. The provided run case adapter owns the concurrent fixture execution; it is not candidate work. Opening balances and deposit amounts are integers, and deposit amounts are nonnegative. Empty deposit lists leave the opening balance unchanged. Each successful call returns None .…

New public brief

Topic tags
Python
Data structures
Communication

xAICoding

Python codingWebAssembly + public tests

Reported interview periodApr 2026

Resumable Iterators

Implement an iterator over a copied list that can serialize its position and later resume at the next not-yet-yielded item. Then compose the same behavior across a sequence of lists.

Practice focusSaved state points to the next item, never the last item returned. A completed iterator restores completed. State must be serializable and restoration must not repeat or skip values. The chain state records both the current group and the position within it. Use exactly {"index": integer} for list state and {"outer": integer, "inner": {"index": integer}} for chain state. Reject malformed keys, booleans/nonintegers and negative or out-of-range active indices with ValueError .…

New public brief

Topic tags
Python
Storage & state

xAICoding

Python codingWebAssembly + public tests

Reported interview periodDec 2025–Apr 2026

In-Memory Database with Expiry and Snapshots

Build an in-memory database in four stages. Each key owns string field/value pairs.

Practice focusMissing values return "" . Scans emit field(value) entries sorted by field. A TTL value is live over [created at, created at + ttl) . A backup captures all live fields and each field's remaining lifetime. Restore selects the newest backup at or before timestamp to restore , replaces current state, and restarts remaining lifetimes at timestamp . Assume timestamps supplied to mutating timed operations are nondecreasing. Backups must not share mutable state with the live database.…

New public brief

Topic tags
Python
Data structures
Storage & state
Networking

xAICoding

Python codingWebAssembly + public tests

Reported interview periodApr 2026

Token-Bucket State Transitions

Each subject has a token bucket stored in a shared key-value cache. A missing bucket begins full. Refill lazily when a request arrives: add elapsed_time * refill_rate, capped at capacity. If time has not advanc…

Practice focusAccept only when the refilled balance covers the full request, then subtract it. A rejected request spends nothing. Persist the refilled state in either case. A backward-moving clock must not create tokens. TokenBucket and the in-memory DistributedCache test double are provided. This base task is sequential; the cache name does not imply atomic distributed behavior. Refill returns a new bucket without editing its argument. Existing buckets retain their stored capacity/refill rate; call configuration initializes missing buckets only.…

New public brief

Topic tags
Python

xAICoding

Python codingWebAssembly + public tests

Reported interview periodFeb 2026

Flatten and Rebuild Nested Values

A nested value contains integer leaves and internal list, tuple, or dict nodes. Dictionaries are traversed in insertion order.

Practice focusImplement: flatten returns leaves in traversal order. unflatten uses structure as a shape template and rebuilds the same container types while replacing leaves from left to right. Raise ValueError when there are too few or too many replacement values, and TypeError for unsupported nodes, including booleans. Empty containers must round-trip. Replacement values must be a list of non-boolean integers; reject other values with TypeError . Do not modify either input.…

Earliest Completion Time in a Data Pipeline

Fixed-Window Token Limiter

Make a Bank Account Thread-Safe

Resumable Iterators

In-Memory Database with Expiry and Snapshots

Token-Bucket State Transitions

Flatten and Rebuild Nested Values

Weighted LRU Cache

Live Audio Room Analytics

Sort Integers with Multiple Workers

Identify Faulty Nodes with Group Tests

Build a token-budget batcher

Sliding-window rate limiter

Interval calendar

Nested dictionary transforms

Command-log store

Dependency-ready scheduler

Streaming metric window

Evaluation slice aggregator

Grade book

Vial log

Comment batches

Launch order

Notes table

Earliest Completion Time in a Data Pipeline

Fixed-Window Token Limiter

Make a Bank Account Thread-Safe

Resumable Iterators

In-Memory Database with Expiry and Snapshots

Token-Bucket State Transitions

Flatten and Rebuild Nested Values

Weighted LRU Cache

Live Audio Room Analytics

Sort Integers with Multiple Workers

Identify Faulty Nodes with Group Tests

Build a token-budget batcher

Sliding-window rate limiter

Interval calendar

Nested dictionary transforms

Command-log store

Dependency-ready scheduler

Streaming metric window

Evaluation slice aggregator

Grade book

Vial log

Comment batches

Launch order

Notes table