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Debugging Season 01 · Oct 15 – Nov 11, 2026

Keep the backend alive.

One focused fix a day. One deeper incident a week. Build a record of work you can come back to.

Today’s rep · Oct 15

Pagination off by one

Page 1 skips the first N items.

About 10 min · Easy · JavaScript

Start today’s rep

This rep works without signing in. Guest work is temporary; sign in before solving to keep your progress.

Your four-week route

Curated from existing executable labs. Focused reps take 8–15 minutes; deeper incidents about 35. Upcoming selections are previews, not newly authored labs.

01Requests & boundariesOct 15 – Oct 21

  1. REP 01 · Oct 15 · TODAY

    Pagination off by one

    10 min · Easy

    Open rep

    Free field notes open after an accepted account solve.

  2. REP 02 · Oct 16

    The Filter That Swallowed False

    15 min · Medium

    Scheduled for Oct 16 · 00:00 UTC

  3. REP 03 · Oct 17

    Unencoded query params

    14 min · Medium

    Scheduled for Oct 17 · 00:00 UTC

  4. REP 04 · Oct 18

    The Missing Header

    13 min · Easy

    Scheduled for Oct 18 · 00:00 UTC

  5. REP 05 · Oct 19

    CORS startsWith() allows lookalike origins

    10 min · Easy

    Scheduled for Oct 19 · 00:00 UTC

  6. REP 06 · Oct 20

    No content with body

    10 min · Easy

    Scheduled for Oct 20 · 00:00 UTC

  7. REP 07 · Oct 21

    Cache TTL: ms vs seconds

    12 min · Easy

    Scheduled for Oct 21 · 00:00 UTC

Weekly Incident · Oct 19

The order total from the other region

Put the week’s ideas to work in a multi-file investigation. About 35 minutes. A free debrief follows acceptance.

Scheduled for Oct 19 · 00:00 UTC

02Work that finishes cleanlyOct 22 – Oct 28

  1. REP 08 · Oct 22

    The missing trace ID

    14 min · Easy

    Scheduled for Oct 22 · 00:00 UTC

  2. REP 09 · Oct 23

    Await Missing in Fire-and-Forget

    12 min · Easy

    Scheduled for Oct 23 · 00:00 UTC

  3. REP 10 · Oct 24

    forEach With an Async Callback

    12 min · Easy

    Scheduled for Oct 24 · 00:00 UTC

  4. REP 11 · Oct 25

    Timeout Not Cleared on Success

    15 min · Easy

    Scheduled for Oct 25 · 00:00 UTC

  5. REP 12 · Oct 26

    The Lock That Left Too Soon

    14 min · Easy

    Scheduled for Oct 26 · 00:00 UTC

  6. REP 13 · Oct 27

    The Stale Lock

    15 min · Easy

    Scheduled for Oct 27 · 00:00 UTC

  7. REP 14 · Oct 28

    The Wallet That Let It Slide

    14 min · Easy

    Scheduled for Oct 28 · 00:00 UTC

Weekly Incident · Oct 26

The pool that never gives connections back

Put the week’s ideas to work in a multi-file investigation. About 35 minutes. A free debrief follows acceptance.

Scheduled for Oct 26 · 00:00 UTC

03Configuration you can trustOct 29 – Nov 4

  1. REP 15 · Oct 29

    The Phantom Env Var

    12 min · Easy

    Scheduled for Oct 29 · 00:00 UTC

  2. REP 16 · Oct 30

    The Case-Sensitive Config

    10 min · Easy

    Scheduled for Oct 30 · 00:00 UTC

  3. REP 17 · Oct 31

    Healthcheck Env Shadow

    12 min · Easy

    Scheduled for Oct 31 · 00:00 UTC

  4. REP 18 · Nov 1

    Dotenv Empty String Truthy

    15 min · Medium

    Scheduled for Nov 1 · 00:00 UTC

  5. REP 19 · Nov 2

    Prod Flag Defaulted On

    12 min · Easy

    Scheduled for Nov 2 · 00:00 UTC

  6. REP 20 · Nov 3

    Frozen Config At Import Time

    15 min · Medium

    Scheduled for Nov 3 · 00:00 UTC

  7. REP 21 · Nov 4

    Deploy Argv Offset

    12 min · Easy

    Scheduled for Nov 4 · 00:00 UTC

Weekly Incident · Nov 2

The token that only fails after deploy

Put the week’s ideas to work in a multi-file investigation. About 35 minutes. A free debrief follows acceptance.

Scheduled for Nov 2 · 00:00 UTC

04Retries without repeatsNov 5 – Nov 11

  1. REP 22 · Nov 5

    Rate-limit window never resets

    12 min · Easy

    Scheduled for Nov 5 · 00:00 UTC

  2. REP 23 · Nov 6

    The Retry That Amplified Outages

    12 min · Easy

    Scheduled for Nov 6 · 00:00 UTC

  3. REP 24 · Nov 7

    The Event That Fired Twice

    15 min · Medium

    Scheduled for Nov 7 · 00:00 UTC

  4. REP 25 · Nov 8

    Idempotency key trimmed too much

    12 min · Easy

    Scheduled for Nov 8 · 00:00 UTC

  5. REP 26 · Nov 9

    Event id mutated by middleware

    14 min · Easy

    Scheduled for Nov 9 · 00:00 UTC

  6. REP 27 · Nov 10

    Dedupe window too short

    14 min · Easy

    Scheduled for Nov 10 · 00:00 UTC

  7. REP 28 · Nov 11

    Partial Success Discarded by Batch

    15 min · Easy

    Scheduled for Nov 11 · 00:00 UTC

Weekly Incident · Nov 9

The job that two workers both completed

Put the week’s ideas to work in a multi-file investigation. About 35 minutes. A free debrief follows acceptance.

Scheduled for Nov 9 · 00:00 UTC

Ready for a longer session?

The backend pack groups five multi-file incidents. Shared labs count in both places, with their original accepted date.

Explore the backend pack

Have only a few minutes?

Daily Mixed Run is a separate five-question drill saved on this device. It does not count as an accepted season fix.

Open Daily Mixed Run