Practice catalogue updated Tue, Sep 08, 2026

Debug the signal.
Ship the fix.

Production-shaped debugging labs across APIs, config, concurrency, and infrastructure. Reproduce the failure, follow the evidence, and prove the smallest safe change.

Ready labs145playable now

Quick starts100under 20 min

Avg. time19minper lab

OSS labs4public history

01Reproducemake the failure visible

02Tracefollow the evidence

03Patchchange the smallest thing

04Verifyprove it holds

Featured labBuglyst · Caching & Performance

Null bypasses cache validation

A short-circuit on falsy role keys returns a hardcoded default and skips the cache entirely.

Hard22 minJavaScript#track#caching-performance#cache

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Choose a failure mode.

Focused bugs are targeted exercises; codebase incidents are deeper, production-shaped investigations with architecture, logs, and multiple layers. Search by symptom, then filter by lab type, source, status, or difficulty.

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4 of 145 labs · source: ossready labs · catalogue order

#LabDifficultyTime

139Open sourceFocused bugHTTP method casing mishandled for non-standard verbsA Request constructor accepts a method option. The library normalizes the six standard HTTP methods (DELETE, GET, HEAD, OPTIONS, POST, PUT) to uppercase regardless of input casing, while preserving every other method string exactly as supplied. Currently, the casing rule applies too broadly, altering method names that should pass through untouched.9 attempts · 0 solved · 0 discussionsEasy20min 140Open sourceFocused bugRepeated trailing slashes in base URLWhen a base URL like 'https://api.github.com///' is joined with a path like '/users', the resulting URL contains an extra slash between the host and the path instead of a single separator.2 attempts · 0 solved · 0 discussionsEasy20min 141Open sourceFocused bugZero-value entries-read argument omitted from XGROUP tokensA caller passes the ENTRIESREAD option set to 0 to XGROUP CREATE or XGROUP SETID. The argument builder should emit the ENTRIESREAD keyword and a '0' token. Instead, when the option value is exactly 0, the trailing ENTRIESREAD and '0' tokens are missing from the produced array.1 attempts · 0 solved · 0 discussionsMed.30min 142Open sourceFocused bugApp.render rejects null and undefined options valuesA caller invokes `app.render(viewName, options, callback)` and expects the callback to deliver the rendered template string even when `options` is `null` or `undefined`. The callback does not deliver the expected rendered output for those cases, while rendering with a normal options object continues to succeed.2 attempts · 0 solved · 0 discussionsEasy10min

discussion · public Practice

What are developers debugging?

Questions, dead ends, and useful investigation clues from real public Practice threads.

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More ways to practice.

Move from a focused drill to open-source history or company-style patterns when you want a different kind of debugging pressure.

Focused exercises

Train code review, system tracing, CI diagnosis, incident response, and fast debugging instincts.

05 drills5 formats · short sessions

Open-source labs

Practice against safely recreated public bug-fix history from the libraries that actually broke.

04 playablereal bug-fix history

Company-style prep

Train backend and infrastructure failure patterns without affiliation claims.

30 patternsPattern-based · unofficial