Dev debugging strategies

Production Debugging Strategies

Systematic patterns for investigating production failures, validated by diagnosing a React Native ML inference failure across 9 competing hypotheses (Feb 2026).

Hypothesis-Driven Framework

For production failures without an obvious cause, structure investigation as numbered hypotheses:

H1: <Short name>
  Confidence: High / Medium / Low
  Category: Environment / Code / Data / Infrastructure
  Failure Mechanism: step-by-step description of how this causes the symptom
  Predicted Symptom: what the user would experience
  Diagnostic Test: cheapest test to confirm/deny (curl, log check, version check)
  Remediation: fix if confirmed

Enumerate all hypotheses first, then rank by diagnostic cost — premature commitment to one hypothesis creates confirmation bias, causing contradicting evidence to be rationalized away.

Temporal Analysis Filter

First question for any regression: "When did this last work?"

Split hypotheses into regression candidates (recent change could explain it) and static bugs (would have been broken from day one) — test regression candidates first. This single question eliminated 4 of 9 hypotheses in one session.

Environment Before Code

For "recently broken" features, check infrastructure before reading application code:

Check	Cost	What It Rules Out
curl -sL "URL" | head -30	2 min	Dead external dependency, wrong URL
Check OS/runtime version compatibility	5 min	Platform-breaking update
Verify CDN / auth tokens unexpired	5 min	Infrastructure regression
Check dependency version matrix	10 min	Package incompatibility after upgrade
Read application code	30+ min	Use only after environment checks pass

Recurring environment traps (jsdom Node 22, PHP display_errors, SQLite/MySQL DDL, Homebrew Python 3.14 ABI, empty CA store) — see reference.md for probes and fallbacks.

CWD-First Diagnostic for Babel/Jest Failures

CWD drift (e.g., cd android && ./gradlew ... leaving shell in a subdirectory) produces identical symptoms to version incompatibilities in Babel/Jest transform errors — check CWD and config resolution before investigating parser/plugin versions. Full probe commands in reference.md.

Deeper Debugging Patterns

• CI vs Local Divergence — fetch real error with gh run view <id> --log-failed, then check SDK/toolchain version mismatch. See reference.md.
• Webhook & Wire-Level — root-cause checklist, wire-level JSON inspection, vendor prompt-injection handling. See reference.md.
• Documentation & Review-Bot Loops — doc-vs-reality drift, review-bot false-positive triage, Playwright visual-verification loop. See reference.md.

Confident False-Negatives (Tool Dialect Traps)

A "0 results" that contradicts a strong prior is a signal to suspect the tool's dialect, not to conclude absence:

• git grep -E with \s: POSIX ERE has no \s, so the pattern silently matches nothing and reports a false "absent." Use a literal space, a bracket class [[:space:]], or grep -P (PCRE).
• curl -w format starting with @: a write-out format string beginning with @ is parsed as a filename ("error reading"), not a format — never lead the -w string with @.

Both produce confident false-negatives that derail diagnosis. When a search result contradicts what you're nearly certain is true, re-run with a different tool/dialect before trusting the zero.

Anti-Patterns

Anti-Pattern	Pattern
Reading code before checking environment for regressions	curl/version checks first; static bugs don't suddenly appear
Trusting a git grep -E '\s' "0 results" as proof of absence	\s is invalid in POSIX ERE — matches nothing; use [[:space:]] or grep -P
Ranking hypotheses by confidence alone	Rank by diagnostic cost; cheap low-confidence tests beat expensive high-confidence ones
Mixing regression hypotheses with always-broken ones	Apply temporal filter first; eliminates ~40% of hypotheses in one step
Fixing the most likely cause without a diagnostic test	Diagnostic confirms before remediation — avoids fixing the wrong thing
Assuming backend involvement without tracing data flow	Trace actual data flow first; mobile apps may run entirely on-device

Tool-specific incident anti-patterns (dispatch direction, FK type asymmetry, pre-commit gitignore loop, Pint casing, Edit on single-line files, PHP display_errors) — see reference.md.

See reference.md for: Investigation Audit Pattern (8 steps), Frontend CSS rules, Reviewer Blocker Triage, Tooling Pitfalls, cost tiers, hypothesis examples, Sanctum triage, Playwright FE↔BE drift + backend-less page.route mocking (OPTIONS preflight, run_code_unsafe realm), git branch & merge hygiene + pre-push commit-contents check + fix-forward recovery for omitted-last-fix, CI-flake, Python extra= trap.