Learn From Fix
After fixing a bug or receiving a correction, capture the lesson to prevent future mistakes.
Usage
/phx:learn-from-fix Fixed N+1 query in user listing - was missing preload /phx:learn-from-fix String vs atom key mismatch in params handling /phx:learn-from-fix LiveView assign_async needs render_async in tests /phx:learn-from-fix --library ical --scope personal ICal.to_ics output needs CRLF line endings /phx:learn-from-fix --library ical --scope project Use method: "PUBLISH" for calendar feeds
Workflow
Step 1: Verify the Lesson
Persist only one of:
- A completed fix verified by tests, reproduction, or user confirmation
- An explicit rule the user taught or asked to save
Stop without writing if neither condition is met.
Do not save a hypothesis, unverified workaround, investigation narrative, or
unsolved error. Use /phx:compound for a detailed completed investigation.
Capture the root cause as a concise actionable rule, not the symptom.
Step 2: Select the Route
When both --library <package> and --scope personal|project are present,
use the Library Route. Require both flags; do not guess scope.
For the skill directory only, trim surrounding whitespace and lowercase the
package name. Preserve underscores when forming directory names
(phoenix_live_view becomes hex-phoenix_live_view).
Validate the normalized name against ^[a-z][a-z0-9_]+$. If it does not match,
ask for a valid Hex package identifier instead of replacing characters or
inventing a name. Use the trimmed original identifier, not the normalized
directory name, to look up mix.lock.
Without --library, use the General Correction Route.
Step 3: Check Existing Knowledge
Check if already documented:
- Grep project CLAUDE.md and
~/.claude/CLAUDE.mdfor the pattern keyword - Check auto-memory files for similar lessons
- For library lessons, inspect both
~/.claude/skills/hex-<package>/SKILL.mdand.claude/skills/hex-<package>/SKILL.md - Read
${CLAUDE_SKILL_DIR}/references/common-mistakes.md(READ-ONLY plugin reference — do NOT edit this file)
Do not duplicate the same lesson across CLAUDE.md, memory, and a package skill. If the same rule exists, merge wording or report it as already captured.
Library Route
CRITICAL: NEVER edit plugin files. Files under
~/.claude/plugins/ are cached and get overwritten on updates.
Package skills are user-owned files, never plugin cache files.
Resolve Scope and Precedence
--scope | Canonical destination |
|---|---|
personal | ~/.claude/skills/hex-<package>/SKILL.md |
project | .claude/skills/hex-<package>/SKILL.md |
Personal skills override same-named project skills. If a personal
hex-<package> skill exists, never unknowingly create or update a shadowed
project skill: explain the conflict and ask whether to update personal scope or
keep distinct project-only knowledge. If project scope already exists before a
personal write, warn that the new skill will shadow it and offer to merge or
move it. Maintain one canonical skill per package unless the user explicitly
needs distinct scope-specific rules; never create one skill per lesson.
Read the Locked Version
Inspect mix.lock for the package. Record the exact locked version when found.
If it is absent, say the lesson is not tied to a locally locked version and do
not invent one. Qualify version-sensitive rules. When two verified rules
conflict across versions, preserve both with explicit version ranges instead of
replacing either.
Create or Safely Update the Skill
For a new package skill, create:
markdown--- name: hex-<package> description: <Package/module/API/task trigger terms for this knowledge> user-invocable: false --- # <Package> Knowledge ## Verified Rules - **<Rule>** (verified with <package> <version>): <actionable guidance>
Make the description precise: include the Hex package, Elixir module names,
important APIs/file formats, and tasks that should trigger this knowledge.
Activation is model-selected from this description; package presence in
mix.lock does not guarantee activation.
Before updating an existing skill, read all of it. Preserve unrelated and hand-authored sections. Merge semantically identical rules. If safe merging is unclear, show the conflict and ask instead of overwriting content.
Do not add paths: to a personal package skill by default: it is a file-path
activation gate, not a dependency predicate. Add paths: only to project scope
when explicitly appropriate and meaningful package-specific paths exist.
General Correction Route
Choose the narrowest non-library destination:
| Scope | Write to | Example |
|---|---|---|
| This project | Project CLAUDE.md | "Never use raw SQL in this app" |
| This project across sessions | Project-keyed auto-memory | "jsonb uses string keys" |
| All your projects | ~/.claude/CLAUDE.md personal instructions | "Prefer explicit error tuples" |
| Detailed completed fix | .claude/solutions/ via /phx:compound | Debugging narrative |
For project or personal instructions, preserve existing content and append
**RULE NAME** — Do NOT [bad]. Instead [good] under the relevant category. Use
personal instructions only for rules that should load in every project. For
auto-memory, append to the project-keyed
~/.claude/projects/{project-hash}/memory/MEMORY.md:
markdown### Lesson: [Title] - **Pattern**: Do NOT [bad] — instead [good] - **Why**: [root cause explanation]
If the lesson is universal to the plugin rather than one project or package, suggest a plugin contribution; never write it into cached plugin files.
Output
After capturing, confirm:
textLesson captured in [location] Pattern: Do NOT [bad pattern] — instead [good pattern] Category: [Ecto/LiveView/OTP/Testing/etc] Version: [locked package version, not found, or not applicable]
For package skills, also state that description-based activation is
model-selected, not guaranteed by mix.lock.
Iron Laws
- NEVER write generated knowledge to plugin cache — use only the user-owned destinations above
- DO NOT duplicate existing lessons — always check CLAUDE.md and memory before writing
- NEVER overwrite hand-authored content — read both package skill scopes and merge safely
- ONLY persist verified knowledge — completed fixes or explicit user-taught rules, never hypotheses
- DISCLOSE scope precedence — a global package skill shadows its project counterpart
References (READ-ONLY — do NOT edit)
${CLAUDE_SKILL_DIR}/references/common-mistakes.md— Common Elixir mistakes reference. Consult when checking for duplicates. Ships with the plugin — NEVER modify.
