Learn From Fix

---
name: learn-from-fix
description: Capture Elixir/Ecto/LiveView lessons and Hex API rules. Use after corrections or when asked to document learning, record a lesson, prevent a fixed mistake, or remember package guidance with --library.
effort: low
argument-hint: "[--library <hex-package> --scope personal|project] <lesson>"
---

# 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](#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](#general-correction-route).

### Step 3: Check Existing Knowledge

Check if already documented:

- Grep project CLAUDE.md and `~/.claude/CLAUDE.md` for the pattern keyword
- Check auto-memory files for similar lessons
- For library lessons, inspect both
  `~/.claude/skills/hex-<package>/SKILL.md` and
  `.claude/skills/hex-<package>/SKILL.md`
- Read `${CLAUDE_SKILL_DIR}/references/common-mistakes.md` (read-only plugin
  reference)

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

Plugin files are read-only: 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:

```text
Lesson 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

1. **NEVER write generated knowledge to plugin cache** — use only the user-owned destinations above
2. **DO NOT duplicate existing lessons** — always check CLAUDE.md and memory before writing
3. **NEVER overwrite hand-authored content** — read both package skill scopes and merge safely
4. **ONLY persist verified knowledge** — completed fixes or explicit user-taught rules, never hypotheses
5. **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.