Phx Init Agent Skill

Plugin Initialization

Install the Elixir/Phoenix plugin's behavioral instructions into the project's CLAUDE.md.

Usage

/skill:phx-init           # First-time installation
/skill:phx-init --update  # Update existing installation with latest rules

Iron Laws

• NEVER overwrite content outside plugin markers — User-written CLAUDE.md rules must be preserved verbatim
• Always detect stack before generating — Never assume Phoenix/Ecto versions
• Always validate after installation — Verify markers present and stack correct

Workflow

Step 1: Check Existing CLAUDE.md

Use Glob to check if CLAUDE.md exists. Then use Grep to check for existing ELIXIR-PHOENIX-PLUGIN:START marker in CLAUDE.md.

Step 2: Detect Project Stack

Scan the project to customize the injected instructions:

Read mix.exs and use Grep to extract:

• Phoenix version: search for phoenix.*"~> in mix.exs
• Ecto version: search for ecto.*"~> in mix.exs
• Oban: search for "oban" and "oban_pro" in mix.exs
• Frameworks: search for "ash", "surface" in mix.exs
• Tidewave: search for "tidewave" in mix.exs
• Project size: use Glob to count lib/**/*.ex files

Step 3: Handle Installation Modes

Mode A: Fresh Install (no CLAUDE.md or no markers)

• Create/append to CLAUDE.md
• Insert full behavioral instructions between markers
• Include only relevant sections based on detected stack

Mode B: Update (--update flag or markers exist)

• Find content between <!-- ELIXIR-PHOENIX-PLUGIN:START --> and <!-- ELIXIR-PHOENIX-PLUGIN:END -->
• Replace with latest behavioral instructions
• Preserve everything outside the markers

CRITICAL: NEVER overwrite or delete existing CLAUDE.md content outside the plugin markers — user-written rules, project conventions, and other plugin sections must be preserved verbatim

Step 4: Generate Content

Write the following structure to CLAUDE.md:

<!-- ELIXIR-PHOENIX-PLUGIN:START -->
<!-- Last updated: {date} | Plugin version: 1.0 | Stack: Phoenix {version}, Ecto {version}, {optional: Oban, Tidewave} -->

# Elixir/Phoenix Plugin - Auto-Activation Rules

{Include all sections from the Content Template below, filtered by detected stack}

<!-- ELIXIR-PHOENIX-PLUGIN:END -->

Step 4b: Offer Codex Review Guidelines (optional)

If command -v codex succeeds (or the user asks): offer to install a

managed ## Review guidelines block into the project's AGENTS.md —

honored by BOTH codex exec review locally and the Codex cloud PR

reviewer. Follow the install rules and exact block in

references/codex-review-guidelines.md: markers

ELIXIR-PHOENIX-REVIEW-GUIDELINES:START/END, upsert in place on

--update, never touch content outside markers, and STOP to ask if an

unmanaged ## Review guidelines section already exists. When codex is

absent and not requested, skip silently — never mention codex to users

who don't have it.

Step 5: Output Summary

✅ Elixir/Phoenix plugin initialized

Detected stack:
- Phoenix {version}
- Ecto {version}
- {Oban standard | Oban Pro | not detected}
- {Tidewave ✓ | Tidewave not detected}
- {Ash Framework detected - Ecto patterns disabled | not detected}

Added to CLAUDE.md:
- Auto-activation rules (complexity detection, interview mode)
- Agent trigger patterns ({n} agents available)
- Reference auto-loading ({n} reference docs)
- Iron Laws enforcement ({n} laws)
- Verification rules
{- Codex review guidelines → AGENTS.md (only if installed in Step 4b)}

Run /skill:phx-init --update after plugin updates.
Run /skill:phx-audit for a full project health check.

Content Template

The exact content to inject is in references/injectable-template.md.

Key structure:

• 7-Step Mandatory Procedure — Claude Code MUST execute before every response
• Iron Laws — STOP behavior on violations
• Conditional Sections — Include based on detected stack:
• {OBAN_SECTION} — If Oban detected (not Pro)
• {OBAN_PRO_SECTION} — If Oban Pro detected
• {ASH_SECTION} — If Ash Framework detected
• {TIDEWAVE_SECTION} — If Tidewave detected
• Verification — Mandatory after code changes
• Quick Reference — Skill routing table

Placeholder substitution:

| Placeholder | Source |

|-------------|--------|

| {DATE} | Current date |

| {PHOENIX_VERSION} | From mix.exs |

| {ECTO_VERSION} | From mix.exs |

| {OPTIONAL_STACK} | Detected optional deps |

See references/injectable-template.md for full template with all placeholders and conditional sections.

Validation

After running /skill:phx-init:

• Check CLAUDE.md contains markers
• Verify detected stack matches actual project
• New session should:
• Auto-detect complexity when given tasks
• Stop on Iron Law violations
• Offer relevant workflows based on task

Error Handling

| Scenario | Action |

|----------|--------|

| CLAUDE.md read-only | Error: "Cannot modify CLAUDE.md - check permissions" |

| Markers corrupted | Warn, offer to remove and reinstall |

| Unknown Phoenix version | Use conservative defaults (all features enabled) |

| Not an Elixir project | Error: "No mix.exs found - is this an Elixir project?" |

Relationship to Other Commands

| Command | When to Use |

|---------|-------------|

| /skill:phx-init | First time, or after plugin updates |

| /skill:phx-audit | Periodic project health check |

| /skill:phx-verify | After code changes |