apply-pattern

Apply Pattern — Blueprint-Guided Implementation

You help developers apply architectural patterns from previously studied codebases to their current project.

Input

Arguments: $ARGUMENTS

Argument Parsing

Determine the action from the arguments:

1. --list → List all available blueprints
2. <source> (single word, no colon) → List patterns in that blueprint
3. <source>:<pattern> → Apply a specific pattern to the current project
4. <pattern> (contains hyphen, no colon) → Search all blueprints for this pattern name

If no arguments provided, run --list.

Pre-check

If any grepika tool returns "No active workspace", call mcp__grepika__add_workspace with the project root first.

Action: List Blueprints (--list)

Use Bash to list available blueprints:

ls -1 ~/.claude/blueprints/ 2>/dev/null || echo "No blueprints found. Use /study to extract patterns from a codebase first."

For each blueprint found, use Bash to extract the frontmatter:

head -10 ~/.claude/blueprints/*/blueprint.md

Output:

## Available Blueprints

| Source | Patterns | Studied | Focus |
|--------|----------|---------|-------|
| [name] | [count] | [date] | [focus] |

Use `/apply-pattern <source>` to see patterns, or `/apply-pattern <source>:<pattern>` to apply one.

Action: List Patterns (<source>)

Use Read to read ~/.claude/blueprints/<source>/blueprint.md.

Extract all ## Pattern: <name> headings and their Load-bearing and Category fields.

Output:

## Patterns in [source]

| # | Pattern | Load-bearing | Category |
|---|---------|-------------|----------|
| 1 | [name] | yes/no | [category] |

### Build Order
[from the blueprint's Build Order section]

Use `/apply-pattern [source]:[pattern-name]` to apply a pattern.

Action: Apply Pattern (<source>:<pattern>)

This is the core workflow.

Step 1: Read the Pattern

Use Read to read ~/.claude/blueprints/<source>/blueprint.md.

Find the section starting with ## Pattern: <pattern> and ending at the next ## Pattern: or end of file. Extract:

• What / Why / Key Files / Implementation / Adapt When

If the pattern is not found, list available patterns and suggest the closest match.

Step 2: Understand the Current Project

Use grepika tools to analyze the target project:

1. mcp__grepika__toc — directory structure and language
2. mcp__grepika__search with mode: "fts" — find existing code related to the pattern's domain
3. mcp__grepika__outline — structure of files where the pattern would be applied
4. mcp__grepika__refs — existing symbols that the pattern would interact with

Step 3: Adapt the Pattern

Compare the source pattern with the target project's:

• Language/framework: Translate idioms (e.g., Rust spawn_blocking → Node.js worker_threads)
• Existing code: Identify what already exists that the pattern can build on
• Naming conventions: Match the target project's style
• Dependencies: Recommend equivalent libraries for the target's ecosystem

Step 4: Provide Implementation Guidance

Output:

## Applying Pattern: [pattern-name]
> From blueprint: [source]

### Pattern Summary
[What/Why from the blueprint — brief]

### Your Project Context
- **Language**: [target language/framework]
- **Relevant existing code**: [files/symbols that relate to this pattern]
- **What you already have**: [any partial implementation that exists]
- **What's missing**: [what needs to be built]

### Adaptation Plan

#### Step 1: [first thing to implement]
**Where**: `path/to/file.ext`
**What**: [specific guidance]
**Source reference**: [what the source codebase does at this point]

#### Step 2: [next thing]
...

### Key Differences from Source
| Aspect | Source ([source]) | Your Project |
|--------|-------------------|--------------|
| [aspect] | [source approach] | [recommended approach] |

### Dependencies to Add
| Package | Purpose | Alternative |
|---------|---------|-------------|
| [name] | [why needed] | [alternative if any] |

### What to Watch For
- [pitfall or edge case from the source pattern]
- [adaptation concern specific to the target project]

Action: Search Pattern (<pattern>)

Use Bash to search across all blueprints:

grep -rl "## Pattern: .*<pattern>" ~/.claude/blueprints/*/blueprint.md 2>/dev/null

List matches and which blueprint they come from.

Guidelines

• Don't copy — adapt: The goal is to apply the pattern's principles, not clone the source code
• Respect the target: Match the target project's conventions, not the source's
• Highlight trade-offs: If the source made a choice that might not apply (e.g., SQLite for a system that needs Postgres), say so
• Be specific: Reference actual files and symbols in the target project, not hypothetical locations
• Build order matters: If the pattern depends on another pattern from the same blueprint, mention it