Writing Technical Blueprints
Purpose
Technical blueprints document how systems work internally. They differ from user documentation (how to use) and README files (project overview).
When to Write Blueprints
Write blueprints for:
- Complex systems with multiple components
- Public APIs and their contracts
- Architecture decisions and their rationale
- Behavior that isn't obvious from code
- Integration points between systems
Skip blueprints for:
- Self-documenting code (simple utilities)
- Test files (tests ARE the documentation)
- External dependencies (link to their docs)
Creating Blueprints
IMPORTANT: Always check for existing blueprints before creating new ones, and use the proper frontmatter format.
# 1. List existing blueprints
Glob("blueprints/*.md")
# 2. Search for related blueprints by keyword
Grep("keyword", path: "blueprints/", output_mode: "files_with_matches")
# 3. Read existing blueprint if updating
Read("blueprints/authentication.md")
# 4. Write the blueprint with frontmatter
Write("blueprints/system-name.md", content_with_frontmatter)The frontmatter format is:
---
name: system-name
summary: Brief one-line description
---Blueprint File Structure
Use this structure for blueprint content:
---
name: system-name
summary: One-line description of what this system does.
---
# System Name
One-line description of what this system does.
## Overview
2-3 paragraphs covering:
- Why this system exists (problem it solves)
- What it does at a high level
- Where it fits in the larger architecture
## Architecture
### Components
List major components:
- **ComponentA** - Purpose and responsibility
- **ComponentB** - Purpose and responsibility
### Data Flow
Describe how data moves through the system:
1. Input arrives via X
2. ComponentA processes and transforms
3. Result passed to ComponentB
4. Output returned/stored
### Dependencies
- **Dependency** - Why it's needed, what it provides
## API / Interface
### Functions
#### `functionName(param1, param2)`
Clear description of what the function does.
**Parameters:**
- `param1` (Type) - What this parameter controls
- `param2` (Type, optional) - Default behavior if omitted
**Returns:** Type - Description of return value
**Throws:** ErrorType - When this error occurs
**Example:**const result = functionName('value', { option: true });
### Configuration
| Option | Type | Default | Description |
| --- | --- | --- | --- |
| `optionA` | boolean | false | What it controls |
## Behavior
### Normal Operation
Step-by-step description of typical execution flow.
### Error Handling
How the system responds to:
- Invalid input
- Missing dependencies
- External failures
### Edge Cases
Document non-obvious behaviors:
- Empty input handling
- Concurrent access
- Resource limits
## Files
Key files and their roles:
- `src/main.ts:1-50` - Entry point, initialization
- `src/processor.ts` - Core processing logic
- `src/types.ts` - Type definitions
## Related Systems
Example format:
- Related System - How they interact
Writing Style
Be Precise
Bad: "The system handles errors gracefully" Good: "Invalid input returns a ValidationError with the field name and reason"
Document Behavior, Not Implementation
Bad: "Uses a for loop to iterate through items" Good: "Processes items sequentially, stopping at the first failure"
Include Examples
Show, don't just tell:
// Good: concrete example
const result = processItems(['a', 'b', 'c'], { parallel: true });
// Returns: { processed: 3, failed: 0 }Keep Current
- Update blueprints alongside code changes
- Remove documentation for deleted features
- Mark deprecated features clearly
Common Mistakes
- Too much detail - Don't document every line of code
- Too little context - Explain why, not just what
- Stale documentation - Outdated docs are worse than none
- Duplicate content - Link instead of copying
- Implementation focus - Document contracts, not internals