Why Vibe Coding Breaks at 500 Lines of Code
The emergence of autonomous AI coding agents (Cursor Composer, Claude Code, Windsurf Cascade, Lovable, Bolt) has supercharged developer velocity. Prototyping an idea in an afternoon feels magical.
However, teams quickly hit the Context Window Decay Curve:
- Silent Scope Mutation: Prompts drift across multi-turn chats; the AI forgets previous API contracts and rewrites existing helpers.
- Architectural Spaghetti: The AI picks incompatible dependencies or mixes ORMs (e.g. Prisma and Drizzle in the same repo).
- Unreproducible State: Chat history is ephemeral. There is no version-controlled artifact defining why a decision was made.
"LLMs require deterministic architectural contracts, not clever conversational prompting."
Anatomy of the Standard 4-File SDD Bundle
In Spec-Driven Development, all product features are governed by four version-controlled markdown files located in your repository's specs/ directory (the MCP server mirrors downloads into a gitignored .specs/ folder, so the committed bundle and your agent's working copy never collide):
Tech stack rules, linting constraints, forbidden dependencies, and security boundaries that the AI agent can never violate.
EARS+ compliant user stories, explicit business logic, edge case handling, and acceptance criteria testable to 100% precision.
Database schemas, API endpoints, component trees, state management diagrams (rendered live via Mermaid.js).
Ordered, unit-testable implementation tasks sized small enough to fit within an LLM's optimal attention window.
SDD vs. TDD vs. Traditional PRDs vs. Vibe Coding
| Methodology | Single Source of Truth | AI Agent Compatibility | Context Drift Risk |
|---|---|---|---|
| Vibe Coding | Chat window history | Poor (High hallucination) | Extremely High |
| Traditional PRD | Confluence / Notion Doc | Medium (Too abstract) | Medium |
| TDD (Test-Driven) | Unit Test Files | Good (Post-architecture) | Low |
| Spec-Driven (SDD) | 4-File Versioned Markdown Bundle | Optimal (Native MCP Context) | Zero (Deterministic) |
How to Implement SDD with Cursor, Claude Code, and Windsurf
Instead of manually copy-pasting markdown files into your chat window, MySpec exposes your project specifications directly over the Model Context Protocol (MCP):
npx -y @myspec/mcp-server
Your AI coding agent can now list projects, read_spec_file the constitution before it writes a line, download_spec_file the whole bundle into .specs/, and push its own edits back as new revisions with update_spec_file - 31 tools in total, with a hosted HTTP variant for agents that have no filesystem. See the MCP server docs.
On Claude Code, the official MySpec plugin sets this up in two commands and adds skills that build the bundle task by task - one test per acceptance criterion, a constitution check, and the task ticked off on MySpec.
Scale that to many agents at once and you have an AI software factory: parallel Claude Code sessions building from one spec, with every pull request verified against its acceptance criteria and a human gate at each milestone.
Every Spec File Type, Explained
Each file in a bundle answers one question and feeds the next. The core MySpec bundle is four files; brownfield work adds a change proposal, and the Spec Kit and OpenSpec methodologies use their own file names for the same jobs. Each guide covers the file's purpose, its section-by-section anatomy, a specimen, the review rules the AI reviewer enforces, and how coding agents consume it.
Vision, principles, and MUST / MUST NOT constraints - the supreme law.
FR-/NFR- requirements with EARS+ acceptance criteria; AR-/BR-/CR- deltas for brownfield.
Architecture, modules, data model, API design, and Mermaid + C4 diagrams.
Milestones of sized, dependency-mapped tasks with a QA verification milestone.
Why / What Changes / Technical Solution / Impact for a brownfield change.
Spec Kit and OpenSpec file types, plus the full file × workflow matrix.
MySpec, Spec Kit, or OpenSpec - Pick Per Session
SDD is a methodology, not a single file layout. MySpec runs three of them natively, so a team already using GitHub's Spec Kit or Fission's OpenSpec keeps its folder conventions and CLI commands while the AI Architect does the interviewing and drafting.
Interview → constitution → requirements → solution → tasks. Brownfield: interview → proposal → requirements delta.
Interview → constitution → specify → plan → tasks, in specs/memory/ and specs/NNN-feature/.
Explore → proposal → delta specs → design → tasks, in openspec/changes/<change-id>/.
Explore the SDD Topic Cluster
constitution, requirements, solution, tasks, proposal, spec, plan, design - one page each.
MySpec, Spec Kit, and OpenSpec: stages, files, folder layouts, and when to use which.
Compare automated Socratic AI architecture vs manual CLI markdown editing.
Master the 6 Easy Approach to Requirements Syntax patterns for AI coding.
How to maintain 10x velocity without building unmaintainable spaghetti code.
Production-ready project constitutions for Next.js and FastAPI.
Compare generic PM PRDs against machine-readable 4-file spec bundles in an interactive simulator.
Stream live specification contracts directly into Cursor and Claude Code.
Install in two commands, sign in once, and let Claude build your spec bundle task by task.
What it is, light vs dark factories, and how to run parallel agents from a spec with human gates.
Ready to Build Your First SDD Spec Bundle?
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