source-driven-development

name	source-driven-development
description	Ground implementation in authoritative docs. Use for current frameworks, libraries, or standards; not when local behavior is the only source needed.

Source-Driven Development

Overview

Every framework-specific code decision must be backed by official documentation. Don't implement from memory — verify, cite, and let the user see your sources. Training data goes stale, APIs get deprecated, best practices evolve. This skill ensures the user gets code they can trust because every pattern traces back to an authoritative source they can check.

When to Use

• The user wants code that follows current best practices for a given framework
• Building boilerplate, starter code, or patterns that will be copied across a project
• The user explicitly asks for documented, verified, or "correct" implementation
• Implementing features where the framework's recommended approach matters (forms, routing, data fetching, state management, auth)
• Reviewing or improving code that uses framework-specific patterns
• Any time you are about to write framework-specific code from memory

When NOT to use:

• Correctness does not depend on a specific version (renaming variables, fixing typos, moving files)
• Pure logic that works the same across all versions (loops, conditionals, data structures)
• The user explicitly wants speed over verification ("just do it quickly")

The Process

NEED ──→ SEARCH ──→ DECIDE ──→ DETECT ──→ FETCH ──→ IMPLEMENT ──→ CITE
  │         │          │          │          │           │            │
  ▼         ▼          ▼          ▼          ▼           ▼            ▼
 What     Packages   Adopt /    Stack &    Official   Follow      Show
 is        skills     extend /   versions  docs       documented  sources
 needed?   APIs       build

Step 0: Search-First Gate (before writing greenfield helpers)

When the ask is "add X functionality", a new utility, integration, or dependency — search before you code.

1. Need analysis — name the capability and language/framework constraints.
2. Parallel search — package registries (npm/PyPI/crates…), existing project modules, installed/agent skills, official docs, GitHub.
3. Evaluate — maintenance, license, API fit, dependency cost, docs quality.
4. Decide with an explicit matrix:

Signal	Action
Exact match, maintained, acceptable license	Adopt — install/use as-is
Partial match, solid base	Extend — thin wrapper around existing tool
Weak scattered matches	Compose — small existing pieces
Nothing suitable after search	Build — minimal custom code, document why

State the decision before implementation:

SEARCH-FIRST:
- need: retry with backoff for HTTP client
- candidates: tenacity (PyPI), existing utils/retry.py
- decision: ADOPT tenacity — maintained, used in sibling service
- sources: https://...

Skipping Step 0 to "just write a helper" is a failure mode unless the user forbade search or the change is a one-line local pure function.

Step 1: Detect Stack and Versions

Read the project's dependency file to identify exact versions:

package.json    → Node/React/Vue/Angular/Svelte
composer.json   → PHP/Symfony/Laravel
requirements.txt / pyproject.toml → Python/Django/Flask
go.mod          → Go
Cargo.toml      → Rust
Gemfile         → Ruby/Rails

State what you found explicitly:

STACK DETECTED:
- React 19.1.0 (from package.json)
- Vite 6.2.0
- Tailwind CSS 4.0.3
→ Fetching official docs for the relevant patterns.

If versions are missing or ambiguous, ask the user. Don't guess — the version determines which patterns are correct.

Step 2: Fetch Official Documentation

Fetch the specific documentation page for the feature you're implementing. Not the homepage, not the full docs — the relevant page.

Source hierarchy (in order of authority):

Priority	Source	Example
1	Official documentation	react.dev, docs.djangoproject.com, symfony.com/doc
2	Official blog / changelog	react.dev/blog, nextjs.org/blog
3	Web standards references	MDN, web.dev, html.spec.whatwg.org
4	Browser/runtime compatibility	caniuse.com, node.green

Not authoritative — never cite as primary sources:

• Stack Overflow answers
• Blog posts or tutorials (even popular ones)
• AI-generated documentation or summaries
• Your own training data (that is the whole point — verify it)

Be precise with what you fetch:

BAD:  Fetch the React homepage
GOOD: Fetch react.dev/reference/react/useActionState

BAD:  Search "django authentication best practices"
GOOD: Fetch docs.djangoproject.com/en/6.0/topics/auth/

After fetching, extract the key patterns and note any deprecation warnings or migration guidance.

When official sources conflict with each other (e.g. a migration guide contradicts the API reference), surface the discrepancy to the user and verify which pattern actually works against the detected version.

Step 3: Implement Following Documented Patterns

Write code that matches what the documentation shows:

• Use the API signatures from the docs, not from memory
• If the docs show a new way to do something, use the new way
• If the docs deprecate a pattern, don't use the deprecated version
• If the docs don't cover something, flag it as unverified

When docs conflict with existing project code:

CONFLICT DETECTED:
The existing codebase uses useState for form loading state,
but React 19 docs recommend useActionState for this pattern.
(Source: react.dev/reference/react/useActionState)

Options:
A) Use the modern pattern (useActionState) — consistent with current docs
B) Match existing code (useState) — consistent with codebase
→ Which approach do you prefer?

Surface the conflict. Don't silently pick one.

Step 4: Cite Your Sources

Every framework-specific pattern gets a citation. The user must be able to verify every decision.

In code comments:

// React 19 form handling with useActionState
// Source: https://react.dev/reference/react/useActionState#usage
const [state, formAction, isPending] = useActionState(submitOrder, initialState);

In conversation:

I'm using useActionState instead of manual useState for the
form submission state. React 19 replaced the manual
isPending/setIsPending pattern with this hook.

Source: https://react.dev/blog/2024/12/05/react-19#actions
"useTransition now supports async functions [...] to handle
pending states automatically"

Citation rules:

• Full URLs, not shortened
• Prefer deep links with anchors where possible (e.g. /useActionState#usage over /useActionState) — anchors survive doc restructuring better than top-level pages
• Quote the relevant passage when it supports a non-obvious decision
• Include browser/runtime support data when recommending platform features
• If you cannot find documentation for a pattern, say so explicitly:

UNVERIFIED: I could not find official documentation for this
pattern. This is based on training data and may be outdated.
Verify before using in production.

Honesty about what you couldn't verify is more valuable than false confidence.

Common Rationalizations

Rationalization	Reality
"I'm confident about this API"	Confidence is not evidence. Training data contains outdated patterns that look correct but break against current versions. Verify.
"Fetching docs wastes tokens"	Hallucinating an API wastes more. The user debugs for an hour, then discovers the function signature changed. One fetch prevents hours of rework.
"The docs won't have what I need"	If the docs don't cover it, that's valuable information — the pattern may not be officially recommended.
"I'll just mention it might be outdated"	A disclaimer doesn't help. Either verify and cite, or clearly flag it as unverified. Hedging is the worst option.
"This is a simple task, no need to check"	Simple tasks with wrong patterns become templates. The user copies your deprecated form handler into ten components before discovering the modern approach exists.

Red Flags

• Reinventing a utility without a search-first decision (adopt/extend/build)
• Writing framework-specific code without checking the docs for that version
• Using "I believe" or "I think" about an API instead of citing the source
• Implementing a pattern without knowing which version it applies to
• Citing Stack Overflow or blog posts instead of official documentation
• Using deprecated APIs because they appear in training data
• Not reading package.json / dependency files before implementing
• Delivering code without source citations for framework-specific decisions
• Fetching an entire docs site when only one page is relevant

Verification

After implementing with source-driven development:

• Search-first decision recorded when adding capabilities (adopt/extend/compose/build)
• Framework and library versions were identified from the dependency file
• Official documentation was fetched for framework-specific patterns
• All sources are official documentation, not blog posts or training data
• Code follows the patterns shown in the current version's documentation
• Non-trivial decisions include source citations with full URLs
• No deprecated APIs are used (checked against migration guides)
• Conflicts between docs and existing code were surfaced to the user
• Anything that could not be verified is explicitly flagged as unverified