Ask Claude to Add Type Annotations to Your Untyped Code

Adding types to an untyped codebase is valuable but tedious. Claude Code reads your functions, traces how they're called, and infers the correct types — then adds the annotations without changing any logic.

Add TypeScript type annotations to all functions in src/utils/helpers.js. 
Infer types from how they're used across the codebase. Don't change any behavior.

Claude searches for call sites, checks what values flow in and out, and adds the right types. A function that accepts a string or undefined and returns a number gets typed correctly because Claude saw the actual usage, not just the function body.

This works across languages:

# Python type hints
Add type hints to all functions in app/services/. Use Union types where 
multiple types are accepted and Optional where None is possible.

# PHP docblocks
Add PHPDoc @param and @return annotations to all public methods in 
app/Http/Controllers/. Check the actual usage to get the types right.

# TypeScript strict mode prep
Find all `any` types in src/ and replace them with specific types 
inferred from how the values are actually used.

For gradual migrations, work directory by directory:

Add TypeScript types to src/utils/ first. After that, show me which 
files in src/components/ have type errors because of the stricter types 
we just added.

Claude propagates the type changes through your codebase, catching cascading type mismatches that a human would need multiple passes to find.

The key is that Claude reads the callers, not just the function — it knows that processUser(data) is called with an object from a specific database query, so it types the parameter to match the actual data shape.

Types are documentation that the compiler enforces — let Claude read your code and write the types it already implies.

via Claude Code