How to Fix TypeScript Module Augmentation in AI-Built Apps
TypeScript module augmentation fails to extend existing types, causing missing properties or duplicate identifier errors. AI-generated declaration files often use incorrect syntax or module paths.
Error messages you might see
Property 'customField' does not exist on type 'Session'Error: Augmentations for the global scope can only be directly nested in external modules or ambient module declarationsWhy this happens in AI-generated code
Missing export in declaration file
AI creates a .d.ts file without an export statement, making it a script instead of a module and breaking augmentation.
Wrong module path in declaration
The `declare module` path doesn't match the actual module name, so the augmentation never applies.
Declaration file not included in tsconfig
AI puts the .d.ts file in a directory not covered by the `include` array in tsconfig.json.
How to fix it
Add an empty export to declaration files
Add `export {}` at the top of .d.ts files to ensure TypeScript treats them as modules, enabling augmentation syntax.
Match the exact module specifier
Use the same string you use in import statements: `declare module 'next-auth'` not `declare module 'next-auth/index'`.
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Frequently asked questions
Where should I put my .d.ts files?
Place them in a `types/` directory and ensure that directory is included in tsconfig.json's `include` array.
How do I extend a third-party library's types?
Create a .d.ts file with `declare module 'library-name' { interface ExistingType { newProp: string; } }` and ensure it has an export.
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