Type error when using `verbatimModuleSyntax: true` in consumer project

[azerum]

Overview

When users use:

• ESM (package.json with `"type": "module")
• verbatimModuleSyntax: true in tsconfig.json

Importing @azure/functions (4.9.0) causes failing type checks, even with skipLibCheck: true.

To reproduce

For quick repo, clone and follow README.md of https://github.com/azerum/azure-functions-type-check-bug

I include files here for completeness:

package.json:

{
  "type": "module",
  "dependencies": {
    "@azure/functions": "4.9.0"
  },
  "devDependencies": {
    "typescript": "5.9.3"
  }
}

tsconfig.json:

{
    "compilerOptions": {
        "module": "node16",
        "moduleResolution": "node16",
        "verbatimModuleSyntax": true,
        "esModuleInterop": true,

        // Note: does not help
        "skipLibCheck": true,
    },

    "include": ["./src/**/*.ts"]
}

src/index.ts:

import {} from "@azure/functions"

Running

npx tsc

should succeed, but instead produces errors:

node_modules/@azure/functions/src/utils/toolProperties.ts:10:1 - error TS1287: A top-level 'export' modifier cannot be used on value declarations in a CommonJS module when 'verbatimModuleSyntax' is enabled.

10 export class ToolPropertyBuilder implements McpToolProperty {
   ~~~~~~

node_modules/@azure/functions/src/utils/toolProperties.ts:162:1 - error TS1287: A top-level 'export' modifier cannot be used on value declarations in a CommonJS module when 'verbatimModuleSyntax' is enabled.

162 export const arg = {
    ~~~~~~

node_modules/@azure/functions/src/utils/toolProperties.ts:225:1 - error TS1287: A top-level 'export' modifier cannot be used on value declarations in a CommonJS module when 'verbatimModuleSyntax' is enabled.

225 export function convertToolProperties(args: Args): McpToolProperty[] {
    ~~~~~~

node_modules/@azure/functions/src/utils/toolProperties.ts:238:1 - error TS1287: A top-level 'export' modifier cannot be used on value declarations in a CommonJS module when 'verbatimModuleSyntax' is enabled.

238 export function isToolProperties(properties: unknown): properties is Args {
    ~~~~~~

node_modules/@azure/functions/src/utils/toolProperties.ts:275:1 - error TS1287: A top-level 'export' modifier cannot be used on value declarations in a CommonJS module when 'verbatimModuleSyntax' is enabled.

275 export function normalizeToolProperties(
    ~~~~~~


Found 5 errors in the same file, starting at: node_modules/@azure/functions/src/utils/toolProperties.ts:10

Reason

index.d.ts of this library imports a .ts (not .d.ts) file on this line. This causes src/utils/toolProperties to be included for compilation in user's project. Compilation fails since:

• @azure/functions is published as CJS
• src/utils/toolProperties uses ESM syntax - export
• verbatimModuleSyntax requires using CJS syntax in CJS files, so it rightfully gives error here

I think library .ts files should not be included in user projects at all, i.e. .d.ts must never import from .ts.

[azerum]

This can cause bugs with tsconfig.json settings other than verbatimModuleSyntax, so I have opened a more generally-worded issue: #397