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This section details on the original issue you should resolve

<issue_title>"use strict" is not injected for CJS builds</issue_title>
<issue_description>I've seen #4607, but I feel it's important to bring this up again.

I think this decision is quite unfortunate.

Is the expectation that developers will add 'use strict' to their ESM/TS source code when dual-bundling as ESM/CJS? Including 'use strict' in ESM code seems counterproductive.

An alternative might be to add 'use strict' to CJS files by default when "type": "module" is used and bundling for CJS.</issue_description>

<agent_instructions>> 1. Should this only apply when the output format is CJS?

This should apply to all formats (other than ESM as it already enables strict mode).

2. Should this behavior only take effect when tsconfig is enabled? If the user does not enable tsconfig, should we avoid emitting "use strict"?

Yes. This behavior should only take effect when tsconfig is enabled.

3. If the user enables tsconfig: 'auto' and we resolve the tsconfig.json from the current working directory, or if they explicitly specify tsconfig: '..', and the include / exclude configuration excludes the entry file, should we still respect the alwaysStrict setting from that tsconfig.json?

No. It should apply the alwaysStrict setting only when the other settings are respected. In other words, we should respect the include / exclude options.
</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@sapphi-red We are going to follow esbuild's behavior, which is to respect tsconfig.json's `strict` (`alwaysStrict`) option. When that is implemented, you should be able to output `"use strict"` by setting `"alwaysStrict": true` in tsconfig.json. If the tsconfig.json has `include` & `exclude`, the entrypoint should be included in them.

For now, if you add `"use strict"` to all entrypoints manually, Rolldown should emit `"use strict"`. @shulaoda I’d like to clarify a few points: 1. Should this only apply when the output format is CJS? 2. Should this behavior only take effect when `tsconfig` is enabled? If the user does not enable `tsconfig`, should we avoid emitting `"use strict"`? 3. If the user enables `tsconfig: 'auto'` and we resolve the `tsconfig.json` from the current working directory, or if they explicitly specify `tsconfig: '..'`, and the `include` / `exclude` configuration excludes the entry file, should we still respect the `alwaysStrict` setting from that `tsconfig.json`? @sapphi-red > 1. Should this only apply when the output format is CJS?

This should apply to all formats (other than ESM as it already enables strict mode).

2. Should this behavior only take effect when tsconfig is enabled? If the user does not enable tsconfig, should we avoid emitting "use strict"?

Yes. This behavior should only take effect when tsconfig is enabled.

3. If the user enables tsconfig: 'auto' and we resolve the tsconfig.json from the current working directory, or if they explicitly specify tsconfig: '..', and the include / exclude configuration excludes the entry file, should we still respect the alwaysStrict setting from that tsconfig.json?

No. It should apply the alwaysStrict setting only when the other settings are respected. In other words, we should respect the include / exclude options.