fix(dev): respect opts.json when false for error handler

[danielroe]

🔗 Linked issue

nuxt/nuxt#33748 (comment)

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

previously it was impossible to force an html response (json: false) if there were certain headers set.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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Project	Deployment	Review	Updated (UTC)
nitro.build	Ready	Preview, Comment	Jan 5, 2026 1:03pm

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

A single file modification in the error handling runtime that replaces a logical OR operator with a nullish coalescing operator when determining JSON response format. This changes how the opts.json flag is evaluated, altering the precedence between explicit false values and header-based content negotiation.

Changes

Cohort / File(s)	Summary
Error Handling Logic src/runtime/internal/error/dev.ts	Changed operator from || to ?? for useJSON computation, modifying response format selection. When opts.json is explicitly false, HTML is now forced; null/undefined values fall back to Accept header logic.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/explicit-json-false

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📜 Recent review details

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 301604f and ed91a8c.

📒 Files selected for processing (1)

• src/runtime/internal/error/dev.ts

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