CORS Error in Next.js API Routes — Fix Guide
Resolve origin mismatches, missing headers, local tunnel edge cases, and browser preflight failures.
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Symptoms
- You see unexpected changes or failures
Root causes
- Misconfigured rules
- Missing permissions or wrong env
- Workspace root mismatch
Fix checklist
- Reduce scope: reproduce with a tiny change
- Ensure rules/config are loaded
- Remove write access temporarily
- Validate with lint/build
Safe debugging prompts
Only diagnose. Do not change code until you identify the most likely root cause.
Provide the smallest fix and list files that will change.
Prevent recurrence
- Add tighter rules for file boundaries
- Add CI checks
FAQ
Should I let the agent run commands?
Prefer explicit, short commands and review outputs before applying changes.
What should I verify after the fix?
Re-run the exact failing path, then verify related pages, env handling, and the production build if relevant.
Should I change multiple files at once?
Prefer the smallest fix first so you can isolate whether the root cause is config, code, or environment.