CSV UTF-8 BOM specific path failure response

When this happens

Use this when only one page, endpoint, static asset, or api route fails. Do not stop at the screen message; validate the route, rewrite rule, API path, host header, and upstream mapping first.

Symptom checklist

• UTF-8 BOM appears repeatedly in the CSV UI or logs.
• BOM, Shift_JIS, delimiter, quote, line break differs between successful and failed requests.
• The issue appears only after separating file format error from data value error.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• UTF-8 BOM specifically changes the investigation surface for CSV: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The file encoding differs from the importer expectation.
• A delimiter, quote, or line break appears inside data.
• Locale-specific dates or timezones are parsed as another region's format.
• Excel rewrites the file with BOM or Shift_JIS behavior.
• One bad row shifts the remaining column count.
• For the specific path failure case, the first useful clue is the route, rewrite rule, API path, host header, and upstream mapping.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare BOM, Shift_JIS, delimiter, quote, line break for success and failure in the same window.
3. Test the hypothesis: parsing or encoding failure caused by charset mismatch, delimiter drift, locale, timezone, row shape, or date format ambiguity.
4. Classify this as specific path failure: the route, rewrite rule, API path, host header, and upstream mapping.
5. Capture current values before changing configuration.

First evidence

Treat UTF-8 BOM as a specific path failure case. First collect evidence for the route, rewrite rule, API path, host header, and upstream mapping.

Output examples

Normal output

Working and failing paths use the intended host, status, and upstream.

Failing output

Only one route changes status, redirect target, content type, or upstream.

Output-to-action branches

• Only one page, endpoint, static asset, or API route fails.
  Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.
• The working and failing outputs differ.
  Act on the differing layer first: For UTF-8 BOM, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not clear every cache before proving the route mapping.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "UTF-8 BOM" ./logs

file input.csv

nkf -g input.csv

python - <<'PY'\nimport csv; print(next(csv.reader(open('input.csv', encoding='utf-8'))))\nPY

wc -l input.csv

file input.csv

nkf -g input.csv

python - <<'PY'
import csv
print(next(csv.reader(open('input.csv', encoding='utf-8'))))
PY

curl -I https://example.com/failing-path && curl -I https://example.com/working-path

Fix order

1. Record the full UTF-8 BOM message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for parsing or encoding failure caused by charset mismatch, delimiter drift, locale, timezone, row shape, or date format ambiguity.
3. Compare the failing case with a successful case before editing settings.
4. If this is the specific path failure branch, Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For UTF-8 BOM, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Detect encoding before import and convert once at the boundary.
• Validate delimiter, quote escaping, and column count before processing.
• Parse dates and timezones with an explicit locale and format.
• Reject bad rows with row numbers instead of silently shifting columns.
• Keep a small fixture file for regression tests.
• For the specific path failure branch, Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.

Evidence links

• RFC 4180 CSV format official
• Python csv module official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for CSV UTF-8 BOM.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change file format error from data value error results.
• Do not trust only the CSV UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for BOM, Shift_JIS, delimiter, quote, line break.
• Add encoding conversion, delimiter pinning, quote escaping, column count checks to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Excel UTF-8 BOM fix note
• Excel UTF-8 BOM fix note 2
• Excel UTF-8 BOM fix note 3
• Excel UTF-8 BOM fix note 4
• Excel UTF-8 BOM fix note 5
• Excel UTF-8 BOM fix note 6
• CSV UTF-8 BOM first triage response
• CSV Shift_JIS first triage response