too_many_fields — Column count exceeds the ingest limit

The row has more columns than the configured maximum. This is a guard against a malformed file consuming unbounded memory.

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Diagnosis

When you hit this

A delimiter mismatch, where a semicolon-separated file read as comma-separated produces one enormous column, or the reverse.
A genuinely wide export from a historian dumping thousands of tags per row.
Unquoted commas inside a text field splitting one column into many.

Resolution

How to fix it

Check the delimiter first
European exports frequently use semicolons. A wrong delimiter is the usual cause and looks nothing like a column-count problem at first.
Quote text fields containing commas
An alarm description with a comma in it splits the row without quoting.
Split very wide exports
Narrow the export to the tags you actually need rather than dumping the full tag list.

Normalization

What Forge does about it

A request that fails on some points and succeeds on others still normalizes. The tags that arrived resolve to canonical fields as usual; the ones that did not come back are listed in unresolved_tags and excluded from coverage_pct, so a partial read reports as partial instead of quietly looking complete.