github/github-mcp-server · info

failed to write CSV header: %w

Error message

failed to write CSV header: %w

What it means

csv.Writer.Write failed while emitting the header row into a bytes.Buffer. The Go csv writer only returns errors from the underlying writer or on field-count/quoting invariants, and a bytes.Buffer's Write never returns an error, so this branch is effectively unreachable defensive code. Reaching it implies memory exhaustion or buffer corruption rather than a data problem.

Source

Thrown at pkg/github/csv_output.go:135

	doc := csvDocument(value)
	if len(doc.metadata) == 0 && len(doc.rows) == 0 {
		return "", nil
	}

	var buf bytes.Buffer
	writeCSVMetadata(&buf, doc.metadata)
	if len(doc.rows) == 0 {
		return buf.String(), nil
	}

	headers := csvHeaders(doc.rows)
	if len(headers) == 0 {
		return buf.String(), nil
	}

	writer := csv.NewWriter(&buf)
	if err := writer.Write(headers); err != nil {
		return "", fmt.Errorf("failed to write CSV header: %w", err)
	}

	for _, row := range doc.rows {
		record := make([]string, len(headers))
		for i, header := range headers {
			record[i] = row[header]
		}
		if err := writer.Write(record); err != nil {
			return "", fmt.Errorf("failed to write CSV row: %w", err)
		}
	}

	writer.Flush()
	if err := writer.Error(); err != nil {
		return "", fmt.Errorf("failed to flush CSV: %w", err)
	}
	return buf.String(), nil
}

View on GitHub (pinned to 0ea1f775a7)

Solutions

1. If converting very large result sets, request smaller pages (lower per_page) so the CSV fits comfortably in memory
2. Check container memory limits and raise them if the output is legitimately huge
3. Otherwise treat as a defensive invariant and report upstream with the error string

if err := writer.Write(headers); err != nil {
	// bytes.Buffer writes cannot fail: treat as fatal invariant, log and abort conversion
	return "", fmt.Errorf("failed to write CSV header: %w", err)
}

When it happens

Trigger: Converting a JSON document with rows to CSV when the in-memory buffer write fails — practically only under severe memory pressure (OOM-adjacent) or with a corrupted buffer in modified builds.

Common situations: Extremely large CSV conversions on memory-constrained containers; not reproducible under normal conditions.

Related errors

• failed to write CSV row: %w
• failed to flush CSV: %w
• failed to unmarshal JSON text: %w
• failed to marshal discussions: %w
• failed to marshal discussion: %w

AI-assisted analysis of github/github-mcp-server@0ea1f775a7 (2026-08-15). Data as JSON: /api/errors/885f3b262abf2ad1. Report an issue: GitHub.