[MODEL] Claude repeatedly uses openpyxl for Excel charts despite it being fundamentally broken for time-series

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Claude made incorrect assumptions about my project

What You Asked Claude to Do

I asked Claude to create Excel line charts showing time-series data (RS485
inverter monitoring:
frequency, current, voltage, power vs wall-clock time HH:MM:SS).

What Claude Actually Did

Claude repeatedly attempted to use openpyxl LineChart with set_categories().
This always generates <c:numRef> in the XML instead of <c:strRef>, causing Excel

to treat each timestamp as a separate data series — producing 80+ colored lines
per chart instead of a single trend line.

Claude tried 6+ different openpyxl-based fixes over 6 hours, none of which
worked,
because the bug is fundamental to openpyxl's chart XML generation and cannot be
patched from user code.

The correct solution (xlsxwriter library) was only found after extensive trial
and error.
Claude should have known openpyxl LineChart is broken for string/time categories
and
recommended xlsxwriter immediately.

Expected Behavior

Claude should immediately recognize that openpyxl LineChart is broken for
string/time-based X-axis categories, and recommend xlsxwriter instead without
requiring the user to discover this through hours of failed attempts.

✻ Cogitated for 4s

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

_No response_

Claude Model

Sonnet

Relevant Conversation

Impact

Critical - Data loss or corrupted project

Claude Code Version

2.1.179 (Claude Code)

Platform

Anthropic API

Additional Context

_No response_