Claude3.5 Sonnet kept hallucinating my Python data frames
Last Thursday, I spent four hours fighting a pandas DataFrame that refused to aggregate correctly. I was building a custom dashboard to track API latency across different regions, and I was using Claude 3.5 Sonnet to handle the heavy lifting of the data cleaning.
The code looked perfect. Syntactically, it was a dream. But every time I ran the script, I got this:
KeyError: 'region_id'
The wild part? I could see the column region_id
right there in my CSV. I checked the spelling. I checked for trailing spaces. Nothing. I fed the error back to the LLM, it apologized, gave me the same code back with a "fix" that didn't change a single line of logic, and told me it had "resolved the issue."
It was a loop of polite incompetence.
The moment I realized the context window was lying to me #
I stopped prompting and actually looked at the raw data. I realized my CSV had a weird BOM (Byte Order Mark) at the start of the file, which meant the first column header wasn't actually region_id
—it was \ufeffregion_id
.
Claude knew the column name was region_id
because I had pasted a snippet of the data earlier, but it wasn't actually reading the file's encoding when it wrote the script. It was hallucinating the success of the code based on the provided snippet rather than the reality of the file on my disk.
I fixed it with a simple encoding='utf-8-sig'
in my read_csv()
call.
import pandas as pd
df = pd.read_csv('latency_data.csv', encoding='utf-8-sig')
print(df.columns) # Now 'region_id' actually exists
That four-hour rabbit hole taught me something critical: AI is a phenomenal co-pilot, but if you stop verifying the "ground truth" of your data, you're just guessing with more expensive tools.
Building a better AI data analysis guide for myself #
After that disaster, I stopped treating the LLM as a magic box and started building a mental framework for how to actually use these things for data work. If you're trying to build an AI data analysis guide for your own workflow, stop asking the AI to "analyze this" and start asking it to "write a verification script for this."
I shifted my process to a three-step loop:
- The Probe: Ask the AI to write a script that prints
df.info()
, df.head()
, and df.columns
.
-
The Execution: Run that locally. Paste the actual output back.
-
The Logic: Only then ask for the analysis code.
This removes the "hallucinated column" problem entirely. It's slower by about 30 seconds, but it saves you four hours of staring at a KeyError
.
| Step | Old Way (Fast but Broken) | New Way (Slow but Stable) |
| :--- | :--- | :--- |
| Input | Raw CSV + "Analyze this" | Raw CSV + "Describe the schema" |
| Verification | Trust the AI's output | Run df.info()
locally |
| Result | Intermittent Hallucinations | Verifiable Data Frames |
| Time spent | 4 hours debugging | 15 mins execution |
Moving from data to decks without the manual grind #
Once I actually got the data right, I hit the second bottleneck: the stakeholder presentation. I hate spending three hours moving charts from a Jupyter Notebook into a slide deck.
I tried a few AI slide generator tools, and honestly, most of them are garbage. They give you generic templates with "Insert Image Here" placeholders and text that sounds like a corporate brochure from 1998.
The only way that actually worked for me was using Claude to generate structured Markdown or VBA code that I could import directly into PowerPoint. I don't want the AI to "design" my slide; I want it to organize my data into a narrative structure that doesn't make me look like an amateur.
For example, I found that providing the AI with the specific insights from my data analysis and asking for a "slide-by-slide outline with a focus on the delta between Q3 and Q4" yielded far better results than any "one-click" slide generator.
Why I stopped struggling in a vacuum #
The real turning point for my productivity wasn't a specific tool, but finding people who had already failed at the things I was currently failing at. That's where a Claude community becomes a force multiplier.
When I first started using MCP (Model Context Protocol) to connect my local database to my LLM, I spent two days trying to figure out why my queries were timing out. I could have spent another two days reading documentation, but instead, I found a thread in a developer community where someone had already mapped out the exact timeout settings for my specific DB version.
The difference between "using AI" and "mastering AI" is usually just knowing which niche forum or community to check. If you're still manually troubleshooting every IndexError
or struggling with AI Coding bottlenecks, you're probably just missing the collective tribal knowledge of people who have already broken the same things.
Joining a community like PromptCube isn't about finding "the perfect prompt"—those don't exist because models change every two weeks. It's about the shared debugging logs. It's about seeing a post that says "Don't use X library with Claude 3.5 because it handles async calls weirdly," and saving yourself a weekend of frustration.
Refining the workflow #
If you want to actually scale your output, you have to treat your AI interaction like a git repository. Version your prompts. Track what failed.
I've started keeping a "failure log" of prompts that led to hallucinations. When I see a pattern—like the BOM encoding issue—I add it to a system prompt that I use across all my data projects.
If you're looking for ways to optimize this, check out the Resources section to see how others are structuring their agentic workflows.
The goal isn't to let the AI do the work. The goal is to use the AI to do the boring parts of the work so you can spend your brainpower on the actual analysis. Stop trusting the "Success!" message from the LLM. Trust the terminal.
Next Using AI for coding shouldn't feel like cheating in 2024 →
a library of Claude prompt techniques, with plenty of directly applicable cases.
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