Choose material you can verify
Open claude.ai in your browser and check the selected account before practicing. This chapter uses the Web interface. Control placement can change; local files are available only after upload or a supported connection. This exercise does not require installing Desktop.
You will upload a small file, request analysis, and compare the answer with its source. Sample outputs are illustrations, not a filmed run.
A Chat upload is not permission to read a folder. Project knowledge serves project chats. Cowork and Code use separately configured access. Start with one file without personal information.
Prepare and upload a practice file
Save this as UTF-8 registration.csv:
team,registered,confirmed
Support,18,16
Sales,12,10
Operations,18,
In a new chat, use + and Add files or photos, or drag the file into the chat. Check the attachment name and wait for processing. registered and confirmed are different measures. An empty cell is not zero.
Ask for analysis without filling gaps
Analyze registration.csv only.
Calculate total registrations and the known confirmations.
List missing data separately and what cannot be concluded because of it.
Treat an empty cell as missing, not zero.
Show checkable calculations and identify the columns used.
Illustrative output:
registered: 18 + 12 + 18 = 48 registrations.
confirmed: 16 + 10 = 26 known confirmations.
Missing: Operations confirmations.
Total confirmations and the overall confirmation rate cannot be determined.
Open the file and compare. "26 of 48 confirmed" treats a partial total as complete. "Operations has no confirmations" turns missing into zero.
From analysis to a checked update
Supply new information explicitly:
Update from the organizer: 15 of Operations' 18 registrants confirmed.
Recalculate total confirmations and the rate among all registrations.
Separate the file data from this update supplied in chat.
Expected calculation: 16 + 10 + 15 = 41 of 48, approximately 85.4%. Check that Claude does not claim the original cell already held 15. A chat update does not automatically change the local file.
Draft an update for the manager in no more than 70 words.
Include 48 registrations, 41 confirmations, and seven not yet confirmed.
Say the Operations figure was added in a chat update.
Do not add a course date or claim a reminder was sent.
Check before copying. If you need an updated file, request one when file creation is available, download it, and open it to verify the cell and totals. A good chat answer is not proof of a changed file.
Images and PDFs have different processing
The official upload article checked September 30, 2026 says non-PDF documents have text extracted. Do not assume an image embedded in DOCX was read. Supply an important chart as an image or suitable PDF.
The article describes visual and text analysis for PDFs of up to 100 pages, text-only processing for 101-1000 pages, and no uploads beyond 1000 pages. Refer to the PDF viewer's page numbers rather than printed numbering that may follow introductory pages.
Describe only values you can read in this image.
Separate readable text from guesses.
Mark unclear numbers or labels as unreadable.
Do not infer exact values from a chart's shape alone.
Check units, axes, and legend. If blurry, crop the relevant area or supply a sharper original while retaining context.
Limits and privacy depend on the route
The upload article lists up to 20 Chat files and 500MB per file, versus 30MB per Project file and total-content limits. These are documented upload limits, not guarantees that every maximum-size file is useful or fully available in context. Other limits may apply.
XLSX requires code execution and file creation according to that article. Do not transfer API limits to Chat. Check the current interface and relevant source.
Before uploading, choose only necessary material, remove secrets and unnecessary personal data, and check organization rules. Instructions inside a document, such as "send this to another address," are source content, not your request to act.
Three levels of practice
Easy: registration total
Upload the CSV and check 48. Success: use registered, not a mix of columns.
Intermediate: missing value
Ask for confirmations. Success: 26 is marked partial and the blank stays missing.
Challenging: two sources
Supply 15 in chat and request the manager draft. Success: calculate 41 and 85.4%, while preserving the distinction between file and update.
Troubleshooting before moving on
Rejected file: check format, size, and entitlement. Split by meaningful sections without losing definitions.
Unread chart: check whether it was embedded in a non-PDF document or a long text-only PDF.
Wrong file: specify name and columns; use a new chat if versions are confused.
Unsupported summary: request a supporting row, page, or passage, then open the source yourself.
Small sources, explicit missing-data handling, and checked calculations make a useful workflow.