Want to access all premium content?
Become a paying subscriber to get access to all courses, guides like this, prompts and skills library, and $500+ worth of AI tools.
Claude's Google Sheets extension lets you inspect a spreadsheet from a sidebar. With Opus 5.5 selected, ask it to trace a total, find incomplete formula ranges, or investigate why a lookup returns #N/A.
We'll audit one spreadsheet: map the tabs, identify problems, approve fixes, and check the results. You'll need a Google account, an eligible Claude plan, and a supported browser. Confirm that Opus 5.5 is available in your sidebar before starting. The setup kit includes a practice budget with five planted errors.
Step 1: Install the extension. Open Claude inside Google Sheets.
Step 2: Map the spreadsheet. Understand the inputs and dependencies.
Step 3: Find the errors. Audit one tab at a time.
Step 4: Review each fix. Check the formula before approving it.
Step 5: Check the results. Reconcile totals and name the audited version.
Step 1: Install the extension
Open Google Workspace Marketplace and find Anthropic's Claude extension. Confirm the publisher and listing before installing; similarly named formula add-ons may appear in search results.
Open your spreadsheet, then go to Extensions, then Claude, then Open Claude. Review the Google permissions and sign in with your Claude account.
Check the available models and select Opus 5.5 if listed. Model availability in the sidebar still needs confirming.
Claude's help page lists Pro, Max, Team, and Enterprise support, with Chrome, Edge, or Safari required. If actions don't complete in Firefox, switch to a supported browser.
For a work Google account, your administrator may need to approve the extension.
Step 2: Map the spreadsheet
Make a copy before starting, or name the current version “Before audit.” This gives you a clear reference for checking changes.
Confirm that “Ask before edits” is enabled. Its location in the sidebar still needs checking.
Paste:
Map this spreadsheet without changing anything.
For each tab, explain its purpose, identify the main input and formula ranges, and list dependencies on other tabs.
Support your explanation with cell references. Flag anything you can't determine.
Compare the map with what you know. Correct missed connections before starting the audit.
For the main total, ask:
Trace the grand total on the Summary tab back to its source inputs.
List the cells, formulas, and tabs involved. Flag references you can't inspect.Step 3: Find the errors
Claude's help page lists a six-minute limit per task. For larger spreadsheets, start with the totals tab and work through the others separately.
Audit the formulas on this tab without editing anything.
Check for:
- Error values such as #REF!, #VALUE!, #DIV/0!, and #N/A
- Circular references
- Hardcoded numbers inside formulas
- Ranges that may omit relevant rows or columns
- Formulas that differ from nearby cells
- Totals that don't reconcile with their components
Rank findings as high, medium, or low severity.
For each, give the cell, evidence, likely impact, and suggested fix. Separate confirmed errors from items needing my review.
Not every unusual formula is wrong. A different calculation or hardcoded value may be deliberate.
Problem | Example | What to check |
|---|---|---|
Error value | A lookup returns | Whether the value is missing, mismatched, or outside the lookup range |
Hardcoded number |
| Whether the rate should come from an input cell |
Incomplete range |
| Whether the extra rows belong in the total |
Pattern difference | One formula differs from its neighbors | Whether that row needs a different calculation |
Repeat on each tab containing formulas. Ask for a combined numbered list that records which tabs and ranges were checked.
Step 4: Review each fix
Start with high-severity findings. Ask for the proposal before the edit:
Review problem #1.
Show the existing formula, proposed replacement, and reason for the change. Identify downstream cells and totals likely to be affected.
Wait for my approval before applying it.
Approve one change, then select the edited cell and check its formula bar.
For repeated hardcoded rates, use a labeled input:
Find formulas using 1.08 as a tax multiplier.
Propose a Tax rate cell on the Inputs tab containing 8%, with formulas referring to it using (1 + that cell).
Confirm that 1.08 has the same meaning in each formula. Show the proposed changes and wait for approval.
If a fix causes another problem, stop and review the change. Version history can restore an earlier state, but restoring a whole version may also remove other edits made since then. Check what would be lost before restoring.
Step 5: Check the results
Request a change report:
List every edit made in this session: cell, previous formula or value, replacement, and reason.
Compare the main totals before and after. Flag changes greater than 5% and explain which edits caused them.
List unresolved findings separately.
Open File, then Version history, then See version history to inspect the earlier values. Claude's help page notes that edits made through the extension appear under your account.
Recalculate representative rows yourself, including one affected by a fix. Reconcile the main totals where practical. One correct row doesn't confirm the rest of the spreadsheet.
Rerun the audit on the affected tabs and review anything still open. Name the current version “Audited” and keep the change report.
Use cases
Once your first audit works, try these four jobs.
Use case #1: Check an inherited budget
Find the areas to review before relying on someone else's file.
Map this budget and identify the five highest-risk formula issues.
Include cell references, evidence, and the possible impact on totals. Don't edit anything.Verify the top finding yourself.
Use case #2: Fix a lookup
Investigate a VLOOKUP or XLOOKUP returning #N/A.
Investigate the #N/A result in D8.
Compare the lookup value, source values, range, and matching settings. Explain the cause and propose a fix without applying it.After an approved fix, check other rows using the same lookup.
Use case #3: Explain a formula
Break a nested formula into readable steps.
Explain F12 one part at a time.
Identify its inputs, conditions, and possible error cases. Describe how changing an input should affect the result.Test an input in your working copy, then restore it.
Use case #4: Find missing rows
Check totals after adding data.
Inspect SUM, SUMIF, and AVERAGE formulas on this tab.
Flag ranges that may omit relevant data. Show the formula range, apparent data boundary, and omitted rows. Don't assume every row belongs in the calculation.Confirm the intended range before extending it.
Confirmed limitations
Confirm Opus 5.5 access in the sidebar. Claude's help page lists the extension as beta, available on eligible paid plans, with Firefox unsupported.
The model can miss errors and propose incorrect fixes. Review changed formulas and important totals yourself.
Tasks stop after six minutes or when the sidebar closes. Work is limited to the open file.
Connected Sheets with live BigQuery data may require an extract before the extension can inspect the rows. Extracting data also creates a snapshot, so check its freshness.
Claude's help page lists triggers, custom menus, and scheduled refreshes as unsupported. Don't treat a formula audit as a review of Apps Script automation.
Closing
Start with the practice budget and complete all five steps. Keep a before-audit version, approve each fix, and check the affected totals before naming the result.
Then repeat the process on a spreadsheet you use regularly. Save the prompt somewhere handy and rerun it after major formula or data changes.
See you in the next one :)