How to compare two documents with ChatGPT properly

Why one upload of both goes wrong

It reads well, which is the problem.

With both documents in view at once, the output merges them. You get statements that are true of one, or of neither, presented as though they applied to both, and no way to trace a claim back to its source without rereading everything.

There is also a length issue. Two long documents together are twice as likely to exceed 2 million tokens, and past that the file is truncated silently. A comparison built on all of document one and half of document two looks exactly like a complete comparison.

The two pass method

1. Decide your dimensions first

   Three to five things you actually care about. For contracts: termination, liability, payment, renewal. For proposals: price, timeline, scope, assumptions. This is the step that decides whether the output is useful.

2. Summarise document one against exactly those dimensions

   "For this document, give me: [dimension 1], [dimension 2], [dimension 3]. One short paragraph each, with the section number each came from. Say 'not covered' where it is absent."

3. Do the same for document two, in a fresh chat

   A fresh chat matters. In the same conversation, the second summary is influenced by the first, and you lose the independence that makes the comparison worth anything.

4. Paste both summaries into a third chat and compare

   "Here are structured summaries of two vendor proposals. Where do they differ materially, which difference carries the most risk for us, and where does either fail to address a dimension?"

Ask for a table, with columns you name

Prose comparisons are hard to scan and easy to hide vagueness in.

Table with columns: dimension, document A, document B, which is better for us and why.

Naming the columns forces a position on every row. A comparison that will not commit to a column is telling you the documents are the same on that dimension, which is itself worth knowing.

The questions worth asking

Beyond simply listing differences.

What is missing from both? Two contracts can share the same gap, and a comparison focused on differences will never surface it.

Which differences actually matter? Most differences are wording. Ask which ones change an obligation, a cost, or a risk.

Where do they contradict each other on the same fact? Two documents describing the same project with different figures is a finding, not a formatting issue.

What would each side argue? For a negotiation, better preparation than a difference list.

Where it is the wrong tool

Exact wording changes. A redline is a character level operation, and a comparison tool does it properly. Use one for what changed, then bring the changed clauses here for what they mean.

Numerical reconciliation. Two spreadsheets that should agree are a job for a spreadsheet. Upload them and ask for the reconciliation logic if you like, but check the arithmetic yourself.

Anything where the layout carries meaning. Images inside documents are discarded on every plan except Enterprise, so a comparison of two visually designed documents is comparing their text only.

Comparing more than two

Same method, more passes. Summarise each into the identical structure, then compare the summaries together.

This scales further than you would expect, because the summaries are short. Ten proposals, summarised into the same five dimensions, is a comfortable amount of text to reason over at once, where ten full proposals is not.