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Giving Claude A File vs Describing It
By Nigel Guy · 2 min read
There's a habit that comes from years of writing emails and briefs to
humans: summarise the document, paste in "the important bit," and describe
the rest in your own words. It feels efficient and it feels like good manners
— you're saving the reader time. Done to Claude, it usually does the opposite
of what you want, because you've quietly become the filter between the
source material and the tool that's meant to be working with it.
The rule: if the document exists as a file, upload it — don't summarise it
into the prompt and hope the summary carried what mattered.
The mechanism: the Upload Threshold
Ask these two questions before you start retyping or paraphrasing:
- Does structure matter? Tables, headings, numbered clauses, line
references, formatting that carries meaning (a contract's clause numbers,
a spreadsheet's columns) — all of this degrades or disappears the moment
you describe it in prose instead of handing over the actual file.
- Could you be wrong about what matters? A summary only includes what
you noticed. If you're asking Claude to check a contract for a specific
type of clause, spot an inconsistency, or catch something you might have
missed, your summary can't contain the thing you're relying on it to
find — that's the entire point of asking.
If the answer to either is yes, upload the file. Reserve describing-in-prose
for genuinely short, low-structure things — "here's roughly what the email
said" — where nothing is lost in translation.
What this changes in practice
- Long documents — upload rather than paste a chunk of text into the
chat box; large pastes are harder to reference precisely and easier to
truncate by accident.
- Spreadsheets — upload the file itself rather than typing out "column A
has dates, column B has amounts." Claude working from the actual columns
can check totals, spot gaps, and reference specific rows; a description
can't be checked against anything.
- Scanned or photographed documents — upload the image or PDF directly
rather than transcribing it yourself; transcription is exactly the kind of
step where a small human error creates a wrong-but-confident answer later.
What to skip
Skip uploading when a plain, short question doesn't need the source at all
("what's a reasonable notice period for this kind of contract, generally?").
And skip re-uploading the same file every session if this is ongoing work —
that's a case for keeping a running project thread instead, so the material
stays in context without repeating the upload.
Guardrails
- Uploading the file doesn't remove the need to check the output — Claude
can still misread a scanned document or misinterpret a spreadsheet's
intent, so treat the source as available for verification, not as proof
the answer is right.
- Sensitive documents deserve the same caution here as anywhere else — see
the checklist on handling sensitive documents before uploading anything
with personal, financial or legally protected information in it.
- File size and format limits exist and change; if an upload fails, that's
usually the reason, not a sign something's gone wrong with the account.
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