Attaching files for context

Pasting the wrong thing — a 40-page document when you needed one paragraph, or a snippet when you needed the full file — is the most common reason a response misses the mark. This lesson draws the line.

What "attaching" actually does

When you attach a file, Claude Desktop reads it into the context window for that conversation. The context window is not unlimited. Claude 3.5 Sonnet has a 200,000-token context window, which is roughly 150,000 words. A short PDF fits easily. A 200-page research report may not — or if it does, a very long document crowds out the rest of the conversation.

Pasting a snippet costs fewer tokens but requires you to paste the right part. If you are uncertain which part matters, attaching the whole file is safer.

When to attach versus paste

Attach the full file when:

• You are asking Claude to find something in a document and you do not know where it is.
• The file is small (under 20 pages, or under 5,000 words).
• You are in a Project with persistent context, where the file sits on the shelf and is reused across sessions.

Paste a snippet when:

• You know exactly which paragraph or function is relevant.
• You are in a one-off chat and do not need the rest of the file.
• You want to reduce token cost on a high-volume workflow.

Walk-through

1. Find a PDF or Word document you have used recently — a report, a spec, a contract.
2. Drag it into the chat window. Confirm the thumbnail appears.
3. Ask: "Summarize the three most important decisions in this document."
4. Now open a second chat. Paste only the section you consider most important (a few paragraphs).
5. Ask the same question.
6. Compare the two responses. Note which one was more precise and why.

If you get stuck

• "The file attachment fails with an error" — Check the file format. Claude Desktop supports PDF, DOCX, TXT, PNG, JPG, CSV. Formats like XLSX, PPTX, or ZIP are not directly supported; convert first.
• "The response ignores part of my document" — Very long documents can cause the model to weight the beginning more heavily than the end. Break the document into sections and ask about each section separately.
• "I cannot drag the file into the chat on Windows" — Use the paperclip icon instead. The drag target is the input field at the bottom, not the message area above.

Cross-reference

For documents that are genuinely too long to fit in one message, "Long documents and chunking" covers the mental model for breaking them up without losing coherence.

Deliverable

You have attached a file and received a substantive response that drew from the document content — not from Claude's general knowledge. The success criterion: the response quotes or paraphrases something specific from the file that you did not include in your question.