[BUG] Pasting multi-line markdown into CLI silently drops/corrupts text chunks

Preflight

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When pasting markdown text (30+ lines) into the Claude Code CLI prompt, chunks of text are silently dropped or corrupted. The corruption is deterministic — pasting the same content multiple times produces the same corruption in the same locations. However, different source texts produce corruption at different positions, suggesting it's position/length-dependent rather than tied to specific characters.

What Should Happen?

Text pasted into the CLI should arrive character-for-character identical to what was copied to the clipboard. We verified via pbcopy/pbpaste that the clipboard contents are correct — the corruption happens between the paste event and what Claude receives as input.

Steps to Reproduce

1. Save this file as /tmp/test-paste.md:

You are a helpful assistant reviewing a shopping list. Be thorough.

**FORMAT:** Each item needs a name and quantity.

**RULES:** Group similar items together. If an item spans two categories, split it.

**Per item:**
- **What:** The item to buy.
- **Quantity:** How many to get
- **Notes:** Any special instructions, isn't decided, or needs checking at store

**My starting draft (challenge as needed):**

1. Bread — whole wheat, sliced
2. Milk — 2%, one gallon
3. Eggs — free range, one dozen
4. Chicken — boneless skinless breasts, about two pounds
5. Rice — basmati, five pound bag
6. Onions — yellow, about three pounds
7. Tomatoes — roma, one pound
8. Olive Oil — extra virgin, one bottle
9. Garlic — one bulb (or pre-minced jar if available)
10. Pasta — penne, one box
11. Cheese — sharp cheddar block
12. Apples — honeycrisp or fuji, about six (e.g., new seasonal variety at farmers market creates better option)
13. Bananas — one bunch
14. Greek Yogurt

**Work interactively:**
0. Challenge the list first — is anything missing or wrong?
1. Suggest better alternatives if any exist
2. Group items by store section
3. Present items for review — don't finalize without input

1. Open the file in an editor, Select All, Copy.
2. Paste into the Claude Code CLI prompt and send.
3. Ask Claude to compare what it received against the source file.

Observed corruption with this file:

• Source line: 0. Challenge the list first — is anything missing or wrong?
• Received by Claude: 0. is anything missing or wrong?
• The text Challenge the list first — was silently dropped.

Additional reproduction notes:

We also tested with a longer (~39 line) markdown document. That document had TWO corruption sites per paste, and the corruption was identical across multiple pastes of the same content:

• Corruption 1: or needs POC/research arrived as ords POC/research
• Corruption 2: A line ending with a two-word phrase, followed by a blank line, followed by **Work interactively:** was collapsed into a garbled mix of the phrase ending and the bold header: [corrupted] interactively:**

What did NOT reproduce: A short (11-line) markdown file with similar structure (numbered list followed by **bold header:**) pasted without corruption. This suggests the bug is length/position-dependent — short pastes survive, longer pastes get corrupted.

Error Messages/Logs

No error messages — the corruption is silent. Claude receives mangled input with no indication that anything went wrong.

Claude Model

Opus

Is this a regression?

I don't know

Claude Code Version

2.1.37 (Claude Code)

Platform

Anthropic API

Operating System

macOS 26.2 (Tahoe), Apple Silicon (arm64)

Terminal/Shell

Cursor IDE integrated terminal (reports as vscode v2.4.28 via $TERM_PROGRAM), zsh

Additional Information

• Node.js v24.4.1
• The clipboard itself is not corrupted — verified by cat file | pbcopy && pbpaste which round-trips perfectly.
• The corruption is deterministic: same source text → same corruption locations → same mangled output, across multiple paste attempts.
• Shorter documents (~11 lines) with the same structural patterns paste correctly. The bug appears to require a minimum text length to trigger.

---
This issue was authored by Claude Code (Opus 4.6) during a debugging session with the user.