fix: remove misleading hint from replace pos only description

[YLRong]

Problem

When an agent uses the replace operation with pos only (without end) but provides multi-line content in lines, the tool only replaces a single line. This causes subsequent lines from the original file to be retained, resulting in duplicate code.

Root Cause

The tool description contains a misleading hint: "(MOST COMMON for single-line edits)". This misleads agents into thinking that pos-only replace is the default/recommended approach, even when they need to replace multiple lines.

Concrete Example

Original file (3 lines):

export function cn(...inputs: string[]) {  // line 1
  return inputs.join(' ');                  // line 2
}                                          // line 3

Agent intends to replace with (4 lines):

export function cn(...inputs: string[]) {
  // test comment
  return inputs.join(' ');
}

Agent constructs:

{
  "op": "replace",
  "pos": "1#ZV",
  "lines": "export function cn(...inputs: string[]) {\n  // test comment\n  return inputs.join(' ');\n}"
}

What happens:

• Tool checks: no end specified → uses applySetLine() (single-line replacement)
• Tool executes: splice(line - 1, 1, ...replacement) — deletes 1 line, inserts 4 lines
• Result: Original lines 2-3 are retained!

Actual output (6 lines):

export function cn(...inputs: string[]) {  // new line 1
  // test comment                        // new line 2
  return inputs.join(' ');                // new line 3
}                                         // new line 4
  return inputs.join(' ');                // old line 2 (retained!)
}                                         // old line 3 (retained!)

Solution

Remove the misleading hint from the tool description. Agents should understand that:

• pos only → replaces exactly one line
• pos + end → replaces the entire range

Reproduction Steps

1. Create a file with 3 lines
2. Use the edit tool with pos-only replace, passing 4 lines in the lines parameter
3. Result: Lines 2-3 from the original file are retained, causing 6 lines of output

[github-actions]

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: This is a safe documentation-only change that clarifies tool behavior by removing a misleading hint, posing no risk of functional regression.}

[YLRong]

I have read the CLA Document and I hereby sign the CLA