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The Text Diff Checker compares two text inputs and highlights the differences between them. It’s essential for code reviews, content comparison, debugging changes, and generating patches.

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Use Cases

  • Code reviews - Compare original vs. modified code
  • Document versioning - Track changes between drafts
  • Configuration audits - Compare production vs. staging configs
  • Debugging - Identify unintended changes in output
  • Patch generation - Create unified diff patches for version control
  • API response comparison - Validate API changes between versions

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How It Works

The tool uses the diff library (same algorithm as Unix diff) to perform line-by-line comparison:
  • Left Input: Original text (“before”)
  • Right Input: Modified text (“after”)
  • Output: Color-coded diff showing additions, deletions, and unchanged lines
The tool compares text line-by-line, not character-by-character. For single-line changes, use a character-level diff tool or inspect the output carefully.

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Actions

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Default (Line Diff)

Shows a simplified diff with markers:
  • + prefix: Lines added in the right input
  • - prefix: Lines removed from the left input
  • (space) prefix: Unchanged lines

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Patch (Unified Diff)

Generates a unified diff patch compatible with git apply, patch, and version control systems: 
--- input
+++ input
@@ -1,3 +1,3 @@
 Line 1
-Old line
+New line
 Line 3

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Input Format

Left Input (Original): 
Hello World
This is line 2
Line 3 remains
Right Input (Modified): 
Hello World
This is line 2 (edited)
Line 3 remains
New line 4

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Output Format

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Line Diff (Default)

  Hello World
- This is line 2
+ This is line 2 (edited)
  Line 3 remains
+ New line 4

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Unified Patch

--- input
+++ input
@@ -1,3 +1,4 @@
 Hello World
-This is line 2
+This is line 2 (edited)
 Line 3 remains
+New line 4

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Examples

Left Input:
function greet(name) {
  console.log("Hello " + name);
}

Right Input:
function greet(name) {
  console.log(`Hello ${name}`);
}

Output (Line Diff):
  function greet(name) {
-   console.log("Hello " + name);
+   console.log(`Hello ${name}`);
  }

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Technical Details

Located in lib/tools/engine.ts:486-495
The tool uses the diff package (v8.0.3) with two functions:

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diffLines (Default Mode)

Performs line-by-line comparison and returns an array of change objects: 
import { diffLines } from 'diff';

const changes = diffLines(leftInput, rightInput);
// Each change has: { value: string, added?: boolean, removed?: boolean }

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createPatch (Patch Mode)

Generates a unified diff patch: 
import { createPatch } from 'diff';

const patch = createPatch('input', leftInput, rightInput);
// Returns a unified diff string compatible with `patch` command

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Performance

  • Algorithm: Myers’ diff algorithm (O(ND) complexity, where N is input size and D is edit distance)
  • Line-based comparison: Fast for large files with localized changes
  • Synchronous processing: All operations run client-side
Large inputs (10,000+ lines) may cause brief UI freezes. For massive files, consider chunking or using a dedicated diff service.

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Understanding Diff Output

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Hunk Headers (Patch Mode)

The @@ lines indicate line ranges: 
@@ -1,3 +1,4 @@
  • -1,3: Original text starts at line 1, spans 3 lines
  • +1,4: Modified text starts at line 1, spans 4 lines

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Context Lines

Both modes show 3 lines of context around changes by default (unchanged lines before/after the diff).

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Common Patterns

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Before/After Code Review

// Before
const sum = (a, b) => a + b;
const result = sum(2, 3);

// After
const sum = (a, b) => a + b;
const multiply = (a, b) => a * b;
const result = multiply(2, 3);

// Diff shows added `multiply` function and changed `sum` to `multiply` call

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Environment Config Comparison

# .env.development
DATABASE_URL=localhost
API_TIMEOUT=5000

# .env.production
DATABASE_URL=prod-db.example.com
API_TIMEOUT=30000
CACHE_TTL=3600

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Git Patch Application

  1. Generate patch with Patch action
  2. Save output to changes.patch
  3. Apply with git apply changes.patch or patch < changes.patch

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Comparison with Other Tools

FeatureText Diffgit diffList Compare
Line-by-line✅✅❌
Patch generation✅✅❌
Set operations❌❌✅
Fuzzy matching❌❌✅
Word-level diff❌✅ (—word-diff)❌
Use Text Diff for code and structured text. Use List Compare for comparing lists of items (emails, IDs, etc.) with set operations.

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