# Diff Checker

**MCP Tool:** `diff_checker`  
**Tier:** Tier 2 — Differentiated  
**Category:** analyzers  
**Endpoint:** https://obfus.link/mcp  
**Price:** $0.015 / call  
**Verification:** ✓ TDD verified  

> Structural diff for text, JSON, YAML, and code

## Atomic Answer

Diff Checker compares two text inputs and outputs a unified, side-by-side, or JSON diff. Structural modes for JSON (by key path), YAML (normalized), and code (by AST declaration) ignore formatting noise so only semantic changes surface. Returns addition, deletion, and unchanged line counts plus a detailed structural change report ready for pipeline processing.

## Description

Compare two text inputs with unified, side-by-side, or JSON diff output. Structural modes diff JSON by key path (reordered keys = no diff), YAML after normalization, and code by AST declaration (reformatting = no diff).

## Agentic Reasoning

USE THIS WHEN: (1) You need to compare two JSON payloads semantically — reordered keys should not register as a diff. (2) You need to verify a code reformatting pass made no semantic changes — code-ast mode diffs by declaration, not by character. (3) You need machine-readable diff output (json format) to process programmatically in an agent pipeline rather than display to a human. DO NOT USE WHEN: inputs exceed 2000 lines — split into smaller chunks first. Do not use for binary file comparison — inputs must be valid UTF-8 text. OVER ALTERNATIVES: prefer this over running git diff (requires a git repo and shell access, not available in sandboxed agent environments) and over manual string comparison (no structural awareness, no stats, no JSON/YAML semantic equality).

## MCP Description

Compares textA and textB and returns a formatted diff plus semantic change metadata. Format options: unified (standard patch), side-by-side (columnar), json (structured array). Structural modes: json (key-path diff — reordered keys produce no diff), yaml (normalized before diff), code-ast (declaration-level diff — reformatting produces no diff). Returns: identical (bool), diff (string), stats (additions/deletions/unchanged), structuralChanges (array).

## Input Schema

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `textA` | `string` | yes | Original text |
| `textB` | `string` | yes | Modified text |
| `format` | `unified` \| `side-by-side` \| `json` | yes |  |
| `language` | `javascript` \| `typescript` \| `python` | no |  |
| `structuralMode` | `text` \| `json` \| `yaml` \| `code-ast` | no |  |
| `ignoreWhitespace` | `boolean` | yes | Ignore trailing whitespace differences |

## Output Schema

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `diff` | `string` | no |  |
| `stats` | `object` | no |  |
| `identical` | `boolean` | no |  |
| `structuralChanges` | `array` | no |  |

## How To Use

1. **Paste original text** — Enter the original (before) text into the Text A field.
2. **Paste modified text** — Enter the modified (after) text into the Text B field.
3. **Choose output format** — Select unified (patch format), side-by-side (columnar), or json (machine-readable).
4. **Select structural mode** — For JSON/YAML/code inputs, select the matching structural mode to diff semantically rather than textually.
5. **Run and review** — Click Compare or press Ctrl+Enter. Review the diff output and structural changes report.

## FAQs

**What does "structural diff" mean for JSON?**

In JSON mode, the diff compares by key path rather than by line position. Two JSON objects with the same keys and values but in different orders are considered identical. Only keys that are added, removed, or have changed values appear in structuralChanges.

**What does "code-ast" structural mode do?**

Code-AST mode extracts top-level function, class, and variable declarations from the source code and diffs at the declaration level. If you run Prettier or Black on the code and nothing changes semantically, the diff will show zero structural changes even if every line was reformatted.

**What is the maximum input size?**

Each text input supports up to 2,000 lines. Inputs exceeding this limit return a PAYLOAD_LIMIT error. For larger files, split them into sections before comparing.

**What do the addition/deletion/unchanged counts represent?**

These count lines: additions are lines present in B but not A, deletions are lines present in A but not B, and unchanged are lines present in both. The counts reflect line-level changes regardless of the structural mode selected.

**Can I use this tool via the MCP API?**

Yes. The tool is registered on the obfus.link MCP server at https://obfus.link/mcp. Call it from any MCP-compatible agent with a Shared Payment Token. The MCP tool name matches the snake_case slug shown in the integration snippet.

## Tags

`diff` · `compare` · `json` · `yaml` · `code` · `patch` · `unified` · `structural`

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*obfus.link — A Subether Labs Infrastructure Project*  
*Canonical URL: https://obfus.link/tool/diff-checker*  
*JSON view: https://obfus.link/tool/diff-checker/json*