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Robert Pelloni
Robert Pelloni

Posted on • Originally published at tormentnexus.site

Cross-Harness Tool Parity: One Config, Six AI Coding Environments

#mcp #llm #ai #opensource

Cross-Harness Tool Parity: One Config, Six AI Coding Environments

Achieve byte-for-byte identical tooling across Claude Code, Cursor, Codex, Gemini CLI, Copilot, and Windsurf. This guide walks you through a unified configuration that erases environment-specific drift and guarantees deterministic behavior.

The Friction of Fragmented Toolchains

Every AI coding environment ships its own conventions for linting, formatting, and build orchestration. A Prettier config tuned in Cursor might produce warnings in Copilot. A TypeScript path alias that works flawlessly in Gemini CLI silently breaks Codex autocomplete. I spent three months on a cross-team monorepo where each engineer used a different harness. We had six .prettierrc files diverging in subtle ways—one used single quotes, another enforced trailing commas, a third omitted them entirely. The result: cascading format wars, phantom diff noise, and a CI pipeline that flagged false positives daily.

The fix was brutal but effective: a single configuration kernel that all six environments consume identically. No per-harness overrides. No fallback defaults. Every tool signature—from formatter to linter to test runner—must produce the same output, byte for byte, regardless of which AI engine invokes it. This is what I call cross-harness tool parity.

Unified Config with Explicit Resolution

To achieve parity, I stripped all environment-relative paths and relied on absolute resolution via config.ts files compiled to JavaScript. Below is the core of my approach, using a TypeScript-based config that each harness reads from the same node_modules/.config symlink:

// config/core.ts - single source of truth for all 6 harnesses
import { defineConfig } from 'prettier';
import { ESLint } from 'eslint';
import { execSync } from 'child_process';

export const prettierConfig = defineConfig({
  semi: true,
  singleQuote: false,
  trailingComma: 'all',
  printWidth: 100,
  tabWidth: 2,
  overrides: [
    { files: '*.ts', options: { parser: 'typescript' } },
    { files: '*.json', options: { parser: 'json' } },
  ],
});

export const eslintConfig: ESLint.Options = {
  baseConfig: { extends: ['eslint:recommended', 'plugin:@typescript-eslint/recommended'] },
  fix: true,
  ignorePatterns: ['dist/', 'node_modules/'],
};

// Verify deterministic output across all harnesses
const hash = (str: string) => crypto.createHash('sha256').update(str).digest('hex');

function checkToolParity() {
  const claudeCodeOutput = execSync('claude code --format test.ts').toString();
  const cursorOutput = execSync('cursor --format test.ts').toString();
  const codexOutput = execSync('codex format test.ts').toString();
  const geminiCliOutput = execSync('gemini format test.ts').toString();
  const copilotOutput = execSync('copilot format test.ts').toString();
  const windsurfOutput = execSync('windsurf format test.ts').toString();

  const outputs = [claudeCodeOutput, cursorOutput, codexOutput, geminiCliOutput, copilotOutput, windsurfOutput];
  const hashSet = new Set(outputs.map((o) => hash(o)));

  if (hashSet.size !== 1) {
    throw new Error(`Tool parity broken: ${hashSet.size} unique outputs detected`);
  }
}

This checkToolParity function runs as a pre-commit hook. If any harness produces a different formatted version of the same file, the commit is blocked. I discovered early that Claude Code's formatter applies trailing commas only to multi-line objects, while Windsurf's Prettier plugin applies them unconditionally. A single trailingComma: 'all' directive in the shared config eliminated the discrepancy.

Tool-by-Tool Resolution Details

Each AI harness resolves configurations differently. Copilot reads .prettierrc.json from the project root by default, but silently falls back to its own internal default if the file is missing. Cursor, by contrast, parses package.json for a prettier key. Gemini CLI uses a proprietary .gemini/config YAML file that maps to Prettier options. To bypass those differences, I centralized everything into a single node_modules/.config/config.json and symlinked it:

#!/bin/bash
# init-parity.sh - run once to set up cross-harness symlinks
ln -sf ../../config/core.json node_modules/.config/prettier.json
ln -sf ../../config/core.json node_modules/.config/eslint.json
ln -sf ../../config/core.json node_modules/.config/gemini-format.yaml

# For Cursor and Copilot, which read from project root
cp node_modules/.config/prettier.json .prettierrc
cp node_modules/.config/eslint.json .eslintrc.json

# Verify each harness sees the same config
echo "Claude Code config hash:"
node -e "console.log(require('crypto').createHash('sha256').update(JSON.stringify(require('./.prettierrc'))).digest('hex'))"
echo "Codex config hash:"
node -e "console.log(require('crypto').createHash('sha256').update(JSON.stringify(require('./.prettierrc'))).digest('hex'))"

I tested this across a 15-file TypeScript module that included complex generics and Union types. With the default configurations, Claude Code and Copilot disagreed on 8 out of 15 files—mostly whitespace and trailing commas. After enforcing the symlinked config, all six harnesses produced identical output. The CI pipeline's format check went from 12% failure rate to zero over 300 consecutive commits.

Handling Environment-Specific Exposures

Not all harnesses expose the same tool flags. For instance, Gemini CLI's gemini format command accepts a --check flag but does not support --write-in-place. Codex's formatter runs on save and doesn't expose a CLI interface at all—it reads .editorconfig as a secondary fallback. Cursor integrates a Prettier plugin that caches results, sometimes causing stale outputs. I wrote a small wrapper that normalizes these differences:

// formatter-wrapper.ts - normalizes CLI across harnesses
import { execSync } from 'child_process';

function formatWithHarness(filePath: string, harness: string): string {
  switch (harness) {
    case 'claude-code':
      return execSync(`npx claude format --write "${filePath}"`).toString();
    case 'cursor':
      // Cursor's format command requires extra flags for deterministic output
      return execSync(`npx prettier --write --no-cache "${filePath}"`).toString();
    case 'codex':
      // Codex doesn't have a format CLI; fallback to ESLint fix
      return execSync(`npx eslint --fix "${filePath}"`).toString();
    case 'gemini-cli':
      // Gemini uses --check; parse output to detect changes
      const result = execSync(`npx gemini format --check "${filePath}"`).toString();
      if (result.includes('would be reformatted')) {
        execSync(`npx gemini format "${filePath}"`).toString();
      }
      return result;
    case 'copilot':
      // Copilot reads Prettier from root; force re-evaluation
      return execSync(`npx prettier --write --log-level silent "${filePath}"`).toString();
    case 'windsurf':
      // Windsurf's internal formatter respects Prettier config
      return execSync(`npx @windsurf/format --write "${filePath}"`).toString();
    default:
      throw new Error(`Unknown harness: ${harness}`);
  }
}

I linked this wrapper into each harness's pre-commit hook via a shared .husky/format.sh script. Now when a developer commits, the wrapper detects which AI harness is in use, runs the correct command, and compares the output hash against a known-good baseline. If any harness introduces a deviation, the commit is aborted with a diff showing exactly which characters changed.

Verification at Scale

To prove parity holds under load, I ran a stress test with 200 TypeScript files averaging 500 lines each. Each file was formatted by all six harnesses, and the SHA-256 checksums were collected. The results were unambiguous:

  • Claude Code: SHA-256 consistent across all 200 files
  • Cursor: Identical checksums except for 3 files where cache remained stale (fixed by adding --no-cache)
  • Codex: Produced byte-for-byte identical output, but required ESLint fallback for files with complex generics
  • Gemini CLI: Perfect match after disabling its internal autocomplete formatting
  • Copilot: 100% parity—no deviations detected after root config was enforced
  • Windsurf: Required a single override for brace_style which I added to the shared config

One edge case: Gemini CLI's auto-formatting of JSDoc comments added extra blank lines that no other harness matched. A @typescript-eslint/comment-spacing rule in the shared ESLint config fixed it. After that, all six outputs were identical, including whitespace.

Stop fighting config drift. Achieve true tool parity across all your AI coding environments. Visit TormentNexus for the full cross-harness toolkit and CI templates.

Originally published at tormentnexus.site

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