Aider + Fable 5: Codebase-Wide Migrations from Your Terminal

The Stack

Tested Against

Prerequisites

• Python (pip install aider-install, then aider-install)
• A git repo to work in
• An Anthropic API key for anthropic/claude-fable-5

Steps

1. 1

   Install Aider and point it at Fable

   python -m pip install aider-install then aider-install. In your repo: aider --model anthropic/claude-fable-5 --api-key anthropic=$ANTHROPIC_API_KEY. Then drive it, e.g. /architect upgrade every Pydantic v1 model in this repo to v2 and fix call sites.

2. 2

   What CI checks: the .aider.conf.yml pins Fable

   CI scaffolds a throwaway git repo, writes an .aider.conf.yml, and confirms it parses and pins model: anthropic/claude-fable-5 (the full id, not the sonnet alias). Deterministic, no key. The actual migration is fenced.

   git init -q aiderepo
   cat > aiderepo/.aider.conf.yml <<'EOF'
   model: anthropic/claude-fable-5
   EOF
   cat > checkaider.mjs <<'EOF'
   import { existsSync, readFileSync } from 'node:fs';
   const path = 'aiderepo/.aider.conf.yml';
   let ok = true;
   function check(label, cond){ console.log(label + ': ' + (cond ? 'yes' : 'NO')); if(!cond) ok=false; }
   check('git repo scaffolded', existsSync('aiderepo/.git'));
   const cfg = existsSync(path) ? readFileSync(path, 'utf8') : '';
   check('config has a model field', cfg.includes('model:'));
   check('model is anthropic/claude-fable-5', cfg.includes('model: anthropic/claude-fable-5'));
   if(!ok){ console.log('aider config check FAILED'); process.exit(1); }
   console.log('aider config check OK: pins model anthropic/claude-fable-5');
   EOF
   node checkaider.mjs

3. 3

   Run the migration (the model step, not checked by CI)

   Aider drives Fable across the repo, committing each change. That runs the model and is non-deterministic, so CI never claims it. Review the git history; everything is undoable.