Tested Claude Sonnet 5 and Fable 5 on two coding tasks.

One was a RAG Debugger added inside the 400K line Open WebUI repo. Other was a full browser based Clash Royale style game.

First, I asked both models to add a RAG Debugger to Open WebUI, which is a codebase with around 400K lines.

The debugger had to show:

* document processing status
* extracted text
* generated chunks
* chunk metadata
* retrieval results
* source details
* retrieval scores

Both models did a good job.

Fable built a debugger that worked across the full Knowledge base. Sonnet focused more on checking one document at a time.

The results:

* Fable 5: 25 minutes, $16.11, 687 lines added
* Sonnet 5: 56 minutes, $12.05, 847 lines added

Sonnet was cheaper here, but Fable was faster and I preferred its approach.

The second task was much bigger.

I asked both models to build a browser based Clash Royale style game with deck building, matchmaking, bot mode, towers, troops, energy, targeting, health, and win conditions.

Fable absolutely cooked.

It built the game in one run, added 31 tests, used Playwright to check the UI, and handled the game logic properly on the server.

Its result:

* 48 minutes
* $27.97
* 5,817 lines added
* 183.5K output tokens
* no major bugs in my testing

Sonnet also made a good looking game. The UI was probably better in some places.

But most of it was broken at first and needed several fixes.

Its result:

* 1 hour 33 minutes
* $33.85
* 7,056 lines added
* 404.9K output tokens
* multiple fixes needed

That is the part I do not get.

Sonnet is supposed to be the cheaper model, but on the harder task it:

* took longer
* wrote more code
* used more than twice the output tokens
* needed more fixes
* still cost more than Fable

Sonnet 5 is clearly better than Sonnet 4.6. It is good at frontend work and tool use.

But for long coding tasks, the cost got ugly fast.

Cheaper per token does not always mean cheaper per completed task.

Complete benchmarks here: [Claude Sonnet 5 vs. Claude Fable 5](https://www.unsiloed.ai/blog/guides/claude-sonnet-5-vs-claude-fable-5-coding-comparison)

Anyone else seeing the same token burn with Sonnet 5?

Sonnet 5 was supposed to be cheaper. It cost me more than Fable 5