[Bug]: Anthropic Fable new finish reason #3624

Bug Description

When running with Fable, it often returns the following error:

 ERROR: Empty completion received - no content, tool calls, or valid finish reason

I'm guessing it's due to Fable returning refusal for some tasks based on it's safety mechanisms

vs

https://platform.claude.com/docs/en/build-with-claude/handling-stop-reasons#refusal

It will apparently return a 200 but with a refusal stop reason and it seems to be up to the harness to retry with a different model.

Steps to Reproduce

Hard to reproduce, but using fable for security related tasks seems to trigger occasional refusal.

Expected Behavior

I believe from the docs the harness is responsible for recognizing the new stop reason and retrying.

Actual Behavior

forge stops responding and retrys, receiving the same error message over and over.

Forge Version

2.13.15

Operating System & Version

macOS

AI Provider

Anthropic

Model

fable-5

Installation Method

curl -fsSL https://forgecode.dev/cli | sh

Configuration