[Bug]: Can't use model without _effort_ support.

[pcastellazzi]

Bug Description

Claude Haiku 4.5 fails with "This model does not support the effort parameter".

Steps to Reproduce

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

Model: claude-haiku-4-5-20251001

# .forge.toml
[reasoning]
enabled = false
effort = "none"

:forge Hello

Expected Behavior

It should say Hello back (or something similar) without producing any errors.

Actual Behavior

● [02:00:33] ERROR: POST https://api.anthropic.com/v1/messages
    Caused by: 400 Bad Request Reason: {"type":"error","error":{"type":"invalid_request_error","message":"This model does not support the effort parameter."},"request_id":"req_xxxxx"}

Forge Version

2.10.0

Operating System & Version

Debian Trixie

AI Provider

Anthropic

Model

claude-haiku-4-5-20251001

Installation Method

Other

Configuration

# .forge.toml
[reasoning]
enabled = false
effort = "none"

[pcastellazzi]

Some extra info:

I tested reasoning.enable and reasoning.effort separately; however, the outcomes were identical. My expectation was that the effort parameter would remain unset in the request if reasoning.effort was set to false, but it appears to be included regardless.

[tivris]

Found the root cause -- it's two bugs interacting:

1. The reasoning config merge uses the agent as base with overwrite_none, so the forge agent's hardcoded enabled: true silently overrides whatever the user sets in .forge.toml. Setting enabled = false has no effect.

2. Effort::None (documented as "skip reasoning entirely") gets mapped to OutputEffort::Low in the Anthropic request builder, which actively enables reasoning instead of skipping it.

Fix in #2941.