fix(agent): let config enabled = false override per-agent reasoning settings (#2994)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: tusharmath

crates/forge_app/src/agent.rs

@@ -145,6 +145,8 @@ impl AgentExt for Agent {
 
         // Apply workflow reasoning configuration to agents.
         // Agent-level fields take priority; config fills in any unset fields.
+        // Exception: config `enabled = false` always wins — it is an explicit
+        // global disable that must override any per-agent setting.
         if let Some(ref config_reasoning) = config.reasoning {
             use forge_config::Effort as ConfigEffort;
             let config_as_domain = ReasoningConfig {
@@ -164,6 +166,11 @@ impl AgentExt for Agent {
             // Start from the agent's own settings and fill unset fields from config.
             let mut merged = agent.reasoning.clone().unwrap_or_default();
             merged.merge(config_as_domain);
+            // If the config explicitly disables reasoning, honour that override
+            // regardless of what the agent definition says.
+            if config_reasoning.enabled == Some(false) {
+                merged.enabled = Some(false);
+            }
             agent.reasoning = Some(merged);
         }
 
@@ -233,4 +240,28 @@ mod tests {
 
         assert_eq!(actual, expected);
     }
+
+    /// When config sets `enabled = false`, it must override the agent's
+    /// `enabled = true`. This prevents reasoning parameters from being sent to
+    /// models that don't support them (e.g. claude-haiku with effort set).
+    #[test]
+    fn test_config_disabled_overrides_agent_enabled() {
+        let config = ForgeConfig::default().reasoning(
+            ConfigReasoningConfig::default()
+                .enabled(false)
+                .effort(ConfigEffort::None),
+        );
+
+        // Agent has reasoning explicitly enabled.
+        let agent = fixture_agent().reasoning(
+            ReasoningConfig::default()
+                .enabled(true)
+                .effort(Effort::High),
+        );
+
+        let actual = agent.apply_config(&config).reasoning;
+
+        // enabled must be false even though the agent said true.
+        assert_eq!(actual.as_ref().and_then(|r| r.enabled), Some(false));
+    }
 }