fix: keep a static empty override from reading caller arguments or the saved effort

backend/windmill-worker/src/ai_executor.rs

@@ -281,12 +281,15 @@ fn override_provider_model(
                     InputTransform::Javascript { .. } | InputTransform::Static { .. }
                 )
             })
-            .map(|_| {
-                local_args
+            .map(|t| match t {
+                // A static null is not evaluated for a tool, which leaves the caller's argument
+                // in its place.
+                InputTransform::Static { value } if value.get() == "null" => String::new(),
+                _ => local_args
                     .get(key)
                     .and_then(|v| serde_json::from_str::<String>(v.get()).ok())
                     .map(|s| s.trim().to_string())
-                    .unwrap_or_default()
+                    .unwrap_or_default(),
             })
     };
     if let Some(model) = authored("model").filter(|m| !m.is_empty()) {
@@ -3015,6 +3018,12 @@ mod tests {
         let sent = serde_json::from_value(json!({ "model": "other" })).unwrap();
         override_provider_model(&mut provider, &sent, &HashMap::new());
         assert_eq!(provider.model, "saved-model");
+        let null = HashMap::from([(
+            "model".to_string(),
+            InputTransform::Static { value: to_raw_value(&serde_json::Value::Null) },
+        )]);
+        override_provider_model(&mut provider, &sent, &null);
+        assert_eq!(provider.model, "saved-model");
 
         assert_eq!(run(json!({})), saved);
         assert_eq!(run(json!({ "model": "" })), saved);

frontend/src/lib/components/flows/conversations/agentChatInputs.test.ts

@@ -541,6 +541,11 @@ describe('a step linked to a saved agent', () => {
 			saved
 		)!
 		expect(wiredFieldValue(wiring, {}, 'reasoning_effort')).toBe('high')
+		const none = resolveAgentModelWiring(
+			[linked({ reasoning_effort: { type: 'static', value: '' } })],
+			saved
+		)!
+		expect(wiredFieldValue(none, {}, 'reasoning_effort')).toBe('')
 		expect(
 			wiredFieldValue(wiring, { model: 'claude-haiku-4-5' }, 'reasoning_effort')
 		).toBeUndefined()

frontend/src/lib/components/flows/conversations/agentChatInputs.ts

@@ -126,7 +126,8 @@ function parseLinkedStep(
 		if (!transform) continue
 		if (transform.type === 'static') {
 			const value = typeof transform.value === 'string' ? transform.value.trim() : ''
-			if (value) fixed[field] = value
+			// An empty model keeps the saved one; an empty effort is the step's choice of none.
+			if (value || field === 'reasoning_effort') fixed[field] = value
 			continue
 		}
 		if (transform.type !== 'javascript') return undefined
@@ -150,10 +151,10 @@ export function wiredFieldValue(
 	if (wiring.whole) return values[wiring.whole]?.[field]
 	const name = wiring.fields[field]
 	const own = name ? values[name] : wiring.fixed[field]
-	if (!wiring.saved || (own !== undefined && own !== '')) return own
-	if (field === 'model') return wiring.saved.model
+	if (!wiring.saved) return own
+	if (field === 'model') return own || wiring.saved.model
 	// An effort the step overrides is the run's whatever it holds: empty there is no effort.
-	if (field === 'reasoning_effort' && !name) {
+	if (field === 'reasoning_effort' && !name && own === undefined) {
 		return wiredFieldValue(wiring, values, 'model') === wiring.saved.model
 			? wiring.saved.reasoning_effort
 			: own