diff --git a/backend/apps/agents/agent_manager.py b/backend/apps/agents/agent_manager.py
index 16551a02..6db1abcb 100644
--- a/backend/apps/agents/agent_manager.py
+++ b/backend/apps/agents/agent_manager.py
@@ -1759,7 +1759,10 @@ class AgentManager:
                     level = "off"
                 if api_type == "anthropic":
                     if level == "off":
-                        options_kwargs["thinking"] = {"type": "disabled"}
+                        # Fable 5 400s on an explicit thinking:disabled; you turn it
+                        # off there by omitting the param (off is Fable's default).
+                        if not (isinstance(resolved_model, str) and "fable" in resolved_model):
+                            options_kwargs["thinking"] = {"type": "disabled"}
                     elif level in ("low", "medium", "high"):
                         options_kwargs["effort"] = level
             except Exception as e:
diff --git a/backend/apps/agents/providers/thinking.py b/backend/apps/agents/providers/thinking.py
index f7a4b09f..48f24b25 100644
--- a/backend/apps/agents/providers/thinking.py
+++ b/backend/apps/agents/providers/thinking.py
@@ -20,6 +20,10 @@ def thinking_params_for(api: str, level: str, model_id: str = "") -> dict | None
 
     if level == "off":
         if api == "anthropic":
+            # Fable 5 400s on an explicit thinking:disabled; omit the param to
+            # turn thinking off (off is its default). Other Claude models accept it.
+            if "fable" in model_id:
+                return None
             return {"thinking": {"type": "disabled"}}
         if api == "codex":
             return {"reasoning": {"effort": "none"}}