diff --git a/pkg/config/latest/types.go b/pkg/config/latest/types.go
index 679a73a70..642d01211 100644
--- a/pkg/config/latest/types.go
+++ b/pkg/config/latest/types.go
@@ -683,6 +683,20 @@ type ThinkingBudget struct {
 	Tokens int `json:"tokens,omitempty"`
 }
 
+// validThinkingEfforts lists all accepted string values for thinking_budget.
+const validThinkingEfforts = "none, minimal, low, medium, high, max, adaptive"
+
+// isValidThinkingEffort reports whether s (case-insensitive, trimmed) is a
+// recognised thinking_budget effort level.
+func isValidThinkingEffort(s string) bool {
+	switch strings.ToLower(strings.TrimSpace(s)) {
+	case "none", "minimal", "low", "medium", "high", "max", "adaptive":
+		return true
+	default:
+		return false
+	}
+}
+
 func (t *ThinkingBudget) UnmarshalYAML(unmarshal func(any) error) error {
 	// Try integer tokens first
 	var n int
@@ -694,6 +708,9 @@ func (t *ThinkingBudget) UnmarshalYAML(unmarshal func(any) error) error {
 	// Try string level
 	var s string
 	if err := unmarshal(&s); err == nil {
+		if !isValidThinkingEffort(s) {
+			return fmt.Errorf("invalid thinking_budget effort %q: must be one of %s", s, validThinkingEfforts)
+		}
 		*t = ThinkingBudget{Effort: s}
 		return nil
 	}
@@ -793,6 +810,9 @@ func (t *ThinkingBudget) UnmarshalJSON(data []byte) error {
 	// Try string level
 	var s string
 	if err := json.Unmarshal(data, &s); err == nil {
+		if !isValidThinkingEffort(s) {
+			return fmt.Errorf("invalid thinking_budget effort %q: must be one of %s", s, validThinkingEfforts)
+		}
 		*t = ThinkingBudget{Effort: s}
 		return nil
 	}
diff --git a/pkg/config/latest/types_test.go b/pkg/config/latest/types_test.go
index 6aede16cb..c69e87100 100644
--- a/pkg/config/latest/types_test.go
+++ b/pkg/config/latest/types_test.go
@@ -1,6 +1,7 @@
 package latest
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/goccy/go-yaml"
@@ -144,6 +145,32 @@ func TestThinkingBudget_IsDisabled(t *testing.T) {
 	}
 }
 
+func TestThinkingBudget_UnmarshalYAML_InvalidEffort(t *testing.T) {
+	t.Parallel()
+
+	input := []byte(`thinking_budget: adaptative`)
+	var config struct {
+		ThinkingBudget *ThinkingBudget `yaml:"thinking_budget"`
+	}
+
+	err := yaml.Unmarshal(input, &config)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), `invalid thinking_budget effort "adaptative"`)
+}
+
+func TestThinkingBudget_UnmarshalJSON_InvalidEffort(t *testing.T) {
+	t.Parallel()
+
+	data := []byte(`{"thinking_budget": "adaptative"}`)
+	var config struct {
+		ThinkingBudget *ThinkingBudget `json:"thinking_budget"`
+	}
+
+	err := json.Unmarshal(data, &config)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), `invalid thinking_budget effort "adaptative"`)
+}
+
 func TestThinkingBudget_IsAdaptive(t *testing.T) {
 	t.Parallel()