diff --git a/internal/apischema/openai/openai.go b/internal/apischema/openai/openai.go
index e2ede0993d..de1e21284f 100644
--- a/internal/apischema/openai/openai.go
+++ b/internal/apischema/openai/openai.go
@@ -869,6 +869,7 @@ type WebSearchLocation struct {
 type ThinkingUnion struct {
 	OfEnabled  *ThinkingEnabled  `json:",omitzero,inline"`
 	OfDisabled *ThinkingDisabled `json:",omitzero,inline"`
+	OfAdaptive *ThinkingAdaptive `json:",omitzero,inline"`
 }
 
 type ThinkingEnabled struct {
@@ -887,6 +888,10 @@ type ThinkingDisabled struct {
 	Type string `json:"type,"`
 }
 
+type ThinkingAdaptive struct {
+	Type string `json:"type,"`
+}
+
 // MarshalJSON implements the json.Marshaler interface for ThinkingUnion.
 func (t *ThinkingUnion) MarshalJSON() ([]byte, error) {
 	if t.OfEnabled != nil {
@@ -895,6 +900,9 @@ func (t *ThinkingUnion) MarshalJSON() ([]byte, error) {
 	if t.OfDisabled != nil {
 		return json.Marshal(t.OfDisabled)
 	}
+	if t.OfAdaptive != nil {
+		return json.Marshal(t.OfAdaptive)
+	}
 	// If both are nil, return an empty object or an error, depending on your desired behavior.
 	return []byte(`{}`), nil
 }
@@ -923,6 +931,12 @@ func (t *ThinkingUnion) UnmarshalJSON(data []byte) error {
 			return err
 		}
 		t.OfDisabled = &disabled
+	case "adaptive":
+		var adaptive ThinkingAdaptive
+		if err := json.Unmarshal(data, &adaptive); err != nil {
+			return err
+		}
+		t.OfAdaptive = &adaptive
 	default:
 		return fmt.Errorf("invalid thinking union type: %s", typeVal)
 	}
diff --git a/internal/apischema/openai/union_test.go b/internal/apischema/openai/union_test.go
index 83e8283687..1b37277ff0 100644
--- a/internal/apischema/openai/union_test.go
+++ b/internal/apischema/openai/union_test.go
@@ -9,6 +9,8 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/require"
+
+	"github.com/envoyproxy/ai-gateway/internal/json"
 )
 
 // TestUnmarshalJSONNestedUnion tests the completion API prompt parsing.
@@ -285,3 +287,110 @@ func TestUnmarshalJSONEmbeddingInput_Errors(t *testing.T) {
 		})
 	}
 }
+
+func TestThinkingUnion_UnmarshalJSON(t *testing.T) {
+	tests := []struct {
+		name   string
+		data   string
+		expect ThinkingUnion
+	}{
+		{
+			name: "enabled",
+			data: `{"type":"enabled","budget_tokens":1024}`,
+			expect: ThinkingUnion{
+				OfEnabled: &ThinkingEnabled{Type: "enabled", BudgetTokens: 1024},
+			},
+		},
+		{
+			name: "disabled",
+			data: `{"type":"disabled"}`,
+			expect: ThinkingUnion{
+				OfDisabled: &ThinkingDisabled{Type: "disabled"},
+			},
+		},
+		{
+			name: "adaptive",
+			data: `{"type":"adaptive"}`,
+			expect: ThinkingUnion{
+				OfAdaptive: &ThinkingAdaptive{Type: "adaptive"},
+			},
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			var got ThinkingUnion
+			err := json.Unmarshal([]byte(tc.data), &got)
+			require.NoError(t, err)
+			require.Equal(t, tc.expect, got)
+		})
+	}
+}
+
+func TestThinkingUnion_UnmarshalJSON_Errors(t *testing.T) {
+	tests := []struct {
+		name        string
+		data        string
+		expectedErr string
+	}{
+		{
+			name:        "missing type field",
+			data:        `{"budget_tokens":1024}`,
+			expectedErr: "thinking config does not have a type",
+		},
+		{
+			name:        "invalid type value",
+			data:        `{"type":"unknown"}`,
+			expectedErr: "invalid thinking union type: unknown",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			var got ThinkingUnion
+			err := json.Unmarshal([]byte(tc.data), &got)
+			require.Error(t, err)
+			require.Contains(t, err.Error(), tc.expectedErr)
+		})
+	}
+}
+
+func TestThinkingUnion_MarshalJSON(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  ThinkingUnion
+		expect string
+	}{
+		{
+			name: "enabled",
+			input: ThinkingUnion{
+				OfEnabled: &ThinkingEnabled{Type: "enabled", BudgetTokens: 1024},
+			},
+			expect: `{"budget_tokens":1024,"type":"enabled"}`,
+		},
+		{
+			name: "disabled",
+			input: ThinkingUnion{
+				OfDisabled: &ThinkingDisabled{Type: "disabled"},
+			},
+			expect: `{"type":"disabled"}`,
+		},
+		{
+			name: "adaptive",
+			input: ThinkingUnion{
+				OfAdaptive: &ThinkingAdaptive{Type: "adaptive"},
+			},
+			expect: `{"type":"adaptive"}`,
+		},
+		{
+			name:   "all nil returns empty object",
+			input:  ThinkingUnion{},
+			expect: `{}`,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := json.Marshal(&tc.input)
+			require.NoError(t, err)
+			require.JSONEq(t, tc.expect, string(got))
+		})
+	}
+}