diff --git a/pkg/providers/anthropic_messages/provider.go b/pkg/providers/anthropic_messages/provider.go
index 6ef74a5b1..ba00c9c89 100644
--- a/pkg/providers/anthropic_messages/provider.go
+++ b/pkg/providers/anthropic_messages/provider.go
@@ -140,15 +140,16 @@ func buildRequestBody(
 	model string,
 	options map[string]any,
 ) (map[string]any, error) {
-	result := map[string]any{
-		"model":      model,
-		"max_tokens": int64(4096),
-		"messages":   []any{},
+	// max_tokens is required and guaranteed by agent loop
+	maxTokens, ok := asInt(options["max_tokens"])
+	if !ok {
+		return nil, fmt.Errorf("max_tokens is required in options")
 	}
 
-	// Set max_tokens from options
-	if mt, ok := asInt(options["max_tokens"]); ok {
-		result["max_tokens"] = int64(mt)
+	result := map[string]any{
+		"model":      model,
+		"max_tokens": int64(maxTokens),
+		"messages":   []any{},
 	}
 
 	// Set temperature from options
diff --git a/pkg/providers/anthropic_messages/provider_test.go b/pkg/providers/anthropic_messages/provider_test.go
index 3fa062360..cea8e1e6c 100644
--- a/pkg/providers/anthropic_messages/provider_test.go
+++ b/pkg/providers/anthropic_messages/provider_test.go
@@ -27,11 +27,13 @@ func TestBuildRequestBody(t *testing.T) {
 			messages: []Message{
 				{Role: "user", Content: "Hello, world!"},
 			},
-			model:   "test-model",
-			options: map[string]any{},
+			model: "test-model",
+			options: map[string]any{
+				"max_tokens": 8192,
+			},
 			want: map[string]any{
 				"model":      "test-model",
-				"max_tokens": int64(4096),
+				"max_tokens": int64(8192),
 				"messages": []any{
 					map[string]any{
 						"role":    "user",
@@ -46,11 +48,13 @@ func TestBuildRequestBody(t *testing.T) {
 				{Role: "user", Content: "What is 2+2?"},
 				{Role: "assistant", Content: "4"},
 			},
-			model:   "test-model",
-			options: map[string]any{},
+			model: "test-model",
+			options: map[string]any{
+				"max_tokens": 8192,
+			},
 			want: map[string]any{
 				"model":      "test-model",
-				"max_tokens": int64(4096),
+				"max_tokens": int64(8192),
 				"messages": []any{
 					map[string]any{
 						"role":    "user",
@@ -74,11 +78,13 @@ func TestBuildRequestBody(t *testing.T) {
 				{Role: "system", Content: "You are a helpful assistant."},
 				{Role: "user", Content: "Hello"},
 			},
-			model:   "test-model",
-			options: map[string]any{},
+			model: "test-model",
+			options: map[string]any{
+				"max_tokens": 8192,
+			},
 			want: map[string]any{
 				"model":      "test-model",
-				"max_tokens": int64(4096),
+				"max_tokens": int64(8192),
 				"system":     "You are a helpful assistant.",
 				"messages": []any{
 					map[string]any{
@@ -110,6 +116,16 @@ func TestBuildRequestBody(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "missing max_tokens returns error",
+			messages: []Message{
+				{Role: "user", Content: "Test"},
+			},
+			model:   "test-model",
+			options: map[string]any{},
+			want:    nil,
+			wantErr: true,
+		},
 		{
 			name: "with tools",
 			messages: []Message{
@@ -132,11 +148,13 @@ func TestBuildRequestBody(t *testing.T) {
 					},
 				},
 			},
-			model:   "test-model",
-			options: map[string]any{},
+			model: "test-model",
+			options: map[string]any{
+				"max_tokens": 8192,
+			},
 			want: map[string]any{
 				"model":      "test-model",
-				"max_tokens": int64(4096),
+				"max_tokens": int64(8192),
 				"messages": []any{
 					map[string]any{
 						"role":    "user",