diff --git a/providers/anthropic/anthropic.go b/providers/anthropic/anthropic.go
index b1bcee24cd40d6751c73dab9317e6c30c97037ed..25ac81dab38230debc3c5ad6fb5d6cb933665a30 100644
--- a/providers/anthropic/anthropic.go
+++ b/providers/anthropic/anthropic.go
@@ -638,7 +638,10 @@ func (a languageModel) toTools(tools []fantasy.Tool, toolChoice *fantasy.ToolCho
 			},
 		}
 	case fantasy.ToolChoiceNone:
-		return anthropicTools, anthropicToolChoice, warnings
+		none := anthropic.NewToolChoiceNoneParam()
+		anthropicToolChoice = &anthropic.ToolChoiceUnionParam{
+			OfNone: &none,
+		}
 	default:
 		anthropicToolChoice = &anthropic.ToolChoiceUnionParam{
 			OfTool: &anthropic.ToolChoiceToolParam{
diff --git a/providers/anthropic/anthropic_test.go b/providers/anthropic/anthropic_test.go
index 51adb7c333110ed1c3a490cce32312ef05df8be9..5f137a78be7a0b2f6fcfbad8403474ac5ece376a 100644
--- a/providers/anthropic/anthropic_test.go
+++ b/providers/anthropic/anthropic_test.go
@@ -1333,3 +1333,34 @@ func TestStream_WebSearchResponse(t *testing.T) {
 	require.NotEmpty(t, textDeltas, "should have text deltas")
 	require.Equal(t, "Here are the results.", textDeltas[0].Delta)
 }
+
+func TestGenerate_ToolChoiceNone(t *testing.T) {
+	t.Parallel()
+
+	server, calls := newAnthropicJSONServer(mockAnthropicGenerateResponse())
+	defer server.Close()
+
+	provider, err := New(
+		WithAPIKey("test-api-key"),
+		WithBaseURL(server.URL),
+	)
+	require.NoError(t, err)
+
+	model, err := provider.LanguageModel(context.Background(), "claude-sonnet-4-20250514")
+	require.NoError(t, err)
+
+	toolChoiceNone := fantasy.ToolChoiceNone
+	_, err = model.Generate(context.Background(), fantasy.Call{
+		Prompt: testPrompt(),
+		Tools: []fantasy.Tool{
+			WebSearchTool(nil),
+		},
+		ToolChoice: &toolChoiceNone,
+	})
+	require.NoError(t, err)
+
+	call := awaitAnthropicCall(t, calls)
+	toolChoice, ok := call.body["tool_choice"].(map[string]any)
+	require.True(t, ok, "request body should have tool_choice")
+	require.Equal(t, "none", toolChoice["type"], "tool_choice should be 'none'")
+}