template: fix args-as-json rendering (#13636)

In #13525, I accidentally broke templates' ability to automatically
render tool call function arguments as JSON.

We do need these to be proper maps because we need templates to be able
to call range, which can't be done on custom types.

template/template.go

@@ -381,6 +381,28 @@ func (t templateTools) String() string {
 	return string(bts)
 }
 
+// templateArgs is a map type with JSON string output for templates.
+type templateArgs map[string]any
+
+func (t templateArgs) String() string {
+	if t == nil {
+		return "{}"
+	}
+	bts, _ := json.Marshal(t)
+	return string(bts)
+}
+
+// templateProperties is a map type with JSON string output for templates.
+type templateProperties map[string]api.ToolProperty
+
+func (t templateProperties) String() string {
+	if t == nil {
+		return "{}"
+	}
+	bts, _ := json.Marshal(t)
+	return string(bts)
+}
+
 // templateTool is a template-compatible representation of api.Tool
 // with Properties as a regular map for template ranging.
 type templateTool struct {
@@ -396,11 +418,11 @@ type templateToolFunction struct {
 }
 
 type templateToolFunctionParameters struct {
-	Type       string                      `json:"type"`
-	Defs       any                         `json:"$defs,omitempty"`
-	Items      any                         `json:"items,omitempty"`
-	Required   []string                    `json:"required,omitempty"`
-	Properties map[string]api.ToolProperty `json:"properties"`
+	Type       string             `json:"type"`
+	Defs       any                `json:"$defs,omitempty"`
+	Items      any                `json:"items,omitempty"`
+	Required   []string           `json:"required,omitempty"`
+	Properties templateProperties `json:"properties"`
 }
 
 // templateToolCall is a template-compatible representation of api.ToolCall
@@ -413,7 +435,7 @@ type templateToolCall struct {
 type templateToolCallFunction struct {
 	Index     int
 	Name      string
-	Arguments map[string]any
+	Arguments templateArgs
 }
 
 // templateMessage is a template-compatible representation of api.Message
@@ -446,7 +468,7 @@ func convertToolsForTemplate(tools api.Tools) templateTools {
 					Defs:       tool.Function.Parameters.Defs,
 					Items:      tool.Function.Parameters.Items,
 					Required:   tool.Function.Parameters.Required,
-					Properties: tool.Function.Parameters.Properties.ToMap(),
+					Properties: templateProperties(tool.Function.Parameters.Properties.ToMap()),
 				},
 			},
 		}
@@ -468,7 +490,7 @@ func convertMessagesForTemplate(messages []*api.Message) []*templateMessage {
 				Function: templateToolCallFunction{
 					Index:     tc.Function.Index,
 					Name:      tc.Function.Name,
-					Arguments: tc.Function.Arguments.ToMap(),
+					Arguments: templateArgs(tc.Function.Arguments.ToMap()),
 				},
 			})
 		}

template/template_test.go

@@ -613,3 +613,159 @@ func TestCollate(t *testing.T) {
 		})
 	}
 }
+
+func TestTemplateArgumentsJSON(t *testing.T) {
+	// Test that {{ .Function.Arguments }} outputs valid JSON, not map[key:value]
+	tmpl := `{{- range .Messages }}{{- range .ToolCalls }}{{ .Function.Arguments }}{{- end }}{{- end }}`
+
+	template, err := Parse(tmpl)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	args := api.NewToolCallFunctionArguments()
+	args.Set("location", "Tokyo")
+	args.Set("unit", "celsius")
+
+	var buf bytes.Buffer
+	err = template.Execute(&buf, Values{
+		Messages: []api.Message{{
+			Role: "assistant",
+			ToolCalls: []api.ToolCall{{
+				Function: api.ToolCallFunction{
+					Name:      "get_weather",
+					Arguments: args,
+				},
+			}},
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := buf.String()
+	// Should be valid JSON, not "map[location:Tokyo unit:celsius]"
+	if strings.HasPrefix(got, "map[") {
+		t.Errorf("Arguments output as Go map format: %s", got)
+	}
+
+	var parsed map[string]any
+	if err := json.Unmarshal([]byte(got), &parsed); err != nil {
+		t.Errorf("Arguments not valid JSON: %s, error: %v", got, err)
+	}
+}
+
+func TestTemplatePropertiesJSON(t *testing.T) {
+	// Test that {{ .Function.Parameters.Properties }} outputs valid JSON
+	// Note: template must reference .Messages to trigger the modern code path that converts Tools
+	tmpl := `{{- range .Messages }}{{- end }}{{- range .Tools }}{{ .Function.Parameters.Properties }}{{- end }}`
+
+	template, err := Parse(tmpl)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	props := api.NewToolPropertiesMap()
+	props.Set("location", api.ToolProperty{Type: api.PropertyType{"string"}, Description: "City name"})
+
+	var buf bytes.Buffer
+	err = template.Execute(&buf, Values{
+		Messages: []api.Message{{Role: "user", Content: "test"}},
+		Tools: api.Tools{{
+			Type: "function",
+			Function: api.ToolFunction{
+				Name:        "get_weather",
+				Description: "Get weather",
+				Parameters: api.ToolFunctionParameters{
+					Type:       "object",
+					Properties: props,
+				},
+			},
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := buf.String()
+	// Should be valid JSON, not "map[location:{...}]"
+	if strings.HasPrefix(got, "map[") {
+		t.Errorf("Properties output as Go map format: %s", got)
+	}
+
+	var parsed map[string]any
+	if err := json.Unmarshal([]byte(got), &parsed); err != nil {
+		t.Errorf("Properties not valid JSON: %s, error: %v", got, err)
+	}
+}
+
+func TestTemplateArgumentsRange(t *testing.T) {
+	// Test that we can range over Arguments in templates
+	tmpl := `{{- range .Messages }}{{- range .ToolCalls }}{{- range $k, $v := .Function.Arguments }}{{ $k }}={{ $v }};{{- end }}{{- end }}{{- end }}`
+
+	template, err := Parse(tmpl)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	args := api.NewToolCallFunctionArguments()
+	args.Set("city", "Tokyo")
+
+	var buf bytes.Buffer
+	err = template.Execute(&buf, Values{
+		Messages: []api.Message{{
+			Role: "assistant",
+			ToolCalls: []api.ToolCall{{
+				Function: api.ToolCallFunction{
+					Name:      "get_weather",
+					Arguments: args,
+				},
+			}},
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := buf.String()
+	if got != "city=Tokyo;" {
+		t.Errorf("Range over Arguments failed, got: %s, want: city=Tokyo;", got)
+	}
+}
+
+func TestTemplatePropertiesRange(t *testing.T) {
+	// Test that we can range over Properties in templates
+	// Note: template must reference .Messages to trigger the modern code path that converts Tools
+	tmpl := `{{- range .Messages }}{{- end }}{{- range .Tools }}{{- range $name, $prop := .Function.Parameters.Properties }}{{ $name }}:{{ $prop.Type }};{{- end }}{{- end }}`
+
+	template, err := Parse(tmpl)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	props := api.NewToolPropertiesMap()
+	props.Set("location", api.ToolProperty{Type: api.PropertyType{"string"}})
+
+	var buf bytes.Buffer
+	err = template.Execute(&buf, Values{
+		Messages: []api.Message{{Role: "user", Content: "test"}},
+		Tools: api.Tools{{
+			Type: "function",
+			Function: api.ToolFunction{
+				Name: "get_weather",
+				Parameters: api.ToolFunctionParameters{
+					Type:       "object",
+					Properties: props,
+				},
+			},
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := buf.String()
+	if got != "location:string;" {
+		t.Errorf("Range over Properties failed, got: %s, want: location:string;", got)
+	}
+}