diff --git a/chat.go b/chat.go
index 609e0c3..9ad31c4 100644
--- a/chat.go
+++ b/chat.go
@@ -62,11 +62,17 @@ type ChatCompletionMessage struct {
 	Name string `json:"name,omitempty"`
 
 	FunctionCall *FunctionCall `json:"function_call,omitempty"`
-	ToolCalls    []ToolCall    `json:"tool_calls,omitempty"`
+
+	// For Role=assistant prompts this may be set to the tool calls generated by the model, such as function calls.
+	ToolCalls []ToolCall `json:"tool_calls,omitempty"`
+
+	// For Role=tool prompts this should be set to the ID given in the assistant's prior request to call a tool.
+	ToolCallID string `json:"tool_call_id,omitempty"`
 }
 
 type ToolCall struct {
 	ID       string       `json:"id"`
+	Type     ToolType     `json:"type"`
 	Function FunctionCall `json:"function"`
 }
 
@@ -84,7 +90,7 @@ const (
 )
 
 type ChatCompletionResponseFormat struct {
-	Type ChatCompletionResponseFormatType `json:"type"`
+	Type ChatCompletionResponseFormatType `json:"type,omitempty"`
 }
 
 // ChatCompletionRequest represents a request structure for chat completion API.
@@ -112,7 +118,7 @@ type ChatCompletionRequest struct {
 	FunctionCall any    `json:"function_call,omitempty"`
 	Tools        []Tool `json:"tools,omitempty"`
 	// This can be either a string or an ToolChoice object.
-	ToolChoiche any `json:"tool_choice,omitempty"`
+	ToolChoice any `json:"tool_choice,omitempty"`
 }
 
 type ToolType string
@@ -126,7 +132,7 @@ type Tool struct {
 	Function FunctionDefinition `json:"function,omitempty"`
 }
 
-type ToolChoiche struct {
+type ToolChoice struct {
 	Type     ToolType     `json:"type"`
 	Function ToolFunction `json:"function,omitempty"`
 }
@@ -182,12 +188,13 @@ type ChatCompletionChoice struct {
 
 // ChatCompletionResponse represents a response structure for chat completion API.
 type ChatCompletionResponse struct {
-	ID      string                 `json:"id"`
-	Object  string                 `json:"object"`
-	Created int64                  `json:"created"`
-	Model   string                 `json:"model"`
-	Choices []ChatCompletionChoice `json:"choices"`
-	Usage   Usage                  `json:"usage"`
+	ID                string                 `json:"id"`
+	Object            string                 `json:"object"`
+	Created           int64                  `json:"created"`
+	Model             string                 `json:"model"`
+	Choices           []ChatCompletionChoice `json:"choices"`
+	Usage             Usage                  `json:"usage"`
+	SystemFingerprint string                 `json:"system_fingerprint"`
 
 	httpHeader
 }
diff --git a/chat_test.go b/chat_test.go
index 5bf1eaf..a8155ed 100644
--- a/chat_test.go
+++ b/chat_test.go
@@ -51,6 +51,20 @@ func TestChatCompletionsWrongModel(t *testing.T) {
 	checks.ErrorIs(t, err, openai.ErrChatCompletionInvalidModel, msg)
 }
 
+func TestChatRequestOmitEmpty(t *testing.T) {
+	data, err := json.Marshal(openai.ChatCompletionRequest{
+		// We set model b/c it's required, so omitempty doesn't make sense
+		Model: "gpt-4",
+	})
+	checks.NoError(t, err)
+
+	// messages is also required so isn't omitted
+	const expected = `{"model":"gpt-4","messages":null}`
+	if string(data) != expected {
+		t.Errorf("expected JSON with all empty fields to be %v but was %v", expected, string(data))
+	}
+}
+
 func TestChatCompletionsWithStream(t *testing.T) {
 	config := openai.DefaultConfig("whatever")
 	config.BaseURL = "http://localhost/v1"