diff --git a/docs/protocol.md b/docs/protocol.md
index 6b33210..5a31669 100644
--- a/docs/protocol.md
+++ b/docs/protocol.md
@@ -680,7 +680,7 @@ Anthropic 流式 `message_start` 写入即时输入估算，最终 `message_delt
 | 18 | missing_thought_signature | 19 | `provider_19` |
 | 其他整数 | `provider_<code>` | | |
 
-generation config、安全设置、工具与转写配置在选择账户前按各账户已载入的模型目录校验，任一账户的条目接受即通过；流式与非流式请求的这类参数错误都在写出响应头前返回 HTTP 400；上游开始响应后的错误以各协议的流内错误事件返回。
+generation config、安全设置、工具与转写配置在选择账户前按各账户已载入的模型目录校验，任一账户的条目接受即通过。流式请求在写出响应头前等待首个事件：参数错误与首个事件之前的上游错误按非流式返回 HTTP 状态与错误对象；10 秒内没有事件时先写出响应头，此后的错误以各协议的流内错误事件返回。
 
 错误响应根形状为 `[null,[code,message,...]]`。协议核心保留 HTTP 状态、协议 code 与 message；公开适配器映射为 OpenAI、Anthropic 或 Gemini 错误对象。Chat、Responses、Anthropic Messages 与 Gemini GenerateContent 将媒体模型的普通文本作为文本结果输出；专用图片端点要求图片结果。HTTP/协议错误或缺失完成帧形成失败；上游 finish reason 作为正常终态保留并映射到各公开协议。
 
@@ -1392,17 +1392,17 @@ Gemini `GET /v1beta/models` 返回 `{"models":[...]}`，单模型路由直接返
 
 | 端点 | 必需字段 | 主要结果 |
 | --- | --- | --- |
-| `/v1/chat/completions` | `model`、非空 `messages` | Chat completion 或增量 chunk |
-| `/v1/responses` | `model`、`input` | Response object 或 `response.*` 事件 |
+| `/v1/chat/completions` | `model` | Chat completion 或增量 chunk |
+| `/v1/responses` | `model` | Response object 或 `response.*` 事件 |
 | `/v1/files` | multipart `file`、`purpose` | OpenAI file object |
-| `/v1/messages` | `model`、非空 `messages`、`max_tokens` | Anthropic message 或 message 事件 |
-| `:generateContent` / `:streamGenerateContent` | 非空 `contents` | Gemini candidates、usage 与 grounding metadata |
+| `/v1/messages` | `model` | Anthropic message 或 message 事件 |
+| `:generateContent` / `:streamGenerateContent` | 路径中的模型 | Gemini candidates、usage 与 grounding metadata |
 | `/v1/images/generations` | `model`、`prompt`、固定 `n=1` | `b64_json` 或 data URL |
 | `/v1/audio/speech` | `model`、`input` | WAV、PCM 或 MP3 body |
 | `/v1/audio/transcriptions` | multipart `file`；`model` 默认 `gemini-3.5-transcribe` | 文本或转录 JSON |
 | `/v1/videos` | `model`、`prompt` | 长任务对象，随后轮询并下载内容 |
 
-Anthropic assistant prefill 以最后一条 `assistant` text message 表示。适配器将前缀放入续写指令，响应正文返回前缀之后的内容。
+生成与计数请求需要系统提示或对话内容中的至少一项，只有系统提示时以它作为 user 轮发送。请求以 assistant 轮结尾时，末尾连续 assistant 轮的正文与代码执行内容作为前缀写入系统指令，模型从前缀之后续写，响应只返回续写部分；末尾未回传结果的函数调用由模型重新生成。各生成入口与计数接口使用同一转换。
 
 四套生成入口共享同一规范请求，输入映射如下：
 
@@ -1459,7 +1459,7 @@ OpenAI Chat 与 Anthropic 省略转换后没有 parts 的空历史消息；纯
 | 字段 | 类型与语义 |
 | --- | --- |
 | `model` | 必需模型 ID |
-| `messages` | 必需非空 message 数组 |
+| `messages` | message 数组 |
 | `stream` | boolean |
 | `stream_options.include_usage` | 在 finish chunk 后发送 usage-only chunk |
 | `tools` | function 或 Google server tool 数组 |
@@ -1675,9 +1675,9 @@ web search 发生时，search call item 排在 message 前；无 grounding query
 | 字段 | 类型与语义 |
 | --- | --- |
 | `model` | 必需模型 ID |
-| `messages` | 必需非空 `{role,content}` 数组；role 为 `user`、`assistant` 或 `system`，`system` 消息在原位置以 `<system-reminder>` 包裹的用户内容发送 |
+| `messages` | `{role,content}` 数组；role 为 `user`、`assistant` 或 `system`，`system` 消息在原位置以 `<system-reminder>` 包裹的用户内容发送 |
 | `system` | string 或 text block 数组 |
-| `max_tokens` | 必需正整数 |
+| `max_tokens` | 正整数，省略时使用模型默认输出上限 |
 | `stop_sequences` | string array |
 | `stream` | boolean |
 | `temperature`、`top_p`、`top_k` | 生成参数 |
diff --git a/internal/aistudio/service.go b/internal/aistudio/service.go
index 01c0b7f..8029547 100644
--- a/internal/aistudio/service.go
+++ b/internal/aistudio/service.go
@@ -502,6 +502,10 @@ func (s *PooledService) CountTokens(ctx context.Context, request TokenCountReque
 	if modelID == "" {
 		return TokenCount{}, fmt.Errorf("%w: CountTokens model 不能为空", ErrInvalidArgument)
 	}
+	request.System, request.Contents = normalizeTurns(request.System, request.Contents)
+	if len(request.Contents) == 0 {
+		return TokenCount{}, fmt.Errorf("%w: 请求没有系统提示或对话内容", ErrInvalidArgument)
+	}
 	modelAccessScope := ModelAccessKey("count-tokens", modelID)
 	selection := AccountSelection{ModelID: modelID, ModelAccessScope: modelAccessScope, Method: "countTokens"}
 	var count TokenCount
@@ -553,6 +557,10 @@ func (s *PooledService) ValidateGenerateRequest(request GenerateRequest) error {
 	if strings.TrimPrefix(strings.TrimSpace(request.Model), "models/") == "" {
 		return fmt.Errorf("%w: GenerateContent model 不能为空", ErrInvalidArgument)
 	}
+	request.System, request.Contents = normalizeTurns(request.System, request.Contents)
+	if len(request.Contents) == 0 {
+		return fmt.Errorf("%w: 请求没有系统提示或对话内容", ErrInvalidArgument)
+	}
 	request, _, err := prepareToolRequest(request)
 	if err != nil {
 		return fmt.Errorf("%w: %v", ErrInvalidArgument, err)
@@ -566,6 +574,10 @@ func (s *PooledService) Generate(ctx context.Context, request GenerateRequest) (
 	if modelID == "" {
 		return nil, fmt.Errorf("%w: GenerateContent model 不能为空", ErrInvalidArgument)
 	}
+	request.System, request.Contents = normalizeTurns(request.System, request.Contents)
+	if len(request.Contents) == 0 {
+		return nil, fmt.Errorf("%w: 请求没有系统提示或对话内容", ErrInvalidArgument)
+	}
 	request, contract, err := prepareToolRequest(request)
 	if err != nil {
 		return nil, fmt.Errorf("%w: %v", ErrInvalidArgument, err)
diff --git a/internal/aistudio/turns.go b/internal/aistudio/turns.go
new file mode 100644
index 0000000..310a489
--- /dev/null
+++ b/internal/aistudio/turns.go
@@ -0,0 +1,51 @@
+package aistudio
+
+import "strings"
+
+// assistantPrefixInstruction 是末尾 assistant 轮改写为续写前缀时追加的系统指令
+const assistantPrefixInstruction = "Continue the assistant response after the following prefix. Return only the continuation, without repeating the prefix."
+
+// normalizeTurns 将末尾连续 assistant 轮改写为系统指令中的续写前缀，没有对话内容时以系统提示作为 user 轮
+func normalizeTurns(system string, contents []Content) (string, []Content) {
+	tail := len(contents)
+	for tail > 0 && contents[tail-1].Role == RoleAssistant {
+		tail--
+	}
+	if prefix := assistantPrefix(contents[tail:]); strings.TrimSpace(prefix) != "" {
+		instruction := assistantPrefixInstruction + "\n<assistant_prefix>" + prefix + "</assistant_prefix>"
+		if system != "" {
+			instruction = system + "\n" + instruction
+		}
+		system = instruction
+	}
+	contents = contents[:tail]
+	if len(contents) == 0 && strings.TrimSpace(system) != "" {
+		return "", []Content{{Role: RoleUser, Parts: []Part{{Text: system}}}}
+	}
+	return system, contents
+}
+
+// assistantPrefix 按顺序拼接 assistant 轮的正文与代码执行内容
+func assistantPrefix(contents []Content) string {
+	var prefix strings.Builder
+	block := func(text string) {
+		if prefix.Len() > 0 && !strings.HasSuffix(prefix.String(), "\n") {
+			prefix.WriteString("\n")
+		}
+		prefix.WriteString(text + "\n")
+	}
+	for _, content := range contents {
+		for _, part := range content.Parts {
+			switch {
+			case part.Thought:
+			case part.Text != "":
+				prefix.WriteString(part.Text)
+			case part.ExecutableCode != nil:
+				block("```" + strings.ToLower(part.ExecutableCode.Language) + "\n" + strings.TrimSuffix(part.ExecutableCode.Code, "\n") + "\n```")
+			case part.CodeExecutionResult != nil:
+				block("```text\n" + strings.TrimSuffix(part.CodeExecutionResult.Output+part.CodeExecutionResult.Error, "\n") + "\n```")
+			}
+		}
+	}
+	return prefix.String()
+}
diff --git a/internal/api/anthropic.go b/internal/api/anthropic.go
index f2e83ea..7f0bc91 100644
--- a/internal/api/anthropic.go
+++ b/internal/api/anthropic.go
@@ -82,8 +82,8 @@ func (s *server) handleAnthropicMessages(w http.ResponseWriter, r *http.Request)
 		writeAnthropicError(w, http.StatusBadRequest, "invalid_request_error", err.Error())
 		return
 	}
-	if request.Model == "" || len(request.Messages) == 0 || request.MaxTokens == nil {
-		writeAnthropicError(w, http.StatusBadRequest, "invalid_request_error", "model, messages and max_tokens are required")
+	if request.Model == "" {
+		writeAnthropicError(w, http.StatusBadRequest, "invalid_request_error", "model is required")
 		return
 	}
 	messageID := newID("msg")
@@ -99,6 +99,15 @@ func (s *server) handleAnthropicMessages(w http.ResponseWriter, r *http.Request)
 	generateRequest.Unary = !request.Stream
 	events, err := s.service.Generate(r.Context(), generateRequest)
 	if err == nil && request.Stream {
+		events, err = awaitStreamStart(r.Context(), events)
+	}
+	if err != nil {
+		if shouldWriteRequestError(r, err) {
+			writeAnthropicError(w, statusFromError(err), anthropicErrorType(err), err.Error())
+		}
+		return
+	}
+	if request.Stream {
 		if err := streamHeaders(w); err != nil {
 			return
 		}
@@ -113,12 +122,6 @@ func (s *server) handleAnthropicMessages(w http.ResponseWriter, r *http.Request)
 		s.streamAnthropic(r, writer, events)
 		return
 	}
-	if err != nil {
-		if shouldWriteRequestError(r, err) {
-			writeAnthropicError(w, statusFromError(err), anthropicErrorType(err), err.Error())
-		}
-		return
-	}
 	result, err := consumeEvents(r.Context(), events, nil)
 	if err != nil {
 		if shouldWriteRequestError(r, err) {
@@ -135,8 +138,8 @@ func (s *server) handleAnthropicCountTokens(w http.ResponseWriter, r *http.Reque
 		writeAnthropicError(w, http.StatusBadRequest, "invalid_request_error", err.Error())
 		return
 	}
-	if request.Model == "" || len(request.Messages) == 0 {
-		writeAnthropicError(w, http.StatusBadRequest, "invalid_request_error", "model and messages are required")
+	if request.Model == "" {
+		writeAnthropicError(w, http.StatusBadRequest, "invalid_request_error", "model is required")
 		return
 	}
 	if err := s.decodeAnthropicSearchHistory(&request); err != nil {
@@ -224,20 +227,6 @@ func (request anthropicRequest) toGenerateRequest(id string) (aistudio.GenerateR
 	if request.OutputConfig != nil {
 		config.ReasoningEffort = request.OutputConfig.Effort
 	}
-	if len(contents) > 0 && contents[len(contents)-1].Role == aistudio.RoleAssistant {
-		last := contents[len(contents)-1]
-		var prefix strings.Builder
-		for _, part := range last.Parts {
-			if part.FunctionCall != nil || part.InlineData != nil || part.File != nil {
-				return aistudio.GenerateRequest{}, fmt.Errorf("assistant prefill requires text content")
-			}
-			if !part.Thought {
-				prefix.WriteString(part.Text)
-			}
-		}
-		contents = contents[:len(contents)-1]
-		system += "\nContinue the assistant response after the following prefix. Return only the continuation, without repeating the prefix.\n<assistant_prefix>" + prefix.String() + "</assistant_prefix>"
-	}
 	return aistudio.GenerateRequest{
 		ID: id, Model: request.Model, System: system, Contents: contents, Config: config, Tools: tools,
 	}, nil
diff --git a/internal/api/gemini.go b/internal/api/gemini.go
index 4707466..e69dd0b 100644
--- a/internal/api/gemini.go
+++ b/internal/api/gemini.go
@@ -286,10 +286,6 @@ func (s *server) handleGeminiAction(w http.ResponseWriter, r *http.Request) {
 		writeGeminiError(w, http.StatusBadRequest, "INVALID_ARGUMENT", err.Error())
 		return
 	}
-	if len(request.Contents) == 0 {
-		writeGeminiError(w, http.StatusBadRequest, "INVALID_ARGUMENT", "contents is required")
-		return
-	}
 	generateRequest, err := request.toGenerateRequest(newID("resp"), model)
 	if err != nil {
 		writeGeminiError(w, http.StatusBadRequest, "INVALID_ARGUMENT", err.Error())
@@ -790,6 +786,9 @@ func (s *server) handleGeminiCountTokens(w http.ResponseWriter, r *http.Request,
 func (s *server) handleGeminiGenerate(w http.ResponseWriter, r *http.Request, request aistudio.GenerateRequest, stream bool) {
 	request.Unary = !stream
 	events, err := s.service.Generate(r.Context(), request)
+	if err == nil && stream {
+		events, err = awaitStreamStart(r.Context(), events)
+	}
 	if err != nil {
 		if shouldWriteRequestError(r, err) {
 			writeGeminiError(w, statusFromError(err), geminiErrorStatus(err), err.Error())
diff --git a/internal/api/generate.go b/internal/api/generate.go
index bb14398..eb8c2aa 100644
--- a/internal/api/generate.go
+++ b/internal/api/generate.go
@@ -100,6 +100,42 @@ func (result *generationResult) apply(event aistudio.Event) error {
 	return nil
 }
 
+// awaitStreamStart 在写出流式响应头前等待首个事件，首个事件为错误时返回该错误，心跳间隔内没有事件时直接返回原事件流
+func awaitStreamStart(ctx context.Context, events <-chan aistudio.Event) (<-chan aistudio.Event, error) {
+	timer := time.NewTimer(streamHeartbeatInterval)
+	defer timer.Stop()
+	var first aistudio.Event
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case <-timer.C:
+		return events, nil
+	case event, ok := <-events:
+		if !ok {
+			return nil, errIncompleteStream
+		}
+		if event.Kind == aistudio.EventError {
+			if event.Err != nil {
+				return nil, event.Err
+			}
+			return nil, errUpstreamStream
+		}
+		first = event
+	}
+	replay := make(chan aistudio.Event)
+	go func() {
+		defer close(replay)
+		for event, ok := first, true; ok; event, ok = <-events {
+			select {
+			case replay <- event:
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+	return replay, nil
+}
+
 func consumeEvents(ctx context.Context, events <-chan aistudio.Event, emit func(aistudio.Event) error) (result generationResult, resultErr error) {
 	return consumeEventsWithHeartbeat(ctx, events, emit, nil)
 }
diff --git a/internal/api/interaction_input.go b/internal/api/interaction_input.go
index d6e2a07..aa5d69f 100644
--- a/internal/api/interaction_input.go
+++ b/internal/api/interaction_input.go
@@ -88,8 +88,8 @@ func interactionList[T any](raw json.RawMessage) ([]T, error) {
 // toGenerateRequest 映射模型、结构化输入、生成参数与函数声明
 func (request interactionRequest) toGenerateRequest(id string) (aistudio.GenerateRequest, error) {
 	generate := aistudio.GenerateRequest{ID: id, Model: strings.TrimPrefix(strings.TrimSpace(request.Model), "models/"), System: request.System}
-	if generate.Model == "" || len(request.Input) == 0 {
-		return generate, fmt.Errorf("model and input are required")
+	if generate.Model == "" {
+		return generate, fmt.Errorf("model is required")
 	}
 	if request.Background {
 		return generate, fmt.Errorf("background must be false")
@@ -99,9 +99,6 @@ func (request interactionRequest) toGenerateRequest(id string) (aistudio.Generat
 	if err != nil {
 		return generate, err
 	}
-	if len(generate.Contents) == 0 {
-		return generate, fmt.Errorf("input must contain content")
-	}
 	generation := request.Generation
 	generate.Config = aistudio.GenerationConfig{
 		Temperature: generation.Temperature, TopP: generation.TopP, TopK: generation.TopK,
@@ -179,7 +176,7 @@ func interactionContents(raw json.RawMessage) ([]aistudio.Content, error) {
 	var text string
 	if json.Unmarshal(raw, &text) == nil {
 		if strings.TrimSpace(text) == "" {
-			return nil, fmt.Errorf("input text is empty")
+			return nil, nil
 		}
 		return []aistudio.Content{{Role: aistudio.RoleUser, Parts: []aistudio.Part{{Text: text}}}}, nil
 	}
diff --git a/internal/api/interactions.go b/internal/api/interactions.go
index ba19400..8da4dc3 100644
--- a/internal/api/interactions.go
+++ b/internal/api/interactions.go
@@ -42,6 +42,9 @@ func (s *server) handleInteraction(w http.ResponseWriter, r *http.Request) {
 	defer cancel()
 	r = r.WithContext(ctx)
 	events, err := s.service.Generate(ctx, generate)
+	if err == nil && request.Stream {
+		events, err = awaitStreamStart(ctx, events)
+	}
 	if err != nil {
 		if shouldWriteRequestError(r, err) {
 			writeGeminiError(w, statusFromError(err), geminiErrorStatus(err), err.Error())
diff --git a/internal/api/openai.go b/internal/api/openai.go
index 55fbb30..93ec2e1 100644
--- a/internal/api/openai.go
+++ b/internal/api/openai.go
@@ -180,8 +180,8 @@ func (s *server) handleChatCompletions(w http.ResponseWriter, r *http.Request) {
 		writeOpenAIError(w, http.StatusBadRequest, "invalid_request", err.Error())
 		return
 	}
-	if request.Model == "" || len(request.Messages) == 0 {
-		writeOpenAIError(w, http.StatusBadRequest, "invalid_request", "model and messages are required")
+	if request.Model == "" {
+		writeOpenAIError(w, http.StatusBadRequest, "invalid_request", "model is required")
 		return
 	}
 	requestID := newID("chatcmpl")
@@ -193,6 +193,9 @@ func (s *server) handleChatCompletions(w http.ResponseWriter, r *http.Request) {
 	generateRequest.Unary = !request.Stream
 	s.thoughtSignatures.Restore(generateRequest.Contents)
 	events, err := s.service.Generate(r.Context(), generateRequest)
+	if err == nil && request.Stream {
+		events, err = awaitStreamStart(r.Context(), events)
+	}
 	if err != nil {
 		if shouldWriteRequestError(r, err) {
 			writeOpenAIError(w, statusFromError(err), openAIErrorCode(err), err.Error())
diff --git a/internal/api/responses.go b/internal/api/responses.go
index 7d3fb8e..9272431 100644
--- a/internal/api/responses.go
+++ b/internal/api/responses.go
@@ -152,8 +152,8 @@ func (s *server) handleResponses(w http.ResponseWriter, r *http.Request) {
 		writeOpenAIError(w, http.StatusBadRequest, "invalid_request", err.Error())
 		return
 	}
-	if request.Model == "" || len(request.Input) == 0 {
-		writeOpenAIError(w, http.StatusBadRequest, "invalid_request", "model and input are required")
+	if request.Model == "" {
+		writeOpenAIError(w, http.StatusBadRequest, "invalid_request", "model is required")
 		return
 	}
 	responseID := newID("resp")
@@ -183,6 +183,9 @@ func (s *server) handleResponses(w http.ResponseWriter, r *http.Request) {
 	}
 	generateRequest.Unary = !request.Stream
 	events, err := s.service.Generate(r.Context(), generateRequest)
+	if err == nil && request.Stream {
+		events, err = awaitStreamStart(r.Context(), events)
+	}
 	if err != nil {
 		if shouldWriteRequestError(r, err) {
 			writeOpenAIError(w, statusFromError(err), openAIErrorCode(err), err.Error())
@@ -314,6 +317,9 @@ func (request responsesRequest) toGenerateRequest(id string) (aistudio.GenerateR
 }
 
 func responsesContents(raw json.RawMessage) ([]aistudio.Content, []string, error) {
+	if value := strings.TrimSpace(string(raw)); value == "" || value == "null" {
+		return nil, nil, nil
+	}
 	var text string
 	if err := json.Unmarshal(raw, &text); err == nil {
 		return []aistudio.Content{{Role: aistudio.RoleUser, Parts: []aistudio.Part{{Text: text}}}}, nil, nil