fix(sdk/go): rewrite max_tokens to max_completion_tokens for newer OpenAI models

sdk/go/ai/client.go

@@ -82,8 +82,8 @@ func (c *Client) CompleteWithMessages(ctx context.Context, messages []Message, o
 }
 
 func (c *Client) doRequest(ctx context.Context, req *Request) (*Response, error) {
-	// Marshal request
-	body, err := json.Marshal(req)
+	// Apply provider-specific parameter rewrites before serialization.
+	body, err := c.marshalRequest(req)
 	if err != nil {
 		return nil, fmt.Errorf("marshal request: %w", err)
 	}
@@ -176,8 +176,8 @@ func (c *Client) StreamComplete(ctx context.Context, prompt string, opts ...Opti
 			}
 		}
 
-		// Marshal request
-		body, err := json.Marshal(req)
+		// Apply provider-specific parameter rewrites before serialization.
+		body, err := c.marshalRequest(req)
 		if err != nil {
 			errCh <- fmt.Errorf("marshal request: %w", err)
 			return

sdk/go/ai/model_params.go

@@ -0,0 +1,101 @@
+package ai
+
+import (
+	"encoding/json"
+	"strings"
+)
+
+// needsMaxCompletionTokens returns true if the model requires
+// max_completion_tokens instead of max_tokens. This applies to OpenAI's
+// newer model families (o1, o3, gpt-4o, etc.) which dropped support for
+// the legacy max_tokens parameter.
+//
+// Reference: https://platform.openai.com/docs/api-reference/chat/create
+func needsMaxCompletionTokens(model string) bool {
+	m := strings.ToLower(strings.TrimSpace(model))
+
+	// Strip provider prefix if present (e.g. "openai/gpt-4o" → "gpt-4o")
+	if idx := strings.LastIndex(m, "/"); idx >= 0 {
+		m = m[idx+1:]
+	}
+
+	// o1, o3, o4 series always use max_completion_tokens
+	if strings.HasPrefix(m, "o1") || strings.HasPrefix(m, "o3") || strings.HasPrefix(m, "o4") {
+		return true
+	}
+
+	// gpt-4o and gpt-4o-mini series use max_completion_tokens
+	if strings.HasPrefix(m, "gpt-4o") {
+		return true
+	}
+
+	// gpt-4.1, gpt-4.5 etc. (newer dot-release models)
+	if strings.HasPrefix(m, "gpt-4.") {
+		return true
+	}
+
+	return false
+}
+
+// isOpenAICompatible returns true if the base URL points to OpenAI or an
+// OpenAI-compatible endpoint (not Anthropic, Cohere, etc.).
+func isOpenAICompatible(baseURL string) bool {
+	lower := strings.ToLower(baseURL)
+
+	// Known non-OpenAI providers (not OpenAI-compatible chat/completions).
+	if strings.Contains(lower, "anthropic") || strings.Contains(lower, "cohere") {
+		return false
+	}
+
+	// OpenAI's own endpoint
+	if strings.Contains(lower, "openai.com") {
+		return true
+	}
+	// OpenRouter proxies to OpenAI models
+	if strings.Contains(lower, "openrouter.ai") {
+		return true
+	}
+	// Azure OpenAI
+	if strings.Contains(lower, "openai.azure.com") {
+		return true
+	}
+	// For unknown/custom endpoints, assume OpenAI-compatible since that's
+	// the most common case for the chat/completions API shape.
+	return true
+}
+
+// marshalRequest serializes the request, applying provider-specific parameter
+// rewrites. For OpenAI-compatible endpoints with newer models, max_tokens is
+// rewritten to max_completion_tokens.
+func (c *Client) marshalRequest(req *Request) ([]byte, error) {
+	model := req.Model
+	if model == "" {
+		model = c.config.Model
+	}
+
+	// If the model needs max_completion_tokens and we have a max_tokens value,
+	// serialize with the rewritten field name.
+	if req.MaxTokens != nil && needsMaxCompletionTokens(model) && isOpenAICompatible(c.config.BaseURL) {
+		return marshalWithMaxCompletionTokens(req)
+	}
+
+	return json.Marshal(req)
+}
+
+// marshalWithMaxCompletionTokens serializes the request with max_completion_tokens
+// instead of max_tokens. We use a shadow struct to avoid modifying the original.
+func marshalWithMaxCompletionTokens(req *Request) ([]byte, error) {
+	type requestAlias Request
+
+	wire := struct {
+		*requestAlias
+		MaxTokens           *int `json:"max_tokens,omitempty"`
+		MaxCompletionTokens *int `json:"max_completion_tokens,omitempty"`
+	}{
+		requestAlias:        (*requestAlias)(req),
+		MaxTokens:           nil, // suppress max_tokens
+		MaxCompletionTokens: req.MaxTokens,
+	}
+
+	return json.Marshal(wire)
+}

sdk/go/ai/model_params_test.go

@@ -0,0 +1,230 @@
+package ai
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestNeedsMaxCompletionTokens(t *testing.T) {
+	tests := []struct {
+		model    string
+		expected bool
+	}{
+		// o1 series
+		{"o1-mini", true},
+		{"o1-preview", true},
+		{"o1", true},
+		// o3 series
+		{"o3-mini", true},
+		{"o3", true},
+		// o4 series
+		{"o4-mini", true},
+		// gpt-4o series
+		{"gpt-4o", true},
+		{"gpt-4o-mini", true},
+		{"gpt-4o-2024-05-13", true},
+		// gpt-4.x dot releases
+		{"gpt-4.1", true},
+		{"gpt-4.5-preview", true},
+		// With provider prefix
+		{"openai/gpt-4o", true},
+		{"openai/o1-mini", true},
+		{"openrouter/openai/gpt-4o-mini", true},
+		// Case insensitive
+		{"GPT-4O", true},
+		{"O1-Mini", true},
+		// Legacy models — should NOT rewrite
+		{"gpt-4", false},
+		{"gpt-4-turbo", false},
+		{"gpt-3.5-turbo", false},
+		{"gpt-4-0613", false},
+		// Non-OpenAI models
+		{"claude-3-opus", false},
+		{"mistral-large", false},
+		{"llama-3-70b", false},
+		// Empty
+		{"", false},
+	}
+
+	for _, tt := range tests {
+		got := needsMaxCompletionTokens(tt.model)
+		if got != tt.expected {
+			t.Errorf("needsMaxCompletionTokens(%q) = %v, want %v", tt.model, got, tt.expected)
+		}
+	}
+}
+
+func TestMarshalRequest_RewritesMaxTokensForNewModels(t *testing.T) {
+	maxTokens := 1000
+	cfg := DefaultConfig()
+	cfg.APIKey = "test-key"
+	cfg.Model = "gpt-4o"
+
+	client, _ := NewClient(cfg)
+
+	req := &Request{
+		Model:     "gpt-4o",
+		MaxTokens: &maxTokens,
+		Messages: []Message{
+			{Role: "user", Content: []ContentPart{{Type: "text", Text: "hello"}}},
+		},
+	}
+
+	body, err := client.marshalRequest(req)
+	if err != nil {
+		t.Fatalf("marshalRequest error: %v", err)
+	}
+
+	var raw map[string]interface{}
+	if err := json.Unmarshal(body, &raw); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if _, ok := raw["max_tokens"]; ok {
+		t.Error("expected max_tokens to be absent for gpt-4o model")
+	}
+	if val, ok := raw["max_completion_tokens"]; !ok {
+		t.Error("expected max_completion_tokens to be present for gpt-4o model")
+	} else if int(val.(float64)) != 1000 {
+		t.Errorf("expected max_completion_tokens=1000, got %v", val)
+	}
+}
+
+func TestMarshalRequest_KeepsMaxTokensForLegacyModels(t *testing.T) {
+	maxTokens := 2000
+	cfg := DefaultConfig()
+	cfg.APIKey = "test-key"
+	cfg.Model = "gpt-3.5-turbo"
+
+	client, _ := NewClient(cfg)
+
+	req := &Request{
+		Model:     "gpt-3.5-turbo",
+		MaxTokens: &maxTokens,
+		Messages: []Message{
+			{Role: "user", Content: []ContentPart{{Type: "text", Text: "hello"}}},
+		},
+	}
+
+	body, err := client.marshalRequest(req)
+	if err != nil {
+		t.Fatalf("marshalRequest error: %v", err)
+	}
+
+	var raw map[string]interface{}
+	if err := json.Unmarshal(body, &raw); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if _, ok := raw["max_completion_tokens"]; ok {
+		t.Error("expected max_completion_tokens to be absent for gpt-3.5-turbo model")
+	}
+	if val, ok := raw["max_tokens"]; !ok {
+		t.Error("expected max_tokens to be present for gpt-3.5-turbo model")
+	} else if int(val.(float64)) != 2000 {
+		t.Errorf("expected max_tokens=2000, got %v", val)
+	}
+}
+
+func TestMarshalRequest_O1MiniUsesMaxCompletionTokens(t *testing.T) {
+	maxTokens := 500
+	cfg := DefaultConfig()
+	cfg.APIKey = "test-key"
+	cfg.Model = "o1-mini"
+
+	client, _ := NewClient(cfg)
+
+	req := &Request{
+		Model:     "o1-mini",
+		MaxTokens: &maxTokens,
+		Messages: []Message{
+			{Role: "user", Content: []ContentPart{{Type: "text", Text: "hello"}}},
+		},
+	}
+
+	body, err := client.marshalRequest(req)
+	if err != nil {
+		t.Fatalf("marshalRequest error: %v", err)
+	}
+
+	var raw map[string]interface{}
+	if err := json.Unmarshal(body, &raw); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if _, ok := raw["max_tokens"]; ok {
+		t.Error("expected max_tokens to be absent for o1-mini")
+	}
+	if val, ok := raw["max_completion_tokens"]; !ok {
+		t.Error("expected max_completion_tokens to be present for o1-mini")
+	} else if int(val.(float64)) != 500 {
+		t.Errorf("expected max_completion_tokens=500, got %v", val)
+	}
+}
+
+func TestMarshalRequest_NilMaxTokensOmitted(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.APIKey = "test-key"
+	cfg.Model = "gpt-4o"
+
+	client, _ := NewClient(cfg)
+
+	req := &Request{
+		Model:     "gpt-4o",
+		MaxTokens: nil,
+		Messages: []Message{
+			{Role: "user", Content: []ContentPart{{Type: "text", Text: "hello"}}},
+		},
+	}
+
+	body, err := client.marshalRequest(req)
+	if err != nil {
+		t.Fatalf("marshalRequest error: %v", err)
+	}
+
+	var raw map[string]interface{}
+	if err := json.Unmarshal(body, &raw); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if _, ok := raw["max_tokens"]; ok {
+		t.Error("expected max_tokens to be absent when nil")
+	}
+	if _, ok := raw["max_completion_tokens"]; ok {
+		t.Error("expected max_completion_tokens to be absent when MaxTokens is nil")
+	}
+}
+
+func TestMarshalRequest_UsesRequestModelOverClientModel(t *testing.T) {
+	maxTokens := 800
+	cfg := DefaultConfig()
+	cfg.APIKey = "test-key"
+	cfg.Model = "gpt-3.5-turbo" // client default is legacy
+
+	client, _ := NewClient(cfg)
+
+	req := &Request{
+		Model:     "o1-preview", // request overrides to new model
+		MaxTokens: &maxTokens,
+		Messages: []Message{
+			{Role: "user", Content: []ContentPart{{Type: "text", Text: "hello"}}},
+		},
+	}
+
+	body, err := client.marshalRequest(req)
+	if err != nil {
+		t.Fatalf("marshalRequest error: %v", err)
+	}
+
+	var raw map[string]interface{}
+	if err := json.Unmarshal(body, &raw); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if _, ok := raw["max_tokens"]; ok {
+		t.Error("expected max_tokens to be absent for o1-preview")
+	}
+	if _, ok := raw["max_completion_tokens"]; !ok {
+		t.Error("expected max_completion_tokens to be present for o1-preview")
+	}
+}