diff --git a/docs/providers/sglang.mdx b/docs/providers/sglang.mdx
index 4c6036fb1..aed2fba93 100644
--- a/docs/providers/sglang.mdx
+++ b/docs/providers/sglang.mdx
@@ -194,6 +194,10 @@ GoModel strips client authorization before forwarding and applies
 
 - Chat completions, streaming, model listing, Responses, and embeddings use
   SGLang's OpenAI-compatible API.
+- SGLang's native `/v1/responses` endpoint rejects tool-bearing requests on
+  some versions. Set `api_mode: chat_compatible` in YAML (or
+  `SGLANG_API_MODE=chat_compatible`) to translate Responses API calls through
+  chat completions instead, which keeps function calling working.
 - Embeddings and model-specific features depend on the model loaded by SGLang.
 - Native batch, file, and stored-response lifecycle interfaces are not yet
   exposed as typed GoModel provider capabilities; use passthrough where the
diff --git a/internal/providers/openai/openai.go b/internal/providers/openai/openai.go
index 677a36b72..46473e3a3 100644
--- a/internal/providers/openai/openai.go
+++ b/internal/providers/openai/openai.go
@@ -2,6 +2,8 @@
 package openai
 
 import (
+	"context"
+	"io"
 	"net/http"
 	"strconv"
 	"strings"
@@ -27,12 +29,29 @@ const (
 	defaultBaseURL = "https://api.openai.com/v1"
 )
 
+// apiModeResponsesViaChat lists the APIMode values that switch the Responses
+// API from native upstream passthrough to translation over chat completions.
+// OpenAI-compatible upstreams such as SGLang or older vLLM builds expose
+// /v1/responses but reject or fail on tool-bearing requests; routing through
+// chat completions keeps function calling working on those upstreams.
+var apiModeResponsesViaChat = map[string]bool{
+	"chat":               true,
+	"chat_completions":   true,
+	"chat-compatible":    true,
+	"chat_compatible":    true,
+	"responses_via_chat": true,
+	"responses-via-chat": true,
+}
+
 // Provider implements the core.Provider interface for OpenAI.
 // Credentials and the realtime base URL are both read live from the embedded
 // CompatibleProvider, so SetBaseURL overrides and key rotation are honored on
 // the realtime websocket dial target too (see realtime.go).
 type Provider struct {
 	*CompatibleProvider
+	// responsesViaChat translates Responses API calls through chat
+	// completions instead of forwarding to the upstream /responses endpoint.
+	responsesViaChat bool
 }
 
 // New creates a new OpenAI provider.
@@ -44,7 +63,27 @@ func New(cfg providers.ProviderConfig, opts providers.ProviderOptions) core.Prov
 			BaseURL:      baseURL,
 			SetHeaders:   setHeaders,
 		}),
+		responsesViaChat: apiModeResponsesViaChat[strings.ToLower(strings.TrimSpace(cfg.APIMode))],
+	}
+}
+
+// Responses sends a Responses API request. With APIMode set to a
+// chat-compatible value the request is translated through chat completions;
+// otherwise it is forwarded to the upstream /responses endpoint.
+func (p *Provider) Responses(ctx context.Context, req *core.ResponsesRequest) (*core.ResponsesResponse, error) {
+	if p.responsesViaChat {
+		return providers.ResponsesViaChat(ctx, p, req)
+	}
+	return p.CompatibleProvider.Responses(ctx, req)
+}
+
+// StreamResponses streams a Responses API request, honoring the same APIMode
+// translation switch as Responses.
+func (p *Provider) StreamResponses(ctx context.Context, req *core.ResponsesRequest) (io.ReadCloser, error) {
+	if p.responsesViaChat {
+		return providers.StreamResponsesViaChat(ctx, p, req, "openai")
 	}
+	return p.CompatibleProvider.StreamResponses(ctx, req)
 }
 
 // NewWithHTTPClient creates a new OpenAI provider with a custom HTTP client.
diff --git a/internal/providers/openai/openai_test.go b/internal/providers/openai/openai_test.go
index acb1b6188..670dda353 100644
--- a/internal/providers/openai/openai_test.go
+++ b/internal/providers/openai/openai_test.go
@@ -37,6 +37,108 @@ func TestNew_ReturnsProvider(t *testing.T) {
 	}
 }
 
+func TestResponses_APIModeChatCompatibleTranslatesViaChat(t *testing.T) {
+	var gotPath string
+	var gotBody map[string]any
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		if err := json.NewDecoder(r.Body).Decode(&gotBody); err != nil {
+			t.Errorf("failed to decode request body: %v", err)
+		}
+		if r.URL.Path == "/responses" {
+			// Simulate an upstream (e.g. SGLang) whose native /responses
+			// endpoint cannot serve tool-bearing requests.
+			w.WriteHeader(http.StatusInternalServerError)
+			_, _ = w.Write([]byte(`Internal Server Error`))
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"id": "chatcmpl_123",
+			"object": "chat.completion",
+			"created": 1677652288,
+			"model": "glm-5.2",
+			"choices": [{
+				"index": 0,
+				"message": {"role": "assistant", "content": "OK"},
+				"finish_reason": "stop"
+			}],
+			"usage": {"prompt_tokens": 10, "completion_tokens": 2, "total_tokens": 12}
+		}`))
+	}))
+	defer server.Close()
+
+	provider := New(providers.ProviderConfig{
+		APIKey:  "test-api-key",
+		BaseURL: server.URL,
+		APIMode: "chat_compatible",
+	}, providers.ProviderOptions{})
+
+	resp, err := provider.Responses(context.Background(), &core.ResponsesRequest{
+		Model: "glm-5.2",
+		Input: "Reply exactly: OK",
+		Tools: []map[string]any{{
+			"type":        "function",
+			"name":        "get_weather",
+			"description": "Get weather",
+			"parameters":  map[string]any{"type": "object"},
+		}},
+	})
+	if err != nil {
+		t.Fatalf("Responses returned error: %v", err)
+	}
+	if gotPath != "/chat/completions" {
+		t.Fatalf("request path = %q, want /chat/completions", gotPath)
+	}
+	tools, ok := gotBody["tools"].([]any)
+	if !ok || len(tools) != 1 {
+		t.Fatalf("translated chat request tools = %v, want exactly one tool", gotBody["tools"])
+	}
+	tool := tools[0].(map[string]any)
+	fn, ok := tool["function"].(map[string]any)
+	if !ok {
+		t.Fatalf("translated tool = %v, want chat-completions function shape", tool)
+	}
+	if fn["name"] != "get_weather" {
+		t.Errorf("translated tool name = %q, want get_weather", fn["name"])
+	}
+	if resp == nil || resp.Model != "glm-5.2" {
+		t.Fatalf("response = %+v, want model glm-5.2", resp)
+	}
+}
+
+func TestResponses_APIModeDefaultKeepsNativePassthrough(t *testing.T) {
+	var gotPath string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{
+			"id": "resp_123",
+			"object": "response",
+			"created_at": 1677652288,
+			"model": "gpt-4o",
+			"status": "completed",
+			"output": []
+		}`))
+	}))
+	defer server.Close()
+
+	provider := New(providers.ProviderConfig{
+		APIKey:  "test-api-key",
+		BaseURL: server.URL,
+	}, providers.ProviderOptions{})
+
+	if _, err := provider.Responses(context.Background(), &core.ResponsesRequest{
+		Model: "gpt-4o",
+		Input: "ping",
+	}); err != nil {
+		t.Fatalf("Responses returned error: %v", err)
+	}
+	if gotPath != "/responses" {
+		t.Fatalf("request path = %q, want native /responses passthrough", gotPath)
+	}
+}
+
 func TestNilRequests_ReturnInvalidRequestError(t *testing.T) {
 	provider := NewWithHTTPClient("test-api-key", nil, llmclient.Hooks{})