feat(provider): support chat completions for openai providers

pkg/config/config.go

@@ -260,6 +260,7 @@ type ProviderOAuthConfig struct {
 }
 
 type ProviderResponsesConfig struct {
+	API                      string   `json:"api,omitempty"`
 	WebSearchEnabled         bool     `json:"web_search_enabled"`
 	WebSearchContextSize     string   `json:"web_search_context_size"`
 	FileSearchVectorStoreIDs []string `json:"file_search_vector_store_ids"`

pkg/config/normalized_test.go

@@ -11,6 +11,9 @@ func TestNormalizedViewProjectsCoreAndRuntime(t *testing.T) {
 		MaxTokens:   12288,
 		Temperature: 0.35,
 		TimeoutSec:  90,
+		Responses: ProviderResponsesConfig{
+			API: "chat_completions",
+		},
 	}
 	cfg.Agents.Subagents["coder"] = SubagentConfig{
 		Enabled:          true,
@@ -40,4 +43,7 @@ func TestNormalizedViewProjectsCoreAndRuntime(t *testing.T) {
 	if got := view.Runtime.Providers["openai"].Temperature; got != 0.35 {
 		t.Fatalf("expected provider temperature in normalized runtime view, got %v", got)
 	}
+	if got := view.Runtime.Providers["openai"].Responses.API; got != "chat_completions" {
+		t.Fatalf("expected provider responses.api in normalized runtime view, got %q", got)
+	}
 }

pkg/config/validate.go

@@ -515,6 +515,13 @@ func validateProviderConfig(path string, p ProviderConfig) []error {
 	if p.OAuth.CooldownSec < 0 {
 		errs = append(errs, fmt.Errorf("%s.oauth.cooldown_sec must be >= 0", path))
 	}
+	if p.Responses.API != "" {
+		switch strings.TrimSpace(p.Responses.API) {
+		case "responses", "chat_completions":
+		default:
+			errs = append(errs, fmt.Errorf("%s.responses.api must be one of: responses, chat_completions", path))
+		}
+	}
 	if p.Responses.WebSearchContextSize != "" {
 		switch p.Responses.WebSearchContextSize {
 		case "low", "medium", "high":

pkg/config/validate_test.go

@@ -247,3 +247,27 @@ func TestValidateProviderHybridRequiresOAuthProvider(t *testing.T) {
 		t.Fatalf("expected oauth.provider validation error, got %v", errs)
 	}
 }
+
+func TestValidateProviderResponsesAPIRejectsUnknownValue(t *testing.T) {
+	t.Parallel()
+
+	cfg := DefaultConfig()
+	pc := cfg.Models.Providers["openai"]
+	pc.Responses.API = "legacy"
+	cfg.Models.Providers["openai"] = pc
+
+	errs := Validate(cfg)
+	if len(errs) == 0 {
+		t.Fatalf("expected validation errors")
+	}
+	found := false
+	for _, err := range errs {
+		if strings.Contains(err.Error(), "models.providers.openai.responses.api") {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("expected responses.api validation error, got %v", errs)
+	}
+}