diff --git a/pkg/model/provider/defaults.go b/pkg/model/provider/defaults.go
index ef084083e..714765d6c 100644
--- a/pkg/model/provider/defaults.go
+++ b/pkg/model/provider/defaults.go
@@ -43,6 +43,11 @@ func isOpenAICompatibleProvider(providerType string) bool {
 	return exists && alias.APIType == "openai"
 }
 
+// IsGithubCopilotProvider returns true if the provider type is "github-copilot".
+func isGithubCopilotProvider(providerType string) bool {
+	return providerType == "github-copilot"
+}
+
 // ---------------------------------------------------------------------------
 // Provider defaults
 // ---------------------------------------------------------------------------
@@ -180,6 +185,9 @@ func cloneModelConfig(cfg *latest.ModelConfig) *latest.ModelConfig {
 //
 // NOTE: max_tokens is NOT set here; see teamloader and runtime/model_switcher.
 func applyModelDefaults(cfg *latest.ModelConfig) {
+	// Set appropriate github copilot api_type.
+	applyGithubCopilotAPIType(cfg)
+
 	// Explicitly disabled → normalise to nil so providers never see it.
 	if cfg.ThinkingBudget.IsDisabled() {
 		cfg.ThinkingBudget = nil
@@ -225,6 +233,18 @@ func ensureInterleavedThinking(cfg *latest.ModelConfig, providerType string) {
 	}
 }
 
+func applyGithubCopilotAPIType(cfg *latest.ModelConfig) {
+	if isGithubCopilotProvider(cfg.Provider) && isCopilotResponsesModel(cfg.Model) {
+		if cfg.ProviderOpts == nil {
+			cfg.ProviderOpts = make(map[string]any)
+		}
+		// If it's not set, or was set to openai_chatcompletions by the generic fallback, override it.
+		if apiType, ok := cfg.ProviderOpts["api_type"].(string); !ok || apiType == "" || apiType == "openai_chatcompletions" {
+			cfg.ProviderOpts["api_type"] = "openai_responses"
+		}
+	}
+}
+
 // ---------------------------------------------------------------------------
 // Model-name predicates
 // ---------------------------------------------------------------------------
@@ -281,3 +301,14 @@ func isGeminiProModel(model string) bool {
 func isGeminiFlashModel(model string) bool {
 	return strings.HasPrefix(gemini3Family(model), "flash")
 }
+
+// isCopilotResponsesModel returns true if the model is a GitHub Copilot model that requires the openai_responses API type.
+func isCopilotResponsesModel(model string) bool {
+	codex := map[string]bool{
+		"gpt-5.3-codex": true,
+		"gpt-5.2-codex": true,
+		"gpt-5.4-mini":  true,
+		"gpt-5.4-nano":  true,
+	}
+	return codex[model]
+}
diff --git a/pkg/model/provider/model_defaults_test.go b/pkg/model/provider/model_defaults_test.go
index fd2e5d268..303109b85 100644
--- a/pkg/model/provider/model_defaults_test.go
+++ b/pkg/model/provider/model_defaults_test.go
@@ -289,3 +289,13 @@ func TestApplyProviderDefaults_DoesNotModifyOriginal(t *testing.T) {
 	// Original custom key must still be there.
 	assert.Equal(t, "original_value", original.ProviderOpts["custom_key"])
 }
+
+func TestIsCopilotResponsesModel(t *testing.T) {
+	t.Parallel()
+
+	assert.True(t, isCopilotResponsesModel("gpt-5.3-codex"))
+	assert.True(t, isCopilotResponsesModel("gpt-5.2-codex"))
+	assert.False(t, isCopilotResponsesModel("gpt-4o"))
+	assert.False(t, isCopilotResponsesModel("claude-sonnet-4-5"))
+	assert.False(t, isCopilotResponsesModel(""))
+}
diff --git a/pkg/model/provider/provider_defaults_test.go b/pkg/model/provider/provider_defaults_test.go
index fd264e0b6..9b51cb42f 100644
--- a/pkg/model/provider/provider_defaults_test.go
+++ b/pkg/model/provider/provider_defaults_test.go
@@ -383,3 +383,40 @@ func TestApplyProviderDefaults_AliasFallback(t *testing.T) {
 	assert.Empty(t, cfg.BaseURL)
 	assert.Empty(t, cfg.TokenKey)
 }
+
+func TestIsGithubCopilotProvider(t *testing.T) {
+	t.Parallel()
+
+	assert.True(t, isGithubCopilotProvider("github-copilot"))
+	assert.False(t, isGithubCopilotProvider("openai"))
+	assert.False(t, isGithubCopilotProvider(""))
+}
+
+func TestGithubCopilotApiType(t *testing.T) {
+	cfg := &latest.ModelConfig{
+		Provider: "github-copilot",
+		Model:    "gpt-5.3-codex",
+	}
+
+	enhancedCfg := applyProviderDefaults(cfg, nil)
+
+	apiType := resolveProviderType(enhancedCfg)
+
+	if apiType != "openai_responses" {
+		t.Errorf("Expected api_type to be 'openai_responses', got '%s'", apiType)
+	}
+
+	// test when it is a custom provider
+	customProviders := map[string]latest.ProviderConfig{
+		"github-copilot": {
+			Provider: "github-copilot",
+		},
+	}
+
+	enhancedCfg2 := applyProviderDefaults(cfg, customProviders)
+	apiType2 := resolveProviderType(enhancedCfg2)
+
+	if apiType2 != "openai_responses" {
+		t.Errorf("Expected api_type to be 'openai_responses', got '%s'", apiType2)
+	}
+}