diff --git a/internal/runtime/executor/github_copilot_executor.go b/internal/runtime/executor/github_copilot_executor.go
index fe724a62..b86146a3 100644
--- a/internal/runtime/executor/github_copilot_executor.go
+++ b/internal/runtime/executor/github_copilot_executor.go
@@ -653,6 +653,7 @@ func normalizeGitHubCopilotChatTools(body []byte) []byte {
 }
 
 func normalizeGitHubCopilotResponsesInput(body []byte) []byte {
+	body = stripGitHubCopilotResponsesUnsupportedFields(body)
 	input := gjson.GetBytes(body, "input")
 	if input.Exists() {
 		// If input is already a string or array, keep it as-is.
@@ -825,6 +826,12 @@ func normalizeGitHubCopilotResponsesInput(body []byte) []byte {
 	return body
 }
 
+func stripGitHubCopilotResponsesUnsupportedFields(body []byte) []byte {
+	// GitHub Copilot /responses rejects service_tier, so always remove it.
+	body, _ = sjson.DeleteBytes(body, "service_tier")
+	return body
+}
+
 func normalizeGitHubCopilotResponsesTools(body []byte) []byte {
 	tools := gjson.GetBytes(body, "tools")
 	if tools.Exists() {
diff --git a/internal/runtime/executor/github_copilot_executor_test.go b/internal/runtime/executor/github_copilot_executor_test.go
index 1055eab6..66d5ce92 100644
--- a/internal/runtime/executor/github_copilot_executor_test.go
+++ b/internal/runtime/executor/github_copilot_executor_test.go
@@ -132,6 +132,19 @@ func TestNormalizeGitHubCopilotResponsesInput_NonStringInputStringified(t *testi
 	}
 }
 
+func TestNormalizeGitHubCopilotResponsesInput_StripsServiceTier(t *testing.T) {
+	t.Parallel()
+	body := []byte(`{"input":"user text","service_tier":"default"}`)
+	got := normalizeGitHubCopilotResponsesInput(body)
+
+	if gjson.GetBytes(got, "service_tier").Exists() {
+		t.Fatalf("service_tier should be removed, got %s", gjson.GetBytes(got, "service_tier").Raw)
+	}
+	if gjson.GetBytes(got, "input").String() != "user text" {
+		t.Fatalf("input = %q, want %q", gjson.GetBytes(got, "input").String(), "user text")
+	}
+}
+
 func TestNormalizeGitHubCopilotResponsesTools_FlattenFunctionTools(t *testing.T) {
 	t.Parallel()
 	body := []byte(`{"tools":[{"type":"function","function":{"name":"sum","description":"d","parameters":{"type":"object"}}},{"type":"web_search"}]}`)