diff --git a/server/adkrest/controllers/runtime.go b/server/adkrest/controllers/runtime.go
index 73259b231..20199e3b5 100644
--- a/server/adkrest/controllers/runtime.go
+++ b/server/adkrest/controllers/runtime.go
@@ -1,4 +1,4 @@
-// Copyright 2025 Google LLC
+// Copyright 2026 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -197,7 +197,9 @@ func (c *RuntimeAPIController) getRunner(req models.RunAgentRequest) (*runner.Ru
 func decodeRequestBody(req *http.Request) (decodedReq models.RunAgentRequest, err error) {
 	var runAgentRequest models.RunAgentRequest
 	defer func() {
-		err = req.Body.Close()
+		if cerr := req.Body.Close(); err == nil && cerr != nil {
+			err = cerr
+		}
 	}()
 	d := json.NewDecoder(req.Body)
 	d.DisallowUnknownFields()
diff --git a/server/adkrest/controllers/runtime_test.go b/server/adkrest/controllers/runtime_test.go
new file mode 100644
index 000000000..0e7d05c15
--- /dev/null
+++ b/server/adkrest/controllers/runtime_test.go
@@ -0,0 +1,57 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package controllers
+
+import (
+	"errors"
+	"io"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// errorCloser is a helper that wraps an io.Reader and returns an error on Close.
+type errorCloser struct {
+	io.Reader
+}
+
+func (errorCloser) Close() error {
+	return errors.New("persistent close error")
+}
+
+func TestDecodeRequestBody_DoesNotOverwriteDecodeErrorOnClose(t *testing.T) {
+	// Invalid JSON should produce a 400 error from decodeRequestBody.
+	// This test reproduces a bug where a deferred Body.Close() overwrites
+	// the decode error (named return value) and causes err to become nil.
+	req := httptest.NewRequest("POST", "http://example/runtime", io.NopCloser(strings.NewReader("{")))
+
+	_, err := decodeRequestBody(req)
+	if err == nil {
+		t.Fatalf("expected decodeRequestBody to return an error for invalid JSON, got nil")
+	}
+}
+
+func TestDecodeRequestBody_ReturnsErrorOnClose(t *testing.T) {
+	// Use valid JSON to ensure decoding succeeds.
+	const validJSON = `{"appName":"test-app","userId":"test-user","sessionId":"test-session","newMessage":{"parts":[{"text":"hello"}]}}`
+	req := httptest.NewRequest("POST", "http://example/runtime", errorCloser{strings.NewReader(validJSON)})
+
+	_, err := decodeRequestBody(req)
+	if err == nil {
+		t.Fatal("expected decodeRequestBody to return an error from Body.Close(), but got nil")
+	}
+	if !strings.Contains(err.Error(), "persistent close error") {
+		t.Fatalf("expected error to contain 'persistent close error', got: %v", err)
+	}
+}