diff --git a/internal/librariangen/generate/generator.go b/internal/librariangen/generate/generator.go
index 2ecf3c05a5..8a1b00e521 100644
--- a/internal/librariangen/generate/generator.go
+++ b/internal/librariangen/generate/generator.go
@@ -27,15 +27,15 @@ import (
 
 	"cloud.google.com/java/internal/librariangen/bazel"
 	"cloud.google.com/java/internal/librariangen/execv"
+	"cloud.google.com/java/internal/librariangen/message"
 	"cloud.google.com/java/internal/librariangen/protoc"
-	"cloud.google.com/java/internal/librariangen/request"
 )
 
 // Test substitution vars.
 var (
 	bazelParse   = bazel.Parse
 	execvRun     = execv.Run
-	requestParse = request.ParseLibrary
+	requestParse = message.ParseLibrary
 	protocBuild  = protoc.Build
 )
 
@@ -113,7 +113,7 @@ func Generate(ctx context.Context, cfg *Config) error {
 // invokeProtoc handles the protoc GAPIC generation logic for the 'generate' CLI command.
 // It reads a request file, and for each API specified, it invokes protoc
 // to generate the client library. It returns the module path and the path to the service YAML.
-func invokeProtoc(ctx context.Context, cfg *Config, generateReq *request.Library) error {
+func invokeProtoc(ctx context.Context, cfg *Config, generateReq *message.Library) error {
 	for _, api := range generateReq.APIs {
 		apiServiceDir := filepath.Join(cfg.SourceDir, api.Path)
 		slog.Info("processing api", "service_dir", apiServiceDir)
@@ -135,7 +135,7 @@ func invokeProtoc(ctx context.Context, cfg *Config, generateReq *request.Library
 // readGenerateReq reads generate-request.json from the librarian-tool input directory.
 // The request file tells librariangen which library and APIs to generate.
 // It is prepared by the Librarian tool and mounted at /librarian.
-func readGenerateReq(librarianDir string) (*request.Library, error) {
+func readGenerateReq(librarianDir string) (*message.Library, error) {
 	reqPath := filepath.Join(librarianDir, "generate-request.json")
 	slog.Debug("librariangen: reading generate request", "path", reqPath)
 
@@ -264,4 +264,4 @@ func unzip(src, dest string) error {
 		}
 	}
 	return nil
-}
\ No newline at end of file
+}
diff --git a/internal/librariangen/request/request.go b/internal/librariangen/message/message.go
similarity index 82%
rename from internal/librariangen/request/request.go
rename to internal/librariangen/message/message.go
index 72734e1edc..fc032bd540 100644
--- a/internal/librariangen/request/request.go
+++ b/internal/librariangen/message/message.go
@@ -12,7 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package request
+// Package message defines data types which the Librarian CLI and language
+// containers exchange.
+// There shouldn't be CLI-specific data type or language container-specific
+// data types in this package.
+// TODO(b/447404382): Move this package to the https://github.com/googleapis/librarian
+// GitHub repository once the interface is finalized.
+package message
 
 import (
 	"encoding/json"
@@ -20,6 +26,16 @@ import (
 	"os"
 )
 
+// ReleaseInitRequest is the structure of the release-init-request.json file.
+type ReleaseInitRequest struct {
+	Libraries []*Library `json:"libraries"`
+}
+
+// ReleaseInitResponse is the structure of the release-init-response.json file.
+type ReleaseInitResponse struct {
+	Error string `json:"error,omitempty"`
+}
+
 // Library is the combination of all the fields used by CLI requests and responses.
 // Each CLI command has its own request/response type, but they all use Library.
 type Library struct {
@@ -75,4 +91,4 @@ func ParseLibrary(path string) (*Library, error) {
 	}
 
 	return &req, nil
-}
\ No newline at end of file
+}
diff --git a/internal/librariangen/request/request_test.go b/internal/librariangen/message/message_test.go
similarity index 99%
rename from internal/librariangen/request/request_test.go
rename to internal/librariangen/message/message_test.go
index 03f05b2b23..2599570313 100644
--- a/internal/librariangen/request/request_test.go
+++ b/internal/librariangen/message/message_test.go
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package request
+package message
 
 import (
 	"os"
@@ -95,4 +95,4 @@ func TestParseLibrary_FileNotFound(t *testing.T) {
 	if err == nil {
 		t.Error("Parse() expected error for non-existent file, got nil")
 	}
-}
\ No newline at end of file
+}