diff --git a/native/native_rust_library/Cargo.lock b/native/native_rust_library/Cargo.lock
--- a/native/native_rust_library/Cargo.lock
+++ b/native/native_rust_library/Cargo.lock
@@ -217,6 +217,12 @@
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "245097e9a4535ee1e3e3931fcfcd55a796a44c643e8596ff6566d68f09b87bbc"
 
+[[package]]
+name = "convert_case"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6245d59a3e82a7fc217c5828a6692dbc6dfb63a0c8c90495621f7b9d79704a0e"
+
 [[package]]
 name = "cpufeatures"
 version = "0.2.5"
@@ -348,6 +354,19 @@
  "syn",
 ]
 
+[[package]]
+name = "derive_more"
+version = "0.99.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fb810d30a7c1953f91334de7244731fc3f3c10d7fe163338a35b9f640960321"
+dependencies = [
+ "convert_case",
+ "proc-macro2",
+ "quote",
+ "rustc_version",
+ "syn",
+]
+
 [[package]]
 name = "digest"
 version = "0.9.0"
@@ -768,6 +787,7 @@
  "comm-opaque2",
  "cxx",
  "cxx-build",
+ "derive_more",
  "lazy_static",
  "opaque-ke 1.2.0",
  "prost",
@@ -1053,6 +1073,15 @@
  "winapi",
 ]
 
+[[package]]
+name = "rustc_version"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bfa0f585226d2e68097d4f95d113b15b83a82e819ab25717ec0590d9584ef366"
+dependencies = [
+ "semver",
+]
+
 [[package]]
 name = "scratch"
 version = "1.0.2"
@@ -1072,6 +1101,12 @@
  "zeroize",
 ]
 
+[[package]]
+name = "semver"
+version = "1.0.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bebd363326d05ec3e2f532ab7660680f3b02130d780c299bca73469d521bc0ed"
+
 [[package]]
 name = "serde"
 version = "1.0.144"
diff --git a/native/native_rust_library/Cargo.toml b/native/native_rust_library/Cargo.toml
--- a/native/native_rust_library/Cargo.toml
+++ b/native/native_rust_library/Cargo.toml
@@ -18,6 +18,7 @@
 tracing = "0.1"
 regex = "1.6"
 comm-opaque2 = {path = "../../shared/comm-opaque2"}
+derive_more = "0.99"
 
 [build-dependencies]
 cxx-build = "1.0"
diff --git a/native/native_rust_library/src/lib.rs b/native/native_rust_library/src/lib.rs
--- a/native/native_rust_library/src/lib.rs
+++ b/native/native_rust_library/src/lib.rs
@@ -105,7 +105,19 @@
     include!("RustCallback.h");
     #[namespace = "comm"]
     #[cxx_name = "stringCallback"]
-    fn string_callback(error: String, counter: u32, ret: String);
+    fn string_callback(error: String, promise_id: u32, ret: String);
+  }
+}
+
+fn handle_string_result_as_callback<E>(
+  result: Result<String, E>,
+  promise_id: u32,
+) where
+  E: std::fmt::Display,
+{
+  match result {
+    Err(e) => string_callback(e.to_string(), promise_id, "".to_string()),
+    Ok(r) => string_callback("".to_string(), promise_id, r),
   }
 }
 
@@ -221,3 +233,13 @@
       .expect("Failed to create Tokio runtime for the Tunnelbroker client"),
   })
 }
+
+#[derive(
+  Debug, derive_more::Display, derive_more::From, derive_more::Error,
+)]
+pub enum Error {
+  #[display(...)]
+  TonicGRPC(Status),
+  #[display(...)]
+  TonicTransport(tonic::transport::Error),
+}