diff --git a/scripts/build-lite-backend-artifact.sh b/scripts/build-lite-backend-artifact.sh
index f83072c..6e600ea 100755
--- a/scripts/build-lite-backend-artifact.sh
+++ b/scripts/build-lite-backend-artifact.sh
@@ -324,6 +324,9 @@ prepare_backend_source() {
     ln -s "$BACKEND_SOURCE_DIR/src" "$prepared_root/src"
     [[ -f "$BACKEND_SOURCE_DIR/Cargo.lock" ]] && ln -s "$BACKEND_SOURCE_DIR/Cargo.lock" "$prepared_root/Cargo.lock"
     [[ -d "$BACKEND_SOURCE_DIR/.cargo" ]] && ln -s "$BACKEND_SOURCE_DIR/.cargo" "$prepared_root/.cargo"
+    # Honor the pinned Rust toolchain (rust-toolchain.toml) inside the prepared root too,
+    # so builds using --silentdragonxlitelib-dir still select rustc 1.63.
+    [[ -f "$BACKEND_SOURCE_DIR/rust-toolchain.toml" ]] && ln -s "$BACKEND_SOURCE_DIR/rust-toolchain.toml" "$prepared_root/rust-toolchain.toml"
     [[ -d "$BACKEND_SOURCE_DIR/libsodium-mingw" ]] && ln -s "$BACKEND_SOURCE_DIR/libsodium-mingw" "$prepared_root/libsodium-mingw"
     # Vendored crate deps (offline builds): the .cargo/config.toml's vendored-sources directory is
     # "vendor" relative to the build root, so expose it inside the prepared root too.
diff --git a/third_party/silentdragonxlite/lib/rust-toolchain.toml b/third_party/silentdragonxlite/lib/rust-toolchain.toml
new file mode 100644
index 0000000..35f85f2
--- /dev/null
+++ b/third_party/silentdragonxlite/lib/rust-toolchain.toml
@@ -0,0 +1,12 @@
+# Rust toolchain pin for the vendored SilentDragonXLite (SDXL) backend.
+#
+# The pinned librustzcash + transitive crates (notably traitobject 0.1.0) rely on
+# pre-1.70 trait-coherence rules and fail to compile on newer rustc with
+#   error[E0119]: conflicting implementations of trait `Trait` for type `(dyn Send + Sync)`
+# so the backend must be built with 1.63 (the toolchain scripts/build-lite-backend-artifact.sh
+# and CLAUDE.md target). rustup auto-selects this when cargo runs in this tree, so no
+# RUSTUP_TOOLCHAIN / `cargo +1.63.0` is needed.
+#
+# Install it once with:  rustup toolchain install 1.63.0
+[toolchain]
+channel = "1.63.0"