diff --git a/cargo/build/protobuf.rs b/cargo/build/protobuf.rs
index 6d279fd85..e3fd67169 100644
--- a/cargo/build/protobuf.rs
+++ b/cargo/build/protobuf.rs
@@ -12,12 +12,49 @@ pub fn collect_protobuf_includes() -> Vec<PathBuf> {
     paths::get_env_paths("PROTOBUF_INCLUDE_PATH")
 }
 
+fn is_protobuf_lib(name: &str) -> bool {
+    // Protobuf's CMake build prefixes these archives with `lib` on every platform,
+    // including Windows, where `collect_static_libs` keeps the file stem as-is.
+    matches!(name.strip_prefix("lib").unwrap_or(name), "protobuf" | "utf8_validity")
+}
+
 /// Returns the paths to the protobuf libraries (to be used as a search path) and a
 /// list of libraries to be linked.
+///
+/// This is an allowlist rather than "everything in the directory", because a protobuf
+/// install tree holds several archives that must not be linked together:
+///
+/// * `protobuf` is the library Crubit uses, and it has undefined references into
+///   `utf8_validity`, so the two belong together.
+/// * `utf8_range` is compiled from the same `utf8_range.c` as `utf8_validity` and
+///   defines the same symbols, so linking both is a duplicate symbol error.
+/// * `protobuf-lite` is a subset of `protobuf`, and would collide with it likewise.
+/// * `protoc` and `upb` are not used by Crubit.
 pub fn collect_protobuf_libs() -> (Vec<PathBuf>, Vec<OsString>) {
-    paths::collect_static_libs("PROTOBUF_LIB_STATIC_PATH", |name| {
-        name.strip_prefix("lib").unwrap_or(name) == "protobuf"
-    })
+    paths::collect_static_libs("PROTOBUF_LIB_STATIC_PATH", is_protobuf_lib)
+}
+
+/// Unit tests for protobuf library allowlist matching.
+///
+/// Note: These tests use standard `#[test]` rather than `googletest` because
+/// `crubit_build` is a build helper crate built via Cargo.
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test] // allow_core_test (see mod tests doc comment)
+    fn test_is_protobuf_lib() {
+        assert!(is_protobuf_lib("protobuf"));
+        assert!(is_protobuf_lib("libprotobuf"));
+        assert!(is_protobuf_lib("utf8_validity"));
+        assert!(is_protobuf_lib("libutf8_validity"));
+        assert!(!is_protobuf_lib("utf8_range"));
+        assert!(!is_protobuf_lib("libutf8_range"));
+        assert!(!is_protobuf_lib("protobuf-lite"));
+        assert!(!is_protobuf_lib("libprotobuf-lite"));
+        assert!(!is_protobuf_lib("protoc"));
+        assert!(!is_protobuf_lib("upb"));
+    }
 }
 
 /// Locates pre-generated .pb.cc C++ source files corresponding to `proto_sources`
diff --git a/docs/cpp/building.md b/docs/cpp/building.md
index 2bf843da2..c88572030 100644
--- a/docs/cpp/building.md
+++ b/docs/cpp/building.md
@@ -94,7 +94,9 @@ export ABSL_INCLUDE_PATH=/path/to/absl/include/dir
 export ABSL_LIB_STATIC_PATH=/path/to/absl/libs
 ## This path contains google/protobuf/ dir with all the includes.
 export PROTOBUF_INCLUDE_PATH=/path/to/protobuf/include/dir
-## This path contains libprotobuf.a
+## This path contains libprotobuf.a and libutf8_validity.a.  Other archives that a
+## protobuf install tree holds (libutf8_range.a, libprotobuf-lite.a, libprotoc.a,
+## libupb.a) are ignored; the first two would collide with the ones above.
 export PROTOBUF_LIB_STATIC_PATH=/path/to/protobuf/libs
 
 cargo build --bin rs_bindings_from_cc