diff --git a/CMakeLists.txt b/CMakeLists.txt
index e5052fa..1621834 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -55,6 +55,10 @@ if (Protobuf_FOUND)
 else()
     message("No installed Protobuf has registered itself with CMake.")
 
+    # TODO: When we upgrade to CMake 3.13 minimum, use its protobuf_generate()
+    # and figure out how to not get it to pass two -I options that then make
+    # protoc claim to succeed but not actually write any files.
+
     # Fall back to FindProtobuf.cmake and hope it is a version that works with the installed Protobuf
     find_package(Protobuf REQUIRED)
 
@@ -65,7 +69,14 @@ else()
 endif()
 
 
-
+# We now have PROTO_SRCS and PROTO_HDRS. But the files for them don't generate
+# until build time. If we add PROTO_SRCS as sources for two different library
+# targets (at least with FindProtobuf.cmake), each library target gets Makefile
+# rules to run protoc, but they try to write to the same output location. If
+# they run in parallel, this causes corruption of the generated files. So we
+# need to sequence things so the Protobuf compiler runs only once, before any
+# of the libraries try to build.
+add_custom_target(run_protoc SOURCES ${PROTO_SRCS} ${PROTO_HDRS} DEPENDS ${PROTO_SRCS} ${PROTO_HDRS})
 
 
 # Find threads
@@ -149,6 +160,10 @@ set_property(TARGET vgio_static PROPERTY POSITION_INDEPENDENT_CODE OFF)
 add_dependencies(vgio link_target)
 add_dependencies(vgio_static link_target)
 
+# Don't build any object files until the protoc-generated sources are generated, once.
+add_dependencies(vgio run_protoc)
+add_dependencies(vgio_static run_protoc)
+
 # Add an alias so that library can be used inside the build tree, e.g. when testing
 add_library(VGio::vgio ALIAS vgio)
 
diff --git a/include/vg/io/alignment_io.hpp b/include/vg/io/alignment_io.hpp
index e9d7bde..f91edaa 100644
--- a/include/vg/io/alignment_io.hpp
+++ b/include/vg/io/alignment_io.hpp
@@ -100,11 +100,19 @@ gafkluge::GafRecord alignment_to_gaf(const HandleGraph& graph,
                                      bool frag_links = true);
 // TODO: These will need to be able to take a forward translation to read named-segment GAF.
 /// Convert a GAF alignment into a vg Alignment. The alignment must be in node ID space.
+///
+/// All tags are preserved in the "tags" annotation, except those interpreted
+/// to construct the Alignment. If the "cg" tag was used to construct the
+/// alignment, it will be annotated with "from_cg" true.
 void gaf_to_alignment(function<size_t(nid_t)> node_to_length,
                       function<string(nid_t, bool)> node_to_sequence,
                       const gafkluge::GafRecord& gaf,
                       Alignment& aln);
 /// Convert a GAF alignment into a vg Alignment. The alignment must be in node ID space.
+///
+/// All tags are preserved in the "tags" annotation, except those interpreted
+/// to construct the Alignment. If the "cg" tag was used to construct the
+/// alignment, it will be annotated with "from_cg" true.
 void gaf_to_alignment(const HandleGraph& graph,
                       const gafkluge::GafRecord& gaf,
                       Alignment& aln);
diff --git a/src/alignment_io.cpp b/src/alignment_io.cpp
index d4dd3da..a0335b2 100644
--- a/src/alignment_io.cpp
+++ b/src/alignment_io.cpp
@@ -1116,6 +1116,7 @@ void gaf_to_alignment(function<size_t(nid_t)> node_to_length,
         }
     }
 
+    std::stringstream extra_tags;
     for (auto opt_it : gaf.opt_fields) {
         if (opt_it.first == "dv") {
             // get the identity from the dv divergence field
@@ -1143,8 +1144,27 @@ void gaf_to_alignment(function<size_t(nid_t)> node_to_length,
         } else if (opt_it.first == "sa") {
             // we use "sa" as opposed to "SA" for our particular articulation of supplementary alignments
             decode_supplementary_tag_value(aln, opt_it.second.second, node_to_length);
+        } else if (opt_it.first == "cs" || opt_it.first == "cg") {
+            // Skip cs and cg fields since we already read them.
+            // So do nothing here.
+        } else {
+            // Other optional fields need to go into the "tags" annotation,
+            // tab-separated, as in SAM.
+            if (extra_tags.tellp() != std::streampos(0)) {
+                // Separate from previous tag with a tab
+                extra_tags << "\t";
+            }
+            // TODO: Deduplicate with serialization in gfakluge.hpp?
+            extra_tags << opt_it.first << ":" << opt_it.second.first << ":" << opt_it.second.second;
         }
     }
+    if (extra_tags.tellp() != std::streampos(0)) {
+        // Attach the tags annotation
+        auto* annotation = aln.mutable_annotation();
+        google::protobuf::Value extra_tags_value;
+        extra_tags_value.set_string_value(extra_tags.str());
+        (*annotation->mutable_fields())["tags"] = extra_tags_value;
+    }
 }
 
 void gaf_to_alignment(const HandleGraph& graph,