diff --git a/data/lm/trie b/data/lm/trie
index a937a226..87dd3a32 100644
--- a/data/lm/trie
+++ b/data/lm/trie
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:a5324f06b27c7b4ef88dd2c8bc3d05d4c718db219c2e007f59061a30c9ac7afa
-size 21627983
+oid sha256:1991108e83cf86830cad118ac9e061aadfc89b73a909a66fe1755f846def556e
+size 24480560
diff --git a/data/smoke_test/vocab.trie b/data/smoke_test/vocab.trie
index 3a5d637f..7237e739 100644
Binary files a/data/smoke_test/vocab.trie and b/data/smoke_test/vocab.trie differ
diff --git a/native_client/ctcdecode/ctc_beam_search_decoder.cpp b/native_client/ctcdecode/ctc_beam_search_decoder.cpp
index 1b7f0e98..e68d778f 100644
--- a/native_client/ctcdecode/ctc_beam_search_decoder.cpp
+++ b/native_client/ctcdecode/ctc_beam_search_decoder.cpp
@@ -12,8 +12,6 @@
 #include "fst/fstlib.h"
 #include "path_trie.h"
 
-using FSTMATCH = fst::SortedMatcher<fst::StdVectorFst>;
-
 DecoderState*
 decoder_init(const Alphabet &alphabet,
              int class_dim,
@@ -41,7 +39,7 @@ decoder_init(const Alphabet &alphabet,
   if (ext_scorer != nullptr && !ext_scorer->is_character_based()) {
     auto dict_ptr = ext_scorer->dictionary->Copy(true);
     root->set_dictionary(dict_ptr);
-    auto matcher = std::make_shared<FSTMATCH>(*dict_ptr, fst::MATCH_INPUT);
+    auto matcher = std::make_shared<fst::SortedMatcher<PathTrie::FstType>>(*dict_ptr, fst::MATCH_INPUT);
     root->set_matcher(matcher);
   }
   
diff --git a/native_client/ctcdecode/path_trie.cpp b/native_client/ctcdecode/path_trie.cpp
index 7fe3fbde..560361ec 100644
--- a/native_client/ctcdecode/path_trie.cpp
+++ b/native_client/ctcdecode/path_trie.cpp
@@ -162,13 +162,12 @@ void PathTrie::remove() {
   }
 }
 
-void PathTrie::set_dictionary(fst::StdVectorFst* dictionary) {
+void PathTrie::set_dictionary(PathTrie::FstType* dictionary) {
   dictionary_ = dictionary;
   dictionary_state_ = dictionary->Start();
   has_dictionary_ = true;
 }
 
-using FSTMATCH = fst::SortedMatcher<fst::StdVectorFst>;
-void PathTrie::set_matcher(std::shared_ptr<FSTMATCH> matcher) {
+void PathTrie::set_matcher(std::shared_ptr<fst::SortedMatcher<FstType>> matcher) {
   matcher_ = matcher;
 }
diff --git a/native_client/ctcdecode/path_trie.h b/native_client/ctcdecode/path_trie.h
index baa27dbe..10a1b687 100644
--- a/native_client/ctcdecode/path_trie.h
+++ b/native_client/ctcdecode/path_trie.h
@@ -14,6 +14,8 @@
  */
 class PathTrie {
 public:
+  using FstType = fst::ConstFst<fst::StdArc>;
+
   PathTrie();
   ~PathTrie();
 
@@ -33,9 +35,9 @@ public:
   void iterate_to_vec(std::vector<PathTrie*>& output);
 
   // set dictionary for FST
-  void set_dictionary(fst::StdVectorFst* dictionary);
+  void set_dictionary(FstType* dictionary);
 
-  void set_matcher(std::shared_ptr<fst::SortedMatcher<fst::StdVectorFst>>);
+  void set_matcher(std::shared_ptr<fst::SortedMatcher<FstType>>);
 
   bool is_empty() { return ROOT_ == character; }
 
@@ -61,10 +63,10 @@ private:
   std::vector<std::pair<int, PathTrie*>> children_;
 
   // pointer to dictionary of FST
-  fst::StdVectorFst* dictionary_;
-  fst::StdVectorFst::StateId dictionary_state_;
+  FstType* dictionary_;
+  FstType::StateId dictionary_state_;
   // true if finding ars in FST
-  std::shared_ptr<fst::SortedMatcher<fst::StdVectorFst>> matcher_;
+  std::shared_ptr<fst::SortedMatcher<FstType>> matcher_;
 };
 
 #endif  // PATH_TRIE_H
diff --git a/native_client/ctcdecode/scorer.cpp b/native_client/ctcdecode/scorer.cpp
index 39b9caf4..92479300 100644
--- a/native_client/ctcdecode/scorer.cpp
+++ b/native_client/ctcdecode/scorer.cpp
@@ -26,8 +26,8 @@
 
 using namespace lm::ngram;
 
-static const int MAGIC = 'TRIE';
-static const int FILE_VERSION = 3;
+static const int32_t MAGIC = 'TRIE';
+static const int32_t FILE_VERSION = 4;
 
 Scorer::Scorer(double alpha,
                double beta,
@@ -123,7 +123,9 @@ void Scorer::setup(const std::string& lm_path, const std::string& trie_path)
 
     if (!is_character_based_) {
       fst::FstReadOptions opt;
-      dictionary.reset(fst::StdVectorFst::Read(fin, opt));
+      opt.mode = fst::FstReadOptions::MAP;
+      opt.source = trie_path;
+      dictionary.reset(FstType::Read(fin, opt));
     }
   }
 
@@ -138,6 +140,8 @@ void Scorer::save_dictionary(const std::string& path)
   fout.write(reinterpret_cast<const char*>(&is_character_based_), sizeof(is_character_based_));
   if (!is_character_based_) {
     fst::FstWriteOptions opt;
+    opt.align = true;
+    opt.source = path;
     dictionary->Write(fout, opt);
   }
 }
@@ -248,6 +252,8 @@ std::vector<std::string> Scorer::make_ngram(PathTrie* prefix)
 
 void Scorer::fill_dictionary(const std::vector<std::string>& vocabulary, bool add_space)
 {
+  // ConstFst is immutable, so we need to use a MutableFst to create the trie,
+  // and then we convert to a ConstFst for the decoder and for storing on disk.
   fst::StdVectorFst dictionary;
   // For each unigram convert to ints and put in trie
   for (const auto& word : vocabulary) {
@@ -262,18 +268,21 @@ void Scorer::fill_dictionary(const std::vector<std::string>& vocabulary, bool ad
    * can greatly increase the size of the FST
    */
   fst::RmEpsilon(&dictionary);
-  fst::StdVectorFst* new_dict = new fst::StdVectorFst;
+  std::unique_ptr<fst::StdVectorFst> new_dict(new fst::StdVectorFst);
 
   /* This makes the FST deterministic, meaning for any string input there's
    * only one possible state the FST could be in.  It is assumed our
    * dictionary is deterministic when using it.
    * (lest we'd have to check for multiple transitions at each state)
    */
-  fst::Determinize(dictionary, new_dict);
+  fst::Determinize(dictionary, new_dict.get());
 
   /* Finds the simplest equivalent fst. This is unnecessary but decreases
    * memory usage of the dictionary
    */
-  fst::Minimize(new_dict);
-  this->dictionary.reset(new_dict);
+  fst::Minimize(new_dict.get());
+
+  // Now we convert the MutableFst to a ConstFst (Scorer::FstType) via its ctor
+  std::unique_ptr<FstType> converted(new FstType(*new_dict));
+  this->dictionary = std::move(converted);
 }
diff --git a/native_client/ctcdecode/scorer.h b/native_client/ctcdecode/scorer.h
index 9a81d4d6..2e881077 100644
--- a/native_client/ctcdecode/scorer.h
+++ b/native_client/ctcdecode/scorer.h
@@ -40,6 +40,8 @@ public:
  *     scorer.get_sent_log_prob({ "WORD1", "WORD2", "WORD3" });
  */
 class Scorer {
+  using FstType = PathTrie::FstType;
+
 public:
   Scorer(double alpha,
          double beta,
@@ -82,7 +84,7 @@ public:
   double beta;
 
   // pointer to the dictionary of FST
-  std::unique_ptr<fst::StdVectorFst> dictionary;
+  std::unique_ptr<FstType> dictionary;
 
 protected:
   // necessary setup: load language model, fill FST's dictionary
diff --git a/native_client/generate_trie.cpp b/native_client/generate_trie.cpp
index 0beb9d41..49944c86 100644
--- a/native_client/generate_trie.cpp
+++ b/native_client/generate_trie.cpp
@@ -3,7 +3,6 @@
 #include <string>
 
 #include "ctcdecode/scorer.h"
-#include "fst/fstlib.h"
 #include "alphabet.h"
 
 using namespace std;
diff --git a/native_client/trie_load.cc b/native_client/trie_load.cc
index ec0073c9..a719a431 100644
--- a/native_client/trie_load.cc
+++ b/native_client/trie_load.cc
@@ -3,7 +3,6 @@
 #include <string>
 
 #include "ctcdecode/scorer.h"
-#include "fst/fstlib.h"
 #include "alphabet.h"
 
 using namespace std;