diff --git a/src/algorithms/chain_items.cpp b/src/algorithms/chain_items.cpp
index c84e399ce1..8bd5a15d53 100644
--- a/src/algorithms/chain_items.cpp
+++ b/src/algorithms/chain_items.cpp
@@ -13,6 +13,7 @@
 
 //#define debug_chaining
 //#define debug_transition
+//#define debug_missing_transition
 //#define debug_dp
 
 namespace vg {
@@ -262,6 +263,18 @@ transition_iterator zip_tree_transition_iterator(const std::vector<SnarlDistance
             generate_zip_tree_transitions(seeds, zip_code_tree, max_graph_lookback_bases,
                                           seed_to_starting, seed_to_ending);
 
+#ifdef debug_missing_transition
+        bool has_missing = \
+        find_missing_zip_tree_transitions(seeds, zip_code_tree, max_graph_lookback_bases,
+                                          seed_to_starting, seed_to_ending, distance_index, 
+                                          all_transitions);
+        if (has_missing) {
+            throw std::runtime_error("Zipcode tree iterator failed to output some transitions");
+        } else {
+            cerr << "No missing transitions" << endl;
+        }
+#endif
+
         std::vector<transition_info> filtered_transitions =
             calculate_transition_read_distances(all_transitions, to_chain, max_read_lookback_bases);
 
@@ -385,6 +398,103 @@ std::vector<transition_info> generate_zip_tree_transitions(
     return all_transitions;
 }
 
+bool find_missing_zip_tree_transitions(
+    const std::vector<SnarlDistanceIndexClusterer::Seed>& seeds,
+    const ZipCodeTree& zip_code_tree,
+    size_t max_graph_lookback_bases,
+    const std::unordered_map<size_t, size_t>& seed_to_starting, 
+    const std::unordered_map<size_t, size_t>& seed_to_ending,
+    const SnarlDistanceIndex& distance_index,
+    const std::vector<transition_info>& all_transitions) {
+    
+    // {source anchor : {dest anchor : dist}}
+    std::unordered_map<size_t, std::unordered_map<size_t, size_t>> found;
+    for (const auto& transition : all_transitions) {
+        size_t dist_to_save = transition.graph_distance;
+        if (!found.count(transition.from_anchor)) {
+            found[transition.from_anchor] = std::unordered_map<size_t, size_t>();
+        }
+        if (found[transition.from_anchor].count(transition.to_anchor)) {
+            // If a transition appears multiple times, remember the min
+            dist_to_save = std::min(transition.graph_distance, 
+                                    found[transition.from_anchor][transition.to_anchor]);
+        }
+        found[transition.from_anchor][transition.to_anchor] = transition.graph_distance;
+    }
+
+    bool has_missing = false;
+
+    // Helper function to check for a distance between two seeds
+    auto check_distance = [&] (const ZipCodeTree::oriented_seed_t& from_seed, bool rev_from,
+                               const ZipCodeTree::oriented_seed_t& to_seed, bool rev_to) {
+        // XOR to get appropriate orientations
+        rev_from ^= from_seed.is_reversed;
+        rev_to ^= to_seed.is_reversed;
+        if (rev_from != rev_to) {
+            // Cannot be compared; incompatible orientations
+            return;
+        }
+
+        // Look up appropriate anchors
+        auto from_anchor_itr = rev_from ? seed_to_starting.find(from_seed.seed)
+                                        : seed_to_ending.find(from_seed.seed);
+        if ((rev_from && from_anchor_itr == seed_to_starting.end())
+            || (!rev_from && from_anchor_itr == seed_to_ending.end())) {
+            // No anchor exists
+            return;
+        }
+        auto to_anchor_itr = rev_to ? seed_to_ending.find(to_seed.seed)
+                                    : seed_to_starting.find(to_seed.seed);
+        if ((rev_to && to_anchor_itr == seed_to_ending.end())
+            || (!rev_to && to_anchor_itr == seed_to_starting.end())) {
+            // No anchor exists
+            return;
+        }
+
+        // Construct seed positions
+        pos_t from_pos = seeds.at(from_seed.seed).pos;
+        size_t from_length = distance_index.minimum_length(distance_index.get_node_net_handle(id(from_pos)));
+        from_pos = rev_from ? reverse(from_pos, from_length)
+                            : from_pos;
+        pos_t to_pos = seeds.at(to_seed.seed).pos;
+        size_t to_length = distance_index.minimum_length(distance_index.get_node_net_handle(id(to_pos)));
+        to_pos = rev_to ? reverse(to_pos, to_length)
+                        : to_pos;
+
+        // Look up true minimum distance
+        size_t true_distance = minimum_nontrivial_distance(distance_index, from_pos, to_pos);
+        if (true_distance <= max_graph_lookback_bases) {
+            // We should've found this transition
+            auto from_anchor = from_anchor_itr->second;
+            auto to_anchor = to_anchor_itr->second;
+            if (!found.count(from_anchor) 
+                || !found[from_anchor].count(to_anchor)
+                || found[from_anchor][to_anchor] != true_distance) {
+                has_missing = true;
+                cerr << "Missing transition " << from_pos << "->" 
+                     << to_pos << " dist " << true_distance << endl;
+            }
+        }
+    };
+    
+    vector<ZipCodeTree::oriented_seed_t> tree_seeds = zip_code_tree.get_all_seeds();
+    for (size_t i = 0; i < tree_seeds.size(); i++) {
+        // Check self-loops
+        check_distance(tree_seeds[i], false, tree_seeds[i], false);
+        check_distance(tree_seeds[i], false, tree_seeds[i], true);
+        check_distance(tree_seeds[i], true, tree_seeds[i], false);
+        for (size_t j = i + 1; j < tree_seeds.size(); j++) {
+            // Check all orientation pairs
+            check_distance(tree_seeds[i], false, tree_seeds[j], false);
+            check_distance(tree_seeds[i], false, tree_seeds[j], true);
+            check_distance(tree_seeds[i], true, tree_seeds[j], false);
+            check_distance(tree_seeds[i], true, tree_seeds[j], true);
+        }
+    }
+
+    return has_missing;
+}
+
 std::vector<transition_info> calculate_transition_read_distances(
     const std::vector<transition_info>& all_transitions,
     const VectorView<Anchor>& to_chain,
diff --git a/src/algorithms/chain_items.hpp b/src/algorithms/chain_items.hpp
index 9ac7b792e0..f4531d4201 100644
--- a/src/algorithms/chain_items.hpp
+++ b/src/algorithms/chain_items.hpp
@@ -477,6 +477,23 @@ std::vector<transition_info> generate_zip_tree_transitions(
     const std::unordered_map<size_t, size_t>& seed_to_starting, 
     const std::unordered_map<size_t, size_t>& seed_to_ending);
 
+/**
+ * Check if all possible transitions were actually found.
+ * 
+ * Iterates over all pairs of seeds and uses the distance index
+ * to determine if there SHOULD have been a transition.
+ * 
+ * Returns if any transitions were missing.
+ */
+bool find_missing_zip_tree_transitions(
+    const std::vector<SnarlDistanceIndexClusterer::Seed>& seeds,
+    const ZipCodeTree& zip_code_tree,
+    size_t max_graph_lookback_bases,
+    const std::unordered_map<size_t, size_t>& seed_to_starting, 
+    const std::unordered_map<size_t, size_t>& seed_to_ending,
+    const SnarlDistanceIndex& distance_index,
+    const std::vector<transition_info>& all_transitions);
+
 /**
  * Calculate read distances for each of the zip tree's transitions.
  * Also filters out transitions that can't be used,