chore: add a unique id to each origin tag

barretenberg/cpp/src/barretenberg/transcript/origin_tag.cpp

@@ -92,7 +92,8 @@ OriginTag::OriginTag(const OriginTag& tag_a, const OriginTag& tag_b)
 {
     // Elements with instant death should not be touched
     if (tag_a.instant_death || tag_b.instant_death) {
-        throw_or_abort("Touched an element that should not have been touched");
+        throw_or_abort("Touched an element that should not have been touched. tag_a id: " +
+                       std::to_string(tag_a.tag_id) + ", tag_b id: " + std::to_string(tag_b.tag_id));
     }
     // If one of the tags is a constant, just use the other tag
     if (tag_a.transcript_index == CONSTANT) {
@@ -107,7 +108,9 @@ OriginTag::OriginTag(const OriginTag& tag_a, const OriginTag& tag_b)
     // A free witness element should not interact with an element that has an origin
     if (tag_a.is_free_witness()) {
         if (!tag_b.is_free_witness() && !tag_b.is_empty()) {
-            throw_or_abort("A free witness element should not interact with an element that has an origin");
+            throw_or_abort(
+                "A free witness element (id: " + std::to_string(tag_a.tag_id) +
+                ") should not interact with an element that has an origin (id: " + std::to_string(tag_b.tag_id) + ")");
         } else {
             // If both are free witnesses or one of them is empty, just use tag_a
             *this = tag_a;
@@ -116,7 +119,9 @@ OriginTag::OriginTag(const OriginTag& tag_a, const OriginTag& tag_b)
     }
     if (tag_b.is_free_witness()) {
         if (!tag_a.is_free_witness() && !tag_a.is_empty()) {
-            throw_or_abort("A free witness element should not interact with an element that has an origin");
+            throw_or_abort(
+                "A free witness element (id: " + std::to_string(tag_b.tag_id) +
+                ") should not interact with an element that has an origin (id: " + std::to_string(tag_a.tag_id) + ")");
         } else {
             // If both are free witnesses or one of them is empty, just use tag_b
             *this = tag_b;
@@ -125,7 +130,10 @@ OriginTag::OriginTag(const OriginTag& tag_a, const OriginTag& tag_b)
     }
     // Elements from different transcripts shouldn't interact
     if (tag_a.transcript_index != tag_b.transcript_index) {
-        throw_or_abort("Tags from different transcripts were involved in the same computation");
+        throw_or_abort("Tags from different transcripts were involved in the same computation. tag_a: { id: " +
+                       std::to_string(tag_a.tag_id) + ", transcript: " + std::to_string(tag_a.transcript_index) +
+                       " } tag_b: { id: " + std::to_string(tag_b.tag_id) +
+                       ", transcript: " + std::to_string(tag_b.transcript_index) + " }");
     }
     // Check that submitted values from different rounds don't mix without challenges
     check_round_provenance(tag_a.round_provenance, tag_b.round_provenance);

barretenberg/cpp/src/barretenberg/transcript/origin_tag.hpp

@@ -16,6 +16,7 @@
 #include "barretenberg/common/assert.hpp"
 #include "barretenberg/common/throw_or_abort.hpp"
 #include "barretenberg/numeric/uint256/uint256.hpp"
+#include <atomic>
 #include <cstddef>
 #include <iterator>
 #include <ostream>
@@ -68,6 +69,11 @@ void check_round_provenance(const uint256_t& provenance_a, const uint256_t& prov
 #ifndef AZTEC_NO_ORIGIN_TAGS
 struct OriginTag {
 
+    // Unique per-object ID for debugging: when a tag merge fails, the error message includes
+    // the IDs of both tags so you can trace which specific field elements were involved.
+    static inline std::atomic<uint64_t> next_tag_id{ 0 };
+    uint64_t tag_id = next_tag_id.fetch_add(1, std::memory_order_relaxed);
+
     static constexpr size_t CONSTANT = static_cast<size_t>(-1);
     static constexpr size_t FREE_WITNESS = static_cast<size_t>(-2);
     // transcript_index represents the index of a unique transcript object that generated the value. It uses
@@ -96,7 +102,7 @@ struct OriginTag {
     OriginTag& operator=(const OriginTag& other) = default;
     OriginTag& operator=(OriginTag&& other) noexcept
     {
-
+        tag_id = other.tag_id;
         transcript_index = other.transcript_index;
         round_provenance = other.round_provenance;
         instant_death = other.instant_death;
@@ -204,8 +210,8 @@ struct OriginTag {
 };
 inline std::ostream& operator<<(std::ostream& os, OriginTag const& v)
 {
-    return os << "{ transcript_idx: " << v.transcript_index << ", round_prov: " << v.round_provenance
-              << ", instadeath: " << v.instant_death << " }";
+    return os << "{ id: " << v.tag_id << ", transcript_idx: " << v.transcript_index
+              << ", round_prov: " << v.round_provenance << ", instadeath: " << v.instant_death << " }";
 }
 
 #else

barretenberg/cpp/src/barretenberg/transcript/origin_tag.test.cpp

@@ -242,4 +242,23 @@ TEST(OriginTag, RetaggingReflectsProtocolConstraints)
     EXPECT_NO_THROW(OriginTag(comm_times_challenge, eval_retagged));
 }
 
+// Test that unique per-object IDs are assigned
+TEST(OriginTag, UniqueTagIds)
+{
+    auto tag_a = OriginTag(0, 0, true);
+    auto tag_b = OriginTag(0, 0, true);
+
+    // Each constructed tag gets a unique ID
+    EXPECT_NE(tag_a.tag_id, tag_b.tag_id);
+
+    // Copy preserves the ID
+    auto tag_a_copy = tag_a;
+    EXPECT_EQ(tag_a_copy.tag_id, tag_a.tag_id);
+
+    // Merge produces a new ID
+    auto merged = OriginTag(tag_a, tag_b);
+    EXPECT_NE(merged.tag_id, tag_a.tag_id);
+    EXPECT_NE(merged.tag_id, tag_b.tag_id);
+}
+
 #endif // AZTEC_NO_ORIGIN_TAGS