Fix removing attachments with reserved keys

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Fix removing attachments with numeric keys, [PR-113](https://github.com/reductstore/reduct-cpp/pull/113)
+- Fix removing attachments with reserved `$`-prefixed keys, [PR-115](https://github.com/reductstore/reduct-cpp/pull/115)
 
 ## 1.18.0 - 2026-02-04
 

src/reduct/bucket.cc

@@ -20,6 +20,7 @@
 #include <optional>
 #include <set>
 #include <thread>
+#include <vector>
 
 #include "reduct/internal/batch_v1.h"
 #include "reduct/internal/batch_v2.h"
@@ -251,11 +252,23 @@ class Bucket : public IBucket {
                           const std::set<std::string>& attachment_keys) const noexcept override {
     QueryOptions options;
     if (!attachment_keys.empty()) {
+      std::vector<std::string> escaped_keys;
+      escaped_keys.reserve(attachment_keys.size());
+      for (const auto& key : attachment_keys) {
+        // Keys starting with '$' must be escaped so the server treats them as literals
+        // instead of interpreting them as messagepack filter operators.
+        if (!key.empty() && key.front() == '$') {
+          escaped_keys.emplace_back(fmt::format("${}", key));
+        } else {
+          escaped_keys.emplace_back(key);
+        }
+      }
+
       nlohmann::json when;
       when["$in"] = nlohmann::json::array();
       when["$in"].push_back({{"&key", {{"$cast", "string"}}}});
-      for (const auto& key : attachment_keys) {
-        when["$in"].push_back(key);
+      for (const auto& escaped_key : escaped_keys) {
+        when["$in"].push_back(escaped_key);
       }
       options.when = when.dump();
     }

tests/reduct/entry_api_test.cc

@@ -782,6 +782,29 @@ TEST_CASE("reduct::IBucket should remove entry attachments with numeric keys", "
   REQUIRE(stored_after.empty());
 }
 
+
+TEST_CASE("reduct::IBucket should remove entry attachments with reserved keys", "[entry_api][1_19]") {
+  Fixture ctx;
+  auto [bucket, _] = ctx.client->CreateBucket(kBucketName);
+  REQUIRE(bucket);
+
+  IBucket::AttachmentMap attachments{
+      {"meta-1", R"({"value":1})"},
+      {"$system", R"({"value":"test"})"},
+      {"$internal", R"({"value":"hidden"})"},
+  };
+
+  REQUIRE(bucket->WriteAttachments("entry-1", attachments) == Error::kOk);
+  REQUIRE(bucket->RemoveAttachments("entry-1", std::set<std::string>{"$system", "$internal"}) == Error::kOk);
+
+  auto [stored, err] = bucket->ReadAttachments("entry-1");
+  REQUIRE(err == Error::kOk);
+  REQUIRE(stored.size() == 1);
+  REQUIRE(stored.contains("meta-1"));
+  REQUIRE(nlohmann::json::parse(stored.at("meta-1")) == nlohmann::json::parse(attachments.at("meta-1")));
+}
+
+
 TEST_CASE("Batch should slice data", "[batch]") {
   auto batch = IBucket::Batch();