diff --git a/contract-tests/server-contract-tests/src/main.cpp b/contract-tests/server-contract-tests/src/main.cpp
index 2422623a3..57520a2a5 100644
--- a/contract-tests/server-contract-tests/src/main.cpp
+++ b/contract-tests/server-contract-tests/src/main.cpp
@@ -50,6 +50,7 @@ int main(int argc, char* argv[]) {
         srv.add_capability("client-prereq-events");
         srv.add_capability("evaluation-hooks");
         srv.add_capability("track-hooks");
+        srv.add_capability("hook-environment-id");
         srv.add_capability("wrapper");
         srv.add_capability("instance-id");
         srv.add_capability("fdv1-fallback");
diff --git a/libs/server-sdk/src/client_impl.cpp b/libs/server-sdk/src/client_impl.cpp
index ba04dda7a..cec42d573 100644
--- a/libs/server-sdk/src/client_impl.cpp
+++ b/libs/server-sdk/src/client_impl.cpp
@@ -378,8 +378,9 @@ void ClientImpl::TrackInternal(Context const& ctx,
     // In this SDK the data is type-safe, and will be enqueued, so it makes
     // minimal functional difference.
     if (!config_.Hooks().empty()) {
-        hooks::TrackSeriesContext series_context(
-            ctx, event_name, metric_value, data, hook_context, std::nullopt);
+        hooks::TrackSeriesContext series_context(ctx, event_name, metric_value,
+                                                 data, hook_context,
+                                                 data_system_->EnvironmentId());
         hooks::ExecuteAfterTrack(config_.Hooks(), series_context, logger_);
     }
 
@@ -487,7 +488,7 @@ EvaluationDetail<Value> ClientImpl::VariationInternal(
     if (!config_.Hooks().empty()) {
         hooks::EvaluationSeriesContext series_context(
             key, context, default_value, method_name, hook_context,
-            std::nullopt);
+            data_system_->EnvironmentId());
         // Executor only created if there are hooks.
         executor.emplace(config_.Hooks(), logger_);
         executor->BeforeEvaluation(series_context);
@@ -501,7 +502,7 @@ EvaluationDetail<Value> ClientImpl::VariationInternal(
         if (executor) {
             hooks::EvaluationSeriesContext series_context(
                 key, context, default_value, method_name, hook_context,
-                std::nullopt);
+                data_system_->EnvironmentId());
             executor->AfterEvaluation(series_context, detail);
         }
 
@@ -523,7 +524,7 @@ EvaluationDetail<Value> ClientImpl::VariationInternal(
         if (executor) {
             hooks::EvaluationSeriesContext series_context(
                 key, context, default_value, method_name, hook_context,
-                std::nullopt);
+                data_system_->EnvironmentId());
             executor->AfterEvaluation(series_context, detail);
         }
 
@@ -539,7 +540,7 @@ EvaluationDetail<Value> ClientImpl::VariationInternal(
     if (executor) {
         hooks::EvaluationSeriesContext series_context(
             key, context, default_value, method_name, hook_context,
-            std::nullopt);
+            data_system_->EnvironmentId());
         executor->AfterEvaluation(series_context, detail);
     }
 
diff --git a/libs/server-sdk/src/data_components/change_notifier/change_notifier.cpp b/libs/server-sdk/src/data_components/change_notifier/change_notifier.cpp
index 76daefe10..511b0fab3 100644
--- a/libs/server-sdk/src/data_components/change_notifier/change_notifier.cpp
+++ b/libs/server-sdk/src/data_components/change_notifier/change_notifier.cpp
@@ -143,6 +143,10 @@ void ChangeNotifier::Apply(
     }
 }
 
+void ChangeNotifier::SetEnvironmentId(std::string environment_id) {
+    sink_.SetEnvironmentId(std::move(environment_id));
+}
+
 bool ChangeNotifier::HasListeners() const {
     std::lock_guard lock{signal_mutex_};
     return !signals_.empty();
diff --git a/libs/server-sdk/src/data_components/change_notifier/change_notifier.hpp b/libs/server-sdk/src/data_components/change_notifier/change_notifier.hpp
index ed0ca1c64..faf491cbd 100644
--- a/libs/server-sdk/src/data_components/change_notifier/change_notifier.hpp
+++ b/libs/server-sdk/src/data_components/change_notifier/change_notifier.hpp
@@ -38,6 +38,7 @@ class ChangeNotifier final : public data_interfaces::ITransactionalDestination,
                 data_model::SegmentDescriptor segment) override;
     void Apply(data_model::ChangeSet<data_interfaces::ChangeSetData> change_set)
         override;
+    void SetEnvironmentId(std::string environment_id) override;
 
     [[nodiscard]] std::string const& Identity() const override;
 
diff --git a/libs/server-sdk/src/data_components/memory_store/memory_store.cpp b/libs/server-sdk/src/data_components/memory_store/memory_store.cpp
index 2eee8ff03..23385c309 100644
--- a/libs/server-sdk/src/data_components/memory_store/memory_store.cpp
+++ b/libs/server-sdk/src/data_components/memory_store/memory_store.cpp
@@ -74,6 +74,19 @@ void MemoryStore::Upsert(std::string const& key,
         std::make_shared<data_model::SegmentDescriptor>(std::move(segment));
 }
 
+void MemoryStore::SetEnvironmentId(std::string environment_id) {
+    std::lock_guard lock{data_mutex_};
+    environment_id_ = std::move(environment_id);
+}
+
+std::optional<std::string> MemoryStore::EnvironmentId() const {
+    std::lock_guard lock{data_mutex_};
+    if (!initialized_) {
+        return std::nullopt;
+    }
+    return environment_id_;
+}
+
 bool MemoryStore::RemoveFlag(std::string const& key) {
     std::lock_guard lock{data_mutex_};
     return flags_.erase(key) == 1;
diff --git a/libs/server-sdk/src/data_components/memory_store/memory_store.hpp b/libs/server-sdk/src/data_components/memory_store/memory_store.hpp
index 3dc73272c..be5e01c51 100644
--- a/libs/server-sdk/src/data_components/memory_store/memory_store.hpp
+++ b/libs/server-sdk/src/data_components/memory_store/memory_store.hpp
@@ -8,6 +8,7 @@
 
 #include <memory>
 #include <mutex>
+#include <optional>
 #include <string>
 #include <unordered_map>
 
@@ -43,6 +44,14 @@ class MemoryStore final : public data_interfaces::IStore,
     void Upsert(std::string const& key,
                 data_model::SegmentDescriptor segment) override;
 
+    void SetEnvironmentId(std::string environment_id) override;
+
+    /**
+     * @return The environment ID reported by LaunchDarkly, if any has been
+     * received.
+     */
+    [[nodiscard]] std::optional<std::string> EnvironmentId() const;
+
     bool RemoveFlag(std::string const& key);
 
     bool RemoveSegment(std::string const& key);
@@ -66,6 +75,7 @@ class MemoryStore final : public data_interfaces::IStore,
                        std::shared_ptr<data_model::SegmentDescriptor>>
         segments_;
     bool initialized_ = false;
+    std::optional<std::string> environment_id_;
     mutable std::mutex data_mutex_;
 };
 
diff --git a/libs/server-sdk/src/data_components/serialization_adapters/json_destination.cpp b/libs/server-sdk/src/data_components/serialization_adapters/json_destination.cpp
index b85f2f978..ac121bf71 100644
--- a/libs/server-sdk/src/data_components/serialization_adapters/json_destination.cpp
+++ b/libs/server-sdk/src/data_components/serialization_adapters/json_destination.cpp
@@ -111,6 +111,8 @@ void JsonDestination::Upsert(std::string const& key,
                     dest_.Upsert(Kinds::Segment, key, Serialize(key, segment)));
 }
 
+void JsonDestination::SetEnvironmentId(std::string) {}
+
 void JsonDestination::LogUpsertResult(
     std::string const& key,
     std::string const& data_type,
diff --git a/libs/server-sdk/src/data_components/serialization_adapters/json_destination.hpp b/libs/server-sdk/src/data_components/serialization_adapters/json_destination.hpp
index 77f4e6a01..fff0c7756 100644
--- a/libs/server-sdk/src/data_components/serialization_adapters/json_destination.hpp
+++ b/libs/server-sdk/src/data_components/serialization_adapters/json_destination.hpp
@@ -70,6 +70,11 @@ class JsonDestination final : public data_interfaces::IDestination {
     void Upsert(std::string const& key,
                 data_model::SegmentDescriptor segment) override;
 
+    /**
+     * @brief No-op; serialized destinations do not store the environment ID.
+     */
+    void SetEnvironmentId(std::string environment_id) override;
+
     /**
      * @return Identity of this destination. Used in logs.
      */
diff --git a/libs/server-sdk/src/data_interfaces/destination/idestination.hpp b/libs/server-sdk/src/data_interfaces/destination/idestination.hpp
index 342acf49c..0bed95354 100644
--- a/libs/server-sdk/src/data_interfaces/destination/idestination.hpp
+++ b/libs/server-sdk/src/data_interfaces/destination/idestination.hpp
@@ -35,6 +35,13 @@ class IDestination {
     virtual void Upsert(std::string const& key,
                         data_model::SegmentDescriptor segment) = 0;
 
+    /**
+     * \brief Record the environment ID that LaunchDarkly reported alongside
+     * the data. Destinations which do not track it ignore the value.
+     * \param environment_id The environment ID.
+     */
+    virtual void SetEnvironmentId(std::string environment_id) = 0;
+
     /**
      * \return Identity of the destination. Used in logs.
      */
diff --git a/libs/server-sdk/src/data_interfaces/source/fdv2_source_result.hpp b/libs/server-sdk/src/data_interfaces/source/fdv2_source_result.hpp
index 53a0cd442..12cb39da7 100644
--- a/libs/server-sdk/src/data_interfaces/source/fdv2_source_result.hpp
+++ b/libs/server-sdk/src/data_interfaces/source/fdv2_source_result.hpp
@@ -98,6 +98,12 @@ struct FDv2SourceResult {
      * Set if the underlying transport observed an FDv1 fallback directive.
      */
     std::optional<FDv1FallbackDirective> fdv1_fallback;
+
+    /**
+     * Set if the underlying transport reported the environment ID (e.g. an
+     * X-LD-EnvID response header).
+     */
+    std::optional<std::string> environment_id;
 };
 
 }  // namespace launchdarkly::server_side::data_interfaces
diff --git a/libs/server-sdk/src/data_interfaces/system/idata_system.hpp b/libs/server-sdk/src/data_interfaces/system/idata_system.hpp
index 0edf778db..f7aaba819 100644
--- a/libs/server-sdk/src/data_interfaces/system/idata_system.hpp
+++ b/libs/server-sdk/src/data_interfaces/system/idata_system.hpp
@@ -2,6 +2,9 @@
 
 #include "../store/istore.hpp"
 
+#include <optional>
+#include <string>
+
 namespace launchdarkly::server_side::data_interfaces {
 
 /**
@@ -21,6 +24,12 @@ class IDataSystem : public IStore {
      */
     virtual void Initialize() = 0;
 
+    /**
+     * @return The environment ID reported by LaunchDarkly alongside the data,
+     * if the system has received one.
+     */
+    [[nodiscard]] virtual std::optional<std::string> EnvironmentId() const = 0;
+
     virtual ~IDataSystem() override = default;
     IDataSystem(IDataSystem const& item) = delete;
     IDataSystem(IDataSystem&& item) = delete;
diff --git a/libs/server-sdk/src/data_systems/background_sync/background_sync_system.cpp b/libs/server-sdk/src/data_systems/background_sync/background_sync_system.cpp
index 1a6de5c0a..7a1c7d7e9 100644
--- a/libs/server-sdk/src/data_systems/background_sync/background_sync_system.cpp
+++ b/libs/server-sdk/src/data_systems/background_sync/background_sync_system.cpp
@@ -47,6 +47,10 @@ std::string const& BackgroundSync::Identity() const {
     return id;
 }
 
+std::optional<std::string> BackgroundSync::EnvironmentId() const {
+    return store_.EnvironmentId();
+}
+
 std::shared_ptr<data_model::FlagDescriptor> BackgroundSync::GetFlag(
     std::string const& key) const {
     return store_.GetFlag(key);
diff --git a/libs/server-sdk/src/data_systems/background_sync/background_sync_system.hpp b/libs/server-sdk/src/data_systems/background_sync/background_sync_system.hpp
index a302b7d0d..620eb3c48 100644
--- a/libs/server-sdk/src/data_systems/background_sync/background_sync_system.hpp
+++ b/libs/server-sdk/src/data_systems/background_sync/background_sync_system.hpp
@@ -55,6 +55,8 @@ class BackgroundSync final : public data_interfaces::IDataSystem {
 
     bool Initialized() const override;
 
+    std::optional<std::string> EnvironmentId() const override;
+
    private:
     data_components::MemoryStore store_;
     data_components::ChangeNotifier change_notifier_;
diff --git a/libs/server-sdk/src/data_systems/background_sync/sources/polling/polling_data_source.cpp b/libs/server-sdk/src/data_systems/background_sync/sources/polling/polling_data_source.cpp
index abb2a5478..cdff2b6b1 100644
--- a/libs/server-sdk/src/data_systems/background_sync/sources/polling/polling_data_source.cpp
+++ b/libs/server-sdk/src/data_systems/background_sync/sources/polling/polling_data_source.cpp
@@ -3,8 +3,8 @@
 #include <launchdarkly/encoding/base_64.hpp>
 #include <launchdarkly/network/http_error_messages.hpp>
 
-#include <launchdarkly/serialization/json_flag.hpp>
 #include <launchdarkly/detail/serialization/json_primitives.hpp>
+#include <launchdarkly/serialization/json_flag.hpp>
 #include <launchdarkly/serialization/json_sdk_data_set.hpp>
 #include <launchdarkly/server_side/data_source_status.hpp>
 
@@ -102,6 +102,13 @@ void PollingDataSource::DoPoll() {
 }
 
 void PollingDataSource::HandlePollResult(network::HttpResult const& res) {
+    if (!res.IsError() && (res.Status() == 200 || res.Status() == 304)) {
+        if (auto const it = res.Headers().find("X-LD-EnvID");
+            it != res.Headers().end() && !it->second.empty()) {
+            sink_->SetEnvironmentId(it->second);
+        }
+    }
+
     auto header_etag = res.Headers().find("etag");
     bool has_etag = header_etag != res.Headers().end();
 
diff --git a/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.cpp b/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.cpp
index 5f7a27a7b..8da169959 100644
--- a/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.cpp
+++ b/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.cpp
@@ -47,6 +47,7 @@ void StreamingDataSource::StartAsync(
     data_model::SDKDataSet const* bootstrap_data) {
     boost::ignore_unused(bootstrap_data);
 
+    sink_ = dest;
     event_handler_.emplace(*dest, logger_, status_manager_);
 
     status_manager_.SetState(DataSourceStatus::DataSourceState::kInitializing);
@@ -125,16 +126,31 @@ void StreamingDataSource::StartAsync(
 
     auto weak_self = weak_from_this();
 
+    client_builder.on_response(
+        [weak_self](boost::beast::http::response_header<> const& headers) {
+            auto self = weak_self.lock();
+            if (!self || headers.result_int() != 200) {
+                return;
+            }
+            if (auto const it = headers.find("X-LD-EnvID");
+                it != headers.end() && !it->value().empty()) {
+                self->sink_->SetEnvironmentId(
+                    std::string(it->value().data(), it->value().size()));
+            }
+        });
+
     client_builder.receiver([weak_self](launchdarkly::sse::Event const& event) {
         if (auto self = weak_self.lock()) {
             auto status =
                 self->event_handler_->HandleMessage(event.type(), event.data());
-            if (status == DataSourceEventHandler::MessageStatus::kInvalidMessage) {
+            if (status ==
+                DataSourceEventHandler::MessageStatus::kInvalidMessage) {
                 // Invalid data received - restart the connection with backoff
                 // to get a fresh stream. The backoff mechanism prevents rapid
                 // reconnection attempts.
                 LD_LOG(self->logger_, LogLevel::kWarn)
-                    << "Received invalid data from stream, restarting connection";
+                    << "Received invalid data from stream, restarting "
+                       "connection";
                 if (self->client_) {
                     self->client_->async_restart("invalid data in stream");
                 }
diff --git a/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.hpp b/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.hpp
index f606686c1..e51509449 100644
--- a/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.hpp
+++ b/libs/server-sdk/src/data_systems/background_sync/sources/streaming/streaming_data_source.hpp
@@ -46,6 +46,9 @@ class StreamingDataSource final
 
     config::built::BackgroundSyncConfig::StreamingConfig streaming_config_;
 
+    // Destination for all data obtained via streaming. Set by StartAsync.
+    data_interfaces::IDestination* sink_ = nullptr;
+
     std::shared_ptr<sse::Client> client_;
 };
 }  // namespace launchdarkly::server_side::data_systems
diff --git a/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.cpp b/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.cpp
index 67bb98766..260e6f60b 100644
--- a/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.cpp
+++ b/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.cpp
@@ -44,6 +44,7 @@ void FDv1AdapterSynchronizer::State::Notify(FDv2SourceResult result) {
         if (closed_future_.IsFinished()) {
             return;
         }
+        result.environment_id = environment_id_;
         if (pending_promise_) {
             promise = std::move(pending_promise_);
             pending_promise_.reset();
@@ -57,6 +58,12 @@ void FDv1AdapterSynchronizer::State::Notify(FDv2SourceResult result) {
     promise->Resolve(std::move(result));
 }
 
+void FDv1AdapterSynchronizer::State::SetEnvironmentId(
+    std::string environment_id) {
+    std::lock_guard lock(mutex_);
+    environment_id_ = std::move(environment_id);
+}
+
 // ----- ConvertingDestination -----
 
 FDv1AdapterSynchronizer::ConvertingDestination::ConvertingDestination(
@@ -113,6 +120,13 @@ void FDv1AdapterSynchronizer::ConvertingDestination::Upsert(
             data_model::Selector{}}}});
 }
 
+void FDv1AdapterSynchronizer::ConvertingDestination::SetEnvironmentId(
+    std::string environment_id) {
+    if (auto state = state_.lock()) {
+        state->SetEnvironmentId(std::move(environment_id));
+    }
+}
+
 std::string const& FDv1AdapterSynchronizer::ConvertingDestination::Identity()
     const {
     static std::string const identity = "FDv1 adapter destination";
diff --git a/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.hpp b/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.hpp
index a382ff90a..2ca25e1c2 100644
--- a/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.hpp
+++ b/libs/server-sdk/src/data_systems/fdv2/fdv1_adapter_synchronizer.hpp
@@ -73,6 +73,9 @@ class FDv1AdapterSynchronizer final
 
         void Notify(data_interfaces::FDv2SourceResult result);
 
+        // Stamped onto subsequently notified results.
+        void SetEnvironmentId(std::string environment_id);
+
        private:
         // Finished once the owning FDv1AdapterSynchronizer's close_promise_
         // is resolved. Read in Notify to drop late results.
@@ -83,6 +86,7 @@ class FDv1AdapterSynchronizer final
         std::optional<async::Promise<data_interfaces::FDv2SourceResult>>
             pending_promise_;
         std::deque<data_interfaces::FDv2SourceResult> result_queue_;
+        std::optional<std::string> environment_id_;
     };
 
     /**
@@ -97,6 +101,7 @@ class FDv1AdapterSynchronizer final
                     data_model::FlagDescriptor flag) override;
         void Upsert(std::string const& key,
                     data_model::SegmentDescriptor segment) override;
+        void SetEnvironmentId(std::string environment_id) override;
         [[nodiscard]] std::string const& Identity() const override;
 
        private:
diff --git a/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.cpp b/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.cpp
index 354fb8850..f28465537 100644
--- a/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.cpp
+++ b/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.cpp
@@ -147,6 +147,10 @@ void FDv2DataSystem::OnInitializerResult(
     data_interfaces::FDv2SourceResult result) {
     using Result = data_interfaces::FDv2SourceResult;
 
+    if (result.environment_id) {
+        change_notifier_.SetEnvironmentId(*result.environment_id);
+    }
+
     bool got_basis = false;
     bool got_shutdown = false;
     bool disconnected = false;
@@ -350,6 +354,10 @@ void FDv2DataSystem::OnSynchronizerResult(
         }
     }
 
+    if (result.environment_id) {
+        change_notifier_.SetEnvironmentId(*result.environment_id);
+    }
+
     bool got_shutdown = false;
     bool advance = false;
     bool disconnected = false;
@@ -487,4 +495,8 @@ bool FDv2DataSystem::Initialized() const {
     return store_.Initialized();
 }
 
+std::optional<std::string> FDv2DataSystem::EnvironmentId() const {
+    return store_.EnvironmentId();
+}
+
 }  // namespace launchdarkly::server_side::data_systems
diff --git a/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.hpp b/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.hpp
index 3c7147ec1..f97259cdb 100644
--- a/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.hpp
+++ b/libs/server-sdk/src/data_systems/fdv2/fdv2_data_system.hpp
@@ -229,6 +229,12 @@ class FDv2DataSystem final : public data_interfaces::IDataSystem {
      */
     bool Initialized() const override;
 
+    /**
+     * Returns the environment ID reported by LaunchDarkly alongside the data,
+     * if data has been received and the transport reported one.
+     */
+    std::optional<std::string> EnvironmentId() const override;
+
    private:
     /**
      * Signals the orchestration loop to stop and closes any active source.
diff --git a/libs/server-sdk/src/data_systems/fdv2/fdv2_polling_impl.cpp b/libs/server-sdk/src/data_systems/fdv2/fdv2_polling_impl.cpp
index 0b668851b..f972eb492 100644
--- a/libs/server-sdk/src/data_systems/fdv2/fdv2_polling_impl.cpp
+++ b/libs/server-sdk/src/data_systems/fdv2/fdv2_polling_impl.cpp
@@ -12,6 +12,7 @@
 
 namespace launchdarkly::server_side::data_systems {
 
+static char const* const kEnvironmentIdHeader = "X-LD-EnvID";
 static char const* const kFDv1FallbackHeader = "X-LD-FD-Fallback";
 static char const* const kFDv1FallbackTtlHeader = "X-LD-FD-Fallback-TTL";
 
@@ -53,6 +54,15 @@ ReadFDv1FallbackDirective(network::HttpResult::HeadersType const& headers) {
     return directive;
 }
 
+static std::optional<std::string> ReadEnvironmentId(
+    network::HttpResult::HeadersType const& headers) {
+    auto const it = headers.find(kEnvironmentIdHeader);
+    if (it == headers.end() || it->second.empty()) {
+        return std::nullopt;
+    }
+    return it->second;
+}
+
 network::HttpRequest MakeFDv2PollRequest(
     std::string const& polling_base_url,
     config::built::HttpProperties const& http_properties,
@@ -210,7 +220,7 @@ data_interfaces::FDv2SourceResult HandleFDv2PollResponse(
                     data_model::ChangeSetType::kNone,
                     {},
                     data_model::Selector{}}},
-            fdv1_fallback};
+            fdv1_fallback, ReadEnvironmentId(res.Headers())};
     }
 
     if (res.Status() == 200) {
@@ -238,6 +248,7 @@ data_interfaces::FDv2SourceResult HandleFDv2PollResponse(
         if (!result.fdv1_fallback) {
             result.fdv1_fallback = fdv1_fallback;
         }
+        result.environment_id = ReadEnvironmentId(res.Headers());
         return result;
     }
 
diff --git a/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.cpp b/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.cpp
index 0f09ef5cf..c0d584ccf 100644
--- a/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.cpp
+++ b/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.cpp
@@ -186,6 +186,15 @@ void FDv2StreamingSynchronizer::State::OnConnect(HttpRequest* req) {
 
 void FDv2StreamingSynchronizer::State::OnResponse(
     HttpResponseHeader const& headers) {
+    if (headers.result_int() == 200) {
+        if (auto const env_it = headers.find("X-LD-EnvID");
+            env_it != headers.end() && !env_it->value().empty()) {
+            std::lock_guard lock(mutex_);
+            environment_id_ =
+                std::string(env_it->value().data(), env_it->value().size());
+        }
+    }
+
     auto const it = headers.find("X-LD-FD-Fallback");
     if (it == headers.end() || !boost::iequals(it->value(), "true")) {
         std::lock_guard lock(mutex_);
@@ -337,6 +346,7 @@ void FDv2StreamingSynchronizer::State::Notify(FDv2SourceResult result) {
         if (!result.fdv1_fallback) {
             result.fdv1_fallback = latest_fdv1_fallback_;
         }
+        result.environment_id = environment_id_;
         if (pending_promise_) {
             promise = std::move(pending_promise_);
             pending_promise_.reset();
diff --git a/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.hpp b/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.hpp
index f1e69ae8b..885456e39 100644
--- a/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.hpp
+++ b/libs/server-sdk/src/data_systems/fdv2/streaming_synchronizer.hpp
@@ -149,6 +149,8 @@ class FDv2StreamingSynchronizer final
         // FDv1 fallback directive from the most recent SSE response.
         std::optional<data_interfaces::FDv1FallbackDirective>
             latest_fdv1_fallback_;
+        // Environment ID reported by the most recent successful SSE response.
+        std::optional<std::string> environment_id_;
         data_model::Selector latest_selector_;
         std::optional<boost::urls::url> base_url_;
         std::shared_ptr<sse::Client> sse_client_;
diff --git a/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.cpp b/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.cpp
index 4235b9676..b9d775e58 100644
--- a/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.cpp
+++ b/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.cpp
@@ -142,6 +142,10 @@ bool LazyLoad::Initialized() const {
     return initialized_.value_or(false);
 }
 
+std::optional<std::string> LazyLoad::EnvironmentId() const {
+    return std::nullopt;
+}
+
 void LazyLoad::RefreshAllFlags() const {
     RefreshAll<data_model::Flag>(Keys::kAllFlags,
                                  data_components::DataKind::kFlag,
diff --git a/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.hpp b/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.hpp
index 9584f60d6..d15c46f01 100644
--- a/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.hpp
+++ b/libs/server-sdk/src/data_systems/lazy_load/lazy_load_system.hpp
@@ -58,6 +58,8 @@ class LazyLoad final : public data_interfaces::IDataSystem {
 
     bool Initialized() const override;
 
+    std::optional<std::string> EnvironmentId() const override;
+
     // Public for usage in tests.
     struct Kinds {
         static integrations::FlagKind const Flag;
diff --git a/libs/server-sdk/src/data_systems/offline.cpp b/libs/server-sdk/src/data_systems/offline.cpp
index 148c10a2d..73cbfd926 100644
--- a/libs/server-sdk/src/data_systems/offline.cpp
+++ b/libs/server-sdk/src/data_systems/offline.cpp
@@ -41,4 +41,8 @@ std::string const& OfflineSystem::Identity() const {
     return ident;
 }
 
+std::optional<std::string> OfflineSystem::EnvironmentId() const {
+    return std::nullopt;
+}
+
 }  // namespace launchdarkly::server_side::data_systems
diff --git a/libs/server-sdk/src/data_systems/offline.hpp b/libs/server-sdk/src/data_systems/offline.hpp
index c9355bd24..ae869de2f 100644
--- a/libs/server-sdk/src/data_systems/offline.hpp
+++ b/libs/server-sdk/src/data_systems/offline.hpp
@@ -22,6 +22,7 @@ class OfflineSystem final : public data_interfaces::IDataSystem {
     [[nodiscard]] bool Initialized() const override;
     [[nodiscard]] std::string const& Identity() const override;
     void Initialize() override;
+    [[nodiscard]] std::optional<std::string> EnvironmentId() const override;
 
    private:
     data_components::DataSourceStatusManager& status_manager_;
diff --git a/libs/server-sdk/tests/fdv1_adapter_synchronizer_test.cpp b/libs/server-sdk/tests/fdv1_adapter_synchronizer_test.cpp
index 26de674b6..f232bbdc8 100644
--- a/libs/server-sdk/tests/fdv1_adapter_synchronizer_test.cpp
+++ b/libs/server-sdk/tests/fdv1_adapter_synchronizer_test.cpp
@@ -115,6 +115,20 @@ TEST(FDv1AdapterSynchronizerTest, FDv1InitProducesFullChangeSet) {
     EXPECT_FALSE(result->fdv1_fallback);
 }
 
+TEST(FDv1AdapterSynchronizerTest, FDv1EnvironmentIdIsReportedWithChangeSet) {
+    MockFDv1Source* source = nullptr;
+    FDv1AdapterSynchronizer adapter(MakeMockBuilder(&source));
+
+    auto future = adapter.Next(data_model::Selector{});
+
+    source->destination_->SetEnvironmentId("env-123");
+    source->destination_->Init(data_model::SDKDataSet{});
+
+    auto result = future.WaitForResult(1s);
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(std::optional<std::string>{"env-123"}, result->environment_id);
+}
+
 TEST(FDv1AdapterSynchronizerTest, FDv1UpsertProducesPartialChangeSet) {
     MockFDv1Source* source = nullptr;
     FDv1AdapterSynchronizer adapter(MakeMockBuilder(&source));
diff --git a/libs/server-sdk/tests/fdv2_polling_impl_test.cpp b/libs/server-sdk/tests/fdv2_polling_impl_test.cpp
index d4ecd9ee6..f87e14c1b 100644
--- a/libs/server-sdk/tests/fdv2_polling_impl_test.cpp
+++ b/libs/server-sdk/tests/fdv2_polling_impl_test.cpp
@@ -162,6 +162,34 @@ TEST(MakeFDv2PollRequestTest, ValidFilterKeyIsIncluded) {
     EXPECT_EQ(req.Url(), "http://example.com/sdk/poll?filter=my-filter_1.0");
 }
 
+TEST(HandleFDv2PollResponseTest, OkReportsEnvironmentId) {
+    auto result =
+        HandleResponse(200, R"({"events":[]})", {{"X-LD-EnvID", "env-123"}});
+    EXPECT_EQ(std::optional<std::string>{"env-123"}, result.environment_id);
+}
+
+TEST(HandleFDv2PollResponseTest, NotModifiedReportsEnvironmentId) {
+    auto result =
+        HandleResponse(304, std::nullopt, {{"X-LD-EnvID", "env-123"}});
+    EXPECT_EQ(std::optional<std::string>{"env-123"}, result.environment_id);
+}
+
+TEST(HandleFDv2PollResponseTest, ErrorStatusDoesNotReportEnvironmentId) {
+    auto result =
+        HandleResponse(503, std::nullopt, {{"X-LD-EnvID", "env-123"}});
+    EXPECT_FALSE(result.environment_id);
+}
+
+TEST(HandleFDv2PollResponseTest, MissingHeaderDoesNotReportEnvironmentId) {
+    auto result = HandleResponse(200, R"({"events":[]})", {});
+    EXPECT_FALSE(result.environment_id);
+}
+
+TEST(HandleFDv2PollResponseTest, EmptyHeaderDoesNotReportEnvironmentId) {
+    auto result = HandleResponse(200, R"({"events":[]})", {{"X-LD-EnvID", ""}});
+    EXPECT_FALSE(result.environment_id);
+}
+
 TEST(MakeFDv2PollRequestTest, InvalidFilterKeyIsDropped) {
     auto logger = MakeNullLogger();
     auto props =
diff --git a/libs/server-sdk/tests/memory_store_test.cpp b/libs/server-sdk/tests/memory_store_test.cpp
index 901282ea2..1ebd30c7e 100644
--- a/libs/server-sdk/tests/memory_store_test.cpp
+++ b/libs/server-sdk/tests/memory_store_test.cpp
@@ -299,6 +299,29 @@ TEST(MemoryStoreTest, CanDeleteExistingSegment) {
     ASSERT_FALSE(store.RemoveSegment("segmentA"));
 }
 
+TEST(MemoryStoreTest, HasNoEnvironmentIdByDefault) {
+    MemoryStore store;
+    store.Init(SDKDataSet());
+    EXPECT_FALSE(store.EnvironmentId());
+}
+
+TEST(MemoryStoreTest, EnvironmentIdIsNotVisibleBeforeInit) {
+    MemoryStore store;
+    store.SetEnvironmentId("env-123");
+    EXPECT_FALSE(store.EnvironmentId());
+
+    store.Init(SDKDataSet());
+    EXPECT_EQ(std::optional<std::string>{"env-123"}, store.EnvironmentId());
+}
+
+TEST(MemoryStoreTest, EnvironmentIdIsRetainedAcrossInit) {
+    MemoryStore store;
+    store.SetEnvironmentId("env-123");
+    store.Init(SDKDataSet());
+    store.Init(SDKDataSet());
+    EXPECT_EQ(std::optional<std::string>{"env-123"}, store.EnvironmentId());
+}
+
 TEST(MemoryStoreTest, CanDeleteExistingFlag) {
     MemoryStore store;