diff --git a/include/datadog/environment.h b/include/datadog/environment.h
index f2846b37..478d6ac3 100644
--- a/include/datadog/environment.h
+++ b/include/datadog/environment.h
@@ -81,7 +81,8 @@ namespace environment {
   MACRO(DD_APM_TRACING_ENABLED, BOOLEAN, true)                                 \
   MACRO(DD_TRACE_RESOURCE_RENAMING_ENABLED, BOOLEAN, false)                    \
   MACRO(DD_TRACE_RESOURCE_RENAMING_ALWAYS_SIMPLIFIED_ENDPOINT, BOOLEAN, false) \
-  MACRO(DD_EXTERNAL_ENV, STRING, "")
+  MACRO(DD_EXTERNAL_ENV, STRING, "")                                           \
+  MACRO(_DD_ROOT_CPP_SESSION_ID, STRING, nullptr)
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) X
 #define WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) ARG,
@@ -95,7 +96,7 @@ enum Variable { DD_LIST_ENVIRONMENT_VARIABLES(WITH_COMMA) };
 #define QUOTED_WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) \
   WITH_COMMA(QUOTED(ARG), TYPE, DEFAULT_VALUE)
 
-inline const char *const variable_names[] = {
+inline const char* const variable_names[] = {
     DD_LIST_ENVIRONMENT_VARIABLES(QUOTED_WITH_COMMA)};
 
 #undef QUOTED
@@ -110,6 +111,10 @@ StringView name(Variable variable);
 // `nullopt` if that variable is not set in the environment.
 Optional<StringView> lookup(Variable variable);
 
+// Set the specified environment `variable` to `value`. Does not overwrite if
+// already set (equivalent to setenv(..., 0) on POSIX).
+void set(Variable variable, StringView value);
+
 std::string to_json();
 
 }  // namespace environment
diff --git a/include/datadog/tracer_config.h b/include/datadog/tracer_config.h
index 2ee4cd02..f342a61b 100644
--- a/include/datadog/tracer_config.h
+++ b/include/datadog/tracer_config.h
@@ -225,6 +225,7 @@ class FinalizedTracerConfig final {
   bool log_on_startup;
   bool generate_128bit_trace_ids;
   Optional<RuntimeID> runtime_id;
+  Optional<std::string> root_session_id;
   Clock clock;
   std::string integration_name;
   std::string integration_version;
diff --git a/include/datadog/tracer_signature.h b/include/datadog/tracer_signature.h
index 042b4d37..73decae7 100644
--- a/include/datadog/tracer_signature.h
+++ b/include/datadog/tracer_signature.h
@@ -33,6 +33,7 @@ namespace tracing {
 
 struct TracerSignature {
   RuntimeID runtime_id;
+  std::string root_session_id;
   std::string default_service;
   std::string default_environment;
   std::string library_version;
@@ -40,8 +41,10 @@ struct TracerSignature {
   StringView library_language_version;
 
   TracerSignature() = delete;
-  TracerSignature(RuntimeID id, std::string service, std::string environment)
+  TracerSignature(RuntimeID id, std::string root_session, std::string service,
+                  std::string environment)
       : runtime_id(id),
+        root_session_id(std::move(root_session)),
         default_service(std::move(service)),
         default_environment(std::move(environment)),
         library_version(tracer_version),
diff --git a/src/datadog/environment.cpp b/src/datadog/environment.cpp
index 4dc3eb3f..4ad7b547 100644
--- a/src/datadog/environment.cpp
+++ b/src/datadog/environment.cpp
@@ -11,19 +11,28 @@ namespace environment {
 StringView name(Variable variable) { return variable_names[variable]; }
 
 Optional<StringView> lookup(Variable variable) {
-  const char *name = variable_names[variable];
-  const char *value = std::getenv(name);
+  const char* name = variable_names[variable];
+  const char* value = std::getenv(name);
   if (!value) {
     return nullopt;
   }
   return StringView{value};
 }
 
+void set(Variable variable, StringView value) {
+  const char* name = variable_names[variable];
+#ifdef _WIN32
+  _putenv_s(name, value.data());
+#else
+  setenv(name, value.data(), /*overwrite=*/0);
+#endif
+}
+
 std::string to_json() {
   auto result = nlohmann::json::object({});
 
-  for (const char *name : variable_names) {
-    if (const char *value = std::getenv(name)) {
+  for (const char* name : variable_names) {
+    if (const char* value = std::getenv(name)) {
       result[name] = value;
     }
   }
diff --git a/src/datadog/telemetry/telemetry.cpp b/src/datadog/telemetry/telemetry.cpp
index 3cd51545..0cac32ce 100644
--- a/src/datadog/telemetry/telemetry.cpp
+++ b/src/datadog/telemetry/telemetry.cpp
@@ -53,10 +53,13 @@ void init(FinalizedConfiguration configuration,
           std::shared_ptr<tracing::HTTPClient> client,
           std::shared_ptr<tracing::EventScheduler> event_scheduler,
           tracing::HTTPClient::URL agent_url, tracing::Clock clock) {
-  instance(Ctor_param{configuration,
-                      tracing::TracerSignature(tracing::RuntimeID::generate(),
-                                               tracing::get_process_name(), ""),
-                      logger, client, event_scheduler, agent_url, clock});
+  auto runtime_id = tracing::RuntimeID::generate();
+  auto root_session_id = runtime_id.string();
+  instance(Ctor_param{
+      configuration,
+      tracing::TracerSignature(runtime_id, std::move(root_session_id),
+                               tracing::get_process_name(), ""),
+      logger, client, event_scheduler, agent_url, clock});
 }
 
 void init(FinalizedConfiguration configuration,
diff --git a/src/datadog/telemetry/telemetry_impl.cpp b/src/datadog/telemetry/telemetry_impl.cpp
index d9464dd7..9072c23f 100644
--- a/src/datadog/telemetry/telemetry_impl.cpp
+++ b/src/datadog/telemetry/telemetry_impl.cpp
@@ -302,13 +302,18 @@ void Telemetry::app_started() {
   auto payload = app_started_payload();
 
   auto on_headers = [payload_size = payload.size(),
-                     debug_enabled = config_.debug](DictWriter& headers) {
+                     debug_enabled = config_.debug,
+                     &sig = tracer_signature_](DictWriter& headers) {
     headers.set("Content-Type", "application/json");
     headers.set("Content-Length", std::to_string(payload_size));
     headers.set("DD-Telemetry-API-Version", "v2");
     headers.set("DD-Client-Library-Language", "cpp");
     headers.set("DD-Client-Library-Version", tracer_version);
     headers.set("DD-Telemetry-Request-Type", "app-started");
+    headers.set("DD-Session-ID", sig.runtime_id.string());
+    if (sig.root_session_id != sig.runtime_id.string()) {
+      headers.set("DD-Root-Session-ID", sig.root_session_id);
+    }
     if (debug_enabled) {
       headers.set("DD-Telemetry-Debug-Enabled", "true");
     }
@@ -356,14 +361,18 @@ void Telemetry::app_closing() {
 
 void Telemetry::send_payload(StringView request_type, std::string payload) {
   auto set_telemetry_headers = [request_type, payload_size = payload.size(),
-                                debug_enabled =
-                                    config_.debug](DictWriter& headers) {
+                                debug_enabled = config_.debug,
+                                &sig = tracer_signature_](DictWriter& headers) {
     headers.set("Content-Type", "application/json");
     headers.set("Content-Length", std::to_string(payload_size));
     headers.set("DD-Telemetry-API-Version", "v2");
     headers.set("DD-Client-Library-Language", "cpp");
     headers.set("DD-Client-Library-Version", tracer_version);
     headers.set("DD-Telemetry-Request-Type", request_type);
+    headers.set("DD-Session-ID", sig.runtime_id.string());
+    if (sig.root_session_id != sig.runtime_id.string()) {
+      headers.set("DD-Root-Session-ID", sig.root_session_id);
+    }
     if (debug_enabled) {
       headers.set("DD-Telemetry-Debug-Enabled", "true");
     }
diff --git a/src/datadog/tracer.cpp b/src/datadog/tracer.cpp
index 7603b92b..2437db43 100644
--- a/src/datadog/tracer.cpp
+++ b/src/datadog/tracer.cpp
@@ -48,8 +48,9 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
     : logger_(config.logger),
       runtime_id_(config.runtime_id ? *config.runtime_id
                                     : RuntimeID::generate()),
-      signature_{runtime_id_, config.defaults.service,
-                 config.defaults.environment},
+      signature_{runtime_id_,
+                 config.root_session_id.value_or(runtime_id_.string()),
+                 config.defaults.service, config.defaults.environment},
       config_manager_(std::make_shared<ConfigManager>(config)),
       collector_(/* see constructor body */),
       span_sampler_(
@@ -64,6 +65,9 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       baggage_extraction_enabled_(false),
       tracing_enabled_(config.tracing_enabled),
       resource_renaming_mode_(config.resource_renaming_mode) {
+  environment::set(environment::_DD_ROOT_CPP_SESSION_ID,
+                   signature_.root_session_id);
+
   telemetry::init(config.telemetry, signature_, logger_, config.http_client,
                   config.event_scheduler, config.agent_url);
   if (config.report_hostname) {
diff --git a/src/datadog/tracer_config.cpp b/src/datadog/tracer_config.cpp
index a10b17a8..adf8cf79 100644
--- a/src/datadog/tracer_config.cpp
+++ b/src/datadog/tracer_config.cpp
@@ -407,6 +407,10 @@ Expected<FinalizedTracerConfig> finalize_config(const TracerConfig &user_config,
     final_config.runtime_id = user_config.runtime_id;
   }
 
+  if (auto val = lookup(environment::_DD_ROOT_CPP_SESSION_ID)) {
+    final_config.root_session_id = std::string{*val};
+  }
+
   final_config.process_tags = user_config.process_tags;
 
   auto agent_finalized =
diff --git a/supported-configurations.json b/supported-configurations.json
index 4ee3558a..85aa9f79 100644
--- a/supported-configurations.json
+++ b/supported-configurations.json
@@ -279,6 +279,13 @@
         "implementation": "A",
         "type": "STRING"
       }
+    ],
+    "_DD_ROOT_CPP_SESSION_ID": [
+      {
+        "default": null,
+        "implementation": "A",
+        "type": "STRING"
+      }
     ]
   },
   "version": "2"
diff --git a/test/remote_config/test_remote_config.cpp b/test/remote_config/test_remote_config.cpp
index 232de5af..9a22bd48 100644
--- a/test/remote_config/test_remote_config.cpp
+++ b/test/remote_config/test_remote_config.cpp
@@ -47,8 +47,10 @@ auto logger = std::make_shared<NullLogger>();
 
 REMOTE_CONFIG_TEST("initial state payload") {
   // Verify the initial payload structure for a remote configuration instance.
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};
 
@@ -92,8 +94,10 @@ REMOTE_CONFIG_TEST("initial state payload") {
 // TODO: test all combination of product and capabilities generation
 
 REMOTE_CONFIG_TEST("response processing") {
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};
 
diff --git a/test/telemetry/test_telemetry.cpp b/test/telemetry/test_telemetry.cpp
index 83590773..35731e63 100644
--- a/test/telemetry/test_telemetry.cpp
+++ b/test/telemetry/test_telemetry.cpp
@@ -89,8 +89,10 @@ TELEMETRY_IMPLEMENTATION_TEST("Tracer telemetry lifecycle") {
   auto client = std::make_shared<MockHTTPClient>();
   auto scheduler = std::make_shared<FakeEventScheduler>();
 
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};
 
@@ -353,6 +355,65 @@ TELEMETRY_IMPLEMENTATION_TEST("Tracer telemetry lifecycle") {
   }
 }
 
+TELEMETRY_IMPLEMENTATION_TEST("session ID headers") {
+  auto logger = std::make_shared<MockLogger>();
+  auto client = std::make_shared<MockHTTPClient>();
+  auto scheduler = std::make_shared<FakeEventScheduler>();
+  auto url = HTTPClient::URL::parse("http://localhost:8000");
+
+  SECTION("root process: DD-Session-ID present, DD-Root-Session-ID absent") {
+    auto rid = RuntimeID::generate();
+    const TracerSignature sig{rid, rid.string(), "testsvc", "test"};
+
+    Telemetry telemetry{*finalize_config(), sig, logger, client,
+                        scheduler,          *url};
+
+    auto it = client->request_headers.items.find("DD-Session-ID");
+    REQUIRE(it != client->request_headers.items.end());
+    CHECK(it->second == rid.string());
+
+    CHECK(client->request_headers.items.find("DD-Root-Session-ID") ==
+          client->request_headers.items.end());
+  }
+
+  SECTION("child process: DD-Root-Session-ID present when different") {
+    auto rid = RuntimeID::generate();
+    auto root_rid = RuntimeID::generate();
+    const TracerSignature sig{rid, root_rid.string(), "testsvc", "test"};
+
+    Telemetry telemetry{*finalize_config(), sig, logger, client,
+                        scheduler,          *url};
+
+    auto session_it = client->request_headers.items.find("DD-Session-ID");
+    REQUIRE(session_it != client->request_headers.items.end());
+    CHECK(session_it->second == rid.string());
+
+    auto root_it = client->request_headers.items.find("DD-Root-Session-ID");
+    REQUIRE(root_it != client->request_headers.items.end());
+    CHECK(root_it->second == root_rid.string());
+  }
+
+  SECTION("heartbeat includes session headers") {
+    auto rid = RuntimeID::generate();
+    auto root_rid = RuntimeID::generate();
+    const TracerSignature sig{rid, root_rid.string(), "testsvc", "test"};
+
+    Telemetry telemetry{*finalize_config(), sig, logger, client,
+                        scheduler,          *url};
+
+    client->request_headers.items.clear();
+    scheduler->trigger_heartbeat();
+
+    auto session_it = client->request_headers.items.find("DD-Session-ID");
+    REQUIRE(session_it != client->request_headers.items.end());
+    CHECK(session_it->second == rid.string());
+
+    auto root_it = client->request_headers.items.find("DD-Root-Session-ID");
+    REQUIRE(root_it != client->request_headers.items.end());
+    CHECK(root_it->second == root_rid.string());
+  }
+}
+
 TELEMETRY_IMPLEMENTATION_TEST("Tracer telemetry API") {
   const Clock clock = [] {
     TimePoint result;
@@ -364,8 +425,10 @@ TELEMETRY_IMPLEMENTATION_TEST("Tracer telemetry API") {
   auto client = std::make_shared<MockHTTPClient>();
   auto scheduler = std::make_shared<FakeEventScheduler>();
 
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};
 
@@ -869,8 +932,10 @@ TELEMETRY_IMPLEMENTATION_TEST("Tracer telemetry configuration") {
   auto client = std::make_shared<MockHTTPClient>();
   auto scheduler = std::make_shared<FakeEventScheduler>();
 
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};
 
diff --git a/test/test_datadog_agent.cpp b/test/test_datadog_agent.cpp
index 92ee9a06..ac7c4a3e 100644
--- a/test/test_datadog_agent.cpp
+++ b/test/test_datadog_agent.cpp
@@ -199,7 +199,8 @@ DATADOG_AGENT_TEST("Remote Configuration") {
   auto finalized = finalize_config(config);
   REQUIRE(finalized);
 
-  const TracerSignature signature(RuntimeID::generate(), "testsvc", "test");
+  auto rid = RuntimeID::generate();
+  const TracerSignature signature(rid, rid.string(), "testsvc", "test");
 
   // TODO: set telemetry mock
   auto config_manager = std::make_shared<ConfigManager>(*finalized);
diff --git a/tools/config-inversion/main.cpp b/tools/config-inversion/main.cpp
index 1bef85dd..c162651b 100644
--- a/tools/config-inversion/main.cpp
+++ b/tools/config-inversion/main.cpp
@@ -11,12 +11,17 @@ namespace fs = std::filesystem;
 namespace env = datadog::tracing::environment;
 
 template <typename T>
-std::string to_string_any(const T& value) {
+nlohmann::json to_json_default(const T& value) {
   std::ostringstream oss;
   oss << value;
   return oss.str();
 }
 
+template <>
+nlohmann::json to_json_default(std::nullptr_t const&) {
+  return nullptr;
+}
+
 nlohmann::json build_configuration() {
   nlohmann::json j;
   j["version"] = "2";
@@ -31,7 +36,7 @@ nlohmann::json build_configuration() {
 #define X(NAME, TYPE, DEFAULT_VALUE)                                       \
   do {                                                                     \
     auto obj = nlohmann::json::object();                                   \
-    obj["default"] = to_string_any(DEFAULT_VALUE);                         \
+    obj["default"] = to_json_default(DEFAULT_VALUE);                       \
     obj["implementation"] = "A";                                           \
     obj["type"] = QUOTED(TYPE);                                            \
     supported_configurations[QUOTED(NAME)] = nlohmann::json::array({obj}); \