diff --git a/CHANGELOG.md b/CHANGELOG.md
index 106cd94..12c4481 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,25 @@
 All notable changes to the Tombstack Native SDK (the `tombstone_*` C ABI and
 the `tombstone` library name are stable — Tombstack is the product name).
 
+## [0.9.2] - 2026-08-25
+
+### Fixed — heartbeat ceiling lowered to 240s (behaviour change; no API/ABI change)
+
+- **`options.heartbeat_interval_s` is now clamped to `[15, 240]`, was `[15, 600]`.** The ceiling is a
+  BILLING constant, not a preference. The server merges consecutive beats into one live session only
+  while the gap stays under its 300s session window, so a 600s cadence shattered a continuously-online
+  player into isolated instants: concurrent sessions stopped overlapping, monthly **peak CCU** read low,
+  and the studio was invoiced on the low figure. An integrator raising the interval to cut telemetry
+  cost — explicitly permitted by the header's own comment — silently reduced their own bill.
+
+  This is bit-for-bit the defect the Unity SDK shipped in 0.19.3. The Tombstack repo's
+  `tests/heartbeat-cadence.test.ts` was written to stop it recurring, but read the C# source **only**,
+  so this SDK kept 600 with a green suite. That guard now parses the ceiling out of this repo's public
+  header as well, and asserts all three SDKs (Unity, Godot, native) agree.
+
+  A caller that passed a value above 240 gets 240 instead. Nothing else about the cadence changes; the
+  default (60s) and the floor (15s) are untouched.
+
 ## [0.9.1] - 2026-07-20
 
 ### Hardening (production audit; no API/ABI change)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f9fb155..407ae18 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,8 @@
 cmake_minimum_required(VERSION 3.16)
-project(tombstone VERSION 0.9.1 LANGUAGES C CXX)
+# NOTE: this version is DUPLICATED as a literal in src/tombstone_api.cpp (`sdk_version`), which is what
+# tombstone_version() returns. Bump both together — 0.9.1's changelog records that the reported version
+# was once stuck at 0.8.0 for exactly this reason. Deriving it from PROJECT_VERSION is the real fix.
+project(tombstone VERSION 0.9.2 LANGUAGES C CXX)
 
 option(TOMBSTONE_BUILD_STATIC "Also build a static tombstone library" OFF)
 option(TOMBSTONE_BUILD_EXAMPLES "Build the example programs" ON)
diff --git a/include/tombstone/tombstone.h b/include/tombstone/tombstone.h
index 3dec3b6..a5f3e0d 100644
--- a/include/tombstone/tombstone.h
+++ b/include/tombstone/tombstone.h
@@ -104,7 +104,11 @@ typedef struct tombstone_options {
      *  "staging", "dev"; clamped to 64). NULL/empty = unset (the server
      *  defaults to "production"). tombstone_set_environment() overrides this. */
     const char *environment;
-    /** Seconds between session heartbeats; clamped to [15, 600]. 0 = default (60). */
+    /** Seconds between session heartbeats; clamped to [15, 240]. 0 = default (60).
+     *  The ceiling is a BILLING constant, not a preference: the server merges
+     *  consecutive beats into one live session only while the gap stays under its
+     *  300s window, so a slower cadence shatters a continuous session into isolated
+     *  instants and the studio's monthly peak-CCU invoice comes out low. */
     int heartbeat_interval_s;
     /** Nonzero (default) = emit periodic session heartbeats. */
     int enable_heartbeats;
diff --git a/src/client.cpp b/src/client.cpp
index 3a9f34d..c4fc293 100644
--- a/src/client.cpp
+++ b/src/client.cpp
@@ -31,7 +31,18 @@ constexpr const char *metrics_batch_path = "/api/v1/ingest/metrics:batch";
 constexpr const char *pull_requests_path = "/api/v1/pull-requests";
 
 constexpr int min_heartbeat_interval_s = 15;
-constexpr int max_heartbeat_interval_s = 600;
+// 240, NOT 600 — the server's session window is 300s and this is a BILLING constant.
+//
+// computePeakConcurrency merges consecutive beats into one live span only while the gap is <= that
+// window. At 600s a player online for an hour produces six isolated instants instead of one span, so
+// concurrent players stop overlapping, the monthly PEAK CCU comes out low, and the studio is invoiced
+// on the low figure. Unity shipped exactly this (0.19.3, MAX_HEARTBEAT_INTERVAL_SECONDS = 600f) and
+// the Tombstack repo carries tests/heartbeat-cadence.test.ts to stop it recurring — but that guard read
+// the C# source only, which is why this side kept 600. It now reads the C header's range as well.
+//
+// The 20% headroom is deliberate: at exactly the window each beat expires as the next arrives, so any
+// scheduling jitter or upload latency opens a gap.
+constexpr int max_heartbeat_interval_s = 240;
 constexpr int default_heartbeat_interval_s = 60;
 
 constexpr const char *identity_file_name = "identity";
diff --git a/src/tombstone_api.cpp b/src/tombstone_api.cpp
index 305385a..c3d151a 100644
--- a/src/tombstone_api.cpp
+++ b/src/tombstone_api.cpp
@@ -12,7 +12,10 @@
 
 namespace {
 
-constexpr const char *sdk_version = "0.9.1";
+// Kept in lockstep with project(... VERSION ...) in CMakeLists.txt BY HAND, and that has failed TWICE:
+// the changelog records this string pinned at "0.1.0" once and stuck at "0.8.0" once. Deriving it from
+// PROJECT_VERSION is the real fix; until then, bump both or ship a third wrong version.
+constexpr const char *sdk_version = "0.9.2";
 
 // The process-wide client. Entry points snapshot the shared_ptr under the
 // mutex and call outside it, so a long flush() neither blocks other calls nor