diff --git a/.changeset/fix-telemetry-test-isolation.md b/.changeset/fix-telemetry-test-isolation.md
new file mode 100644
index 0000000000..c783d48838
--- /dev/null
+++ b/.changeset/fix-telemetry-test-isolation.md
@@ -0,0 +1,5 @@
+---
+"@core/sync-service": patch
+---
+
+Test-only: make the StackSupervisor telemetry test synchronous to avoid serve_shape counter contamination from concurrently running tests. No runtime changes.
diff --git a/packages/sync-service/test/electric/stack_supervisor/telemetry_test.exs b/packages/sync-service/test/electric/stack_supervisor/telemetry_test.exs
index 0b658eae75..5c543abf87 100644
--- a/packages/sync-service/test/electric/stack_supervisor/telemetry_test.exs
+++ b/packages/sync-service/test/electric/stack_supervisor/telemetry_test.exs
@@ -1,6 +1,10 @@
 if Electric.telemetry_enabled?() and Code.ensure_loaded?(ElectricTelemetry.Reporters.Prometheus) do
   defmodule Electric.StackSupervisor.TelemetryTest do
-    use ExUnit.Case, async: true
+    # Not async: the Prometheus reporter attaches a global :telemetry handler for
+    # [:electric, :plug, :serve_shape], so concurrent async tests that emit that
+    # event (via the real serve_shape plug) leak into this reporter's per-status
+    # counters and make the exact-count assertions below non-deterministic.
+    use ExUnit.Case, async: false
 
     alias Electric.StackSupervisor.Telemetry