diff --git a/servers/sentry.go b/servers/sentry.go
index 55281a3..4e92cc2 100644
--- a/servers/sentry.go
+++ b/servers/sentry.go
@@ -47,6 +47,17 @@ var sentryStartupProbeTimeout = 5 * time.Second
 // mutates it.
 var sentryDialCheckTimeout = 3 * time.Second
 
+// sentryTransportOverride, when non-nil, replaces the transport of the client
+// setupSentry constructs. Test-only injection seam (#179): the probe-stall
+// test needs a transport whose Flush deterministically reports not-drained,
+// and binding a stub onto the hub cannot reach the client setupSentry binds
+// itself. Nil in production — the SDK builds its real HTTP transport and
+// nothing here changes. Caveat: a non-nil transport flips sentry-go (v0.46.2)
+// into its legacy-transport client mode (with batchMeter), whereas
+// production's nil selects the telemetry-processor mode — stub-transport tests
+// therefore exercise the legacy flush path; re-check on SDK bumps.
+var sentryTransportOverride sentry.Transport
+
 // setupSentry initialises Sentry error capture (errors only, no tracing) when
 // SENTRY_DSN is present in the environment. Without a DSN nothing is
 // initialised — no client, no capture, local/dev unaffected; the only side
@@ -89,6 +100,9 @@ func setupSentry() bool {
 		// Belt-and-braces scrubbing at the choke point — see scrubEvent for
 		// the exact (request-material-only) scope of the guarantee.
 		BeforeSend: scrubEvent,
+		// Nil in production (SDK default transport); see
+		// sentryTransportOverride for the test-only seam.
+		Transport: sentryTransportOverride,
 	})
 	if err != nil {
 		// Telemetry must never take the API down: log and run without it.
diff --git a/servers/sentry_test.go b/servers/sentry_test.go
index c88502f..af96787 100644
--- a/servers/sentry_test.go
+++ b/servers/sentry_test.go
@@ -5,9 +5,6 @@ import (
 	"context"
 	"encoding/json"
 	"net"
-	"net/http"
-	"net/http/httptest"
-	"net/url"
 	"os"
 	"sync"
 	"syscall"
@@ -65,6 +62,16 @@ func bindStubClient(t *testing.T, flushDrains bool) *stubTransport {
 	return transport
 }
 
+// setSentryTransportOverride points the sentryTransportOverride seam (#179) at
+// tr for the duration of the test, restoring the production nil afterwards.
+// The reset rides along with the set, so a set-without-reset is unwritable
+// through this helper.
+func setSentryTransportOverride(t *testing.T, tr sentry.Transport) {
+	t.Helper()
+	sentryTransportOverride = tr
+	t.Cleanup(func() { sentryTransportOverride = nil })
+}
+
 func TestSetupSentry_NoDSN_Disabled(t *testing.T) {
 	viper.Set("SENTRY_DSN", "")
 	defer viper.Set("SENTRY_DSN", "")
@@ -210,30 +217,33 @@ func TestSentryStartupProbe_ClientSideDrop_WarnsAndFails(t *testing.T) {
 }
 
 // TestSetupSentry_ProbeStall_WarnsAndWithholdsEnabledLine pins that
-// setupSentry actually INVOKES the startup probe: the DSN points at a local
-// server that holds the canary's HTTP request past the shrunk probe window,
-// so the real transport cannot drain. Deleting the sentryStartupProbe() call
-// from setupSentry turns both assertions red (no warning, and the enabled
-// line prints). No real network involved.
+// setupSentry actually INVOKES the startup probe against the client it just
+// configured: a stub transport injected through the test seam reports
+// Flush=not-drained, so the stalled-transport premise holds by construction —
+// no socket dial manufactures the stall, and machine load cannot invert the
+// outcome (#179: a refused dial is a fast send outcome that drains the queue,
+// so the old local-server stall lost the timing race under load). Binding a
+// stub onto the hub is not enough here — setupSentry binds its own client, so
+// the seam must reach the client it constructs. Deleting the
+// sentryStartupProbe() call from setupSentry turns this red (no stall
+// warning, the enabled line prints, no canary reaches the transport).
 func TestSetupSentry_ProbeStall_WarnsAndWithholdsEnabledLine(t *testing.T) {
-	release := make(chan struct{})
-	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		<-release // hold the connection past the probe window
-	}))
-	defer srv.Close()
-	defer close(release) // LIFO: unblock the handler before Close waits on it
-
-	srvURL, err := url.Parse(srv.URL)
-	require.NoError(t, err)
-	viper.Set("SENTRY_DSN", "http://public@"+srvURL.Host+"/1")
+	transport := &stubTransport{flushDrains: false}
+	setSentryTransportOverride(t, transport)
+
+	// 127.0.0.1:1 refuses instantly — the suite's deterministic stand-in for
+	// the dial-check pre-check, which is frozen and irrelevant to the stall
+	// premise. Shrink its window anyway so a pathological environment cannot
+	// stall the suite. The probe window itself no longer needs shrinking: the
+	// stub's Flush reports not-drained immediately, regardless of load.
+	viper.Set("SENTRY_DSN", "http://public@127.0.0.1:1/1")
 	defer func() {
 		viper.Set("SENTRY_DSN", "")
 		sentry.CurrentHub().BindClient(nil)
 	}()
-
-	restore := sentryStartupProbeTimeout
-	sentryStartupProbeTimeout = 100 * time.Millisecond
-	defer func() { sentryStartupProbeTimeout = restore }()
+	restoreDial := sentryDialCheckTimeout
+	sentryDialCheckTimeout = 500 * time.Millisecond
+	defer func() { sentryDialCheckTimeout = restoreDial }()
 
 	var warnBuf, infoBuf bytes.Buffer
 	Warn.SetOutput(&warnBuf)
@@ -248,6 +258,10 @@ func TestSetupSentry_ProbeStall_WarnsAndWithholdsEnabledLine(t *testing.T) {
 		"a transport that cannot drain within the window must be loud at boot")
 	assert.NotContains(t, infoBuf.String(), "Sentry error capture enabled",
 		"the success line must be withheld when the probe could not drain")
+	events := transport.Events()
+	require.Len(t, events, 1,
+		"the canary must reach the transport of the client setupSentry just configured — the probe ran against THAT client, not some pre-bound stub")
+	assert.Equal(t, "sentry startup probe", events[0].Message)
 }
 
 // TestSentryDebugEnabled_StrictParse pins the operator-trap fix: viper's