diff --git a/desktop/src/shared/lib/channelSwitchPerf.test.mjs b/desktop/src/shared/lib/channelSwitchPerf.test.mjs
index 957c86efed..a83471a129 100644
--- a/desktop/src/shared/lib/channelSwitchPerf.test.mjs
+++ b/desktop/src/shared/lib/channelSwitchPerf.test.mjs
@@ -248,6 +248,27 @@ test("an undisturbed settle records exactly one measure", async () => {
   });
 });
 
+test("abandoned switches never accumulate start marks", async () => {
+  await withSettleHarness(async ({ abandon, begin }) => {
+    const { CHANNEL_SWITCH_START_MARK } = await import(
+      "./channelSwitchPerf.ts"
+    );
+    performance.clearMarks?.(CHANNEL_SWITCH_START_MARK);
+    // Traces that die without recording (forum visits, route exits, drops)
+    // never reach record()'s buffer clearing — begin() must bound the
+    // buffer itself or weeks-long sessions accumulate a mark per abandon.
+    for (const channelId of ["aaaa", "bbbb", "cccc", "dddd"]) {
+      begin(channelId);
+      abandon(channelId);
+    }
+    assert.equal(
+      performance.getEntriesByName(CHANNEL_SWITCH_START_MARK).length,
+      1,
+    );
+    performance.clearMarks?.(CHANNEL_SWITCH_START_MARK);
+  });
+});
+
 test("a settle in a hidden window drops the trace instead of recording", async () => {
   await withSettleHarness(
     async ({ begin, settle, flush, measures }) => {
diff --git a/desktop/src/shared/lib/channelSwitchPerf.ts b/desktop/src/shared/lib/channelSwitchPerf.ts
index 163658a05e..bbe09bd09a 100644
--- a/desktop/src/shared/lib/channelSwitchPerf.ts
+++ b/desktop/src/shared/lib/channelSwitchPerf.ts
@@ -201,6 +201,12 @@ export function beginChannelSwitchTrace(channelId: string): void {
     windowFetch: null,
     membersFetch: null,
   };
+  // Clear the previous start mark here, not only in record(): traces that
+  // die without recording (forum visits, route exits, drops) never reach
+  // record()'s buffer clearing, and weeks-long sessions would accumulate a
+  // stray mark per abandon. Clearing at begin bounds the buffer to one
+  // start mark no matter how the previous trace ended.
+  performance.clearMarks(CHANNEL_SWITCH_START_MARK);
   performance.mark(CHANNEL_SWITCH_START_MARK, { detail: { channelId } });
 }