diff --git a/runner/docs/adr/0041-observability-stack.md b/runner/docs/adr/0041-observability-stack.md
index 39220e41f..9cb185697 100644
--- a/runner/docs/adr/0041-observability-stack.md
+++ b/runner/docs/adr/0041-observability-stack.md
@@ -563,7 +563,8 @@ starting values, tuned after launch: preview-ready below 97 % (Tier-1) or 95 % (
 over 1 h, evaluated per tier only with at least 10 non-abandoned previews in that hour;
 session start p95 above 20 s, evaluated only with at least 20 `ready` starts in the hour
 (below either floor the rule is not firing, so a firing alert resolves through the normal path); `at_capacity` above 5/h; 5xx above 1 % over
-15 min; LiteLLM errors above 5 %; compile errors on one `ht_major` doubling day over
+15 min, evaluated only with at least 100 requests after the exclusions (at 1 % one error exceeds the threshold only below 100 requests, so a single 500 cannot page);
+LiteLLM errors above 5 % over 1 h, evaluated only with at least 20 non-denied calls (same reasoning at 5 %); compile errors on one `ht_major` doubling day over
 day; snapshot builds failing above 50 % per framework over 30 min with at least 10
 failed; an embed above 20 % errors with more than 50 views in 24 h; backlog older
 than 2 h.
diff --git a/runner/pipeline/o11y-alerts.test.mjs b/runner/pipeline/o11y-alerts.test.mjs
index 313537d51..de847ffc0 100644
--- a/runner/pipeline/o11y-alerts.test.mjs
+++ b/runner/pipeline/o11y-alerts.test.mjs
@@ -18,6 +18,7 @@ const { evaluateAndNotify, slackPoster, escapeSlackMrkdwn, writeNewFingerprintPo
 const {
   atCapacityRule,
   fiveXxRateRule,
+  FIVE_XX_MIN_REQUESTS,
   previewReadyRateRule,
   PREVIEW_READY_MIN_SAMPLES,
   sessionStartP95Rule,
@@ -25,6 +26,7 @@ const {
   embedErrorRateRule,
   compileErrorDoublingRule,
   litellmErrorRateRule,
+  LITELLM_MIN_CALLS,
   snapshotBuildFailedRateRule,
   backlogAgeRule,
   rejectedKeyRule,
@@ -988,6 +990,58 @@ test("fiveXxRateRule: points written before the status column existed (no reason
   assert.equal(result.firing, false, result.detail);
 });
 
+function apiRows(ok, bad) {
+  return [
+    { metric: "api.request", route_class: "api/demos", outcome: "2xx", count: ok },
+    { metric: "api.request", route_class: "api/demos", outcome: "5xx", count: bad },
+  ];
+}
+
+// The fake engine only proves the SQL passes the guard and the arithmetic is right, not that Analytics Engine accepts it.
+test("fiveXxRateRule sample floor: below 100 requests even 100% 5xx does not fire, at 100 it fires above 1%", async () => {
+  assert.equal(FIVE_XX_MIN_REQUESTS, 100);
+  const single = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(0, 1)).queryFn);
+  assert.equal(single.firing, false, "one 500 out of one request is 100% but says nothing");
+  assert.match(single.detail, /only 1 request\(s\), below the 100 needed/);
+
+  const allBad = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(0, 99)).queryFn);
+  assert.equal(allBad.firing, false, allBad.detail);
+
+  const oneInNinetyNine = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(98, 1)).queryFn);
+  assert.equal(oneInNinetyNine.firing, false, "1/99 is 1.01% but below the floor");
+
+  const atFloorOne = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(99, 1)).queryFn);
+  assert.equal(atFloorOne.firing, false, "1/100 is exactly 1%, not above it: a single error never fires at the floor");
+
+  const atFloor = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(98, 2)).queryFn);
+  assert.equal(atFloor.firing, true, atFloor.detail);
+  assert.match(atFloor.detail, /^2\.00% 5xx over the last 15 min \(2\/100,/);
+
+  const healthy = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(500, 0)).queryFn);
+  assert.equal(healthy.firing, false);
+});
+
+test("fiveXxRateRule sample floor: the floor counts the ADJUSTED total, not the raw one", async () => {
+  const rows = [
+    ...apiRows(0, 5),
+    { metric: "api.request", route_class: "api/session", outcome: "5xx", count: 200 },
+    { metric: "session.start", outcome: "at_capacity", count: 200 },
+  ];
+  const result = await fiveXxRateRule({}, makeFakeAeQuery(rows).queryFn);
+  assert.equal(result.firing, false, "205 raw requests but only 5 after exclusions");
+});
+
+test("fiveXxRateRule sample floor: a firing alert resolves once traffic drops below the floor", async () => {
+  const writer = fakeInboxWriter();
+  const slackCalls = [];
+  const deps = { inboxWriter: writer, postSlack: async (t) => slackCalls.push(t), aeSink: fakeAeSink(), commonAttrs: COMMON_ATTRS, nowMs: 1000 };
+  const firing = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(95, 5)).queryFn);
+  assert.equal(await evaluateAndNotify(firing, deps), "fired");
+  const quiet = await fiveXxRateRule({}, makeFakeAeQuery(apiRows(0, 1)).queryFn);
+  assert.equal(await evaluateAndNotify(quiet, { ...deps, nowMs: 2000 }), "resolved");
+  assert.match(slackCalls[1], /resolved/);
+});
+
 test("previewReadyRateRule: tier 1 below 97% fires, tier 2 within threshold does not (mixed)", async () => {
   const fake = makeFakeAeQuery([
     { metric: "preview.ready_ms", tier: "1", outcome: "ready", count: 90 },
@@ -1308,6 +1362,51 @@ test("litellmErrorRateRule: chat.answer + theme.ai combined over 5% fires; under
   assert.equal(underResult.firing, false, underResult.detail); // 2/200 = 1%
 });
 
+function gatewayRows(chatOk, chatErr, extra = []) {
+  return [
+    { metric: "chat.answer", outcome: "answered", count: chatOk },
+    { metric: "chat.answer", outcome: "error", count: chatErr },
+    ...extra,
+  ];
+}
+
+test("litellmErrorRateRule sample floor: below 20 calls even 100% errors does not fire, at 20 it fires above 5%", async () => {
+  assert.equal(LITELLM_MIN_CALLS, 20);
+  const single = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(0, 1)).queryFn);
+  assert.equal(single.firing, false);
+  assert.match(single.detail, /only 1 call\(s\), below the 20 needed/);
+
+  const oneInNineteen = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(18, 1)).queryFn);
+  assert.equal(oneInNineteen.firing, false, "1/19 is 5.26% but below the floor");
+
+  const oneAtFloor = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(19, 1)).queryFn);
+  assert.equal(oneAtFloor.firing, false, "1/20 is exactly 5%: a single error never fires at the floor");
+
+  const atFloor = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(18, 2)).queryFn);
+  assert.equal(atFloor.firing, true, atFloor.detail);
+  assert.match(atFloor.detail, /^10\.00% gateway errors over the last hour \(2\/20,/);
+
+  const healthy = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(100, 0)).queryFn);
+  assert.equal(healthy.firing, false);
+});
+
+test("litellmErrorRateRule sample floor: denied requests do not count toward the floor", async () => {
+  const rows = gatewayRows(0, 2, [{ metric: "chat.answer", outcome: "denied", count: 500 }]);
+  const result = await litellmErrorRateRule({}, makeFakeAeQuery(rows).queryFn);
+  assert.equal(result.firing, false, "2 real calls plus 500 denied is still below the floor");
+});
+
+test("litellmErrorRateRule sample floor: a firing alert resolves once calls drop below the floor", async () => {
+  const writer = fakeInboxWriter();
+  const slackCalls = [];
+  const deps = { inboxWriter: writer, postSlack: async (t) => slackCalls.push(t), aeSink: fakeAeSink(), commonAttrs: COMMON_ATTRS, nowMs: 1000 };
+  const firing = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(18, 2)).queryFn);
+  assert.equal(await evaluateAndNotify(firing, deps), "fired");
+  const quiet = await litellmErrorRateRule({}, makeFakeAeQuery(gatewayRows(0, 1)).queryFn);
+  assert.equal(await evaluateAndNotify(quiet, { ...deps, nowMs: 2000 }), "resolved");
+  assert.match(slackCalls[1], /resolved/);
+});
+
 // `denied` (a rate-limit/budget refusal at `index.ts`'s own gate — never
 // reaches the LiteLLM gateway) must be excluded from the denominator.
 // Including it only ever drags the computed error% down, which can mask a
diff --git a/runner/workers/o11y/src/alerts/rules.ts b/runner/workers/o11y/src/alerts/rules.ts
index 24b79c825..a9a2d8ad5 100644
--- a/runner/workers/o11y/src/alerts/rules.ts
+++ b/runner/workers/o11y/src/alerts/rules.ts
@@ -176,6 +176,9 @@ export async function atCapacityRule(env: Env, queryFn: AeQueryFn = runAeQuery):
  *  `reason`, and only the 503 is dropped. A build-failed 500 there still counts. */
 const FIVE_XX_STILL_BUILDING_ROUTE_CLASSES = ["d/:id", "embed/:id"];
 
+/** Requests (after the exclusions below) in the 15-minute window under which the rule is not evaluated: at a 1 % threshold one error only exceeds it when the total is under 100, so a floor of 100 means a single 500 can never page. */
+export const FIVE_XX_MIN_REQUESTS = 100;
+
 export async function fiveXxRateRule(env: Env, queryFn: AeQueryFn = runAeQuery): Promise<RuleResult> {
   const windowMs = 15 * 60 * 1000;
   const routeClassCol = col("route_class");
@@ -208,12 +211,14 @@ export async function fiveXxRateRule(env: Env, queryFn: AeQueryFn = runAeQuery):
   const adjustedFiveXx = Math.max(0, fiveXx - deliberate);
   const adjustedTotal = Math.max(0, total - deliberate);
   const pct = ratio(adjustedFiveXx, adjustedTotal) * 100;
+  const enoughSamples = adjustedTotal >= FIVE_XX_MIN_REQUESTS;
   return {
     rule: "api-5xx-rate",
-    firing: adjustedTotal > 0 && pct > 1,
+    firing: enoughSamples && pct > 1,
     detail:
       `${pct.toFixed(2)}% 5xx over the last 15 min (${adjustedFiveXx}/${adjustedTotal}, threshold 1%, ` +
-      `excludes at-capacity/container-starting/chat-theme-gateway refusals and still-building 503s)`,
+      `excludes at-capacity/container-starting/chat-theme-gateway refusals and still-building 503s)` +
+      (enoughSamples ? "" : `; only ${adjustedTotal} request(s), below the ${FIVE_XX_MIN_REQUESTS} needed to evaluate the rate`),
   };
 }
 
@@ -376,6 +381,9 @@ export async function snapshotBuildFailedRateRule(env: Env, queryFn: AeQueryFn =
 // ---- LiteLLM errors: above 5% (chat.answer + theme.ai, both gateway -------
 // call sites; window: 1h, same reasoning as session-start) ------------------
 
+/** Gateway calls (denied excluded) in the hour under which the rule is not evaluated: at a 5 % threshold one error only exceeds it when the total is under 20, so a floor of 20 means a single gateway error can never page. */
+export const LITELLM_MIN_CALLS = 20;
+
 export async function litellmErrorRateRule(env: Env, queryFn: AeQueryFn = runAeQuery): Promise<RuleResult> {
   const windowMs = HOUR_MS;
   const [chat, theme] = await Promise.all([
@@ -390,10 +398,13 @@ export async function litellmErrorRateRule(env: Env, queryFn: AeQueryFn = runAeQ
     .reduce((sum, [, c]) => sum + c, 0);
   const errors = (chat.get("error") ?? 0) + (theme.get("error") ?? 0);
   const pct = ratio(errors, total) * 100;
+  const enoughSamples = total >= LITELLM_MIN_CALLS;
   return {
     rule: "litellm-error-rate",
-    firing: total > 0 && pct > 5,
-    detail: `${pct.toFixed(2)}% gateway errors over the last hour (${errors}/${total}, threshold 5%)`,
+    firing: enoughSamples && pct > 5,
+    detail:
+      `${pct.toFixed(2)}% gateway errors over the last hour (${errors}/${total}, threshold 5%)` +
+      (enoughSamples ? "" : `; only ${total} call(s), below the ${LITELLM_MIN_CALLS} needed to evaluate the rate`),
   };
 }