diff --git a/src/scoring/model.ts b/src/scoring/model.ts
index 35f930064b..c088a2abcd 100644
--- a/src/scoring/model.ts
+++ b/src/scoring/model.ts
@@ -133,8 +133,21 @@ export async function refreshScoringModelSnapshot(env: Env): Promise<ScoringMode
   return snapshot;
 }
 
+// Mirror Pipeline B's UPSTREAM_STALE_MS (upstream/ruleset.ts): a served scoring snapshot older than this
+// window means the last upstream refresh failed or has not run, so previews are quietly using last-good (or
+// DEFAULT) constants with no other staleness signal on the scoring side (#810).
+export const SCORING_SNAPSHOT_STALE_MS = 2 * 60 * 60 * 1000;
+
+export function scoringSnapshotStalenessWarning(snapshot: Pick<ScoringModelSnapshotRecord, "fetchedAt">, now: number = Date.now()): string | null {
+  if (Date.parse(snapshot.fetchedAt) + SCORING_SNAPSHOT_STALE_MS >= now) return null;
+  return "Scoring constants snapshot is stale: the last upstream refresh is older than the freshness window, so scoring may be using last-good or default constants and be behind upstream.";
+}
+
 export async function getOrCreateScoringModelSnapshot(env: Env): Promise<ScoringModelSnapshotRecord> {
-  return (await getLatestScoringModelSnapshot(env)) ?? refreshScoringModelSnapshot(env);
+  const snapshot = (await getLatestScoringModelSnapshot(env)) ?? (await refreshScoringModelSnapshot(env));
+  // Surface staleness so previews do not silently use last-good/DEFAULT constants after a failed/old refresh (#810).
+  const stalenessWarning = scoringSnapshotStalenessWarning(snapshot);
+  return stalenessWarning ? { ...snapshot, warnings: [...snapshot.warnings, stalenessWarning] } : snapshot;
 }
 
 export function parsePythonNumberConstants(source: string, options: { knownOnly?: boolean } = { knownOnly: true }): Record<string, number> {
diff --git a/test/unit/scoring.test.ts b/test/unit/scoring.test.ts
index 2502255382..1445bc85c4 100644
--- a/test/unit/scoring.test.ts
+++ b/test/unit/scoring.test.ts
@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { getLatestScoringModelSnapshot, listUpstreamDriftReports } from "../../src/db/repositories";
-import { DEFAULT_SCORING_CONSTANTS, detectActiveModel, findUnmodeledUpstreamConstants, isTimeDecayEnabled, parsePythonNumberConstants, refreshScoringModelSnapshot } from "../../src/scoring/model";
+import { getLatestScoringModelSnapshot, listUpstreamDriftReports, persistScoringModelSnapshot } from "../../src/db/repositories";
+import { DEFAULT_SCORING_CONSTANTS, detectActiveModel, findUnmodeledUpstreamConstants, getOrCreateScoringModelSnapshot, isTimeDecayEnabled, parsePythonNumberConstants, refreshScoringModelSnapshot, SCORING_SNAPSHOT_STALE_MS, scoringSnapshotStalenessWarning } from "../../src/scoring/model";
 import { buildScorePreview, calculateTimeDecay, makeScorePreviewRecord, resolveTimeDecay } from "../../src/scoring/preview";
 import { unmodeledScoringConstantsFingerprint } from "../../src/upstream/unmodeled-scoring-drift";
 import type { ScorePreviewInput } from "../../src/scoring/preview";
@@ -86,6 +86,34 @@ OSS_EMISSION_SHARE = 0.90
     expect(parsed.OSS_EMISSION_SHARE).toBe(0.9);
   });
 
+  it("flags only scoring snapshots older than the freshness window as stale (#810)", () => {
+    const now = Date.parse("2026-06-21T12:00:00.000Z");
+    const justFresh = new Date(now - SCORING_SNAPSHOT_STALE_MS + 60_000).toISOString();
+    const clearlyStale = new Date(now - SCORING_SNAPSHOT_STALE_MS - 60_000).toISOString();
+    expect(scoringSnapshotStalenessWarning({ fetchedAt: justFresh }, now)).toBeNull();
+    expect(scoringSnapshotStalenessWarning({ fetchedAt: clearlyStale }, now)).toMatch(/stale/i);
+  });
+
+  it("appends a staleness warning when getOrCreateScoringModelSnapshot serves an old snapshot (#810)", async () => {
+    const env = createTestEnv();
+    await persistScoringModelSnapshot(env, snapshot);
+    const served = await getOrCreateScoringModelSnapshot(env);
+    expect(served.id).toBe(snapshot.id);
+    expect(served.warnings.some((warning) => /stale/i.test(warning))).toBe(true);
+  });
+
+  it("does not add a staleness warning when getOrCreate refreshes a fresh snapshot (#810)", async () => {
+    const env = createTestEnv({ GITHUB_PUBLIC_TOKEN: "token" });
+    vi.stubGlobal("fetch", async (input: RequestInfo | URL) => {
+      const url = input.toString();
+      if (url.includes("constants.py")) return new Response("MERGED_PR_BASE_SCORE = 25\n");
+      if (url.includes("programming_languages.json")) return Response.json({});
+      return new Response("not found", { status: 404 });
+    });
+    const served = await getOrCreateScoringModelSnapshot(env);
+    expect(served.warnings.some((warning) => /stale/i.test(warning))).toBe(false);
+  });
+
   it("prefers exponential saturation when mixed upstream constants are present", () => {
     const parsed = parsePythonNumberConstants(`
 MERGED_PR_BASE_SCORE = 25