diff --git a/config/quality/quality-baseline.json b/config/quality/quality-baseline.json
index b1cd01d12b..ddec82469c 100644
--- a/config/quality/quality-baseline.json
+++ b/config/quality/quality-baseline.json
@@ -120,7 +120,7 @@
       "_rebaseline_2026_06_26_v3837_release": "343->345. v3.8.37 cycle drift surfaced by the release-green pre-flight (the Quality Ratchet does NOT run on PR->release fast-gates, so warnings/complexity accrued unmeasured across this cycle's 76 commits — provider adds DGrid/Pioneer/xAI, headroom proxy lifecycle #4649, ~50 SSE/translator fixes, Engine Combos #5062). Trust-but-verify: this release-finalize working tree touches ONLY CHANGELOG.md, docs/i18n/*/CHANGELOG.md mirrors, and these baselines — 0 production-code change, so all drift is inherited cycle drift (`any` warn-allowed in open-sse/ + tests/). Tighten via --require-tighten next cycle."
     },
     "cognitiveComplexity": {
-      "value": 882,
+      "value": 883,
       "_rebaseline_2026_07_07_v3846_release_close": "877->882 (+5). v3.8.46 release close (generate-release Phase 0 pre-flight): drift herdado do merge burst do ciclo. Trust-but-verify: os fixes de base-red do captain (agentSkills path.resolve #6366, catalogo cache #6408, tipagem de teste, MitmProxyTab suppression) sao cognitive-net-zero — check:cognitive-complexity mede 882 identico com e sem os fixes (a catraca NAO roda no fast-path PR->release). Tighten via --update next cycle.",
       "_rebaseline_2026_07_03_v3844_ipfilter_release_green": "861->867 (+6). v3.8.44 cycle drift measured on release tip 32e4c906e during the #6131/#5975 release-green rebaseline. Inherited from the merge burst (Quality Ratchet does not run on PR->release fast-gates). route-edge-coverage +7 is my #5975 test comment; the rest is parallel-session drift. Tighten via --update next cycle.",
       "_rebaseline_2026_07_03_v3844_review_prs_fix_batch": "860->861 (+1). Inherited v3.8.44 cycle drift surfaced by the release-green pre-flight during the /review-prs fix-batch round; check:cognitive-complexity measures 861 on the release tip 72ee80649. Negligible +1 from the round's / parallel-session merge burst (cognitive-complexity does NOT run on PR->release fast-gates). Structural shrink tracked in #3501. Tighten via --update next cycle.",
@@ -137,7 +137,8 @@
       "_rebaseline_2026_06_22_v3833_release": "793→797 (+4) — pre-existing cycle drift on origin/release/v3.8.33.",
       "_rebaseline_2026_06_26_v3837_release": "816->826. v3.8.37 cycle drift surfaced by the release-green pre-flight (the Quality Ratchet does NOT run on PR->release fast-gates, so warnings/complexity accrued unmeasured across this cycle's 76 commits — provider adds DGrid/Pioneer/xAI, headroom proxy lifecycle #4649, ~50 SSE/translator fixes, Engine Combos #5062). Trust-but-verify: this release-finalize working tree touches ONLY CHANGELOG.md, docs/i18n/*/CHANGELOG.md mirrors, and these baselines — 0 production-code change, so all drift is inherited cycle drift (`any` warn-allowed in open-sse/ + tests/). Tighten via --require-tighten next cycle.",
       "_rebaseline_2026_06_26_v3838_release": "826->833. v3.8.38 release base measures 833 locally on origin/release/v3.8.38 (800b04ad6) while the committed baseline still says 826. This PR measures the same 833 after refactoring jsonToSse helpers back under the sonarjs/cognitive-complexity threshold, so it does not add a net cognitive-complexity violation. The baseline bump records inherited release-base drift only; structural shrink remains tracked by the existing chatCore decomposition work.",
-      "_rebaseline_2026_07_06_v3845_release_close": "867->877 (+10). v3.8.45 cycle drift measured by check:release-green (hermetic) on release tip 5ecca12aa5 during the /generate-release Phase 0 pre-flight. Inherited from the cycle's merge burst (cognitive-complexity does not run on PR->release fast-gates); the captain's pre-flight fixes are gate/test/workflow changes (complexity-neutral). Tighten via --update next cycle."
+      "_rebaseline_2026_07_06_v3845_release_close": "867->877 (+10). v3.8.45 cycle drift measured by check:release-green (hermetic) on release tip 5ecca12aa5 during the /generate-release Phase 0 pre-flight. Inherited from the cycle's merge burst (cognitive-complexity does not run on PR->release fast-gates); the captain's pre-flight fixes are gate/test/workflow changes (complexity-neutral). Tighten via --update next cycle.",
+      "_rebaseline_2026_07_07_6519_chirag_fallback_reasons": "882->883 (+1). PR #6519 (@chirag127, #6461): the preview route's fallbackReasons dedup loop adds one function over the cognitive threshold. Owner-approved rebaseline (contributor own-growth). Tighten via --update next cycle."
     },
     "typeCoveragePct": {
       "value": 92.17,
@@ -376,4 +377,4 @@
   "_zizmor_rebaseline_2026_06_19_r1_redundancy": "zizmorFindings 139 -> 145. Quebra: +3 drift PRE-EXISTENTE da base release/v3.8.30 a23d0d678 (medido com minhas mudancas stashed = 142 > 139; o fast-path do release nao roda check:workflows --ratchet) + 3 do novo workflow mutation-redundancy.yml (R1 disableBail): exatamente 3 unpinned-uses de actions/checkout@v7, actions/setup-node@v6, actions/upload-artifact@v7 — a MESMA convencao @vN deliberada e INTOCADA de todos os workflows (ver _scanner_harden_workflows_2026_06_16), identica ao nightly-mutation.yml. SHA-pinar so este workflow violaria a convencao. NOTA DE COLISAO CROSS-PR: o PR #4321 (a11y) tambem rebaselina este metric 139->145 (+3 do job a11y) off a MESMA base — se ambos mergearem, o total real vira 148 (142 base + 3 a11y + 3 r1) e o segundo a mergear precisa reconciliar zizmorFindings -> 148 (mesmo padrao release-volatil dos baselines de complexity/eslint).",
   "_zizmor_rebaseline_2026_06_19_a11y_148_reconcile": "RECONCILIACAO CROSS-PR (release-volatil) ao mergear #4321 (a11y) APOS #4322 (R1): zizmorFindings 145 -> 148. O #4322 ja rebaselinou 139->145 (drift base 142 + 3 unpinned-uses do mutation-redundancy.yml). Este PR adiciona +3 unpinned-uses @vN do novo job 'a11y' (nightly-resilience.yml): actions/checkout@v7, actions/setup-node@v6, actions/cache@v5.0.5 — MESMA convencao @vN deliberada e INTOCADA de todos os workflows (ver _scanner_harden_workflows_2026_06_16). Total = 142 base + 3 r1 + 3 a11y = 148, MEDIDO com `node scripts/check/check-workflows.mjs --ratchet` na arvore release(com #4322)+#4321 = 148 exato. Nenhum template-injection/artipacked/cache-poisoning novo.",
   "_zizmor_rebaseline_2026_06_20_ci_build_artifact_reuse": "zizmorFindings 148 -> 152. Drift legitimo deste PR ao reutilizar o artefato next-build do job Build em package-artifact/electron-package-smoke e ao separar o build de compatibilidade Node 26: +4 unpinned-uses novos (2x actions/download-artifact@v8, actions/checkout@v7, actions/setup-node@v6). Mantida a convencao deliberada @vN dos workflows (sem SHA-pinning/manual update burden), conforme precedentes _scanner_harden_workflows_2026_06_16 e _zizmor_rebaseline_2026_06_19_*. Sem novos findings de template-injection/artipacked/cache-poisoning; medido localmente com zizmor 1.25.2 via `npm run check:workflows -- --ratchet` = 152."
-}
+}
\ No newline at end of file
diff --git a/src/app/api/compression/preview/route.ts b/src/app/api/compression/preview/route.ts
index 3153298ffa..cc8fc7e360 100644
--- a/src/app/api/compression/preview/route.ts
+++ b/src/app/api/compression/preview/route.ts
@@ -230,6 +230,31 @@ export async function POST(req: Request) {
       ? summarizeEncoderCandidates(messages, DEFAULT_MIN_ROWS, countTextTokens)
       : null;
 
+    // #6461: when fallbackApplied=true, synthesize a deduped reason list from data the
+    // pipeline already produces on result.stats (engineBreakdown[].rejectReason,
+    // validationErrors, and inflation-guard entries in validationWarnings). Non-fallback
+    // runs return []/null — zero change on the happy path.
+    const fallbackReasons: string[] = [];
+    if (diff.fallbackApplied) {
+      const seen = new Set<string>();
+      const push = (s: unknown) => {
+        if (typeof s === "string" && s.length > 0 && !seen.has(s)) {
+          seen.add(s);
+          fallbackReasons.push(s);
+        }
+      };
+      for (const step of engineBreakdown) {
+        if ((step as { rejected?: boolean }).rejected === true) {
+          push((step as { rejectReason?: string }).rejectReason);
+        }
+      }
+      for (const err of diff.validationErrors ?? []) push(err);
+      for (const warn of diff.validationWarnings ?? []) {
+        if (typeof warn === "string" && warn.startsWith("pipeline-inflation-guard:")) push(warn);
+      }
+    }
+    const fallbackReason = fallbackReasons[0] ?? null;
+
     return NextResponse.json({
       encoderComparison,
       original: originalText,
@@ -246,7 +271,7 @@ export async function POST(req: Request) {
       mode: effectiveMode,
       intensity: null,
       outputMode: null,
-      skippedReasons: [],
+      skippedReasons: fallbackReasons,
       diff: diff.segments,
       preservedBlocks: diff.preservedBlocks,
       ruleRemovals: diff.ruleRemovals,
@@ -261,7 +286,10 @@ export async function POST(req: Request) {
       validationWarnings: diff.validationWarnings,
       validationErrors: diff.validationErrors,
       fallbackApplied: diff.fallbackApplied,
-      ...(diff.fallbackReason && { fallbackReason: diff.fallbackReason }),
+      // Prefer the pipeline's canonical `diff.fallbackReason`; fall back to the
+      // first synthesized reason (#6461) when the pipeline did not set one.
+      fallbackReason: diff.fallbackReason ?? fallbackReason,
+      fallbackReasons,
       ...(diff.heatmap ? { heatmap: diff.heatmap } : {}),
     });
   } catch (err: unknown) {
diff --git a/tests/unit/compression/preview-fallback-reasons-6461.test.ts b/tests/unit/compression/preview-fallback-reasons-6461.test.ts
new file mode 100644
index 0000000000..4de93f128f
--- /dev/null
+++ b/tests/unit/compression/preview-fallback-reasons-6461.test.ts
@@ -0,0 +1,56 @@
+/**
+ * #6461 — the compression preview response must surface WHY a run fell back
+ * (deduped `fallbackReasons[]`, mirrored into `skippedReasons`) instead of the
+ * previously hard-coded `skippedReasons: []`. Non-fallback runs return [] on
+ * both — zero change on the happy path (regression guard, Rule #18).
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import { join } from "node:path";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+
+const TEST_DATA_DIR = mkdtempSync(join(tmpdir(), "preview-fallback-6461-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.API_KEY_SECRET = process.env.API_KEY_SECRET ?? "test-secret-32-chars-min-aaaaaaaa";
+delete process.env.INITIAL_PASSWORD;
+
+const core = await import("../../../src/lib/db/core.ts");
+const route = await import("../../../src/app/api/compression/preview/route.ts");
+
+function makeReq(body: unknown) {
+  return new Request("http://localhost/api/compression/preview", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+test.beforeEach(() => core.resetDbInstance());
+test.after(() => {
+  core.resetDbInstance();
+  rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+test("#6461 preview exposes fallbackReasons and mirrors it into skippedReasons", async () => {
+  const res = await route.POST(
+    makeReq({
+      messages: [{ role: "user", content: "$ git status\nOn branch main\nnothing to commit" }],
+      engineId: "rtk",
+    })
+  );
+  assert.equal(res.status, 200);
+  const body = await res.json();
+  // Structural contract added by #6461 (absent on the pre-patch response).
+  assert.ok(Array.isArray(body.fallbackReasons), "fallbackReasons must be an array");
+  assert.deepEqual(
+    body.skippedReasons,
+    body.fallbackReasons,
+    "skippedReasons must mirror fallbackReasons"
+  );
+  // Happy path (no fallback): both empty, and fallbackReason is null.
+  if (body.fallbackApplied !== true) {
+    assert.deepEqual(body.fallbackReasons, []);
+    assert.equal(body.fallbackReason ?? null, null);
+  }
+});