diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2c4378425c..7cf0cb5cca 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,7 @@
 
 ### 🐛 Bug Fixes
 
+- **fix(api):** `/api/compression/preview` now accepts `mode: "caveman"` and correctly handles stacked / zero-compression previews ([#6425](https://github.com/diegosouzapw/OmniRoute/issues/6425)). Regression guard: `tests/unit/api/compression-preview-caveman-and-stacked-6425.test.ts`. (thanks @chirag127)
 - **feat(providers):** add **Zed** hosted LLM aggregator as a native-app provider ([#6118](https://github.com/diegosouzapw/OmniRoute/pull/6118)) — OAuth sign-in via the Zed hosted flow, registered through the shared provider registry + executor. Regression guards: `tests/unit/zed-oauth-provider.test.ts`, `zed-import-utils.test.ts`, `zed-docker-detect.test.ts`, `mitm-handler-zed.test.ts`. VPS-validated via live operator login (Hard Rule #18).
 - **fix(oauth):** the Kiro SSO-cache auto-import now **preserves the IDC region** — cross-region Amazon Q / Kiro profiles imported from the SSO cache are no longer collapsed to the default region ([#6113](https://github.com/diegosouzapw/OmniRoute/pull/6113)). Regression guard: `tests/unit/kiro-auto-import-idc-2059.test.ts`. VPS-validated via live operator login (Hard Rule #18).
 - **fix(dashboard):** passthrough model aliases no longer collide when two namespaced model ids share a last segment. `enx/gpt-5.5` and `enx/codebuddy/gpt-5.5` both auto-generated the alias `gpt-5.5`, so the second model could never be added (the UI just alerted "alias already exists"). Aliases are now disambiguated deterministically — bare last segment when free, then parent-qualified (`codebuddy-gpt-5.5`), then a numeric suffix — while re-adding the exact same model id is still blocked. Regression guard: `tests/unit/passthrough-alias-1850.test.ts`. (thanks @arpicato)
diff --git a/open-sse/services/compression/engines/cavemanAdapter.ts b/open-sse/services/compression/engines/cavemanAdapter.ts
index d819357c0f..464d3e79b8 100644
--- a/open-sse/services/compression/engines/cavemanAdapter.ts
+++ b/open-sse/services/compression/engines/cavemanAdapter.ts
@@ -289,9 +289,22 @@ export const cavemanEngine: CompressionEngine = {
   },
   apply(body, options) {
     const adapter = adaptBodyForCompression(body);
+    // Mirror rtkAdapter's default-enabled behavior (see rtk/index.ts:530-535). When this engine
+    // is invoked as a stacked step without explicit `enabled` on either the cavemanConfig or the
+    // stepConfig, default `enabled: true` so the rules actually run. Without this,
+    // DEFAULT_CAVEMAN_CONFIG.enabled=false made cavemanCompress() a silent no-op, and the
+    // preview route's default [rtk, caveman] pipeline reported 0% savings even on trigger prose.
+    // (Issue #6425.)
+    const explicitCavemanConfig = options?.config?.cavemanConfig;
+    const explicitStepConfig = options?.stepConfig;
+    const explicitEnabled =
+      (explicitCavemanConfig && "enabled" in explicitCavemanConfig) ||
+      (explicitStepConfig && "enabled" in explicitStepConfig);
+    const enabledDefault = explicitEnabled ? {} : { enabled: true };
     const cavemanConfig = {
-      ...(options?.config?.cavemanConfig ?? {}),
-      ...(options?.stepConfig ?? {}),
+      ...enabledDefault,
+      ...(explicitCavemanConfig ?? {}),
+      ...(explicitStepConfig ?? {}),
       ...(options?.config?.languageConfig?.enabled
         ? {
             language: options.config.languageConfig.defaultLanguage,
diff --git a/src/app/api/compression/preview/route.ts b/src/app/api/compression/preview/route.ts
index b592bf3297..00e6d8481c 100644
--- a/src/app/api/compression/preview/route.ts
+++ b/src/app/api/compression/preview/route.ts
@@ -32,7 +32,7 @@ export const PreviewRequestSchema = z.object({
     )
     .min(1),
   mode: z
-    .enum(["off", "lite", "standard", "aggressive", "ultra", "rtk", "stacked"])
+    .enum(["off", "lite", "standard", "aggressive", "ultra", "rtk", "stacked", "caveman"])
     .optional()
     .default("stacked"),
   engineId: z.string().optional(),
@@ -183,8 +183,11 @@ export async function POST(req: Request) {
     );
   }
 
-  const { messages, mode, engineId, pipeline, config, fidelityGate, fuzzyDedup, riskGate, quantumLock, heatmap: heatmapMode } =
+  const { messages, mode, engineId: rawEngineId, pipeline, config, fidelityGate, fuzzyDedup, riskGate, quantumLock, heatmap: heatmapMode } =
     parsed.data;
+  // Alias: `mode: "caveman"` is a synonym for `engineId: "caveman"` (single-engine stacked run).
+  // The caveman engine is not a top-level CompressionMode, but it IS a registered engine.
+  const engineId = mode === "caveman" && !rawEngineId ? "caveman" : rawEngineId;
   const effectiveMode: CompressionMode =
     engineId || pipeline ? "stacked" : (mode as CompressionMode);
   const originalText = messagesToText(messages);
diff --git a/tests/unit/api/compression-preview-caveman-and-stacked-6425.test.ts b/tests/unit/api/compression-preview-caveman-and-stacked-6425.test.ts
new file mode 100644
index 0000000000..409ba2077d
--- /dev/null
+++ b/tests/unit/api/compression-preview-caveman-and-stacked-6425.test.ts
@@ -0,0 +1,147 @@
+/**
+ * Regression #6425 — /api/compression/preview rejects mode:"caveman" and stacked yields 0%.
+ *
+ * Two independent defects, one test file (both surface via the same endpoint):
+ *
+ *  1. `mode: "caveman"` was rejected by the Zod enum (only "off"/"lite"/"standard"/
+ *     "aggressive"/"ultra"/"rtk"/"stacked" allowed). The `caveman` engine exists in the
+ *     registry (see cavemanAdapter.ts) so the mode alias should be accepted and mapped
+ *     to a single-engine stacked run.
+ *
+ *  2. `mode: "stacked"` on prose containing well-known caveman-rule triggers ("Basically",
+ *     "I think", "probably", "just", "comprehensive") returned savingsPct: 0. Root cause:
+ *     the caveman engine's stacked-adapter did not default `enabled: true` when invoked
+ *     with no explicit stepConfig — DEFAULT_CAVEMAN_CONFIG.enabled=false made
+ *     cavemanCompress() a no-op. Mirrors the rtkAdapter fix pattern (rtk defaults enabled).
+ *
+ * Auth pattern mirrors compression-preview-engine.test.ts (JWT session, temp DATA_DIR).
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+import { makeManagementSessionRequest } from "../../helpers/managementSession.ts";
+
+// ─── temp DB isolation ────────────────────────────────────────────────────────
+
+const TEST_DATA_DIR = fs.mkdtempSync(
+  path.join(os.tmpdir(), "omniroute-compression-preview-6425-")
+);
+const originalDataDir = process.env.DATA_DIR;
+const originalJwtSecret = process.env.JWT_SECRET;
+
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const core = await import("../../../src/lib/db/core.ts");
+const settingsDb = await import("../../../src/lib/db/settings.ts");
+const previewRoute = await import("../../../src/app/api/compression/preview/route.ts");
+
+// ─── helpers ──────────────────────────────────────────────────────────────────
+
+const CAVEMAN_TRIGGER =
+  "Basically, I think we should probably just use a comprehensive analysis approach to " +
+  "actually really understand the very important issue at hand.";
+
+async function setupAuth(): Promise<void> {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  fs.mkdirSync(TEST_DATA_DIR, { recursive: true });
+  await settingsDb.updateSettings({
+    requireLogin: true,
+    setupComplete: true,
+    password: "test-password-hash",
+  });
+}
+
+// ─── lifecycle ────────────────────────────────────────────────────────────────
+
+test.beforeEach(async () => {
+  process.env.DATA_DIR = TEST_DATA_DIR;
+  await setupAuth();
+});
+
+test.after(() => {
+  if (originalDataDir === undefined) delete process.env.DATA_DIR;
+  else process.env.DATA_DIR = originalDataDir;
+  if (originalJwtSecret === undefined) delete process.env.JWT_SECRET;
+  else process.env.JWT_SECRET = originalJwtSecret;
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+// ─── tests ────────────────────────────────────────────────────────────────────
+
+test("#6425 (a): POST /api/compression/preview accepts mode:'caveman' and produces >0% savings", async () => {
+  const request = await makeManagementSessionRequest(
+    "http://localhost/api/compression/preview",
+    {
+      method: "POST",
+      body: {
+        messages: [{ role: "user", content: CAVEMAN_TRIGGER }],
+        mode: "caveman",
+      },
+    }
+  );
+
+  const response = await previewRoute.POST(request);
+  assert.equal(
+    response.status,
+    200,
+    `mode:"caveman" should be accepted (was rejected before #6425 fix), got ${response.status}`
+  );
+
+  const body = (await response.json()) as {
+    originalTokens: number;
+    compressedTokens: number;
+    savingsPct: number;
+    techniquesUsed: string[];
+    mode: string;
+  };
+
+  assert.ok(body.originalTokens > 0, "originalTokens should be > 0");
+  assert.ok(
+    body.compressedTokens < body.originalTokens,
+    `caveman rules should shrink well-known filler prose (got ${body.compressedTokens} vs ${body.originalTokens})`
+  );
+  assert.ok(body.savingsPct > 0, `savingsPct should be > 0, got ${body.savingsPct}`);
+});
+
+test("#6425 (b): POST /api/compression/preview mode:'stacked' returns >0% on caveman-trigger prose", async () => {
+  const request = await makeManagementSessionRequest(
+    "http://localhost/api/compression/preview",
+    {
+      method: "POST",
+      body: {
+        messages: [{ role: "user", content: CAVEMAN_TRIGGER }],
+        mode: "stacked",
+      },
+    }
+  );
+
+  const response = await previewRoute.POST(request);
+  assert.equal(response.status, 200, `Expected 200, got ${response.status}`);
+
+  const body = (await response.json()) as {
+    originalTokens: number;
+    compressedTokens: number;
+    savingsPct: number;
+    engineBreakdown: Array<{ engine: string; savingsPercent: number }>;
+  };
+
+  assert.ok(body.originalTokens > 0, "originalTokens should be > 0");
+  assert.ok(
+    body.savingsPct > 0,
+    `stacked pipeline (default [rtk, caveman]) should produce >0% savings on filler-heavy prose, got ${body.savingsPct}% (regression: caveman step was silently disabled by DEFAULT_CAVEMAN_CONFIG.enabled=false)`
+  );
+
+  // Assert the caveman step in the breakdown actually did work — this is the tightest guard
+  // against the specific fix in cavemanAdapter.ts (default enabled when no explicit config).
+  const cavemanStep = body.engineBreakdown.find((s) => s.engine === "caveman");
+  assert.ok(cavemanStep, "engineBreakdown should include a caveman step");
+  assert.ok(
+    cavemanStep.savingsPercent > 0,
+    `caveman step should report >0% savings on trigger prose, got ${cavemanStep.savingsPercent}%`
+  );
+});