diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef0ca1d484..09d7cd2203 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 
 ### 🐛 Bug Fixes
 
+- **fix(api):** the `X-OmniRoute-Compression` response header is now echoed on `/v1/chat/completions` and `/v1/completions` ([#6422](https://github.com/diegosouzapw/OmniRoute/issues/6422)). Regression guard: `tests/unit/compression-header-echo-6422.test.ts`. (thanks @chirag127)
 - **fix(api):** concurrent `GET /v1/models` requests are coalesced into a single catalog build ([#6408](https://github.com/diegosouzapw/OmniRoute/issues/6408)). Regression guard: `tests/unit/v1-models-concurrent-6408.test.ts`. (thanks @chirag127)
 - **fix(api):** `/v1/completions` now echoes the requested `body.model` in its JSON + streamed responses. Regression guard: `tests/unit/completions-body-model-echo.test.ts`. (thanks @chirag127)
 - **fix(api):** env-var master keys now see the full `/v1/models` catalog ([#6406](https://github.com/diegosouzapw/OmniRoute/issues/6406)). Regression guard: `tests/unit/models-catalog-envkey-6406.test.ts`. (thanks @chirag127)
diff --git a/src/app/api/v1/chat/completions/route.ts b/src/app/api/v1/chat/completions/route.ts
index 0bc62035d7..5edf8da0d8 100644
--- a/src/app/api/v1/chat/completions/route.ts
+++ b/src/app/api/v1/chat/completions/route.ts
@@ -8,6 +8,10 @@ import { acceptHeaderForcesStream } from "@omniroute/open-sse/utils/aiSdkCompat.
 import { withEarlyStreamKeepalive } from "@omniroute/open-sse/utils/earlyStreamKeepalive";
 import { resolveKeepaliveThreshold } from "@omniroute/open-sse/utils/keepaliveThreshold";
 import { checkChatAdmission } from "@/shared/middleware/chatBodyAdmission";
+import {
+  readCompressionRequestHeader,
+  withCompressionHeaderEcho,
+} from "@/shared/utils/compressionHeaderEcho";
 
 let initPromise = null;
 
@@ -99,14 +103,26 @@ export async function POST(request) {
     parsedBodyIsRecord && acceptHeaderForcesStream(acceptHeader, parsedBody.stream);
   const wantsStreaming = (parsedBodyIsRecord && parsedBody.stream === true) || acceptForcesStream;
 
+  // #6422 — capture the compression request header once so we can echo it back
+  // on the response when internal early-returns (idempotency cache, some combo
+  // paths) drop the meta the docs promise.
+  const compressionRequestHeader = readCompressionRequestHeader(request);
+
   if (wantsStreaming) {
     const reqId = generateRequestId();
-    return await withEarlyStreamKeepalive(handleChat(request, null, parsedBody, reqId), {
-      signal: request.signal,
-      thresholdMs: resolveKeepaliveThreshold(parsedBody?.model),
-      extraHeaders: { "X-Correlation-Id": reqId },
-    });
+    const streamedResponse = await withEarlyStreamKeepalive(
+      handleChat(request, null, parsedBody, reqId),
+      {
+        signal: request.signal,
+        thresholdMs: resolveKeepaliveThreshold(parsedBody?.model),
+        extraHeaders: { "X-Correlation-Id": reqId },
+      }
+    );
+    return withCompressionHeaderEcho(streamedResponse, compressionRequestHeader);
   }
 
-  return await handleChat(request, null, parsedBody);
+  return withCompressionHeaderEcho(
+    await handleChat(request, null, parsedBody),
+    compressionRequestHeader
+  );
 }
diff --git a/src/app/api/v1/completions/route.ts b/src/app/api/v1/completions/route.ts
index 07b577be5a..d6a2254b9c 100644
--- a/src/app/api/v1/completions/route.ts
+++ b/src/app/api/v1/completions/route.ts
@@ -3,6 +3,10 @@ import { buildClientRawRequest, handleChat } from "@/sse/handlers/chat";
 import { initTranslators } from "@omniroute/open-sse/translator/index.ts";
 import { createInjectionGuard } from "@/middleware/promptInjectionGuard";
 import { asTextCompletionResponse } from "./textCompletionTransform.ts";
+import {
+  readCompressionRequestHeader,
+  withCompressionHeaderEcho,
+} from "@/shared/utils/compressionHeaderEcho";
 
 let initPromise = null;
 const injectionGuard = createInjectionGuard();
@@ -40,6 +44,10 @@ export async function OPTIONS() {
 export async function POST(request: Request) {
   await ensureInitialized();
 
+  // #6422 — capture the compression request header once so we can echo it back
+  // on the response when internal early-returns drop the meta the docs promise.
+  const compressionRequestHeader = readCompressionRequestHeader(request);
+
   // Prompt injection guard
   try {
     const cloned = request.clone();
@@ -78,10 +86,14 @@ export async function POST(request: Request) {
         // #3571 — translate the chat-pipeline response back to the legacy
         // text-completion shape so OpenAI Completion clients (e.g. TabbyML) work.
         // Thread `body.model` so response `body.model` echoes the caller's
-        // requested identifier, matching the `x-omniroute-model` header.
-        return await asTextCompletionResponse(
-          await handleChat(newRequest, buildClientRawRequest(request, body)),
-          typeof body.model === "string" ? body.model : undefined
+        // requested identifier, matching the `x-omniroute-model` header, and
+        // echo the compression header on the way out.
+        return withCompressionHeaderEcho(
+          await asTextCompletionResponse(
+            await handleChat(newRequest, buildClientRawRequest(request, body)),
+            typeof body.model === "string" ? body.model : undefined
+          ),
+          compressionRequestHeader
         );
       }
     }
@@ -101,5 +113,8 @@ export async function POST(request: Request) {
   } catch {
     // ignore — asTextCompletionResponse falls back to upstream body.model
   }
-  return await asTextCompletionResponse(await handleChat(request), requestedModel);
+  return withCompressionHeaderEcho(
+    await asTextCompletionResponse(await handleChat(request), requestedModel),
+    compressionRequestHeader
+  );
 }
diff --git a/src/shared/utils/compressionHeaderEcho.ts b/src/shared/utils/compressionHeaderEcho.ts
new file mode 100644
index 0000000000..e5ec4beee5
--- /dev/null
+++ b/src/shared/utils/compressionHeaderEcho.ts
@@ -0,0 +1,59 @@
+/**
+ * Compression response header echo (#6422).
+ *
+ * When a request carries `x-omniroute-compression`, docs promise the response echoes
+ * `X-OmniRoute-Compression: <mode>; source=<source>`. Internal paths (idempotency
+ * cache short-circuit, some combo/fusion assembly paths) build response headers
+ * without threading `compressionResponseMeta` — so the promised echo silently
+ * disappears. This helper is the outermost safety net: if the response is missing
+ * the header and the request supplied one, echo a best-effort value derived
+ * directly from the request header. Existing header values from the inner pipeline
+ * (which carry richer `tokens=...; rules: ...` annotations) are never overwritten.
+ */
+import { OMNIROUTE_RESPONSE_HEADERS } from "@/shared/constants/headers";
+
+const COMPRESSION_REQUEST_HEADER = "x-omniroute-compression";
+const COMPRESSION_RESPONSE_HEADER = OMNIROUTE_RESPONSE_HEADERS.compression;
+
+function normalizeRequestValue(raw: string): string {
+  const trimmed = raw.trim();
+  const lower = trimmed.toLowerCase();
+  if (lower === "off" || lower === "default") return lower;
+  if (lower.startsWith("engine:")) return lower;
+  // Named combo — preserve the operator's casing on the mode field. Source is always
+  // request-header when the header drove the choice.
+  return trimmed;
+}
+
+/**
+ * Read the compression request header (case-insensitive). Returns null on absent/blank.
+ */
+export function readCompressionRequestHeader(request: {
+  headers: { get(name: string): string | null };
+}): string | null {
+  const raw = request.headers.get(COMPRESSION_REQUEST_HEADER);
+  return typeof raw === "string" && raw.trim() ? raw : null;
+}
+
+/**
+ * Wrap a Response so it carries `X-OmniRoute-Compression: <mode>; source=request-header`
+ * when the request supplied `x-omniroute-compression` and the inner pipeline did not
+ * already set it. Never overwrites an existing value — the inner pipeline may have
+ * attached a richer annotation. A best-effort echo covers idempotency-cache,
+ * fusion-envelope, and any other early-return path that dropped the meta.
+ */
+export function withCompressionHeaderEcho(
+  response: Response,
+  requestHeaderValue: string | null
+): Response {
+  if (!requestHeaderValue) return response;
+  if (response.headers.has(COMPRESSION_RESPONSE_HEADER)) return response;
+  const mode = normalizeRequestValue(requestHeaderValue);
+  const headers = new Headers(response.headers);
+  headers.set(COMPRESSION_RESPONSE_HEADER, `${mode}; source=request-header`);
+  return new Response(response.body, {
+    status: response.status,
+    statusText: response.statusText,
+    headers,
+  });
+}
diff --git a/tests/unit/compression-header-echo-6422.test.ts b/tests/unit/compression-header-echo-6422.test.ts
new file mode 100644
index 0000000000..d60e1ea43e
--- /dev/null
+++ b/tests/unit/compression-header-echo-6422.test.ts
@@ -0,0 +1,89 @@
+/**
+ * #6422 — X-OmniRoute-Compression response header echo.
+ *
+ * Docs promise that when a request supplies `x-omniroute-compression`, the response
+ * echoes `X-OmniRoute-Compression: <mode>; source=<source>`. Internal early-returns
+ * (idempotency-cache short-circuit, some combo/fusion assembly paths) omit that
+ * header, so the outermost route layer echoes a best-effort value from the request.
+ * These tests lock the helper's contract so a future refactor cannot silently drop
+ * the echo again.
+ */
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import {
+  readCompressionRequestHeader,
+  withCompressionHeaderEcho,
+} from "../../src/shared/utils/compressionHeaderEcho";
+
+function makeRequest(headerValue: string | null): {
+  headers: { get(name: string): string | null };
+} {
+  return {
+    headers: {
+      get(name: string) {
+        if (name.toLowerCase() === "x-omniroute-compression") return headerValue;
+        return null;
+      },
+    },
+  };
+}
+
+describe("compressionHeaderEcho (#6422)", () => {
+  it("reads the request header (case-insensitive, trimmed)", () => {
+    assert.equal(readCompressionRequestHeader(makeRequest("engine:rtk")), "engine:rtk");
+    assert.equal(readCompressionRequestHeader(makeRequest("  off  ")), "  off  ");
+    assert.equal(readCompressionRequestHeader(makeRequest(null)), null);
+    assert.equal(readCompressionRequestHeader(makeRequest("")), null);
+    assert.equal(readCompressionRequestHeader(makeRequest("   ")), null);
+  });
+
+  it("echoes the request header value onto the response when missing", () => {
+    const inner = new Response("body", { status: 200 });
+    const wrapped = withCompressionHeaderEcho(inner, "engine:rtk");
+    assert.equal(wrapped.headers.get("X-OmniRoute-Compression"), "engine:rtk; source=request-header");
+    assert.equal(wrapped.status, 200);
+  });
+
+  it("normalizes off / default / engine:* to lowercase", () => {
+    assert.equal(
+      withCompressionHeaderEcho(new Response(""), "OFF").headers.get("X-OmniRoute-Compression"),
+      "off; source=request-header"
+    );
+    assert.equal(
+      withCompressionHeaderEcho(new Response(""), "Default").headers.get("X-OmniRoute-Compression"),
+      "default; source=request-header"
+    );
+    assert.equal(
+      withCompressionHeaderEcho(new Response(""), "Engine:Rtk").headers.get(
+        "X-OmniRoute-Compression"
+      ),
+      "engine:rtk; source=request-header"
+    );
+  });
+
+  it("preserves operator casing on named-combo values", () => {
+    const wrapped = withCompressionHeaderEcho(new Response(""), "  MyCombo  ");
+    assert.equal(wrapped.headers.get("X-OmniRoute-Compression"), "MyCombo; source=request-header");
+  });
+
+  it("never overwrites an existing X-OmniRoute-Compression header set by the inner pipeline", () => {
+    const inner = new Response("", {
+      headers: {
+        "X-OmniRoute-Compression": "stacked; source=routing; tokens=100->42; rules: rtk-nl x2",
+      },
+    });
+    const wrapped = withCompressionHeaderEcho(inner, "engine:rtk");
+    assert.equal(
+      wrapped.headers.get("X-OmniRoute-Compression"),
+      "stacked; source=routing; tokens=100->42; rules: rtk-nl x2"
+    );
+  });
+
+  it("is a no-op when the request did not supply the header", () => {
+    const inner = new Response("", { headers: { "X-Other": "keep" } });
+    const wrapped = withCompressionHeaderEcho(inner, null);
+    assert.equal(wrapped, inner);
+    assert.equal(wrapped.headers.get("X-OmniRoute-Compression"), null);
+    assert.equal(wrapped.headers.get("X-Other"), "keep");
+  });
+});