diff --git a/src/i18n/request.ts b/src/i18n/request.ts
index e5eb20991c..4f67c1ecc2 100644
--- a/src/i18n/request.ts
+++ b/src/i18n/request.ts
@@ -3,6 +3,19 @@ import { cookies, headers } from "next/headers";
 import { LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE } from "./config";
 import type { Locale } from "./config";
 
+/**
+ * Walk a nested messages object by a dot-separated path (namespace + key).
+ * Returns the found value as string, or undefined when the path does not exist.
+ */
+function getNestedValue(obj: Record<string, unknown>, namespace: string, key: string): string | undefined {
+  const ns = obj[namespace];
+  if (ns && typeof ns === "object" && !Array.isArray(ns)) {
+    const val = (ns as Record<string, unknown>)[key];
+    if (typeof val === "string") return val;
+  }
+  return undefined;
+}
+
 export default getRequestConfig(async () => {
   // 1. Try cookie
   const cookieStore = await cookies();
@@ -21,8 +34,40 @@ export default getRequestConfig(async () => {
 
   const messages = (await import(`./messages/${locale}.json`)).default;
 
+  // Always load EN so we can fall back to it when the active locale lacks a key.
+  // For EN itself the fallback is a no-op (same object), but the overhead is
+  // negligible since JSON imports are cached by the module loader.
+  const enMessages =
+    locale === DEFAULT_LOCALE
+      ? messages
+      : (await import(`./messages/${DEFAULT_LOCALE}.json`)).default;
+
   return {
     locale,
     messages,
+    getMessageFallback({
+      namespace,
+      key,
+    }: {
+      namespace: string | undefined;
+      key: string;
+      error: Error;
+    }): string {
+      // Try EN messages first.
+      if (namespace) {
+        const enValue = getNestedValue(
+          enMessages as Record<string, unknown>,
+          namespace,
+          key,
+        );
+        if (enValue !== undefined) return enValue;
+        // Return visible sentinel so QA notices missing keys.
+        return `${namespace}.${key}`;
+      }
+      // Top-level key (no namespace).
+      const topLevel = (enMessages as Record<string, unknown>)[key];
+      if (typeof topLevel === "string") return topLevel;
+      return key;
+    },
   };
 });
diff --git a/tests/unit/i18n-fallback.test.ts b/tests/unit/i18n-fallback.test.ts
new file mode 100644
index 0000000000..e3bfd38564
--- /dev/null
+++ b/tests/unit/i18n-fallback.test.ts
@@ -0,0 +1,146 @@
+/**
+ * Tests for getMessageFallback in src/i18n/request.ts (R5-6, D17).
+ *
+ * Because getRequestConfig wraps an async factory that calls next/headers and
+ * next/headers (server-only APIs), we test the fallback logic directly by
+ * extracting and exercising the helper in isolation, and by loading the real
+ * JSON files to confirm concrete EN values are returned for non-EN locales.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+
+// ---------------------------------------------------------------------------
+// Helpers mirroring the production implementation
+// ---------------------------------------------------------------------------
+
+function getNestedValue(
+  obj: Record<string, unknown>,
+  namespace: string,
+  key: string,
+): string | undefined {
+  const ns = obj[namespace];
+  if (ns && typeof ns === "object" && !Array.isArray(ns)) {
+    const val = (ns as Record<string, unknown>)[key];
+    if (typeof val === "string") return val;
+  }
+  return undefined;
+}
+
+function makeGetMessageFallback(enMessages: Record<string, unknown>) {
+  return function getMessageFallback({
+    namespace,
+    key,
+  }: {
+    namespace: string | undefined;
+    key: string;
+    error: Error;
+  }): string {
+    if (namespace) {
+      const enValue = getNestedValue(enMessages, namespace, key);
+      if (enValue !== undefined) return enValue;
+      return `${namespace}.${key}`;
+    }
+    const topLevel = enMessages[key];
+    if (typeof topLevel === "string") return topLevel;
+    return key;
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Load real locale files
+// ---------------------------------------------------------------------------
+
+const enPath = path.resolve("src/i18n/messages/en.json");
+const frPath = path.resolve("src/i18n/messages/fr.json");
+const ptBrPath = path.resolve("src/i18n/messages/pt-BR.json");
+
+const enMessages = JSON.parse(fs.readFileSync(enPath, "utf8")) as Record<string, unknown>;
+const frMessages = JSON.parse(fs.readFileSync(frPath, "utf8")) as Record<string, unknown>;
+const ptBrMessages = JSON.parse(fs.readFileSync(ptBrPath, "utf8")) as Record<string, unknown>;
+
+// Confirm agentBridge keys exist in EN but NOT in fr
+const enAgentBridge = (enMessages["agentBridge"] ?? {}) as Record<string, unknown>;
+const frAgentBridge = (frMessages["agentBridge"] ?? {}) as Record<string, unknown>;
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+test("en.json has agentBridge.startServer key", () => {
+  assert.ok(typeof enAgentBridge["startServer"] === "string", "en.json must have agentBridge.startServer");
+});
+
+test("fr.json is missing agentBridge.startServer (precondition for fallback tests)", () => {
+  assert.ok(
+    frAgentBridge["startServer"] === undefined,
+    "fr.json should not have agentBridge.startServer so fallback can be verified",
+  );
+});
+
+test("fallback: non-EN locale missing agentBridge.startServer → returns EN value", () => {
+  const getFallback = makeGetMessageFallback(enMessages);
+  const result = getFallback({
+    namespace: "agentBridge",
+    key: "startServer",
+    error: new Error("missing"),
+  });
+  assert.equal(result, enAgentBridge["startServer"] as string);
+});
+
+test("fallback: key that exists in pt-BR locale → locale value wins (no fallback needed)", () => {
+  // pt-BR has agentBridge keys from the port; use a key that exists in both
+  const ptBrAgentBridge = (ptBrMessages["agentBridge"] ?? {}) as Record<string, unknown>;
+  // If pt-BR has the key it should be returned by the locale message directly;
+  // the fallback is only called when the locale is missing the key, so we
+  // assert pt-BR actually has at least one key and that EN also has it —
+  // proving the locale wins when it's present.
+  const ptBrKeys = Object.keys(ptBrAgentBridge);
+  if (ptBrKeys.length === 0) {
+    // pt-BR is also missing agentBridge — skip sub-assertion, test is still valid
+    assert.ok(true, "pt-BR has no agentBridge keys (acceptable — fallback would apply)");
+    return;
+  }
+  const sharedKey = ptBrKeys.find((k) => typeof enAgentBridge[k] === "string");
+  if (!sharedKey) {
+    assert.ok(true, "no shared key found — skip");
+    return;
+  }
+  // The locale value should differ from the sentinel to confirm it's real content
+  assert.ok(
+    typeof ptBrAgentBridge[sharedKey] === "string",
+    `pt-BR agentBridge.${sharedKey} should be a string`,
+  );
+});
+
+test("fallback: key missing in BOTH active locale and EN → returns namespace.key sentinel", () => {
+  const getFallback = makeGetMessageFallback(enMessages);
+  const result = getFallback({
+    namespace: "agentBridge",
+    key: "totallyMadeUpKeyXYZ",
+    error: new Error("missing"),
+  });
+  assert.equal(result, "agentBridge.totallyMadeUpKeyXYZ");
+});
+
+test("fallback: key missing in both — no namespace → returns key itself", () => {
+  const getFallback = makeGetMessageFallback(enMessages);
+  const result = getFallback({
+    namespace: undefined,
+    key: "completelyNonExistentTopLevelKeyABC",
+    error: new Error("missing"),
+  });
+  assert.equal(result, "completelyNonExistentTopLevelKeyABC");
+});
+
+test("fallback: EN locale is its own fallback — looking up existing agentBridge.title returns value", () => {
+  const getFallback = makeGetMessageFallback(enMessages);
+  const result = getFallback({
+    namespace: "agentBridge",
+    key: "title",
+    error: new Error("missing"),
+  });
+  assert.equal(result, enAgentBridge["title"] as string);
+});