diff --git a/package-lock.json b/package-lock.json
index 049e1ff80..626cb053a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -28,6 +28,7 @@
         "node-pty": "1.1.0",
         "pino": "9.14.0",
         "plist": "5.0.0",
+        "semver": "7.8.5",
         "smol-toml": "^1.7.0",
         "write-file-atomic": "7.0.0",
         "ws": "8.21.0",
@@ -40,6 +41,7 @@
         "@fast-check/vitest": "0.4.1",
         "@modelcontextprotocol/client": "2.0.0-beta.4",
         "@types/node": "24.10.1",
+        "@types/semver": "7.7.1",
         "@types/write-file-atomic": "4.0.3",
         "@types/ws": "8.18.1",
         "@vitest/coverage-v8": "4.1.10",
@@ -1718,6 +1720,13 @@
         "undici-types": "~7.16.0"
       }
     },
+    "node_modules/@types/semver": {
+      "version": "7.7.1",
+      "resolved": "https://registry.npmjs.org/@types/semver/-/semver-7.7.1.tgz",
+      "integrity": "sha512-FmgJfu+MOcQ370SD0ev7EI8TlCAfKYU+B4m5T3yXc1CiRN94g/SZPtsCkk506aUDtlMnFZvasDwHHUcZUEaYuA==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/unist": {
       "version": "3.0.3",
       "resolved": "https://registry.npmjs.org/@types/unist/-/unist-3.0.3.tgz",
@@ -4347,7 +4356,6 @@
       "version": "7.8.5",
       "resolved": "https://registry.npmjs.org/semver/-/semver-7.8.5.tgz",
       "integrity": "sha512-Y7/KDsb8LjooZpwaqGyulO6DQlksgCncchHGk+sZIY4SBvUocMBEFH5Ur1fI4dV+Jvl0w6cjvucaIi40puRioA==",
-      "dev": true,
       "license": "ISC",
       "bin": {
         "semver": "bin/semver.js"
diff --git a/package.json b/package.json
index e3a3148a4..329e662c7 100644
--- a/package.json
+++ b/package.json
@@ -107,6 +107,7 @@
     "node-pty": "1.1.0",
     "pino": "9.14.0",
     "plist": "5.0.0",
+    "semver": "7.8.5",
     "smol-toml": "^1.7.0",
     "write-file-atomic": "7.0.0",
     "ws": "8.21.0",
@@ -116,6 +117,7 @@
     "@fast-check/vitest": "0.4.1",
     "@modelcontextprotocol/client": "2.0.0-beta.4",
     "@types/node": "24.10.1",
+    "@types/semver": "7.7.1",
     "@types/write-file-atomic": "4.0.3",
     "@types/ws": "8.18.1",
     "@vitest/coverage-v8": "4.1.10",
diff --git a/src/application/Upgrade.ts b/src/application/Upgrade.ts
index f8a8b9704..756c5d5e4 100644
--- a/src/application/Upgrade.ts
+++ b/src/application/Upgrade.ts
@@ -4,6 +4,7 @@ import { basename, dirname, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 import { promisify } from "node:util";
 
+import { gt, valid } from "semver";
 import { z } from "zod";
 
 import { PRODUCT_IDENTITY } from "../identity.js";
@@ -59,7 +60,7 @@ export type UpgradeResult =
       readonly status: "failed";
       readonly currentVersion: string;
       readonly latestVersion: string | null;
-      readonly reason: "unknown-install-method" | "install";
+      readonly reason: "unknown-install-method" | "version-check" | "install";
       readonly remediation: string;
     };
 
@@ -70,7 +71,20 @@ export const runUpgrade = async (
   output: UpgradeOutput = "human",
 ): Promise<UpgradeResult> => {
   const latestVersion = await host.latestVersion();
-  if (latestVersion === currentVersion)
+  const currentIsValid = valid(currentVersion) !== null;
+  const latestIsValid =
+    latestVersion !== undefined && valid(latestVersion) !== null;
+  if (!currentIsValid || !latestIsValid)
+    return {
+      status: "failed",
+      currentVersion,
+      latestVersion: latestVersion ?? null,
+      reason: "version-check",
+      remediation:
+        "REA could not verify a newer registry release. Check npm registry access and version metadata, then rerun rea upgrade.",
+    };
+
+  if (!gt(latestVersion, currentVersion))
     return {
       status: "current",
       currentVersion,
@@ -100,8 +114,8 @@ export const runUpgrade = async (
   return {
     status: "upgraded",
     previousVersion: currentVersion,
-    latestVersion: latestVersion ?? null,
-    versionCheck: latestVersion === undefined ? "unavailable" : "available",
+    latestVersion,
+    versionCheck: "available",
     installMethod: "npm",
     command: UPGRADE_COMMAND,
     clientRestartRequired: true,
diff --git a/tests/upgrade.test.ts b/tests/upgrade.test.ts
index 9344d186a..f3399c855 100644
--- a/tests/upgrade.test.ts
+++ b/tests/upgrade.test.ts
@@ -61,18 +61,62 @@ describe("CLI upgrade", () => {
     expect(host.installs).toBe(1);
   });
 
-  it("continues with npm latest when the registry check is unavailable", async () => {
+  it("does not mutate when the registry check is unavailable", async () => {
     const host = new FakeUpgradeHost();
     host.latest = undefined;
 
     expect(await runUpgrade("0.5.0", host)).toMatchObject({
-      status: "upgraded",
+      status: "failed",
       latestVersion: null,
-      versionCheck: "unavailable",
+      reason: "version-check",
+    });
+    expect(host.installs).toBe(0);
+  });
+
+  it.each([
+    ["1.10.0", "1.9.0"],
+    ["1.0.0", "1.0.0-rc.1"],
+  ])(
+    "does not downgrade installed version %s to registry version %s",
+    async (currentVersion, latestVersion) => {
+      const host = new FakeUpgradeHost();
+      host.latest = latestVersion;
+
+      expect(await runUpgrade(currentVersion, host)).toEqual({
+        status: "current",
+        currentVersion,
+        latestVersion,
+        installMethod: "npm",
+      });
+      expect(host.installs).toBe(0);
+    },
+  );
+
+  it("upgrades a prerelease when the final release is available", async () => {
+    const host = new FakeUpgradeHost();
+    host.latest = "1.0.0";
+
+    expect(await runUpgrade("1.0.0-rc.1", host)).toMatchObject({
+      status: "upgraded",
+      previousVersion: "1.0.0-rc.1",
+      latestVersion: "1.0.0",
     });
     expect(host.installs).toBe(1);
   });
 
+  it("rejects invalid registry version metadata without mutation", async () => {
+    const host = new FakeUpgradeHost();
+    host.latest = "latest";
+
+    expect(await runUpgrade("1.0.0", host)).toMatchObject({
+      status: "failed",
+      currentVersion: "1.0.0",
+      latestVersion: "latest",
+      reason: "version-check",
+    });
+    expect(host.installs).toBe(0);
+  });
+
   it("preserves stdout for an explicitly structured CLI response", async () => {
     const host = new FakeUpgradeHost();