diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b5aed90..5ae573b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,6 +15,10 @@ jobs:
         with:
           node-version: "22"
       - run: npm ci
+      - run: npm ci
+        working-directory: mcp-server
+      - run: npm test
+        working-directory: mcp-server
       - run: node scripts/check-schema-drift.mjs
       - run: npm test
       - run: npx tsc --noEmit
diff --git a/docs/0-requirements.ja.md b/docs/0-requirements.ja.md
index 768f649..d793806 100644
--- a/docs/0-requirements.ja.md
+++ b/docs/0-requirements.ja.md
@@ -391,6 +391,9 @@ Requirements:
 - MCP client user を認証する
 - installation 済み repository に access する
 - GitHub API read に token を使う
+- cached Dynamic Client Registration は、登録済み redirect URI 集合が今回の
+  OAuth flow で要求する localhost callback URI を全て含む場合だけ再利用し、
+  含まない場合は authorization 前に client registration を置き換える
 
 ## Storage Rules
 
diff --git a/docs/0-requirements.md b/docs/0-requirements.md
index eec4b8a..20690ac 100644
--- a/docs/0-requirements.md
+++ b/docs/0-requirements.md
@@ -393,6 +393,9 @@ Requirements:
 - authenticate the MCP client user
 - access repositories through the installed app
 - use GitHub tokens for API reads
+- reuse a cached Dynamic Client Registration only when its registered redirect
+  URI set covers every localhost callback URI requested for the current OAuth
+  flow; otherwise register a replacement client before authorization
 
 ## Storage Rules
 
diff --git a/mcp-server/README.md b/mcp-server/README.md
index 750f2a2..600a71f 100644
--- a/mcp-server/README.md
+++ b/mcp-server/README.md
@@ -84,6 +84,11 @@ OAuth client registration and tokens are stored in:
 - `~/.github-rag-mcp/oauth-client.json` (dynamic client registration)
 - `~/.github-rag-mcp/oauth-tokens.json` (access + refresh tokens)
 
+The proxy reuses a cached client registration only when it includes every
+redirect URI required by the current localhost callback listener. Because the
+listener uses a random port, a registration from a previous authorization may
+be replaced automatically before the browser opens.
+
 Delete these files to force a fresh authorization flow.
 
 ## Tools exposed
@@ -101,7 +106,8 @@ The `type` filter accepts: `issue`, `pull_request`, `release`, `doc`, `wiki_doc`
 ## Authentication flow
 
 1. On first tool call, the proxy discovers OAuth metadata at `${RAG_WORKER_URL}/.well-known/oauth-authorization-server`.
-2. It performs Dynamic Client Registration (RFC 7591) if no client is cached.
+2. It performs Dynamic Client Registration (RFC 7591) if no compatible client
+   is cached for the current localhost callback URIs.
 3. It starts a one-shot localhost HTTP listener on a random port and opens the browser to the Worker's authorization endpoint.
 4. After you approve, the Worker redirects to `http://127.0.0.1:<port>/callback` with an authorization code.
 5. The proxy exchanges the code for tokens (PKCE S256) and saves them.
@@ -112,6 +118,9 @@ The browser callback never leaves your machine; the authorization code is delive
 ## Troubleshooting
 
 - **Browser does not open.** The proxy logs the authorization URL to stderr; copy it into a browser manually.
+- **`redirect_uri is not associated with this application`.** Upgrade the
+  proxy. Current versions replace cached client registrations whose redirect
+  URI set does not cover the callback port selected for this authorization.
 - **`OAuth callback timed out after 5 minutes`.** Re-invoke any tool to restart the flow.
 - **`Failed to reach worker`.** Check that `RAG_WORKER_URL` is correct and reachable from your machine.
 - **Stale credentials.** Remove `~/.github-rag-mcp/oauth-tokens.json` (and optionally `oauth-client.json`) and retry.
diff --git a/mcp-server/package.json b/mcp-server/package.json
index 936e2c3..a34d634 100644
--- a/mcp-server/package.json
+++ b/mcp-server/package.json
@@ -14,7 +14,7 @@
   ],
   "scripts": {
     "start": "node server/index.js",
-    "test": "node --check server/index.js",
+    "test": "node --check server/index.js && node --test test/oauth-client-registration.test.js",
     "pack:mcpb": "mcpb pack"
   },
   "dependencies": {
diff --git a/mcp-server/server/index.js b/mcp-server/server/index.js
index 6afe41e..0cf692c 100644
--- a/mcp-server/server/index.js
+++ b/mcp-server/server/index.js
@@ -23,6 +23,7 @@ import { homedir } from "node:os";
 import { join } from "node:path";
 import { exec } from "node:child_process";
 import { createRequire } from "node:module";
+import { supportsRedirectUris } from "./oauth-client-registration.js";
 
 const require = createRequire(import.meta.url);
 const { version: PACKAGE_VERSION } = require("../package.json");
@@ -89,7 +90,7 @@ async function saveClientRegistration(reg) {
 
 async function ensureClientRegistration(metadata, redirectUris) {
   const existing = await loadClientRegistration();
-  if (existing) return existing;
+  if (supportsRedirectUris(existing, redirectUris)) return existing;
 
   if (!metadata.registration_endpoint) {
     throw new Error("OAuth server does not support dynamic client registration");
diff --git a/mcp-server/server/oauth-client-registration.js b/mcp-server/server/oauth-client-registration.js
new file mode 100644
index 0000000..ad743fc
--- /dev/null
+++ b/mcp-server/server/oauth-client-registration.js
@@ -0,0 +1,21 @@
+/**
+ * Return true only when a cached Dynamic Client Registration can be reused for
+ * every redirect URI requested by the current OAuth callback listener.
+ */
+export function supportsRedirectUris(registration, redirectUris) {
+  if (
+    registration === null ||
+    typeof registration !== "object" ||
+    typeof registration.client_id !== "string" ||
+    registration.client_id.length === 0 ||
+    !Array.isArray(registration.redirect_uris)
+  ) {
+    return false;
+  }
+
+  const registeredUris = new Set(
+    registration.redirect_uris.filter((uri) => typeof uri === "string"),
+  );
+
+  return redirectUris.every((uri) => registeredUris.has(uri));
+}
diff --git a/mcp-server/test/oauth-client-registration.test.js b/mcp-server/test/oauth-client-registration.test.js
new file mode 100644
index 0000000..18cb487
--- /dev/null
+++ b/mcp-server/test/oauth-client-registration.test.js
@@ -0,0 +1,80 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { supportsRedirectUris } from "../server/oauth-client-registration.js";
+
+const REQUIRED_URIS = [
+  "http://127.0.0.1:43123/callback",
+  "http://localhost:43123/callback",
+];
+
+test("reuses a registration that covers every requested redirect URI", () => {
+  assert.equal(
+    supportsRedirectUris(
+      {
+        client_id: "client-id",
+        redirect_uris: [...REQUIRED_URIS].reverse(),
+      },
+      REQUIRED_URIS,
+    ),
+    true,
+  );
+});
+
+test("reuses a registration whose redirect URI set is a superset", () => {
+  assert.equal(
+    supportsRedirectUris(
+      {
+        client_id: "client-id",
+        redirect_uris: [
+          ...REQUIRED_URIS,
+          "http://127.0.0.1:49999/callback",
+        ],
+      },
+      REQUIRED_URIS,
+    ),
+    true,
+  );
+});
+
+test("rejects a stale registration from a different callback port", () => {
+  assert.equal(
+    supportsRedirectUris(
+      {
+        client_id: "client-id",
+        redirect_uris: [
+          "http://127.0.0.1:40000/callback",
+          "http://localhost:40000/callback",
+        ],
+      },
+      REQUIRED_URIS,
+    ),
+    false,
+  );
+});
+
+test("rejects a registration missing one requested redirect URI", () => {
+  assert.equal(
+    supportsRedirectUris(
+      {
+        client_id: "client-id",
+        redirect_uris: [REQUIRED_URIS[0]],
+      },
+      REQUIRED_URIS,
+    ),
+    false,
+  );
+});
+
+test("rejects malformed cached registrations", () => {
+  assert.equal(supportsRedirectUris(null, REQUIRED_URIS), false);
+  assert.equal(supportsRedirectUris({}, REQUIRED_URIS), false);
+  assert.equal(
+    supportsRedirectUris({ client_id: "", redirect_uris: REQUIRED_URIS }, REQUIRED_URIS),
+    false,
+  );
+  assert.equal(
+    supportsRedirectUris({ client_id: "client-id", redirect_uris: "not-an-array" }, REQUIRED_URIS),
+    false,
+  );
+});