diff --git a/packages/cli/src/index.ts b/packages/cli/src/index.ts
index 9c38a82..f3abab4 100644
--- a/packages/cli/src/index.ts
+++ b/packages/cli/src/index.ts
@@ -285,6 +285,7 @@ range syntax (main..feature, main...feature) also work.`)
         diffArgs,
         description,
         effectiveRef,
+        pinnedRef: prBase?.oid,
         version: pkg.version,
         registryInfo: { repoRoot, repoHash, repoName },
       });
diff --git a/packages/cli/src/server.ts b/packages/cli/src/server.ts
index 018a9c4..e00476d 100644
--- a/packages/cli/src/server.ts
+++ b/packages/cli/src/server.ts
@@ -116,6 +116,11 @@ interface ServerOptions {
   diffArgs: string[];
   description?: string;
   effectiveRef?: string;
+  /**
+   * When the instance was started for a pull request, the commit its diff must be taken
+   * from. A `ref` in the URL that disagrees is corrected rather than honoured.
+   */
+  pinnedRef?: string;
   version?: string;
   registryInfo?: {
     repoRoot: string;
@@ -166,6 +171,7 @@ export function startServer(options: ServerOptions): Promise<ServerResult> {
     effectiveRef,
     version,
     registryInfo,
+    pinnedRef,
   } = options;
 
   const includeUntracked = diffArgs.length === 0;
@@ -225,6 +231,16 @@ export function startServer(options: ServerOptions): Promise<ServerResult> {
           return;
         }
 
+        // A pull request's diff has to match the one GitHub shows, so a stale ref in the URL —
+        // a tab left open from an earlier session, a bookmark — is corrected on load. Switching
+        // revision inside the UI is a client-side navigation and never reaches this.
+        if (pinnedRef && pathname === '/diff' && url.searchParams.get('ref') !== pinnedRef) {
+          url.searchParams.set('ref', pinnedRef);
+          res.writeHead(302, { Location: `${pathname}?${url.searchParams.toString()}` });
+          res.end();
+          return;
+        }
+
         if (pathname === '/api/revert-file' && req.method === 'POST') {
           try {
             const body = JSON.parse(await readBody(req));