From e8e436d8e28d303639cec023e4d8a40827b53c0f Mon Sep 17 00:00:00 2001
From: "Fredrik Liljegren (Claude Code Claude Opus 5)"
 <fredrik.liljegren@naturalcycles.com>
Date: Fri, 21 Aug 2026 10:53:23 +0200
Subject: [PATCH] fix(server): pin a pull request's diff to its base commit
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The ref comes from the URL, so a tab left open from an earlier session — or a bookmark, or
a hand-edited address — kept rendering a different diff than the pull request's, under a
header that still read "Changes from master". A pull request's diff has to be the one
GitHub shows.

A /diff request whose ref disagrees with the pull request's base commit is now redirected
to it, with the other query parameters preserved. Switching revision inside the UI is a
client-side navigation and never reaches the server, so exploring other revisions in an
open session still works; reloading returns to the pull request.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018PkYQzbsnMihHesafWvXKs
---
 packages/cli/src/index.ts  |  1 +
 packages/cli/src/server.ts | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

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));