fix(github): diff a PR from its base commit, not the local branch

package.json

@@ -14,7 +14,7 @@
   "scripts": {
     "build": "tsx scripts/build.ts",
     "build:skills": "tsx scripts/build-skills.ts",
-    "test": "npm run test -w @diffity/git && npm run test -w @diffity/parser && npm run test -w @diffity/ui && npm run test:scripts",
+    "test": "npm run test -w @diffity/git && npm run test -w @diffity/github && npm run test -w @diffity/parser && npm run test -w @diffity/ui && npm run test:scripts",
     "link-dev": "tsx scripts/link-dev.ts",
     "dev": "tsx scripts/dev.ts",
     "release:patch": "npm run build && tsx scripts/release.ts patch && npm publish -w packages/cli",

packages/cli/src/index.ts

@@ -4,11 +4,12 @@ import { createRequire } from 'node:module';
 import open from 'open';
 import pc from 'picocolors';
 import { isGitRepo, isValidGitRef, getRepoRoot, getRepoName, normalizeRef, WORKING_TREE_REFS } from '@diffity/git';
+import type { PrBase } from '@diffity/github';
 import {
   isGitHubPrUrl,
   parseGitHubPrUrl,
   checkoutPr,
-  getPrBaseRef,
+  getPrBase,
   isCliInstalled,
   isAuthenticated,
   detectRemote,
@@ -100,6 +101,8 @@ range syntax (main..feature, main...feature) also work.`)
       }
     }
 
+    let prBase: PrBase | null = null;
+
     if (refs.length === 1 && isGitHubPrUrl(refs[0])) {
       const parsed = parseGitHubPrUrl(refs[0]);
       if (!parsed) {
@@ -147,15 +150,14 @@ range syntax (main..feature, main...feature) also work.`)
         process.exit(1);
       }
 
-      let baseRef: string;
       try {
-        baseRef = getPrBaseRef(parsed.number);
+        prBase = getPrBase(parsed.number);
       } catch {
         console.error(pc.red(`Error: Could not determine base branch for PR #${parsed.number}.`));
         process.exit(1);
       }
 
-      refs[0] = baseRef;
+      refs[0] = prBase.oid;
     }
 
     // --base/--compare flags take precedence over positional args
@@ -221,6 +223,10 @@ range syntax (main..feature, main...feature) also work.`)
       description = 'Unstaged changes';
     }
 
+    if (prBase) {
+      description = `Changes from ${prBase.name}`;
+    }
+
     let effectiveRef: string;
     if (refs.length > 0) {
       effectiveRef = refs.length === 2 ? `${refs[0]}..${refs[1]}` : refs[0];

packages/github/package.json

@@ -13,13 +13,16 @@
   },
   "scripts": {
     "build": "tsc",
-    "dev": "tsc --watch"
+    "dev": "tsc --watch",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "files": [
     "dist"
   ],
   "devDependencies": {
     "@types/node": "^25.5.0",
-    "typescript": "^5.9.3"
+    "typescript": "^5.9.3",
+    "vitest": "^4.1.0"
   }
 }

packages/github/src/index.ts

@@ -1,4 +1,4 @@
-export type { GitHubRemote, GitHubDetails, PrComment, PushResult, PulledThread } from './types.js';
+export type { GitHubRemote, GitHubDetails, PrBase, PrComment, PushResult, PulledThread } from './types.js';
 export { detectRemote, fetchDetails, isCliInstalled, isAuthenticated } from './detection.js';
 export { getFiles, getComments, getCommentCount, pushComments, pullComments } from './pr.js';
-export { isGitHubPrUrl, parseGitHubPrUrl, checkoutPr, getPrBaseRef } from './pr-url.js';
+export { isGitHubPrUrl, parseGitHubPrUrl, checkoutPr, getPrBase, parsePrBase } from './pr-url.js';

packages/github/src/pr-url.ts

@@ -1,4 +1,5 @@
 import { exec } from './exec.js';
+import type { PrBase } from './types.js';
 
 const PR_URL_REGEX = /(?:https?:\/\/)?github\.com\/([^/]+)\/([^/]+)\/pull\/(\d+)/;
 
@@ -23,6 +24,19 @@ export function checkoutPr(prNumber: number): void {
   exec(`gh pr checkout ${prNumber}`);
 }
 
-export function getPrBaseRef(prNumber: number): string {
-  return exec(`gh pr view ${prNumber} --json baseRefName --jq '.baseRefName'`);
+export function parsePrBase(json: string): PrBase {
+  const { baseRefName, baseRefOid } = JSON.parse(json) as {
+    baseRefName?: string;
+    baseRefOid?: string;
+  };
+
+  if (!baseRefName || !baseRefOid) {
+    throw new Error('Pull request response is missing baseRefName or baseRefOid');
+  }
+
+  return { name: baseRefName, oid: baseRefOid };
+}
+
+export function getPrBase(prNumber: number): PrBase {
+  return parsePrBase(exec(`gh pr view ${prNumber} --json baseRefName,baseRefOid`));
 }

packages/github/src/types.ts

@@ -12,6 +12,16 @@ export interface GitHubDetails {
   commentCount: number;
 }
 
+export interface PrBase {
+  /** The base branch's name, for display. */
+  name: string;
+  /**
+   * The commit the pull request is based on. The diff must be taken from this, not from the
+   * local branch of the same name, which is usually behind the remote.
+   */
+  oid: string;
+}
+
 export interface PulledThreadComment {
   body: string;
   authorName: string;

packages/github/tests/pr-base.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { parsePrBase } from '../src/pr-url.js';
+
+describe('parsePrBase', () => {
+  it('returns the base branch name and the commit it points at', () => {
+    const json = JSON.stringify({
+      baseRefName: 'master',
+      baseRefOid: '24e3eeab4c62927d05341e3eb9347c272fa7e3af',
+    });
+
+    expect(parsePrBase(json)).toEqual({
+      name: 'master',
+      oid: '24e3eeab4c62927d05341e3eb9347c272fa7e3af',
+    });
+  });
+
+  it('rejects a response without the oid, rather than diffing against a local branch', () => {
+    const json = JSON.stringify({ baseRefName: 'master' });
+
+    expect(() => parsePrBase(json)).toThrow(/baseRefName or baseRefOid/);
+  });
+
+  it('rejects a response without the branch name', () => {
+    const json = JSON.stringify({ baseRefOid: '24e3eeab4c' });
+
+    expect(() => parsePrBase(json)).toThrow(/baseRefName or baseRefOid/);
+  });
+});