diff --git a/README.md b/README.md
index 8bc2d20..5908cb4 100644
--- a/README.md
+++ b/README.md
@@ -47,6 +47,18 @@ Review the current branch against a target branch:
 codiff main
 ```
 
+Review the current branch against the branch its pull request or merge request
+targets, which is handy for stacked changes where the base is not `main`:
+
+```bash
+codiff base
+```
+
+Codiff asks GitHub through `gh` or GitLab through `glab` (chosen from the
+repository's remote) for the current branch's PR/MR base branch, refreshes that
+branch from the remote, and diffs against it. When there is no open request, or
+the relevant CLI is unavailable, it falls back to the remote's default branch.
+
 Review a GitHub pull request or GitLab merge request using the current repository remote:
 
 ```bash
diff --git a/bin/arguments.js b/bin/arguments.js
index 475b738..2edd405 100644
--- a/bin/arguments.js
+++ b/bin/arguments.js
@@ -91,6 +91,7 @@ export const usageExamples = [
   { command: 'codiff', description: 'Review staged and unstaged changes.' },
   { command: 'codiff /path/to/repo', description: 'Review changes in a specific repository.' },
   { command: 'codiff main', description: 'Review the current branch against main.' },
+  { command: 'codiff base', description: "Review against the current branch's PR/MR base branch." },
   { command: 'codiff a1b2c3d', description: 'Review a specific commit.' },
   { command: "codiff '#75'", description: 'Review pull request #75.' },
   { command: 'codiff pr 75', description: 'Review pull request #75 (alternate syntax).' },
@@ -216,6 +217,8 @@ const getReviewProviderMarker = (arg) =>
       ? 'gitlab'
       : null;
 
+const isBaseReviewMarker = (arg) => /^base$/i.test(arg);
+
 const isPullRequestUrlArgument = (arg) => parseReviewUrl(arg) != null;
 
 export const resolvePullRequestUrl = (repositoryPath, number, provider) =>
@@ -250,6 +253,7 @@ export const parseArguments = (args) => {
   let pullRequestProvider = null;
   let pullRequestUrl = null;
   let requestedPath = null;
+  let reviewBase = false;
   let sourceCandidate = null;
   let rangeCandidate = null;
   const walkthroughContextPath =
@@ -314,13 +318,17 @@ export const parseArguments = (args) => {
         : null;
   }
   if (!range && !commitRef && !branchRef && sourceCandidate) {
-    const source = resolveSourceCandidate(repositoryPath, sourceCandidate);
-    if (source?.branchRef) {
-      branchRef = source.branchRef;
-    } else if (source?.commitRef) {
-      commitRef = source.commitRef;
-    } else if (requestedPath == null) {
-      requestedPath = sourceCandidate;
+    if (isBaseReviewMarker(sourceCandidate)) {
+      reviewBase = true;
+    } else {
+      const source = resolveSourceCandidate(repositoryPath, sourceCandidate);
+      if (source?.branchRef) {
+        branchRef = source.branchRef;
+      } else if (source?.commitRef) {
+        commitRef = source.commitRef;
+      } else if (requestedPath == null) {
+        requestedPath = sourceCandidate;
+      }
     }
   }
 
@@ -338,6 +346,7 @@ export const parseArguments = (args) => {
     pullRequestNumber,
     ...(pullRequestProvider ? { pullRequestProvider } : {}),
     pullRequestUrl,
+    ...(reviewBase ? { reviewBase: true } : {}),
     requestedPath: resolve(requestedPath ?? process.cwd()),
     ...(values.share === true ? { share: true } : {}),
     version: values.version === true,
diff --git a/bin/codiff-app b/bin/codiff-app
index 1dc0b50..d0c2a0c 100755
--- a/bin/codiff-app
+++ b/bin/codiff-app
@@ -51,6 +51,7 @@ show_help() {
   printf '  %-32s%s%s%s\n' "codiff" "$gray" "Review staged and unstaged changes." "$reset"
   printf '  %-32s%s%s%s\n' "codiff /path/to/repo" "$gray" "Review changes in a specific repository." "$reset"
   printf '  %-32s%s%s%s\n' "codiff main" "$gray" "Review the current branch against main." "$reset"
+  printf '  %-32s%s%s%s\n' "codiff base" "$gray" "Review against the current branch's PR/MR base branch." "$reset"
   printf '  %-32s%s%s%s\n' "codiff a1b2c3d" "$gray" "Review a specific commit." "$reset"
   printf '  %-32s%s%s%s\n' "codiff '#75'" "$gray" "Review pull request #75." "$reset"
   printf '  %-32s%s%s%s\n' "codiff pr 75" "$gray" "Review pull request #75 (alternate syntax)." "$reset"
@@ -64,20 +65,22 @@ show_help() {
 }
 
 for arg in "$@"; do
-  if [ "$arg" = "--share" ]; then
-    runtime="${CODIFF_NODE_COMMAND:-}"
-    if [ -z "$runtime" ]; then
-      electron_exec="$(ls "$app_path/Contents/MacOS/" 2>/dev/null | head -n 1)"
-      if [ -n "$electron_exec" ]; then
-        runtime="$app_path/Contents/MacOS/$electron_exec"
+  case "$arg" in
+    --share|[Bb][Aa][Ss][Ee])
+      runtime="${CODIFF_NODE_COMMAND:-}"
+      if [ -z "$runtime" ]; then
+        electron_exec="$(ls "$app_path/Contents/MacOS/" 2>/dev/null | head -n 1)"
+        if [ -n "$electron_exec" ]; then
+          runtime="$app_path/Contents/MacOS/$electron_exec"
+        fi
       fi
-    fi
-    if [ -z "$runtime" ] || [ ! -x "$runtime" ]; then
-      printf '%s\n' "codiff: could not find the bundled runtime for --share." >&2
-      exit 1
-    fi
-    ELECTRON_RUN_AS_NODE=1 exec "$runtime" "$script_dir/codiff.js" "$@"
-  fi
+      if [ -z "$runtime" ] || [ ! -x "$runtime" ]; then
+        printf '%s\n' "codiff: could not find the bundled runtime." >&2
+        exit 1
+      fi
+      ELECTRON_RUN_AS_NODE=1 exec "$runtime" "$script_dir/codiff.js" "$@"
+      ;;
+  esac
 done
 
 repository_path=""
diff --git a/bin/codiff.js b/bin/codiff.js
index 570356f..9a14846 100755
--- a/bin/codiff.js
+++ b/bin/codiff.js
@@ -1,6 +1,6 @@
 #!/usr/bin/env node
 
-import { spawn } from 'node:child_process';
+import { execFileSync, spawn } from 'node:child_process';
 import { existsSync, mkdtempSync, readFileSync, rmSync } from 'node:fs';
 import http from 'node:http';
 import https from 'node:https';
@@ -19,6 +19,7 @@ const {
   shareWalkthroughFile,
 } = require('../electron/headless-walkthrough-share.cjs');
 const { sharePlanFile } = require('../electron/headless-plan-share.cjs');
+const { resolveBaseBranchRef } = require('../electron/review-source.cjs');
 
 // The renderer is the built dist/ by default. When Codiff's own Vite dev server
 // is running, use it instead so source edits hot-reload without a rebuild. The
@@ -119,7 +120,6 @@ const run = async () => {
 
   const {
     agentBackend,
-    branchRef,
     claudeSessionId,
     codexSessionId,
     commitRef,
@@ -130,12 +130,26 @@ const run = async () => {
     pullRequestProvider,
     range,
     requestedPath,
+    reviewBase,
     share,
     walkthrough,
     walkthroughContextPath,
     walkthroughFilePath,
   } = parsedArguments;
-  let { pullRequestUrl } = parsedArguments;
+  let { branchRef, pullRequestUrl } = parsedArguments;
+
+  if (reviewBase && !branchRef && !commitRef && !range) {
+    const { base, ref, remote } = resolveBaseBranchRef(requestedPath);
+    branchRef = ref;
+    try {
+      execFileSync('git', ['-C', requestedPath, 'fetch', remote, base, '--quiet'], {
+        stdio: 'ignore',
+        timeout: 30_000,
+      });
+    } catch {
+      // Keep going with the local remote ref when offline.
+    }
+  }
 
   if (planFilePath && (!existsSync(planFilePath) || !/\.md$/i.test(planFilePath))) {
     process.stderr.write(`codiff: plan file not found or not Markdown: ${planFilePath}\n`);
diff --git a/core/__tests__/codiff-cli.test.ts b/core/__tests__/codiff-cli.test.ts
index d16c692..90668ab 100644
--- a/core/__tests__/codiff-cli.test.ts
+++ b/core/__tests__/codiff-cli.test.ts
@@ -10,6 +10,7 @@ import {
   truncate,
   writeFile,
 } from 'node:fs/promises';
+import { createRequire } from 'node:module';
 import { tmpdir } from 'node:os';
 import { join, resolve } from 'node:path';
 import { promisify } from 'node:util';
@@ -20,6 +21,48 @@ import { createFakeCommandLogger, createFakeOpenLogger } from './helpers/cli.ts'
 
 const execFileAsync = promisify(execFile);
 
+const require = createRequire(import.meta.url);
+const { resolveBaseBranchRef } = require('../../electron/review-source.cjs') as {
+  resolveBaseBranchRef: (repositoryPath: string) => {
+    base: string;
+    ref: string;
+    remote: string;
+  };
+};
+
+const writeFakeExecutable = async (directory: string, name: string, body: string) => {
+  const path = join(directory, name);
+  await writeFile(path, body);
+  await chmod(path, 0o755);
+  return path;
+};
+
+const createTemporaryDirectory = async (prefix: string) => {
+  const directory = await realpath(await mkdtemp(join(tmpdir(), prefix)));
+  return {
+    directory,
+    [Symbol.asyncDispose]: () => rm(directory, { force: true, recursive: true }),
+  };
+};
+
+const overridePath = (value: string) => {
+  const previousPath = process.env.PATH;
+  process.env.PATH = value;
+  return {
+    [Symbol.dispose]: () => {
+      process.env.PATH = previousPath;
+    },
+  };
+};
+
+const createDisposableFakeCommandLogger = async (
+  prefix: string,
+  commandName: string,
+): Promise<Awaited<ReturnType<typeof createFakeCommandLogger>> & AsyncDisposable> => {
+  const logger = await createFakeCommandLogger(prefix, commandName);
+  return { ...logger, [Symbol.asyncDispose]: logger.cleanup };
+};
+
 const git = async (repo: string, args: ReadonlyArray<string>) => {
   await execFileAsync(
     'git',
@@ -329,6 +372,103 @@ test('parseArguments treats GitLab MR marker values as review sources', () => {
   });
 });
 
+test('parseArguments treats the base keyword as a PR/MR base review', () => {
+  expect(parseArguments(['base'])).toEqual({
+    commitRef: null,
+    help: false,
+    pullRequestNumber: null,
+    pullRequestUrl: null,
+    requestedPath: resolve(process.cwd()),
+    reviewBase: true,
+    version: false,
+    walkthrough: false,
+  });
+
+  expect(parseArguments(['BASE'])).toMatchObject({ reviewBase: true });
+});
+
+test('parseArguments still lets --branch base review a literal base branch', async () => {
+  await withCwd(refRepositoryPath, () => {
+    expect(parseArguments(['--branch', 'base'])).toMatchObject({
+      branchRef: 'base',
+      commitRef: null,
+    });
+    expect(parseArguments(['--branch', 'base'])).not.toHaveProperty('reviewBase');
+  });
+});
+
+test('resolveBaseBranchRef reads the GitHub PR base branch through gh', async () => {
+  await using temporaryDirectory = await createTemporaryDirectory('codiff-base-github-');
+  const repositoryPath = join(temporaryDirectory.directory, 'repo');
+  const fakeBin = join(temporaryDirectory.directory, 'bin');
+
+  await mkdir(repositoryPath);
+  await mkdir(fakeBin);
+  await git(repositoryPath, ['init']);
+  await git(repositoryPath, ['remote', 'add', 'origin', 'git@github.com:owner/repo.git']);
+  await writeFakeExecutable(fakeBin, 'gh', '#!/bin/sh\nprintf "feature-base\\n"\n');
+  using pathOverride = overridePath(`${fakeBin}:${process.env.PATH ?? ''}`);
+  void pathOverride;
+
+  expect(resolveBaseBranchRef(repositoryPath)).toEqual({
+    base: 'feature-base',
+    ref: 'origin/feature-base',
+    remote: 'origin',
+  });
+});
+
+test('resolveBaseBranchRef reads the GitLab MR target branch through glab', async () => {
+  await using temporaryDirectory = await createTemporaryDirectory('codiff-base-gitlab-');
+  const repositoryPath = join(temporaryDirectory.directory, 'repo');
+  const fakeBin = join(temporaryDirectory.directory, 'bin');
+
+  await mkdir(repositoryPath);
+  await mkdir(fakeBin);
+  await git(repositoryPath, ['init']);
+  await git(repositoryPath, [
+    'remote',
+    'add',
+    'origin',
+    'git@gitlab.example.com:group/project.git',
+  ]);
+  await writeFakeExecutable(fakeBin, 'glab', '#!/bin/sh\nprintf "release\\n"\n');
+  using pathOverride = overridePath(`${fakeBin}:${process.env.PATH ?? ''}`);
+  void pathOverride;
+
+  expect(resolveBaseBranchRef(repositoryPath)).toEqual({
+    base: 'release',
+    ref: 'origin/release',
+    remote: 'origin',
+  });
+});
+
+test('resolveBaseBranchRef falls back to the remote default branch without a request', async () => {
+  await using temporaryDirectory = await createTemporaryDirectory('codiff-base-fallback-');
+  const repositoryPath = join(temporaryDirectory.directory, 'repo');
+  const fakeBin = join(temporaryDirectory.directory, 'bin');
+
+  await mkdir(repositoryPath);
+  await mkdir(fakeBin);
+  await git(repositoryPath, ['init']);
+  await git(repositoryPath, ['config', 'user.email', 'codiff@example.com']);
+  await git(repositoryPath, ['config', 'user.name', 'Codiff Test']);
+  await git(repositoryPath, ['remote', 'add', 'origin', 'git@github.com:owner/repo.git']);
+  await git(repositoryPath, ['commit', '--allow-empty', '-m', 'init']);
+  const { stdout } = await execFileAsync('git', ['-C', repositoryPath, 'rev-parse', 'HEAD'], {
+    encoding: 'utf8',
+  });
+  await git(repositoryPath, ['update-ref', 'refs/remotes/origin/main', stdout.trim()]);
+  await writeFakeExecutable(fakeBin, 'gh', '#!/bin/sh\nexit 1\n');
+  using pathOverride = overridePath(`${fakeBin}:${process.env.PATH ?? ''}`);
+  void pathOverride;
+
+  expect(resolveBaseBranchRef(repositoryPath)).toEqual({
+    base: 'main',
+    ref: 'origin/main',
+    remote: 'origin',
+  });
+});
+
 test('parseArguments accepts nested GitLab merge request URLs', () => {
   expect(
     parseArguments(['https://gitlab.example.com/group/subgroup/project/-/merge_requests/23']),
@@ -418,6 +558,19 @@ test('packaged terminal helper forwards GitLab MR markers to Electron', async ()
   }
 });
 
+test('packaged terminal helper runs `base` through the bundled Node entry point', async () => {
+  await using logger = await createDisposableFakeCommandLogger('codiff-packaged-base-', 'runtime');
+
+  await execFileAsync(resolve('bin/codiff-app'), ['base'], {
+    env: {
+      ...logger.env,
+      CODIFF_NODE_COMMAND: logger.commandPath,
+    },
+  });
+
+  expect(await logger.readArgs()).toEqual([resolve('bin/codiff.js'), 'base']);
+});
+
 test('packaged terminal helper forwards HEAD^1 to Electron as a commit', async () => {
   const logger = await createFakeOpenLogger();
 
diff --git a/electron/review-source.cjs b/electron/review-source.cjs
index f74902d..5771da4 100644
--- a/electron/review-source.cjs
+++ b/electron/review-source.cjs
@@ -104,6 +104,64 @@ const remotePriority = (remote) =>
       ? 2
       : 3;
 
+const tryCommandOutput = (command, args, cwd) => {
+  try {
+    return execFileSync(command, args, {
+      cwd,
+      encoding: 'utf8',
+      stdio: ['ignore', 'pipe', 'ignore'],
+      timeout: 15_000,
+    }).trim();
+  } catch {
+    return '';
+  }
+};
+
+/** @param {string} repositoryPath */
+const getPrimaryReviewRemote = (repositoryPath) =>
+  readReviewRemotes(repositoryPath).sort(
+    (left, right) => remotePriority(left) - remotePriority(right),
+  )[0];
+
+/** @param {string} repositoryPath @param {string} remoteName */
+const resolveDefaultRemoteBranch = (repositoryPath, remoteName) => {
+  const head = tryCommandOutput(
+    'git',
+    ['symbolic-ref', '--quiet', '--short', `refs/remotes/${remoteName}/HEAD`],
+    repositoryPath,
+  );
+  const prefix = `${remoteName}/`;
+  if (head.startsWith(prefix)) {
+    return head.slice(prefix.length);
+  }
+
+  return 'main';
+};
+
+/**
+ * @param {string} repositoryPath
+ * @returns {{ base: string; ref: string; remote: string }}
+ */
+const resolveBaseBranchRef = (repositoryPath) => {
+  const remote = getPrimaryReviewRemote(repositoryPath);
+  const remoteName = remote.name;
+  const base =
+    remote.provider === 'gitlab'
+      ? tryCommandOutput(
+          'glab',
+          ['mr', 'view', '--output', 'json', '--jq', '.target_branch'],
+          repositoryPath,
+        )
+      : tryCommandOutput(
+          'gh',
+          ['pr', 'view', '--json', 'baseRefName', '--jq', '.baseRefName'],
+          repositoryPath,
+        );
+
+  const branch = base || resolveDefaultRemoteBranch(repositoryPath, remoteName);
+  return { base: branch, ref: `${remoteName}/${branch}`, remote: remoteName };
+};
+
 /**
  * @param {string} repositoryPath
  * @param {number} number
@@ -137,5 +195,6 @@ module.exports = {
   parseRemoteUrl,
   parseReviewUrl,
   readReviewRemotes,
+  resolveBaseBranchRef,
   resolveReviewUrl,
 };