diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index 20ab58f..7446b17 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -53,10 +53,12 @@ A cron job runs the sweep every minute (`*/1 * * * *`); the sweep self-locks so
    failed heads are throttled in local state instead.) The marker check falls back to "any comment" when
    `gh api user` is unavailable (a GitHub-App integration 403s on it), so dedup never silently re-reviews forever.
 4. **Build memory** from the remaining comments (see below).
-5. **Review.** The _runner_ fetches the diff (`gh pr diff`) and feeds it to `codex exec` over **stdin**, in a
-   `workspace-write` sandbox restricted to `/tmp` with **network off and no `gh`**. Codex reads the rubric +
-   corpus + the inlined diff and writes the review to a temp file ending in the marker; the _runner_, not Codex,
-   posts it with `gh pr comment`.
+5. **Review.** The _runner_ fetches the diff via GitHub's compare API (`baseRefOid...headRefOid`), so stacked PRs
+   whose base is another feature branch diff against that base, not `main`. It spins a detached worktree at the PR
+   head SHA (`$STUPIFY_HOME/worktrees/<n>-<sha>`) so codex reads the same tree the diff describes, then feeds the
+   diff to `codex exec` over **stdin**, in a `workspace-write` sandbox restricted to `/tmp` with **network off and
+   no `gh`**. Codex reads the rubric + corpus + the inlined diff and writes the review to a temp file ending in the
+   marker; the _runner_, not Codex, posts it with `gh pr comment`.
    Candidates are collected serially (all the cheap gh gates), then reviewed by a pool of up to `CODEX_JOBS`
    (default 3) concurrent codex runs — a busy sweep's wall-clock is the slowest review, not the sum of them. A
    quota wall from any run stops new launches while in-flight runs drain.
diff --git a/src/cli.ts b/src/cli.ts
index 473ec12..fe2704c 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -8,7 +8,7 @@
  * `stupify run [--dry]` → run one review sweep right now.
  */
 import { spawnSync } from 'node:child_process'
-import { copyFileSync, existsSync, mkdirSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
+import { appendFileSync, copyFileSync, existsSync, mkdirSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
 import { homedir } from 'node:os'
 import { dirname, join, relative, resolve } from 'node:path'
 import { fileURLToPath } from 'node:url'
@@ -74,6 +74,20 @@ function die(message: string): never {
   process.exit(1)
 }
 
+/** Trust an extra directory in ~/.codex/config.toml (stacked-PR worktrees live outside the main checkout). */
+function appendCodexProjectTrust(dir: string): void {
+  const file = join(process.env.CODEX_HOME ?? join(homedir(), '.codex'), 'config.toml')
+  if (!existsSync(file)) {
+    return
+  }
+  const body = readFileSync(file, 'utf8')
+  const key = `[projects."${dir}"]`
+  if (body.includes(key)) {
+    return
+  }
+  appendFileSync(file, `\n${key}\ntrust_level = "trusted"\n`)
+}
+
 async function installSweepEngine(dest = join(HOME, 'review-sweep.ts')): Promise<void> {
   const built = await Bun.build({
     entrypoints: [join(PKG_DIR, 'review-sweep.ts')],
@@ -450,10 +464,13 @@ async function setup(argv: {
     .join('\n')
   writeFileSync(join(HOME, 'config.env'), `${cfg}\n`)
   if (host) {
+    const worktrees = join(HOME, 'worktrees')
+    mkdirSync(worktrees, { recursive: true })
     writeCodexGatewayConfig({
       ...(argv.codexHost === undefined ? {} : { gatewayHost: argv.codexHost }),
       trustDir: join(HOME, 'repo'),
     })
+    appendCodexProjectTrust(worktrees)
   } // exe.dev VM: route Codex through the no-key exe-llm gateway
   try {
     installCron({
diff --git a/src/review-sweep.test.ts b/src/review-sweep.test.ts
index 4acf8f6..9139220 100644
--- a/src/review-sweep.test.ts
+++ b/src/review-sweep.test.ts
@@ -66,9 +66,11 @@ const cfg = (): Config => ({
   codexJobs: 3,
 })
 
-const pr = (number: number, sha: string): Pr => ({
+const pr = (number: number, sha: string, base = 'main', baseSha = 'a'.repeat(40)): Pr => ({
   number,
   headRefOid: sha,
+  baseRefOid: baseSha,
+  baseRefName: base,
   isDraft: false,
   author: { login: 'someone', is_bot: false },
   labels: [{ name: 'codex-review' }],
diff --git a/src/sweep/codex.ts b/src/sweep/codex.ts
index 590fa77..b330a32 100644
--- a/src/sweep/codex.ts
+++ b/src/sweep/codex.ts
@@ -79,7 +79,9 @@ export async function runReview(
   priorThread: string,
   diff: string,
   dismissed: string[] = [],
+  workDir?: string,
 ): Promise<ReviewOutcome> {
+  const cwd = workDir ?? cfg.repoDir
   const outPath = reviewOutPath(cfg, pr)
   rmSync(outPath, { force: true }) // clear any stale file so we never read a previous run's review
   const schemaPath = join(cfg.stateDir, 'review-schema.json')
@@ -87,7 +89,7 @@ export async function runReview(
   const codexArgs = [
     'exec',
     '--cd',
-    cfg.repoDir,
+    cwd,
     '--output-schema',
     schemaPath, // the provider enforces ReviewOutput on the final message...
     '--output-last-message',
@@ -110,7 +112,7 @@ export async function runReview(
   codexArgs.push('-') // read the prompt from STDIN, not argv — the inlined corpus + diff would blow ARG_MAX (E2BIG)
 
   const cx = await execAsync('codex', codexArgs, {
-    cwd: cfg.repoDir,
+    cwd,
     timeoutMs: 1_200_000,
     input: reviewPrompt(cfg, pr, priorThread, diff, dismissed),
   })
diff --git a/src/sweep/diff.ts b/src/sweep/diff.ts
index 8150385..9935842 100644
--- a/src/sweep/diff.ts
+++ b/src/sweep/diff.ts
@@ -1,14 +1,15 @@
 // Fetching and measuring PR diffs. The RUNNER fetches the diff (not codex) so codex needs no network or gh —
-// it reviews the diff straight from the prompt, sandboxed.
+// it reviews the diff straight from the prompt, sandboxed. Stacked PRs diff base..head via the compare API, not main.
 import { exec } from '@bevyl-ai/agent-tools'
 
 import { type Config } from './config'
+import { type Pr } from './prs'
 
 // GitHub's diff endpoint 406s past EITHER of these, so a big enough PR can't even be MEASURED. They are GitHub's
 // limits, not ours — DIFF_LINE_CAP can be set above the line one but never reached.
 export const GH_DIFF_LIMITS = '20000-line / 300-file'
 
-/** A `gh pr diff` failure that is GitHub's size refusal rather than a transient error — retrying can never fix it.
+/** A compare/diff failure that is GitHub's size refusal rather than a transient error — retrying can never fix it.
  *  Matches on gh's stable `too_large` code first: the prose differs per limit (lines vs files) and can be reworded,
  *  but both variants carry the code. Missing one variant is what kept #8338/#8241 looping after the first fix. */
 export const isDiffTooLarge = (output: string): boolean =>
@@ -19,8 +20,14 @@ export const isDiffTooLarge = (output: string): boolean =>
 // 'too-large' is terminal, and conflating them is what left oversized PRs re-fetched every 60s forever.
 type DiffRead = { ok: true; diff: string } | { ok: false; reason: 'unreadable' | 'too-large' }
 
-export function getDiff(cfg: Config, number: number): DiffRead {
-  const r = exec('gh', ['pr', 'diff', String(number), '--repo', cfg.slug])
+/** Diff the PR's head against its base (not defaultBranch) — correct for stacked PRs. */
+export function getDiff(cfg: Config, pr: Pick<Pr, 'baseRefOid' | 'headRefOid'>): DiffRead {
+  const r = exec('gh', [
+    'api',
+    `repos/${cfg.slug}/compare/${pr.baseRefOid}...${pr.headRefOid}`,
+    '-H',
+    'Accept: application/vnd.github.diff',
+  ])
   if (r.ok) {
     return { ok: true, diff: r.stdout }
   }
diff --git a/src/sweep/prs.ts b/src/sweep/prs.ts
index 3ed7a4e..1146d76 100644
--- a/src/sweep/prs.ts
+++ b/src/sweep/prs.ts
@@ -10,6 +10,8 @@ import { type Config, log } from './config'
 export const Pr = z.object({
   number: z.number(),
   headRefOid: z.string(),
+  baseRefOid: z.string(),
+  baseRefName: z.string(),
   isDraft: z.boolean(),
   author: z.object({ login: z.string(), is_bot: z.boolean() }).nullable(), // is_bot flags GitHub App bots (app/dependabot) the [bot] suffix misses
   labels: z.array(z.object({ name: z.string() })),
@@ -26,7 +28,7 @@ const PR_LIST_LIMIT = 500
 
 export function listPrs(cfg: Config): Pr[] | null {
   // Filter the PR list directly rather than `gh pr list --label` — that search index lags behind labelling.
-  const fields = 'number,headRefOid,isDraft,author,labels,title,body'
+  const fields = 'number,headRefOid,baseRefOid,baseRefName,isDraft,author,labels,title,body'
   const r = exec('gh', [
     'pr',
     'list',
diff --git a/src/sweep/review-one.ts b/src/sweep/review-one.ts
index 094aa07..781b325 100644
--- a/src/sweep/review-one.ts
+++ b/src/sweep/review-one.ts
@@ -1,8 +1,9 @@
 // `stupify review <pr>` — review ONE pull request on demand (no cron, no checkout, no lock) and print it,
 // or `--post` it. Always a FRESH perspective: no prior-review memory, so you get the full take.
+import { existsSync } from 'node:fs'
 import { join } from 'node:path'
 
-import { exec } from '@bevyl-ai/agent-tools'
+import { detectRepo, exec } from '@bevyl-ai/agent-tools'
 
 import { runReview } from './codex'
 import { type Config } from './config'
@@ -10,6 +11,7 @@ import { getDiff, GH_DIFF_LIMITS } from './diff'
 import { postReview } from './github'
 import { hasMachinery } from './prompt'
 import { Pr } from './prs'
+import { prepareHeadWorktree, removeHeadWorktree } from './worktree'
 
 /** Accepts a PR URL or `owner/repo#123` (the CLI resolves a bare `#123` against the cwd repo before calling here). */
 export async function reviewOne(cfg: Config, ref: string, post: boolean): Promise<void> {
@@ -29,15 +31,25 @@ export async function reviewOne(cfg: Config, ref: string, post: boolean): Promis
     console.error('stupify review: no taste found. Run `stupify taste` (or add a .review/ to this repo) first.')
     process.exit(1)
   }
-  // No checkout for an ad-hoc review — codex reviews from the inlined diff. Run it in the current directory (codex
-  // needs a real workspace to operate in); if you're standing in the target repo it gets useful file context for free.
+  // Ad-hoc review runs in cwd. When cwd is the target repo we spin a head worktree for file context; otherwise codex
+  // reviews from the inlined diff alone (cross-repo refs can't fetch into a foreign checkout).
   cfg.repoDir = process.cwd()
-  const head = exec('gh', ['pr', 'view', String(number), '--repo', slug, '--json', 'headRefOid,title,body'])
+  const head = exec('gh', [
+    'pr',
+    'view',
+    String(number),
+    '--repo',
+    slug,
+    '--json',
+    'headRefOid,baseRefOid,baseRefName,title,body',
+  ])
   if (!head.ok) {
     console.error(`stupify review: couldn't read ${slug}#${number} via gh (auth? does it exist?).`)
     process.exit(1)
   }
-  const meta = Pr.pick({ headRefOid: true, title: true, body: true }).parse(JSON.parse(head.stdout))
+  const meta = Pr.pick({ headRefOid: true, baseRefOid: true, baseRefName: true, title: true, body: true }).parse(
+    JSON.parse(head.stdout),
+  )
   const pr = {
     number,
     ...meta,
@@ -45,7 +57,7 @@ export async function reviewOne(cfg: Config, ref: string, post: boolean): Promis
     author: { login: '', is_bot: false },
     labels: [],
   }
-  const read = getDiff(cfg, number)
+  const read = getDiff(cfg, pr)
   if (!read.ok) {
     console.error(
       read.reason === 'too-large'
@@ -55,8 +67,26 @@ export async function reviewOne(cfg: Config, ref: string, post: boolean): Promis
     process.exit(1)
   }
   const { diff } = read
-  console.error(`reviewing ${slug}#${number} …`) // progress on stderr; stdout stays just the review
-  const r = await runReview(cfg, pr, '', diff) // no memory: a manual review is always a fresh, full take
+  const localRepo = detectRepo()
+  const canWorktree =
+    localRepo !== null && localRepo.toLowerCase() === slug.toLowerCase() && existsSync(join(cfg.repoDir, '.git'))
+  console.error(`reviewing ${slug}#${number} (base ${pr.baseRefName}) …`) // progress on stderr; stdout stays just the review
+  let workDir: string | undefined
+  if (canWorktree) {
+    workDir = prepareHeadWorktree(cfg.repoDir, pr) ?? undefined
+    if (workDir === undefined) {
+      console.error(`stupify review: couldn't checkout head for ${slug}#${number} (git fetch/worktree failed).`)
+      process.exit(1)
+    }
+  }
+  let r
+  try {
+    r = await runReview(cfg, pr, '', diff, [], workDir)
+  } finally {
+    if (canWorktree && workDir !== undefined) {
+      removeHeadWorktree(cfg.repoDir, pr)
+    }
+  }
   if (r.kind === 'limit' || r.kind === 'fail') {
     console.error(
       `stupify review: ${r.kind === 'limit' ? 'codex is out of credits / rate-limited' : "codex couldn't produce a review"} — ${r.reason}`,
diff --git a/src/sweep/review-pr.ts b/src/sweep/review-pr.ts
index 6679e59..9065dd4 100644
--- a/src/sweep/review-pr.ts
+++ b/src/sweep/review-pr.ts
@@ -8,6 +8,7 @@ import { type Config, log } from './config'
 import { postNote, postReview, resolveThreads } from './github'
 import { type Pr } from './prs'
 import { FIXED_NOTE, STILL_NOTE } from './verdict'
+import { prepareHeadWorktree, removeHeadWorktree } from './worktree'
 
 // A posted review carries its token spend and its blocking-finding count — zero blocking reads as a green status.
 export type SweepReviewResult = { tokens: number; blocking: number } | 'limit' | 'clean' | 'fixed' | 'open' | null
@@ -47,8 +48,18 @@ export async function reviewPr(
   openThreadIds: string[],
   dismissed: string[],
 ): Promise<SweepReviewResult> {
-  log(`reviewing PR #${pr.number} @ ${pr.headRefOid.slice(0, 8)}`)
-  const r = await runReview(cfg, pr, priorThread, diff, dismissed)
+  log(`reviewing PR #${pr.number} @ ${pr.headRefOid.slice(0, 8)} (base ${pr.baseRefName})`)
+  const workDir = prepareHeadWorktree(cfg.repoDir, pr)
+  if (workDir === null) {
+    log(`  review FAILED for #${pr.number} — couldn't checkout head for file context`)
+    return null
+  }
+  let r
+  try {
+    r = await runReview(cfg, pr, priorThread, diff, dismissed, workDir)
+  } finally {
+    removeHeadWorktree(cfg.repoDir, pr)
+  }
   if (r.kind === 'limit' || r.kind === 'fail') {
     log(`  review FAILED for #${pr.number} — ${r.reason}`)
     if (r.kind === 'limit') {
diff --git a/src/sweep/sweep.ts b/src/sweep/sweep.ts
index 17be025..b52c0d8 100644
--- a/src/sweep/sweep.ts
+++ b/src/sweep/sweep.ts
@@ -118,7 +118,7 @@ export function collectCandidates(
     }
 
     // Fetch the diff once, here in the runner — codex reviews it from the prompt with no network/gh of its own.
-    const read = getDiff(cfg, pr.number)
+    const read = getDiff(cfg, pr)
     if (!read.ok && read.reason === 'too-large') {
       // Terminal: gh will never hand us this diff, so there is nothing to retry and nothing to measure. Say so
       // plainly — the old wording promised a retry that could not possibly succeed.
diff --git a/src/sweep/worktree.ts b/src/sweep/worktree.ts
new file mode 100644
index 0000000..50a0c1f
--- /dev/null
+++ b/src/sweep/worktree.ts
@@ -0,0 +1,32 @@
+// Detached worktree at a PR's head SHA so codex reads the same tree the diff describes — required for stacked
+// PRs whose base is not main (the shared checkout stays on defaultBranch for refreshRepo).
+import { rmSync } from 'node:fs'
+import { dirname, join } from 'node:path'
+
+import { exec } from '@bevyl-ai/agent-tools'
+
+import { type Pr } from './prs'
+
+export function headWorktreePath(repoDir: string, pr: Pr): string {
+  return join(dirname(repoDir), 'worktrees', `${pr.number}-${pr.headRefOid.slice(0, 8)}`)
+}
+
+/** Fetch base+head and add a detached worktree at the PR head. Returns null on failure. */
+export function prepareHeadWorktree(repoDir: string, pr: Pr): string | null {
+  const dir = headWorktreePath(repoDir, pr)
+  rmSync(dir, { recursive: true, force: true })
+  exec('git', ['worktree', 'prune'], { cwd: repoDir })
+  if (
+    !exec('git', ['fetch', '-q', 'origin', pr.baseRefOid, pr.headRefOid], { cwd: repoDir }).ok ||
+    !exec('git', ['worktree', 'add', '--detach', dir, pr.headRefOid], { cwd: repoDir }).ok
+  ) {
+    return null
+  }
+  return dir
+}
+
+export function removeHeadWorktree(repoDir: string, pr: Pr): void {
+  const dir = headWorktreePath(repoDir, pr)
+  exec('git', ['worktree', 'remove', '--force', dir], { cwd: repoDir })
+  rmSync(dir, { recursive: true, force: true })
+}