diff --git a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/github.ts b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/github.ts
index d1de3fb3..53e237ef 100644
--- a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/github.ts
+++ b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/github.ts
@@ -513,10 +513,15 @@ export class GhGitHubReader implements T.GitHubReader {
       "--limit",
       "300",
       "--json",
-      "number,headRefName,baseRefName",
+      "number,headRefName,baseRefName,isCrossRepository",
     ]);
     return list(value, "open PRs").map((item, index) => {
       const object = record(item, `open PRs[${index}]`);
+      if (typeof object.isCrossRepository !== "boolean")
+        missing(
+          `open PRs[${index}].isCrossRepository`,
+          object.isCrossRepository
+        );
       return {
         number: parsePrNumber(object.number, `open PRs[${index}].number`),
         headRefName: string(
@@ -527,6 +532,7 @@ export class GhGitHubReader implements T.GitHubReader {
           object.baseRefName,
           `open PRs[${index}].baseRefName`
         ),
+        isCrossRepository: object.isCrossRepository,
       };
     });
   }
@@ -705,7 +711,11 @@ export function orderStack(
   open: readonly T.OpenPullRequest[]
 ): T.NonEmpty<T.PrContext> {
   const byNumber = new Map(open.map((pr) => [pr.number, pr]));
-  const byHead = new Map(open.map((pr) => [pr.headRefName, pr]));
+  const byHead = new Map(
+    open
+      .filter((pr) => pr.isCrossRepository !== true)
+      .map((pr) => [pr.headRefName, pr])
+  );
   const children = new Map<string, T.OpenPullRequest[]>();
   for (const pr of open)
     children.set(pr.baseRefName, [...(children.get(pr.baseRefName) ?? []), pr]);
@@ -717,7 +727,8 @@ export function orderStack(
   let current = start;
   while (byHead.has(current.baseRefName)) {
     const parent = byHead.get(current.baseRefName);
-    if (parent === undefined) break;
+    if (parent === undefined || parent === start || down.includes(parent))
+      break;
     down.push(parent);
     current = parent;
   }
@@ -727,6 +738,7 @@ export function orderStack(
   ]);
   const up: T.OpenPullRequest[] = [];
   const visit = (parent: T.OpenPullRequest): void => {
+    if (parent.isCrossRepository === true) return;
     for (const child of children.get(parent.headRefName) ?? []) {
       if (seen.has(child.number)) continue;
       seen.add(child.number);
diff --git a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/stack.test.ts b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/stack.test.ts
new file mode 100644
index 00000000..3b5bc94c
--- /dev/null
+++ b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/stack.test.ts
@@ -0,0 +1,133 @@
+// Stack discovery must end. A PR from a fork's main into main has the head
+// branch name main, which is also the base of every PR stacked on main, and
+// two PRs whose branches point at each other form a cycle.
+import { afterEach, describe, expect, it } from "bun:test";
+import { spawnSync } from "node:child_process";
+import {
+  chmodSync,
+  mkdirSync,
+  mkdtempSync,
+  rmSync,
+  writeFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { orderStack } from "./github.ts";
+import { parsePrNumber } from "./types.ts";
+
+const context = (number: number) => ({
+  owner: "owner",
+  repo: "repo",
+  number: parsePrNumber(number),
+});
+const pr = (
+  number: number,
+  headRefName: string,
+  baseRefName: string,
+  isCrossRepository = false
+) => ({
+  number: parsePrNumber(number),
+  headRefName,
+  baseRefName,
+  isCrossRepository,
+});
+const numbers = (stack: readonly { readonly number: number }[]) =>
+  stack.map((item) => Number(item.number));
+
+describe("stack discovery", () => {
+  const forkMain = pr(7, "main", "main", true);
+
+  it("does not take a fork PR from main as the parent of a PR on main", () => {
+    expect(
+      numbers(
+        orderStack(context(42), [
+          forkMain,
+          pr(42, "feature", "main"),
+          pr(43, "upstack", "feature"),
+        ])
+      )
+    ).toEqual([42, 43]);
+  });
+
+  it("does not stack the PRs on main above a fork PR from main", () => {
+    expect(
+      numbers(orderStack(context(7), [forkMain, pr(42, "feature", "main")]))
+    ).toEqual([7]);
+  });
+
+  it("stops at a cycle of branches", () => {
+    expect(
+      numbers(orderStack(context(42), [pr(41, "a", "b"), pr(42, "b", "a")]))
+    ).toEqual([41, 42]);
+  });
+});
+
+describe("the gh reader on a repository with an open PR from a fork's main", () => {
+  const directories: string[] = [];
+  afterEach(() => {
+    for (const dir of directories.splice(0))
+      rmSync(dir, { recursive: true, force: true });
+  });
+
+  it("watches the stack without the fork PR", () => {
+    const dir = mkdtempSync(join(tmpdir(), "watch-fork-main-"));
+    directories.push(dir);
+    const bin = join(dir, "bin");
+    mkdirSync(bin);
+    const gh = join(bin, "gh");
+    writeFileSync(
+      gh,
+      `#!${process.execPath}
+const args = process.argv.slice(2);
+const fail = (stderr) => { console.error(stderr); process.exit(1); };
+const graphql = (name) => args.some((a) => a.includes('query ' + name));
+const commits = (commit) => ({ data: { repository: { pullRequest: { commits: { nodes: [{ commit }] } } } } });
+let value;
+if (args[0] === 'pr' && args[1] === 'list') {
+  value = [
+    { number: 7, headRefName: 'main', baseRefName: 'main', isCrossRepository: true },
+    { number: 1, headRefName: 'feature', baseRefName: 'main', isCrossRepository: false },
+  ];
+} else if (args[0] === 'pr' && args[1] === 'view') {
+  value = { mergeable: 'MERGEABLE', mergeStateStatus: 'CLEAN', reviewDecision: '', headRefOid: 'head', headRefName: 'feature', baseRefName: 'main', state: 'OPEN', mergedAt: null, isDraft: false };
+} else if (args[0] === 'pr' && args[1] === 'checks') {
+  fail("no checks reported on the 'feature' branch");
+} else if (graphql('PrCheckRollup')) {
+  value = commits({ statusCheckRollup: null });
+} else if (graphql('PrCommitStatuses')) {
+  value = commits({ oid: 'head', statusCheckRollup: null });
+} else if (graphql('ReviewThreads')) {
+  value = { data: { repository: { pullRequest: { reviewThreads: { nodes: [], pageInfo: { hasNextPage: false, endCursor: null } } } } } };
+} else {
+  fail('unexpected fixture command: ' + JSON.stringify(args));
+}
+console.log(JSON.stringify(value));
+`
+    );
+    chmodSync(gh, 0o755);
+    const entry = join(dir, "entry.ts");
+    writeFileSync(
+      entry,
+      `import { main } from ${JSON.stringify(join(import.meta.dir, "cli.ts"))}; process.exitCode = await main(process.argv.slice(2));\n`
+    );
+    const result = spawnSync(
+      process.execPath,
+      [
+        entry,
+        ...["--owner", "owner", "--repo", "repo", "--pr", "1"],
+        ...["--stack", "--status-only", "--settle", "0"],
+      ],
+      {
+        encoding: "utf8",
+        timeout: 10_000,
+        env: { PATH: `${bin}:${process.env.PATH}` },
+      }
+    );
+    expect(result.signal).toBeNull();
+    expect(result.status).toBe(0);
+    expect(JSON.parse(result.stdout)).toMatchObject({
+      kind: "STATUS",
+      rows: [{ kind: "open", context: context(1) }],
+    });
+  });
+});
diff --git a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/types.ts b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/types.ts
index 1cc2a895..ff5754ab 100644
--- a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/types.ts
+++ b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/types.ts
@@ -55,6 +55,8 @@ export interface OpenPullRequest {
   readonly number: PrNumber;
   readonly headRefName: string;
   readonly baseRefName: string;
+  // A head branch in another repository is never the base of a PR here.
+  readonly isCrossRepository?: boolean;
 }
 export interface ReviewComment {
   readonly authorLogin: string | null;
diff --git a/tools/forks.json b/tools/forks.json
index 086fa801..5a8a2841 100644
--- a/tools/forks.json
+++ b/tools/forks.json
@@ -50,7 +50,7 @@
     },
     "plugins/pstack/skills/poteto-mode/scripts/watch-pr/github.ts": {
       "kind": "policy",
-      "why": "Gives the gh reader the reviewActivity method that settle.ts uses, and carries two fixes from Michael Denyer's pstack-claude: a completed check that gh buckets as pending, such as STALE, reads as failed instead of pending forever (pstack-claude #97), and a head with no checks configured reads as such when gh reports none and the head has no rollup (pstack-claude #153).",
+      "why": "Gives the gh reader the reviewActivity method that settle.ts uses, and carries two fixes from Michael Denyer's pstack-claude: a completed check that gh buckets as pending, such as STALE, reads as failed instead of pending forever (pstack-claude #97), and a head with no checks configured reads as such when gh reports none and the head has no rollup (pstack-claude #153); its stack discovery never takes a PR whose head is in another repository as a parent and stops at a branch cycle, so an open PR from a fork's main cannot loop it forever.",
       "since": "2.0.0-rc.1",
       "upstream": "not-proposed"
     },
@@ -74,7 +74,7 @@
     },
     "plugins/pstack/skills/poteto-mode/scripts/watch-pr/types.ts": {
       "kind": "policy",
-      "why": "Adds the settle state, decision, and reader method that settle.ts defines, and the review-required and merge-blocked gates, the no-checks read and CI state, and the confirming-block wait and timeout reason that the fixes from Michael Denyer's pstack-claude add.",
+      "why": "Adds the settle state, decision, and reader method that settle.ts defines, and the review-required and merge-blocked gates, the no-checks read and CI state, and the confirming-block wait and timeout reason that the fixes from Michael Denyer's pstack-claude add; it also adds the isCrossRepository flag on an open PR that stack discovery reads.",
       "since": "2.0.0-rc.1",
       "upstream": "not-proposed"
     },