diff --git a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping-cli.test.ts b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping-cli.test.ts
index 70bb8afd..565fe5fe 100644
--- a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping-cli.test.ts
+++ b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping-cli.test.ts
@@ -40,25 +40,12 @@ function fixture(
   writeFileSync(
     gh,
     `#!${process.execPath}
-import { readFileSync, writeFileSync, appendFileSync } from 'node:fs';
+import { readFileSync, writeFileSync } from 'node:fs';
+import { fakeGitHub } from ${JSON.stringify(join(import.meta.dir, "shipping.test-helper.ts"))};
 const state = JSON.parse(readFileSync(process.env.SHIPPING_STATE, 'utf8'));
-const args = process.argv.slice(2);
-const query = args.find(arg => arg.startsWith('query='));
-let result;
-if (query.includes('disablePullRequestAutoMerge')) {
-  state.autoMergeRequest = null;
-  result = { disablePullRequestAutoMerge: { clientMutationId: null } };
-} else if (query.includes('dequeuePullRequest')) {
-  if (!query.includes('dequeuePullRequest(input:{id:$id})')) {
-    console.error("DequeuePullRequestInput requires id");
-    process.exit(1);
-  }
-  state.mergeQueueEntry = null;
-  result = { dequeuePullRequest: { clientMutationId: null } };
-} else result = { repository: { pullRequest: state } };
-appendFileSync(process.env.SHIPPING_STATE + '.calls', query + '\\n');
+const response = fakeGitHub(state, process.argv.slice(2));
 writeFileSync(process.env.SHIPPING_STATE, JSON.stringify(state));
-console.log(JSON.stringify({ data: result }));
+console.log(JSON.stringify(response));
 `
   );
   chmodSync(gh, 0o755);
diff --git a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test-helper.ts b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test-helper.ts
new file mode 100644
index 00000000..c65ca787
--- /dev/null
+++ b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test-helper.ts
@@ -0,0 +1,48 @@
+// The slice of GitHub's GraphQL schema the shipping service depends on: the
+// one required input field of each mutation, and the pull request field the
+// mutation clears. Regenerate the input names with
+//   gh api graphql -f query='{ __type(name:"DequeuePullRequestInput") { inputFields { name } } }'
+const MUTATIONS: Record<string, { input: string; clears: string }> = {
+  disablePullRequestAutoMerge: {
+    input: "pullRequestId",
+    clears: "autoMergeRequest",
+  },
+  dequeuePullRequest: { input: "id", clears: "mergeQueueEntry" },
+};
+
+const MUTATION =
+  /mutation\s*\w*\s*\(\s*\$id\s*:\s*ID!\s*\)\s*\{\s*(\w+)\(\s*input\s*:\s*\{\s*(\w+)\s*:\s*\$id\s*\}\s*\)/;
+
+/**
+ * Answer one `gh api graphql` call the way GitHub would.
+ *
+ * A query returns only the pull request fields it selects. A mutation is
+ * refused unless it declares `$id` as `ID!`, names a known mutation and its
+ * required input field, and carries this pull request's node id.
+ */
+export function fakeGitHub(
+  pullRequest: Record<string, unknown>,
+  argv: readonly string[]
+): { data: Record<string, unknown> } {
+  const query = argv.find((arg) => arg.startsWith("query=")) ?? "";
+  if (!query.includes("mutation")) {
+    // Drop nested selections, so `mergeQueueEntry { id }` does not count as
+    // selecting the pull request's own `id`.
+    const selection = query.replace(/\{[^{}]*\}/g, "");
+    const selected = Object.entries(pullRequest).filter(([field]) =>
+      new RegExp(`\\b${field}\\b`).test(selection)
+    );
+    return {
+      data: { repository: { pullRequest: Object.fromEntries(selected) } },
+    };
+  }
+  const [, name = "", input = ""] = MUTATION.exec(query) ?? [];
+  const contract = MUTATIONS[name];
+  if (!contract) throw new Error(`not a mutation GitHub accepts: ${query}`);
+  if (input !== contract.input)
+    throw new Error(`${name} input requires ${contract.input}, got ${input}`);
+  if (!argv.includes(`id=${pullRequest.id}`))
+    throw new Error("Could not resolve to a node with the given id");
+  pullRequest[contract.clears] = null;
+  return { data: { [name]: { clientMutationId: null } } };
+}
diff --git a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test.ts b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test.ts
index 6d698335..cd36f6a0 100644
--- a/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test.ts
+++ b/plugins/pstack/skills/poteto-mode/scripts/watch-pr/shipping.test.ts
@@ -8,6 +8,7 @@ import {
   type LandingRecord,
   type ShippingService,
 } from "./shipping.ts";
+import { fakeGitHub } from "./shipping.test-helper.ts";
 
 const context = { owner: "owner", repo: "repo", number: parsePrNumber(1) };
 const empty: LandingRecord = {
@@ -227,25 +228,14 @@ describe("shipping GitHub boundary", () => {
   });
 
   it("uses separate mutations for both pending mechanisms", async () => {
-    let autoMergeRequest: { enabledAt: string } | null = { enabledAt: "now" };
-    let mergeQueueEntry: { id: string } | null = { id: "queue" };
-    const service = new GhShippingService(async (args) => {
-      const query = args.find((arg) => arg.startsWith("query=")) ?? "";
-      if (query.includes("disablePullRequestAutoMerge")) {
-        expect(args).toContain("id=pr-id");
-        autoMergeRequest = null;
-        return {
-          data: { disablePullRequestAutoMerge: { clientMutationId: null } },
-        };
-      }
-      if (query.includes("dequeuePullRequest")) {
-        expect(query).toContain("dequeuePullRequest(input:{id:$id})");
-        expect(args).toContain("id=pr-id");
-        mergeQueueEntry = null;
-        return { data: { dequeuePullRequest: { clientMutationId: null } } };
-      }
-      return response({ ...raw, autoMergeRequest, mergeQueueEntry });
-    });
+    const pullRequest = {
+      ...raw,
+      autoMergeRequest: { enabledAt: "now" },
+      mergeQueueEntry: { id: "queue" },
+    };
+    const service = new GhShippingService(async (args) =>
+      fakeGitHub(pullRequest, args)
+    );
     const expected = await service.inspect(context);
     expect(await cancelPending(service, expected)).toEqual({
       kind: "cancelled",