diff --git a/pr-checks/post-comment.test.ts b/pr-checks/post-comment.test.ts
index 492c45f34..db6d45539 100644
--- a/pr-checks/post-comment.test.ts
+++ b/pr-checks/post-comment.test.ts
@@ -10,7 +10,9 @@ import { describe, it } from "node:test";
 import { type ApiClient } from "./api-client";
 import {
   findExistingComment,
+  type Options,
   parseOptions,
+  performAction,
   resolveToken,
 } from "./post-comment";
 
@@ -211,3 +213,87 @@ describe("findExistingComment", async () => {
     assert.equal(id, undefined);
   });
 });
+
+/** A baseline set of `Options`, for use in `performAction` tests. */
+const baseOptions: Options = {
+  body: "hello",
+  marker: "<!-- marker -->",
+  actionCondition: true,
+  actionIfTrue: "insert",
+  actionIfFalse: "delete",
+  issueId: 42,
+  repository: { owner: "owner", repo: "repo" },
+};
+
+describe("performAction", async () => {
+  await it("does nothing for 'none'", async () => {
+    const { client, calls } = fakeClient([]);
+    await performAction(client, "none", baseOptions);
+    assert.deepEqual(calls, {
+      createComment: [],
+      updateComment: [],
+      deleteComment: [],
+    });
+  });
+
+  await it("creates a new comment for 'insert', with the marker prepended", async () => {
+    const { client, calls } = fakeClient([]);
+    await performAction(client, "insert", baseOptions);
+    assert.deepEqual(calls.createComment, [
+      {
+        owner: "owner",
+        repo: "repo",
+        issue_number: 42,
+        body: "<!-- marker -->\nhello",
+      },
+    ]);
+    assert.deepEqual(calls.updateComment, []);
+    assert.deepEqual(calls.deleteComment, []);
+  });
+
+  await it("creates a new comment for 'upsert' when no existing comment is found", async () => {
+    const { client, calls } = fakeClient([{ id: 1, body: "unrelated" }]);
+    await performAction(client, "upsert", baseOptions);
+    assert.deepEqual(calls.createComment, [
+      {
+        owner: "owner",
+        repo: "repo",
+        issue_number: 42,
+        body: "<!-- marker -->\nhello",
+      },
+    ]);
+    assert.deepEqual(calls.updateComment, []);
+  });
+
+  await it("updates the existing comment for 'upsert' when one is found", async () => {
+    const { client, calls } = fakeClient([
+      { id: 7, body: "<!-- marker -->\nold" },
+    ]);
+    await performAction(client, "upsert", baseOptions);
+    assert.deepEqual(calls.updateComment, [
+      {
+        owner: "owner",
+        repo: "repo",
+        comment_id: 7,
+        body: "<!-- marker -->\nhello",
+      },
+    ]);
+    assert.deepEqual(calls.createComment, []);
+  });
+
+  await it("does nothing for 'delete' when no existing comment is found", async () => {
+    const { client, calls } = fakeClient([{ id: 1, body: "unrelated" }]);
+    await performAction(client, "delete", baseOptions);
+    assert.deepEqual(calls.deleteComment, []);
+  });
+
+  await it("deletes the existing comment for 'delete' when one is found", async () => {
+    const { client, calls } = fakeClient([
+      { id: 9, body: "<!-- marker -->\nold" },
+    ]);
+    await performAction(client, "delete", baseOptions);
+    assert.deepEqual(calls.deleteComment, [
+      { owner: "owner", repo: "repo", comment_id: 9 },
+    ]);
+  });
+});
diff --git a/pr-checks/post-comment.ts b/pr-checks/post-comment.ts
index 01b36cac7..6c1a1e5e5 100644
--- a/pr-checks/post-comment.ts
+++ b/pr-checks/post-comment.ts
@@ -172,16 +172,17 @@ export async function findExistingComment(
   return comments.find((comment) => comment.body?.startsWith(marker))?.id;
 }
 
-async function main(): Promise<number> {
-  const options = parseOptions();
-  const action = options.actionCondition
-    ? options.actionIfTrue
-    : options.actionIfFalse;
-  console.info(`Resolved action: ${action}`);
-
-  const token = resolveToken(process.env);
+/**
+ * Performs the resolved `action` against a (possibly pre-existing) marked comment, using
+ * `client` to talk to the GitHub API. This is the core dispatch logic of this script, extracted
+ * from `main` so it can be unit-tested with an injected (fake) `client`.
+ */
+export async function performAction(
+  client: ApiClient,
+  action: Action,
+  options: Options,
+): Promise<void> {
   const { issueId, marker, repository } = options;
-  const client = getApiClient(token);
   const body = `${marker}\n${options.body}`;
 
   let existingCommentId: number | undefined;
@@ -235,6 +236,19 @@ async function main(): Promise<number> {
       }
       break;
   }
+}
+
+async function main(): Promise<number> {
+  const options = parseOptions();
+  const action = options.actionCondition
+    ? options.actionIfTrue
+    : options.actionIfFalse;
+  console.info(`Resolved action: ${action}`);
+
+  const token = resolveToken(process.env);
+  const client = getApiClient(token);
+
+  await performAction(client, action, options);
 
   return 0;
 }