More test cases:

src/index.ts

@@ -64,7 +64,11 @@ class UpdateFlakeLockAction extends DetSysAction {
     const commitMessage =
       this.commitMessage !== ""
         ? this.commitMessage
-        : renderCommitMessage(this.commitMessageTemplate, flakeDotLock);
+        : renderCommitMessage(
+            this.commitMessageTemplate,
+            flakeDir,
+            flakeDotLock,
+          );
 
     // Nix command of this form:
     // nix ${maybe nix options} flake ${"update" or "lock"} ${maybe --update-input flags} --commit-lock-file --commit-lockfile-summary ${commit message}

src/template.test.ts

@@ -1,31 +1,58 @@
-import { renderCommitMessage } from "./template.js";
-import { describe, expect, test } from "vitest";
+import { renderCommitMessage, renderPullRequestBody } from "./template.js";
+import { template } from "handlebars";
+import { Test, describe, expect, test } from "vitest";
 
 describe("templating", () => {
   test("commit message", () => {
     type TestCase = {
       template: string;
+      flakeDotLockDir: string;
       flakeDotLock: string;
       expected: string;
     };
 
     const testCases: TestCase[] = [
       {
-        template: "Updating lockfile at {{ flake_dot_lock }}",
+        template: "Updating flake.lock in dir {{ flake_dot_lock_dir }}",
+        flakeDotLockDir: ".",
         flakeDotLock: "./flake.lock",
-        expected: "Updating lockfile at ./flake.lock",
+        expected: "Updating flake.lock in dir .",
       },
       {
         template:
           "Here I go doing some updating of my pristine flake.lock at {{ flake_dot_lock }}",
+        flakeDotLockDir: "subflake",
         flakeDotLock: "subflake/flake.lock",
         expected:
           "Here I go doing some updating of my pristine flake.lock at subflake/flake.lock",
       },
+      {
+        template: "This variable doesn't exist: {{ foo }}",
+        flakeDotLockDir: ".",
+        flakeDotLock: "./flake.lock",
+        expected: "This variable doesn't exist: ",
+      },
     ];
 
-    testCases.forEach(({ template, flakeDotLock, expected }) => {
-      expect(renderCommitMessage(template, flakeDotLock)).toEqual(expected);
+    testCases.forEach(
+      ({ template, flakeDotLockDir, flakeDotLock, expected }) => {
+        expect(
+          renderCommitMessage(template, flakeDotLockDir, flakeDotLock),
+        ).toEqual(expected);
+      },
+    );
+  });
+
+  test("pull request body", () => {
+    type TestCase = {
+      template: string;
+      expected: string;
+    };
+
+    const testCases: TestCase[] = [];
+
+    testCases.forEach(({ template, expected }) => {
+      expect(renderPullRequestBody(template)).toEqual(expected);
     });
   });
 });

src/template.ts

@@ -1,10 +1,19 @@
 import Handlebars from "handlebars";
 
+export function renderPullRequestBody(template: string): string {
+  const tpl = Handlebars.compile(template);
+  return tpl({});
+}
+
 export function renderCommitMessage(
   template: string,
+  flakeDotLockDir: string,
   flakeDotLock: string,
 ): string {
-  return render(template, { flake_dot_lock: flakeDotLock });
+  return render(template, {
+    flake_dot_lock_dir: flakeDotLockDir,
+    flake_dot_lock: flakeDotLock,
+  });
 }
 
 function render(template: string, inputs: Record<string, string>): string {