diff --git a/src/components/AddRepoButton.svelte b/src/components/AddRepoButton.svelte
index ca88e30da..04ae13185 100644
--- a/src/components/AddRepoButton.svelte
+++ b/src/components/AddRepoButton.svelte
@@ -9,8 +9,9 @@
 
   import { nodeRunning } from "@app/lib/events";
   import { invoke } from "@app/lib/invoke";
+  import { seedTarget } from "@app/lib/seedRepo";
   import useLocalStorage from "@app/lib/useLocalStorage.svelte";
-  import { parseRepositoryId, twemoji } from "@app/lib/utils";
+  import { twemoji } from "@app/lib/utils";
 
   import { announce } from "@app/components/AnnounceSwitch.svelte";
   import Button from "@app/components/Button.svelte";
@@ -56,20 +57,25 @@
   );
 
   async function submit() {
-    const trimmedRid = rid.trim();
-
-    if (trimmedRid === "") {
+    if (rid.trim() === "") {
       return;
     }
 
-    validationMessage = validate(trimmedRid);
-
-    if (validationMessage === undefined) {
-      await seed(trimmedRid);
-      await reload();
-      rid = "";
-      closeFocused();
+    const target = seedTarget(
+      rid,
+      seededNotReplicated,
+      repos.map(r => r.rid),
+    );
+    if ("error" in target) {
+      validationMessage = target.error;
+      return;
     }
+
+    validationMessage = undefined;
+    await seed(target.rid);
+    await reload();
+    rid = "";
+    closeFocused();
   }
 
   async function seed(rid: string) {
@@ -82,20 +88,6 @@
       console.error("Seeding failed", error);
     }
   }
-
-  function validate(rid: string): string | undefined {
-    const parsedRid = parseRepositoryId(rid);
-    if (parsedRid === undefined) {
-      return "RID is not valid";
-    }
-
-    if (seededNotReplicated.includes(rid)) {
-      return "This repo is already queued for fetching";
-    }
-    if (repos.map(r => r.rid).includes(rid)) {
-      return "This repo is already seeded";
-    }
-  }
 </script>
 
 <style>
diff --git a/src/lib/seedRepo.ts b/src/lib/seedRepo.ts
new file mode 100644
index 000000000..3d74793d8
--- /dev/null
+++ b/src/lib/seedRepo.ts
@@ -0,0 +1,23 @@
+import { parseRepositoryId } from "@app/lib/utils";
+
+// Validates a RID typed into the "Seed a repo" form. The form accepts a bare
+// `z…` id as well as `rad:z…`, so the result is the RID in the `rad:` form the
+// seeded and queued lists use.
+export function seedTarget(
+  input: string,
+  queued: string[],
+  seeded: string[],
+): { rid: string } | { error: string } {
+  const parsed = parseRepositoryId(input.trim());
+  if (parsed === undefined) {
+    return { error: "RID is not valid" };
+  }
+  const rid = `${parsed.prefix}${parsed.pubkey}`;
+  if (queued.includes(rid)) {
+    return { error: "This repo is already queued for fetching" };
+  }
+  if (seeded.includes(rid)) {
+    return { error: "This repo is already seeded" };
+  }
+  return { rid };
+}
diff --git a/tests/unit/seedRepo.test.ts b/tests/unit/seedRepo.test.ts
new file mode 100644
index 000000000..e3f6c5274
--- /dev/null
+++ b/tests/unit/seedRepo.test.ts
@@ -0,0 +1,31 @@
+import { describe, expect, test } from "vitest";
+
+import { seedTarget } from "@app/lib/seedRepo";
+
+const rid = "rad:z3fpY7nttPPa6MBnAv2DccHzQJnqe";
+const bare = "z3fpY7nttPPa6MBnAv2DccHzQJnqe";
+
+describe("seedTarget", () => {
+  test.each([rid, bare, `  ${rid}  `])("accepts %j as %j", input => {
+    expect(seedTarget(input, [], [])).toEqual({ rid });
+  });
+
+  test.each(["", "rad:", "rad:z0OIl", "not a rid", "rad:zabc"])(
+    "rejects %j",
+    input => {
+      expect(seedTarget(input, [], [])).toEqual({ error: "RID is not valid" });
+    },
+  );
+
+  test.each([rid, bare])("reports %j as already queued", input => {
+    expect(seedTarget(input, [rid], [])).toEqual({
+      error: "This repo is already queued for fetching",
+    });
+  });
+
+  test.each([rid, bare])("reports %j as already seeded", input => {
+    expect(seedTarget(input, [], [rid])).toEqual({
+      error: "This repo is already seeded",
+    });
+  });
+});