diff --git a/frontend/packages/swarm-ui/package.json b/frontend/packages/swarm-ui/package.json
index dae581ff..d83a859b 100644
--- a/frontend/packages/swarm-ui/package.json
+++ b/frontend/packages/swarm-ui/package.json
@@ -6,7 +6,8 @@
   "type": "module",
   "scripts": {
     "build": "node ./build.mjs",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "test": "node --test test/*.test.mjs"
   },
   "dependencies": {
     "@hive/shared": "*",
diff --git a/frontend/packages/swarm-ui/src/pages/review-settings/SettingsTypes.ts b/frontend/packages/swarm-ui/src/pages/review-settings/SettingsTypes.ts
index 0453e3ac..0e30e61f 100644
--- a/frontend/packages/swarm-ui/src/pages/review-settings/SettingsTypes.ts
+++ b/frontend/packages/swarm-ui/src/pages/review-settings/SettingsTypes.ts
@@ -8,8 +8,7 @@ export type Effort = (typeof EFFORTS)[number];
 
 /** Which runtime a classification runs on: the claude CLI on the Anthropic
  * API key, or opencode on an ACP provider. */
-export const RUNTIMES = ["api-key", "acp"] as const;
-export type Runtime = (typeof RUNTIMES)[number];
+export type Runtime = "api-key" | "acp";
 
 /** What the provider charges, in USD per million tokens. A cache price left
  * out counts as `input`. */
diff --git a/frontend/packages/swarm-ui/test/settings-types.test.mjs b/frontend/packages/swarm-ui/test/settings-types.test.mjs
new file mode 100644
index 00000000..e76cddfa
--- /dev/null
+++ b/frontend/packages/swarm-ui/test/settings-types.test.mjs
@@ -0,0 +1,119 @@
+// The classifier settings form's pure logic: the draft a stored document
+// becomes, the checks before a save, the request body, and what counts as a
+// change. Run by `npm test`, which Node runs with its built-in test runner and
+// type stripping, so the `.ts` module is imported as it is.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import {
+  changed,
+  draftOf,
+  maxValue,
+  priceValue,
+  settingsProblem,
+  toSettings,
+} from "../src/pages/review-settings/SettingsTypes.ts";
+
+const apiKey = {
+  model: "haiku",
+  effort: null,
+  max_concurrent_reviews: 2,
+  runtime: "api-key",
+  acp: null,
+};
+
+const acpStored = {
+  ...apiKey,
+  runtime: "acp",
+  acp: {
+    base_url: "https://llm.example/v1",
+    model: "Qwen/Qwen3-35B",
+    prices: { input: 0.5, output: 2, cache_read: 0.1 },
+  },
+};
+
+// An ACP draft over the API key settings, with every provider field filled.
+function acpDraft(over = {}) {
+  return {
+    ...draftOf(apiKey),
+    runtime: "acp",
+    acpUrl: "https://llm.example/v1",
+    acpModel: "Qwen/Qwen3-35B",
+    priceInput: "0.5",
+    priceOutput: "2",
+    ...over,
+  };
+}
+
+test("an unedited draft is valid and changes nothing", () => {
+  const d = draftOf(apiKey);
+  assert.equal(settingsProblem(d), null);
+  assert.equal(changed(d, apiKey), false);
+});
+
+test("the API key request leaves the provider out, so the stored one stays", () => {
+  const body = toSettings({ ...draftOf(acpStored), runtime: "api-key" });
+  assert.deepEqual(body, {
+    model: "haiku",
+    effort: null,
+    max_concurrent_reviews: 2,
+    runtime: "api-key",
+  });
+  assert.equal("acp" in body, false);
+});
+
+test("the ACP request carries the provider and its prices", () => {
+  const body = toSettings(acpDraft({ priceCacheRead: "0.1" }));
+  assert.equal(body.runtime, "acp");
+  assert.deepEqual(body.acp, {
+    base_url: "https://llm.example/v1",
+    model: "Qwen/Qwen3-35B",
+    prices: { input: 0.5, output: 2, cache_read: 0.1 },
+  });
+});
+
+test("a cache price left empty is left out of the request", () => {
+  const { prices } = toSettings(acpDraft()).acp;
+  assert.equal("cache_read" in prices, false);
+  assert.equal("cache_write" in prices, false);
+});
+
+test("the draft of a stored provider round-trips to the same request", () => {
+  const body = toSettings(draftOf(acpStored));
+  assert.deepEqual(body.acp, acpStored.acp);
+  assert.equal(changed(draftOf(acpStored), acpStored), false);
+});
+
+test("the provider fields are checked only on the ACP runtime", () => {
+  const empty = { ...draftOf(apiKey), acpUrl: "http://nope" };
+  assert.equal(settingsProblem(empty), null);
+  assert.match(settingsProblem({ ...empty, runtime: "acp" }), /https:\/\//);
+});
+
+test("the ACP checks name the field at fault", () => {
+  assert.match(settingsProblem(acpDraft({ acpUrl: "http://x" })), /https:\/\//);
+  assert.match(settingsProblem(acpDraft({ acpModel: "a b" })), /model/);
+  assert.match(settingsProblem(acpDraft({ priceOutput: "-1" })), /price/);
+  assert.match(settingsProblem(acpDraft({ priceInput: "" })), /price/);
+  assert.match(settingsProblem(acpDraft({ priceCacheWrite: "x" })), /cache/);
+  assert.equal(settingsProblem(acpDraft()), null);
+});
+
+test("switching the runtime is a change; edits to the hidden provider are not", () => {
+  const saved = apiKey;
+  assert.equal(changed({ ...draftOf(saved), runtime: "acp" }, saved), true);
+  assert.equal(
+    changed({ ...draftOf(saved), acpUrl: "https://x" }, saved),
+    false,
+  );
+  assert.equal(changed(acpDraft({ priceOutput: "3" }), acpStored), true);
+});
+
+test("the reviews limit and a price parse as the form needs", () => {
+  assert.equal(maxValue("8"), 8);
+  assert.equal(maxValue("9"), null);
+  assert.equal(maxValue("1.5"), null);
+  assert.equal(priceValue("0"), 0);
+  assert.equal(priceValue("0.25"), 0.25);
+  assert.equal(priceValue("-1"), null);
+  assert.equal(priceValue("1e3"), null);
+});
diff --git a/nix/checks.nix b/nix/checks.nix
index 6314207e..1bcf7fc3 100644
--- a/nix/checks.nix
+++ b/nix/checks.nix
@@ -369,6 +369,10 @@ in
     buildPhase = ''
       runHook preBuild
       npm run typecheck --workspace=packages/swarm-ui
+      # The package's own tests (`test/`, Node's built-in runner) ride this
+      # derivation: it already has the workspace installed, and a second
+      # `npm ci` for the same tree would only repeat it.
+      npm run test --workspace=packages/swarm-ui
       runHook postBuild
     '';