diff --git a/frontend/src/shared/state/listThunksRejectOnError.test.ts b/frontend/src/shared/state/listThunksRejectOnError.test.ts
new file mode 100644
index 00000000..4ca21540
--- /dev/null
+++ b/frontend/src/shared/state/listThunksRejectOnError.test.ts
@@ -0,0 +1,87 @@
+// Run: node --test (via frontend/scripts/run-tests.mjs)
+//
+// The chokepoint half of ENG-277. Every list thunk did:
+//
+//     const res = await fetch(url);
+//     const data = await res.json();
+//     return data.things;
+//
+// On a 401 that parses the ERROR body, `data.things` is undefined, the thunk RESOLVES, and the
+// reducer's `for (const t of action.payload)` throws inside immer. Guarding the reducer stops the
+// crash; guarding the thunk stops the bad value existing at all, which is the higher rung.
+//
+// These run the real payload creators against a stubbed fetch, so they test behaviour rather than
+// the presence of a line of source.
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { fetchTools } from './toolsSlice.ts';
+import { fetchSkills } from './skillsSlice.ts';
+import { fetchModes } from './modesSlice.ts';
+import { fetchOutputs } from './outputsSlice.ts';
+import { fetchWorkflows } from './workflowsSlice.ts';
+
+type Thunk = { (arg?: unknown): (d: unknown, g: unknown, e: unknown) => Promise<{ type: string; payload?: unknown }> };
+
+const LIST_THUNKS: Array<[string, Thunk]> = [
+  ['fetchTools', fetchTools as unknown as Thunk],
+  ['fetchSkills', fetchSkills as unknown as Thunk],
+  ['fetchModes', fetchModes as unknown as Thunk],
+  ['fetchOutputs', fetchOutputs as unknown as Thunk],
+  ['fetchWorkflows', fetchWorkflows as unknown as Thunk],
+];
+
+/** Run a thunk's payload creator with fetch stubbed, and report which lifecycle action it ended on. */
+async function runWith(thunk: Thunk, response: unknown): Promise<string> {
+  const realFetch = globalThis.fetch;
+  (globalThis as { fetch: unknown }).fetch = async () => response;
+  try {
+    // createAsyncThunk ignores getState here except for `condition`; loading:false lets it run.
+    const getState = () => ({
+      tools: { loading: false }, skills: { loading: false }, modes: { loading: false },
+      outputs: { loading: false }, workflows: { loading: false, items: {} },
+    });
+    const action = await thunk(undefined)(() => {}, getState, undefined);
+    return action?.type ?? 'no-action';
+  } finally {
+    (globalThis as { fetch: unknown }).fetch = realFetch;
+  }
+}
+
+const unauthorized = {
+  ok: false,
+  status: 401,
+  json: async () => ({ detail: 'Unauthorized' }),
+};
+
+for (const [name, thunk] of LIST_THUNKS) {
+  test(`${name} REJECTS on 401 instead of resolving undefined into the reducer`, async () => {
+    const type = await runWith(thunk, unauthorized);
+    assert.ok(
+      type.endsWith('/rejected'),
+      `${name} ended on "${type}"; a fulfilled action here hands the reducer an undefined payload`,
+    );
+  });
+}
+
+// The negative half: a healthy response must still reach the reducer, or "it rejects" would pass
+// on a thunk that had been broken into rejecting always.
+test('a healthy 200 still fulfils, for every list thunk', async () => {
+  const okBody = {
+    ok: true,
+    status: 200,
+    json: async () => ({
+      tools: [], skills: [], modes: [], builtin_defaults: {}, outputs: [], workflows: [],
+    }),
+  };
+  for (const [name, thunk] of LIST_THUNKS) {
+    const type = await runWith(thunk, okBody);
+    assert.ok(type.endsWith('/fulfilled'), `${name} ended on "${type}" for a good response`);
+  }
+});
+
+test('a 500 rejects too, so this is about status and not about the 401 shape', async () => {
+  const type = await runWith(fetchTools as unknown as Thunk, {
+    ok: false, status: 500, json: async () => ({ detail: 'boom' }),
+  });
+  assert.ok(type.endsWith('/rejected'), `ended on "${type}"`);
+});
diff --git a/frontend/src/shared/state/modesSlice.ts b/frontend/src/shared/state/modesSlice.ts
index 9a294d64..3679eb2a 100644
--- a/frontend/src/shared/state/modesSlice.ts
+++ b/frontend/src/shared/state/modesSlice.ts
@@ -29,6 +29,7 @@ export const fetchModes = createAsyncThunk(
   'modes/fetch',
   async () => {
     const res = await fetch(`${MODES_API}/list`);
+    if (!res.ok) throw new Error(`Modes list failed: ${res.status}`);
     const data = await res.json();
     return { modes: data.modes as Mode[], builtinDefaults: (data.builtin_defaults ?? {}) as Record<string, Mode> };
   },
diff --git a/frontend/src/shared/state/outputsSlice.ts b/frontend/src/shared/state/outputsSlice.ts
index 7ad98e62..0706490b 100644
--- a/frontend/src/shared/state/outputsSlice.ts
+++ b/frontend/src/shared/state/outputsSlice.ts
@@ -96,6 +96,7 @@ export const fetchOutputs = createAsyncThunk(
   'outputs/fetch',
   async () => {
     const res = await fetch(`${OUTPUTS_API}/list`);
+    if (!res.ok) throw new Error(`Outputs list failed: ${res.status}`);
     const data = await res.json();
     return data.outputs as Output[];
   },
diff --git a/frontend/src/shared/state/skillsSlice.ts b/frontend/src/shared/state/skillsSlice.ts
index cd6bec0c..22759c76 100644
--- a/frontend/src/shared/state/skillsSlice.ts
+++ b/frontend/src/shared/state/skillsSlice.ts
@@ -36,6 +36,7 @@ export const fetchSkills = createAsyncThunk(
   'skills/fetch',
   async () => {
     const res = await fetch(`${SKILLS_API}/list`);
+    if (!res.ok) throw new Error(`Skills list failed: ${res.status}`);
     const data = await res.json();
     return data.skills as Skill[];
   },
diff --git a/frontend/src/shared/state/toolsSlice.ts b/frontend/src/shared/state/toolsSlice.ts
index fabea840..a61ac51e 100644
--- a/frontend/src/shared/state/toolsSlice.ts
+++ b/frontend/src/shared/state/toolsSlice.ts
@@ -41,6 +41,7 @@ export const fetchTools = createAsyncThunk(
   'tools/fetch',
   async () => {
     const res = await fetch(`${TOOLS_API}/list`);
+    if (!res.ok) throw new Error(`Tools list failed: ${res.status}`);
     const data = await res.json();
     return data.tools as ToolDefinition[];
   },
diff --git a/frontend/src/shared/state/workflowsSlice.ts b/frontend/src/shared/state/workflowsSlice.ts
index 9dfb0d33..acba632c 100644
--- a/frontend/src/shared/state/workflowsSlice.ts
+++ b/frontend/src/shared/state/workflowsSlice.ts
@@ -282,6 +282,7 @@ export const fetchWorkflows = createAsyncThunk(
   async (dashboardId?: string) => {
     const url = dashboardId ? `${API}/list?dashboard_id=${encodeURIComponent(dashboardId)}` : `${API}/list`;
     const res = await fetch(url);
+    if (!res.ok) throw new Error(`Workflows list failed: ${res.status}`);
     const data = await res.json();
     return data.workflows as Workflow[];
   },