diff --git a/website/cypress/contract/oasst_api_contract_tests.cy.ts b/website/cypress/contract/oasst_api_contract_tests.cy.ts
index ff5bb156..2abbdf41 100644
--- a/website/cypress/contract/oasst_api_contract_tests.cy.ts
+++ b/website/cypress/contract/oasst_api_contract_tests.cy.ts
@@ -1,4 +1,4 @@
-import OasstApiClient from "src/lib/oasst_api_client";
+import OasstApiClient, { OasstError } from "src/lib/oasst_api_client";
 
 describe("Contract test for Oasst API", function () {
   // Assumes this is running the mock server.
@@ -23,7 +23,68 @@ describe("Contract test for Oasst API", function () {
     expect(await oasstApiClient.ackTask(task.id, "321")).to.be.null;
   });
 
-  // TODO(#354): Add test for 204
-  // TODO(#354): Add test for parsing >=300, throwing an OasstError
-  // TODO(#354): Add test for parsing >=300, throwing a generic error
+  // Note: Below here are unittests for OasstApiClient, not contract tests. They still fit here because
+  // the other contract tests are also testing the OasstApiClient.
+  const callApiMethod = () => {
+    return oasstApiClient.ackTask("", "");
+  };
+
+  it("should return null for a 204 response", async () => {
+    const mockFetch = cy.stub(global, "fetch").resolves({
+      status: 204,
+    });
+
+    const result = await callApiMethod();
+    assert.isNull(result);
+
+    mockFetch.restore();
+  });
+
+  it("should throw an OasstError with data from the response for a non-2XX response with a valid OasstError response", async () => {
+    const mockFetch = cy.stub(global, "fetch").resolves({
+      status: 400,
+      text: () =>
+        // Note: this is vulnerable to interface changes in the Oasst API.
+        // The python tests use a Pydantic model to ensure this object is always valid,
+        // but we don't have that here.
+        // This could be a case for generating Zod schemas from OpenAPI.
+        Promise.resolve(
+          JSON.stringify({
+            message: "error message",
+            error_code: 1000,
+          })
+        ),
+    });
+
+    try {
+      await callApiMethod();
+      assert.fail();
+    } catch (error) {
+      assert.instanceOf(error, OasstError);
+      if (error instanceof OasstError) {
+        assert.equal(error.errorCode, 1000);
+        assert.equal(error.message, "error message");
+        assert.equal(error.httpStatusCode, 400);
+      }
+    }
+
+    mockFetch.restore();
+  });
+
+  it("should throw a generic OasstError with the text from the response for a non-2XX response with an unknown format", async () => {
+    const mockFetch = cy.stub(global, "fetch").resolves({
+      status: 400,
+      text: () => Promise.resolve("error message"),
+    });
+
+    try {
+      await callApiMethod();
+      assert.fail();
+    } catch (error) {
+      assert.instanceOf(error, OasstError);
+      assert.equal(error.message, "error message");
+    }
+
+    mockFetch.restore();
+  });
 });