diff --git a/src/suite.ts b/src/suite.ts
index 214c139..f2644bb 100644
--- a/src/suite.ts
+++ b/src/suite.ts
@@ -1,6 +1,6 @@
 import { readFile } from "node:fs/promises";
 import { parse as parseYaml } from "yaml";
-import type { EvalCase, EvalSuite, ScorerSpec } from "./types.js";
+import type { EvalCase, EvalSuite, Message, ScorerSpec } from "./types.js";
 
 /** Raised when a suite fails structural validation. */
 export class SuiteValidationError extends Error {
@@ -32,6 +32,25 @@ function assert(condition: unknown, message: string): asserts condition {
   if (!condition) throw new SuiteValidationError(message);
 }
 
+const MESSAGE_ROLES = new Set<Message["role"]>(["system", "user", "assistant"]);
+
+function validateMessages(
+  messages: unknown[],
+  caseId: string,
+): EvalCase["input"]["messages"] {
+  assert(messages.length > 0, `case "${caseId}" input.messages must not be empty`);
+  return messages.map((m, i) => {
+    assert(m && typeof m === "object", `case "${caseId}" messages[${i}] must be an object`);
+    const msg = m as Record<string, unknown>;
+    assert(
+      typeof msg.role === "string" && MESSAGE_ROLES.has(msg.role as Message["role"]),
+      `case "${caseId}" messages[${i}].role must be system, user, or assistant`,
+    );
+    assert(typeof msg.content === "string", `case "${caseId}" messages[${i}].content must be a string`);
+    return { role: msg.role as Message["role"], content: msg.content };
+  });
+}
+
 /** Validate an arbitrary object into a typed {@link EvalSuite}. */
 export function validateSuite(data: unknown): EvalSuite {
   assert(data && typeof data === "object", "suite must be an object");
@@ -104,6 +123,9 @@ function validateCase(data: unknown, index: number, ids: Set<string>): EvalCase
   const hasPrompt = typeof input.prompt === "string";
   const hasMessages = Array.isArray(input.messages);
   assert(hasPrompt || hasMessages, `case "${c.id}" input needs a prompt or messages`);
+  if (hasPrompt) {
+    assert((input.prompt as string).length > 0, `case "${c.id}" input.prompt must not be empty`);
+  }
 
   assert(Array.isArray(c.scorers), `case "${c.id}" requires a scorers array`);
   assert((c.scorers as unknown[]).length > 0, `case "${c.id}" needs at least one scorer`);
@@ -115,7 +137,7 @@ function validateCase(data: unknown, index: number, ids: Set<string>): EvalCase
     description: typeof c.description === "string" ? c.description : undefined,
     input: {
       prompt: hasPrompt ? (input.prompt as string) : undefined,
-      messages: hasMessages ? (input.messages as EvalCase["input"]["messages"]) : undefined,
+      messages: hasMessages ? validateMessages(input.messages as unknown[], c.id as string) : undefined,
     },
     model: typeof c.model === "string" ? c.model : undefined,
     provider: typeof c.provider === "string" ? c.provider : undefined,
diff --git a/tests/suite.test.ts b/tests/suite.test.ts
index 70dcc91..63bf023 100644
--- a/tests/suite.test.ts
+++ b/tests/suite.test.ts
@@ -97,6 +97,39 @@ cases:
     ).toThrow(new RegExp(`suite\\.${field}`));
   });
 
+  it("rejects an empty-string prompt", () => {
+    expect(() =>
+      validateSuite({
+        name: "d",
+        cases: [{ id: "x", input: { prompt: "" }, scorers: [{ type: "regex" }] }],
+      }),
+    ).toThrow(/prompt/);
+  });
+
+  it("rejects an empty messages array", () => {
+    expect(() =>
+      validateSuite({
+        name: "d",
+        cases: [{ id: "x", input: { messages: [] }, scorers: [{ type: "regex" }] }],
+      }),
+    ).toThrow(/messages/);
+  });
+
+  it("rejects unknown message roles", () => {
+    expect(() =>
+      validateSuite({
+        name: "d",
+        cases: [
+          {
+            id: "x",
+            input: { messages: [{ role: "bogus", content: "hi" }] },
+            scorers: [{ type: "regex" }],
+          },
+        ],
+      }),
+    ).toThrow(/role/);
+  });
+
   it.each([
     ["temperature", Number.NaN],
     ["maxTokens", -1],