diff --git a/src/suite.ts b/src/suite.ts
index 214c139..5a006f8 100644
--- a/src/suite.ts
+++ b/src/suite.ts
@@ -91,6 +91,13 @@ function validateSamplingOptions(data: Record<string, unknown>, label: string):
   }
 }
 
+function validateTags(tags: unknown, caseId: string): string[] | undefined {
+  if (tags === undefined) return undefined;
+  if (typeof tags === "string") return [tags];
+  if (Array.isArray(tags)) return (tags as unknown[]).map(String);
+  throw new SuiteValidationError(`case "${caseId}" tags must be a string or array of strings`);
+}
+
 function validateCase(data: unknown, index: number, ids: Set<string>): EvalCase {
   assert(data && typeof data === "object", `cases[${index}] must be an object`);
   const c = data as Record<string, unknown>;
@@ -123,7 +130,7 @@ function validateCase(data: unknown, index: number, ids: Set<string>): EvalCase
     maxTokens: typeof c.maxTokens === "number" ? c.maxTokens : undefined,
     expected: typeof c.expected === "string" ? c.expected : undefined,
     scorers,
-    tags: Array.isArray(c.tags) ? (c.tags as string[]).map(String) : undefined,
+    tags: validateTags(c.tags, c.id as string),
     vars: isVars(c.vars) ? c.vars : undefined,
   };
 }
diff --git a/tests/suite.test.ts b/tests/suite.test.ts
index 70dcc91..4368605 100644
--- a/tests/suite.test.ts
+++ b/tests/suite.test.ts
@@ -54,6 +54,21 @@ cases:
     expect(() => validateSuite({ cases: [] })).toThrow(SuiteValidationError);
   });
 
+  it("coerces string tags to a single-element array", () => {
+    const suite = validateSuite({
+      name: "d",
+      cases: [
+        {
+          id: "arith",
+          tags: "arith",
+          input: { prompt: "2+2" },
+          scorers: [{ type: "regex", pattern: "4" }],
+        },
+      ],
+    });
+    expect(suite.cases[0]!.tags).toEqual(["arith"]);
+  });
+
   it("rejects duplicate case ids", () => {
     expect(() =>
       validateSuite({