diff --git a/src/compare.ts b/src/compare.ts
index df7726c..071d160 100644
--- a/src/compare.ts
+++ b/src/compare.ts
@@ -36,6 +36,14 @@ export interface CompareResult {
   cases: CaseDelta[];
 }
 
+/** Raised when compare options fail validation. */
+export class CompareValidationError extends Error {
+  constructor(message: string) {
+    super(`[evalgate] invalid compare options: ${message}`);
+    this.name = "CompareValidationError";
+  }
+}
+
 /** Options for {@link compareRuns}. */
 export interface CompareOptions {
   /**
@@ -65,6 +73,9 @@ export function compareRuns(
   options: CompareOptions = {},
 ): CompareResult {
   const tolerance = options.tolerance ?? 0;
+  if (!Number.isFinite(tolerance) || tolerance < 0) {
+    throw new CompareValidationError("tolerance must be a non-negative finite number");
+  }
   const failOnPassFlip = options.failOnPassFlip !== false;
 
   const baseMap = byId(baseline.cases);
diff --git a/tests/compare.test.ts b/tests/compare.test.ts
index 093f6c6..67ac3d2 100644
--- a/tests/compare.test.ts
+++ b/tests/compare.test.ts
@@ -74,4 +74,16 @@ describe("compareRuns", () => {
     expect(changes.a).toBe("removed");
     expect(changes.b).toBe("added");
   });
+
+  it("rejects negative tolerance", () => {
+    const base = run([caseResult("a", 1)]);
+    const head = run([caseResult("a", 0.5)]);
+    expect(() => compareRuns(base, head, { tolerance: -0.5 })).toThrow(/tolerance/);
+  });
+
+  it("rejects NaN tolerance", () => {
+    const base = run([caseResult("a", 1)]);
+    const head = run([caseResult("a", 0.5)]);
+    expect(() => compareRuns(base, head, { tolerance: NaN })).toThrow(/tolerance/);
+  });
 });