diff --git a/packages/crypto-config/source/configuration.test.ts b/packages/crypto-config/source/configuration.test.ts
index 2e74520b4..b0a52c318 100644
--- a/packages/crypto-config/source/configuration.test.ts
+++ b/packages/crypto-config/source/configuration.test.ts
@@ -42,6 +42,21 @@ describe<{
 		);
 	});
 
+	const withRoundValidators = (value: number) => ({
+		...cryptoJson,
+		milestones: cryptoJson.milestones.map((milestone, index) =>
+			index === 1 ? { ...milestone, roundValidators: value } : milestone,
+		),
+	});
+
+	it("should reject a milestone with roundValidators above the 64 wire-format cap", ({ configManager }) => {
+		assert.throws(() => configManager.setConfig(withRoundValidators(65)));
+	});
+
+	it("should accept a milestone with roundValidators at the 64 cap", ({ configManager }) => {
+		assert.not.throws(() => configManager.setConfig(withRoundValidators(64)));
+	});
+
 	it("should throw on set before config is initialized", () => {
 		const fresh = new Configuration();
 
diff --git a/packages/crypto-config/source/schemas.ts b/packages/crypto-config/source/schemas.ts
index 9905d0e47..ceebb8161 100644
--- a/packages/crypto-config/source/schemas.ts
+++ b/packages/crypto-config/source/schemas.ts
@@ -77,7 +77,10 @@ const milestone: AnySchemaObject = {
 		height: { minimum: 0, type: "integer" },
 		p2p: milestoneP2p,
 		reward: { type: "string" },
-		roundValidators: { minimum: 0, type: "integer" },
+		// Hard ceiling: the consensus wire format packs the signer bitmap into a fixed uint64
+		// (serializer.ts / validatorSetPack), so index 64 (the 65th validator) overflows on
+		// serialize. Guard it here until the wire format is redesigned as a variable bitset.
+		roundValidators: { maximum: 64, minimum: 0, type: "integer" },
 		satoshi: milestoneSatoshi,
 		snapshot: milestoneSnapshot,
 		timeouts: milestoneTimeouts,