diff --git a/src/lib/__tests__/packetBuilder.spec.ts b/src/lib/__tests__/packetBuilder.spec.ts
new file mode 100644
index 00000000..c4a69a76
--- /dev/null
+++ b/src/lib/__tests__/packetBuilder.spec.ts
@@ -0,0 +1,120 @@
+import type { ISerializableCommand } from '../../commands'
+import { ProtocolVersion } from '../../enums'
+import { PacketBuilder } from '../packetBuilder'
+
+class FakeCommand implements ISerializableCommand {
+	static readonly rawName: string = 'FAKE'
+
+	constructor(public readonly length: number, public readonly value: number = 1) {}
+
+	public get lengthWithHeader(): number {
+		return this.length + 8
+	}
+
+	serialize = jest.fn((_version: ProtocolVersion): Buffer => {
+		return Buffer.alloc(this.length, this.value)
+	})
+}
+
+describe('PacketBuilder', () => {
+	it('No commands', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+		expect(builder.getPackets()).toHaveLength(0)
+	})
+
+	it('Single command', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const cmd = new FakeCommand(10)
+		builder.addCommand(cmd)
+
+		expect(builder.getPackets()).toHaveLength(1)
+		expect(builder.getPackets()).toHaveLength(1) // Ensure that calling it twice doesnt affect the output
+		expect(builder.getPackets()[0]).toHaveLength(cmd.lengthWithHeader)
+	})
+
+	it('Once finished cant add more commands', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const cmd = new FakeCommand(10)
+		builder.addCommand(cmd)
+
+		expect(builder.getPackets()).toHaveLength(1)
+
+		expect(() => builder.addCommand(cmd)).toThrow('finished')
+	})
+
+	it('Repeated command', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const cmd = new FakeCommand(10)
+		for (let i = 0; i < 5; i++) {
+			builder.addCommand(cmd)
+		}
+
+		expect(builder.getPackets()).toHaveLength(1)
+		expect(builder.getPackets()[0]).toHaveLength(cmd.lengthWithHeader * 5)
+	})
+
+	it('Repeated command spanning multiple packets', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const cmd = new FakeCommand(10)
+		for (let i = 0; i < 60; i++) {
+			builder.addCommand(cmd)
+		}
+
+		expect(cmd.lengthWithHeader).toBe(18)
+		expect(builder.getPackets()).toHaveLength(3)
+
+		expect(builder.getPackets()[0]).toHaveLength(cmd.lengthWithHeader * 27)
+		expect(builder.getPackets()[1]).toHaveLength(cmd.lengthWithHeader * 27)
+		expect(builder.getPackets()[2]).toHaveLength(cmd.lengthWithHeader * 6)
+	})
+
+	it('Command too large to fit a packets', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const cmd = new FakeCommand(501)
+		expect(() => builder.addCommand(cmd)).toThrow('too large')
+	})
+
+	it('Command same size as packet', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const cmd = new FakeCommand(500 - 8)
+		expect(cmd.lengthWithHeader).toBe(500)
+
+		builder.addCommand(cmd)
+		expect(builder.getPackets()).toHaveLength(1)
+		expect(builder.getPackets()[0]).toHaveLength(cmd.lengthWithHeader)
+	})
+
+	it('Commands of mixed sizes', () => {
+		const builder = new PacketBuilder(500, ProtocolVersion.V8_1_1)
+
+		const largeCmd = new FakeCommand(400)
+		const mediumCmd = new FakeCommand(80)
+		const smallCmd = new FakeCommand(10)
+
+		// packet 0:
+		builder.addCommand(mediumCmd)
+		builder.addCommand(smallCmd)
+
+		// packet 1:
+		builder.addCommand(largeCmd)
+		builder.addCommand(mediumCmd)
+
+		// packet 2:
+		builder.addCommand(smallCmd)
+		builder.addCommand(smallCmd)
+		builder.addCommand(largeCmd)
+
+		expect(builder.getPackets()).toHaveLength(3)
+		expect(builder.getPackets()[0]).toHaveLength(mediumCmd.lengthWithHeader + smallCmd.lengthWithHeader)
+		expect(builder.getPackets()[1]).toHaveLength(largeCmd.lengthWithHeader + mediumCmd.lengthWithHeader)
+		expect(builder.getPackets()[2]).toHaveLength(
+			smallCmd.lengthWithHeader + smallCmd.lengthWithHeader + largeCmd.lengthWithHeader
+		)
+	})
+})
diff --git a/src/lib/packetBuilder.ts b/src/lib/packetBuilder.ts
index 3a9945ab..d4db48ed 100644
--- a/src/lib/packetBuilder.ts
+++ b/src/lib/packetBuilder.ts
@@ -7,6 +7,7 @@ export class PacketBuilder {
 
 	readonly #completedBuffers: Buffer[] = []
 
+	#finished = false
 	#currentPacketBuffer: Buffer
 	#currentPacketFilled: number
 
@@ -19,16 +20,17 @@ export class PacketBuilder {
 	}
 
 	public addCommand(cmd: ISerializableCommand): void {
+		if (this.#finished) throw new Error('Packets have been finished')
+
 		if (typeof cmd.serialize !== 'function') {
 			throw new Error(`Command ${cmd.constructor.name} is not serializable`)
 		}
 
-		const payload = cmd.serialize(this.#protocolVersion)
-
 		const rawName: string = (cmd.constructor as any).rawName
+		const payload = cmd.serialize(this.#protocolVersion)
 
 		const totalLength = payload.length + 8
-		if (totalLength >= this.#maxPacketSize) {
+		if (totalLength > this.#maxPacketSize) {
 			throw new Error(`Comamnd ${cmd.constructor.name} is too large for a single packet`)
 		}
 
@@ -37,11 +39,9 @@ export class PacketBuilder {
 			this.#finishBuffer()
 		}
 
-		// Command name
+		// Add to packet
 		this.#currentPacketBuffer.writeUInt16BE(payload.length + 8, this.#currentPacketFilled + 0)
 		this.#currentPacketBuffer.write(rawName, this.#currentPacketFilled + 4, 4)
-
-		// Body
 		payload.copy(this.#currentPacketBuffer, this.#currentPacketFilled + 8)
 
 		this.#currentPacketFilled += totalLength
@@ -50,11 +50,13 @@ export class PacketBuilder {
 	public getPackets(): Buffer[] {
 		this.#finishBuffer(true)
 
+		this.#finished = true
+
 		return this.#completedBuffers
 	}
 
 	#finishBuffer(skipCreateNext?: boolean) {
-		if (this.#currentPacketFilled === 0) return
+		if (this.#currentPacketFilled === 0 || this.#finished) return
 
 		this.#completedBuffers.push(this.#currentPacketBuffer.subarray(0, this.#currentPacketFilled))