diff --git a/internal/protocol/constants.go b/internal/protocol/constants.go
new file mode 100644
index 0000000..015853c
--- /dev/null
+++ b/internal/protocol/constants.go
@@ -0,0 +1,31 @@
+package protocol
+
+// MessageType represents the type of protocol message
+type MessageType byte
+
+const (
+	ChallengeRequestType  MessageType = 0x01
+	SolutionRequestType   MessageType = 0x03
+	// Response types (for responses.go)
+	ChallengeResponseType MessageType = 0x02
+	QuoteResponseType     MessageType = 0x04
+	ErrorResponseType     MessageType = 0x05
+)
+
+// Error codes as defined in protocol specification
+const (
+	ErrMalformedMessage    = "MALFORMED_MESSAGE"
+	ErrInvalidChallenge    = "INVALID_CHALLENGE"
+	ErrInvalidSolution     = "INVALID_SOLUTION"
+	ErrExpiredChallenge    = "EXPIRED_CHALLENGE"
+	ErrRateLimited         = "RATE_LIMITED"
+	ErrServerError         = "SERVER_ERROR"
+	ErrTooManyConnections  = "TOO_MANY_CONNECTIONS"
+	ErrDifficultyTooHigh   = "DIFFICULTY_TOO_HIGH"
+)
+
+// Protocol constants
+const (
+	MaxPayloadSize = 8 * 1024 // 8KB maximum payload size
+	HeaderSize     = 5        // 1 byte type + 4 bytes length
+)