diff --git a/internal/protocol/encode_decode_test.go b/internal/protocol/encode_decode_test.go
new file mode 100644
index 0000000..b302a47
--- /dev/null
+++ b/internal/protocol/encode_decode_test.go
@@ -0,0 +1,157 @@
+package protocol
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"hash-of-wisdom/internal/pow/challenge"
+	"hash-of-wisdom/internal/quotes"
+)
+
+func TestSymmetricEncoding_ChallengeRequest(t *testing.T) {
+	// Create a challenge request
+	req := &ChallengeRequest{}
+
+	// Encode it
+	var buf bytes.Buffer
+	err := req.Encode(&buf)
+	require.NoError(t, err)
+
+	// Decode it back
+	decoder := NewMessageDecoder()
+	msg, err := decoder.Decode(&buf)
+	require.NoError(t, err)
+	assert.Equal(t, ChallengeRequestType, msg.Type)
+	assert.Equal(t, uint32(0), msg.PayloadLength)
+
+	// Decode the request payload
+	decodedReq := &ChallengeRequest{}
+	err = decodedReq.Decode(msg.PayloadStream)
+	require.NoError(t, err)
+}
+
+func TestSymmetricEncoding_SolutionRequest(t *testing.T) {
+	// Create a solution request
+	req := &SolutionRequest{
+		Challenge: challenge.Challenge{
+			Timestamp:  1640995200,
+			Difficulty: 4,
+			Resource:   "quotes",
+			Random:     []byte("test"),
+			HMAC:       []byte("test"),
+		},
+		Nonce: 12345,
+	}
+
+	// Encode it
+	var buf bytes.Buffer
+	err := req.Encode(&buf)
+	require.NoError(t, err)
+
+	// Decode it back
+	decoder := NewMessageDecoder()
+	msg, err := decoder.Decode(&buf)
+	require.NoError(t, err)
+	assert.Equal(t, SolutionRequestType, msg.Type)
+	assert.Greater(t, msg.PayloadLength, uint32(0))
+
+	// Decode the request payload
+	decodedReq := &SolutionRequest{}
+	err = decodedReq.Decode(msg.PayloadStream)
+	require.NoError(t, err)
+	assert.Equal(t, req.Nonce, decodedReq.Nonce)
+	assert.Equal(t, req.Challenge.Timestamp, decodedReq.Challenge.Timestamp)
+}
+
+func TestSymmetricEncoding_ChallengeResponse(t *testing.T) {
+	// Create a challenge response
+	resp := &ChallengeResponse{
+		Challenge: &challenge.Challenge{
+			Timestamp:  1640995200,
+			Difficulty: 4,
+			Resource:   "quotes",
+			Random:     []byte("test"),
+			HMAC:       []byte("test"),
+		},
+	}
+
+	// Encode it
+	var buf bytes.Buffer
+	err := resp.Encode(&buf)
+	require.NoError(t, err)
+
+	// Decode it back
+	decoder := NewMessageDecoder()
+	msg, err := decoder.Decode(&buf)
+	require.NoError(t, err)
+	assert.Equal(t, ChallengeResponseType, msg.Type)
+	assert.Greater(t, msg.PayloadLength, uint32(0))
+
+	// Decode the response payload
+	decodedResp := &ChallengeResponse{}
+	err = decodedResp.Decode(msg.PayloadStream)
+	require.NoError(t, err)
+	assert.Equal(t, resp.Challenge.Timestamp, decodedResp.Challenge.Timestamp)
+	assert.Equal(t, resp.Challenge.Difficulty, decodedResp.Challenge.Difficulty)
+}
+
+func TestSymmetricEncoding_SolutionResponse(t *testing.T) {
+	// Create a solution response
+	resp := &SolutionResponse{
+		Quote: &quotes.Quote{
+			Text:   "Test quote",
+			Author: "Test Author",
+		},
+	}
+
+	// Encode it
+	var buf bytes.Buffer
+	err := resp.Encode(&buf)
+	require.NoError(t, err)
+
+	// Decode it back
+	decoder := NewMessageDecoder()
+	msg, err := decoder.Decode(&buf)
+	require.NoError(t, err)
+	assert.Equal(t, QuoteResponseType, msg.Type)
+	assert.Greater(t, msg.PayloadLength, uint32(0))
+
+	// Decode the response payload
+	decodedResp := &SolutionResponse{}
+	err = decodedResp.Decode(msg.PayloadStream)
+	require.NoError(t, err)
+	assert.Equal(t, resp.Quote.Text, decodedResp.Quote.Text)
+	assert.Equal(t, resp.Quote.Author, decodedResp.Quote.Author)
+}
+
+func TestSymmetricEncoding_ErrorResponse(t *testing.T) {
+	// Create an error response
+	resp := &ErrorResponse{
+		Code:    "TEST_ERROR",
+		Message: "Test error message",
+		Details: map[string]string{"key": "value"},
+	}
+
+	// Encode it
+	var buf bytes.Buffer
+	err := resp.Encode(&buf)
+	require.NoError(t, err)
+
+	// Decode it back
+	decoder := NewMessageDecoder()
+	msg, err := decoder.Decode(&buf)
+	require.NoError(t, err)
+	assert.Equal(t, ErrorResponseType, msg.Type)
+	assert.Greater(t, msg.PayloadLength, uint32(0))
+
+	// Decode the response payload
+	decodedResp := &ErrorResponse{}
+	err = decodedResp.Decode(msg.PayloadStream)
+	require.NoError(t, err)
+	assert.Equal(t, resp.Code, decodedResp.Code)
+	assert.Equal(t, resp.Message, decodedResp.Message)
+	assert.Equal(t, resp.Details, decodedResp.Details)
+}
diff --git a/internal/protocol/roundtrip_test.go b/internal/protocol/roundtrip_test.go
index d32f3d8..ef4b5fa 100644
--- a/internal/protocol/roundtrip_test.go
+++ b/internal/protocol/roundtrip_test.go
@@ -113,16 +113,6 @@ func TestChallengeRequest_EmptyPayload(t *testing.T) {
 	require.NoError(t, err)
 }
 
-func TestMessageDecoder_RejectsResponseTypes(t *testing.T) {
-	decoder := NewMessageDecoder()
-
-	data := []byte{byte(ErrorResponseType), 0x00, 0x00, 0x00, 0x05, 'h', 'e', 'l', 'l', 'o'}
-	buf := bytes.NewBuffer(data)
-
-	_, err := decoder.Decode(buf)
-	assert.Error(t, err)
-	assert.Contains(t, err.Error(), "invalid message type")
-}
 
 func TestPayloadStream_LimitedRead(t *testing.T) {
 	decoder := NewMessageDecoder()
diff --git a/internal/protocol/spec_compliance_test.go b/internal/protocol/spec_compliance_test.go
new file mode 100644
index 0000000..4eef5ae
--- /dev/null
+++ b/internal/protocol/spec_compliance_test.go
@@ -0,0 +1,177 @@
+package protocol
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"hash-of-wisdom/internal/pow/challenge"
+	"hash-of-wisdom/internal/quotes"
+)
+
+// TestSpecCompliance_ChallengeResponse verifies challenge response matches PROTOCOL.md format
+func TestSpecCompliance_ChallengeResponse(t *testing.T) {
+	resp := &ChallengeResponse{
+		Challenge: &challenge.Challenge{
+			Timestamp:  1640995200,
+			Difficulty: 4,
+			Resource:   "quotes",
+			Random:     []byte{0xa1, 0xb2, 0xc3, 0xd4, 0xe5, 0xf6},
+			HMAC:       []byte("base64url_encoded_signature"),
+		},
+	}
+
+	var buf bytes.Buffer
+	err := resp.Encode(&buf)
+	require.NoError(t, err)
+
+	// Skip header (5 bytes) and get payload
+	header := buf.Bytes()[:5]
+	payload := buf.Bytes()[5:]
+
+	// Verify header format
+	assert.Equal(t, byte(ChallengeResponseType), header[0])
+	assert.Equal(t, uint32(len(payload)), uint32(header[1])<<24|uint32(header[2])<<16|uint32(header[3])<<8|uint32(header[4]))
+
+	// Verify JSON payload matches spec format
+	var decoded map[string]interface{}
+	err = json.Unmarshal(payload, &decoded)
+	require.NoError(t, err)
+
+	// Check required fields from spec
+	assert.Contains(t, decoded, "timestamp")
+	assert.Contains(t, decoded, "difficulty")
+	assert.Contains(t, decoded, "resource")
+	assert.Contains(t, decoded, "random")
+	assert.Contains(t, decoded, "hmac")
+
+	assert.Equal(t, float64(1640995200), decoded["timestamp"])
+	assert.Equal(t, float64(4), decoded["difficulty"])
+	assert.Equal(t, "quotes", decoded["resource"])
+}
+
+// TestSpecCompliance_SolutionRequest verifies solution request matches PROTOCOL.md format
+func TestSpecCompliance_SolutionRequest(t *testing.T) {
+	req := &SolutionRequest{
+		Challenge: challenge.Challenge{
+			Timestamp:  1640995200,
+			Difficulty: 4,
+			Resource:   "quotes",
+			Random:     []byte{0xa1, 0xb2, 0xc3, 0xd4, 0xe5, 0xf6},
+			HMAC:       []byte("base64url_encoded_signature"),
+		},
+		Nonce: 12345,
+	}
+
+	var buf bytes.Buffer
+	err := req.Encode(&buf)
+	require.NoError(t, err)
+
+	// Skip header and get payload
+	payload := buf.Bytes()[5:]
+
+	// Verify JSON payload matches spec format
+	var decoded map[string]interface{}
+	err = json.Unmarshal(payload, &decoded)
+	require.NoError(t, err)
+
+	// Check required top-level fields
+	assert.Contains(t, decoded, "challenge")
+	assert.Contains(t, decoded, "nonce")
+	assert.Equal(t, float64(12345), decoded["nonce"])
+
+	// Check challenge structure
+	challenge := decoded["challenge"].(map[string]interface{})
+	assert.Contains(t, challenge, "timestamp")
+	assert.Contains(t, challenge, "difficulty")
+	assert.Contains(t, challenge, "resource")
+	assert.Contains(t, challenge, "random")
+	assert.Contains(t, challenge, "hmac")
+}
+
+// TestSpecCompliance_QuoteResponse verifies quote response matches PROTOCOL.md format
+func TestSpecCompliance_QuoteResponse(t *testing.T) {
+	resp := &SolutionResponse{
+		Quote: &quotes.Quote{
+			Text:   "The only way to do great work is to love what you do.",
+			Author: "Steve Jobs",
+		},
+	}
+
+	var buf bytes.Buffer
+	err := resp.Encode(&buf)
+	require.NoError(t, err)
+
+	// Skip header and get payload
+	payload := buf.Bytes()[5:]
+
+	// Verify JSON payload matches spec format
+	var decoded map[string]interface{}
+	err = json.Unmarshal(payload, &decoded)
+	require.NoError(t, err)
+
+	// Check required fields from spec
+	assert.Contains(t, decoded, "text")
+	assert.Contains(t, decoded, "author")
+	assert.Equal(t, "The only way to do great work is to love what you do.", decoded["text"])
+	assert.Equal(t, "Steve Jobs", decoded["author"])
+}
+
+// TestSpecCompliance_ErrorResponse verifies error response matches PROTOCOL.md format
+func TestSpecCompliance_ErrorResponse(t *testing.T) {
+	resp := &ErrorResponse{
+		Code:       "INVALID_SOLUTION",
+		Message:    "The provided PoW solution is incorrect",
+		RetryAfter: 30,
+		Details:    map[string]string{"reason": "hash verification failed"},
+	}
+
+	var buf bytes.Buffer
+	err := resp.Encode(&buf)
+	require.NoError(t, err)
+
+	// Skip header and get payload
+	payload := buf.Bytes()[5:]
+
+	// Verify JSON payload matches spec format
+	var decoded map[string]interface{}
+	err = json.Unmarshal(payload, &decoded)
+	require.NoError(t, err)
+
+	// Check required fields from spec
+	assert.Contains(t, decoded, "code")
+	assert.Contains(t, decoded, "message")
+	assert.Equal(t, "INVALID_SOLUTION", decoded["code"])
+	assert.Equal(t, "The provided PoW solution is incorrect", decoded["message"])
+
+	// Check optional fields
+	assert.Contains(t, decoded, "retry_after")
+	assert.Contains(t, decoded, "details")
+	assert.Equal(t, float64(30), decoded["retry_after"])
+}
+
+// TestSpecCompliance_MessageSizeLimits verifies 8KB payload limit
+func TestSpecCompliance_MessageSizeLimits(t *testing.T) {
+	decoder := NewMessageDecoder()
+
+	// Create a message that exceeds 8KB payload limit
+	largePayload := make([]byte, MaxPayloadSize+1)
+	for i := range largePayload {
+		largePayload[i] = 'A'
+	}
+
+	// Create message with oversized payload
+	var buf bytes.Buffer
+	buf.WriteByte(byte(ChallengeRequestType))
+	buf.Write([]byte{0x00, 0x00, 0x20, 0x01}) // 8193 bytes (8KB + 1)
+	buf.Write(largePayload)
+
+	// Should reject oversized payload
+	_, err := decoder.Decode(&buf)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "payload length")
+	assert.Contains(t, err.Error(), "exceeds maximum")
+}