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feat(messages): enforce 2000 character message length limit #324

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2248974
feat(messages): enforce 2000 character message length limit
Jun 28, 2026
9531dad
fix: use utf8.RuneCountInString instead of len([]rune()) for efficiency
Jun 28, 2026
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5 changes: 5 additions & 0 deletions pkg/hub/handlers_agent_messaging.go
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@ import (
"net/http"
"strings"
"time"
"unicode/utf8"

"github.com/GoogleCloudPlatform/scion/pkg/agent/state"
"github.com/GoogleCloudPlatform/scion/pkg/api"
Expand Down Expand Up @@ -68,6 +69,10 @@ func (s *Server) handleAgentOutboundMessage(w http.ResponseWriter, r *http.Reque
ValidationError(w, "msg is required", nil)
return
}
if msgLen := utf8.RuneCountInString(req.Msg); msgLen > messages.MaxMessageLength {
ValidationError(w, fmt.Sprintf("message exceeds %d character limit (current: %d chars). Consider splitting into multiple messages using multiple scion message invocations", messages.MaxMessageLength, msgLen), nil)
return
}
if req.Type == "" {
req.Type = "input-needed"
}
Expand Down
6 changes: 6 additions & 0 deletions pkg/messages/types.go
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,9 @@ import (
// Schema version for the structured message format.
const Version = 1

// Maximum length of the Msg field in characters for outbound messages.
const MaxMessageLength = 2000

// Maximum size of the Msg field in bytes.
const MaxMsgSize = 64 * 1024 // 64KB

Expand Down Expand Up @@ -127,6 +130,9 @@ func (m *StructuredMessage) Validate() error {
if m.Msg == "" {
return fmt.Errorf("msg field is required")
}
if len([]rune(m.Msg)) > MaxMessageLength {
return fmt.Errorf("message exceeds %d character limit (current: %d chars). Consider splitting into multiple messages using multiple scion message invocations", MaxMessageLength, len([]rune(m.Msg)))
}
Comment on lines +133 to +135

@gemini-code-assist gemini-code-assist Bot Jun 28, 2026

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high

The comment for MaxMessageLength on line 28 states:

// Maximum length of the Msg field in characters for outbound messages.

However, StructuredMessage.Validate() enforces this 2000-character limit unconditionally on all messages. This includes inbound user-to-agent messages (e.g., instructions from user:alice to agent:backend-dev). Since user prompts, code snippets, or logs can easily exceed 2000 characters, this unconditional limit will block valid inbound messages.

If the limit is indeed only intended for outbound messages (agent-to-user or agent-to-agent), we should conditionally apply this check in Validate(), for example by checking if the sender is an agent using SenderPrefix(m.Sender) == "agent".

Additionally, converting the string to []rune twice (once for the check and once for the error message) is inefficient and causes redundant memory allocations. We should use utf8.RuneCountInString(m.Msg) and store the result in a variable.

Note: You will need to import "unicode/utf8" in this file. Also, since validMsg() in types_test.go uses Sender: "user:alice", you will need to update the test cases ("msg exceeds character limit" and "msg at character limit") to set m.Sender = "agent:some-agent" so that the validation is triggered during the tests.

	if SenderPrefix(m.Sender) == "agent" {
		if msgLen := utf8.RuneCountInString(m.Msg); msgLen > MaxMessageLength {
			return fmt.Errorf("message exceeds %d character limit (current: %d chars). Consider splitting into multiple messages using multiple scion message invocations", MaxMessageLength, msgLen)
		}
	}

if len(m.Msg) > MaxMsgSize {
return fmt.Errorf("msg exceeds maximum size of %d bytes", MaxMsgSize)
}
Expand Down
19 changes: 19 additions & 0 deletions pkg/messages/types_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,25 @@ func TestStructuredMessage_Validate(t *testing.T) {
}
})

t.Run("msg exceeds character limit", func(t *testing.T) {
m := validMsg()
m.Msg = strings.Repeat("x", MaxMessageLength+1)
err := m.Validate()
if err == nil {
t.Error("expected error for msg exceeding character limit")
} else if !strings.Contains(err.Error(), "character limit") {
t.Errorf("error should mention character limit, got: %v", err)
}
})

t.Run("msg at character limit", func(t *testing.T) {
m := validMsg()
m.Msg = strings.Repeat("x", MaxMessageLength)
if err := m.Validate(); err != nil {
t.Errorf("unexpected error for msg at character limit: %v", err)
}
})

t.Run("msg too large", func(t *testing.T) {
m := validMsg()
m.Msg = strings.Repeat("x", MaxMsgSize+1)
Expand Down
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