fix: do not reuse message IDs

Author: johnstcn

x/acpio/acp_conversation.go

@@ -30,6 +30,7 @@ type ACPConversation struct {
 	cancel            context.CancelFunc
 	agentIO           ChunkableAgentIO
 	messages          []st.ConversationMessage
+	nextID            int  // monotonically increasing message ID
 	prompting         bool // true while agent is processing
 	streamingResponse strings.Builder
 	logger            *slog.Logger
@@ -104,18 +105,20 @@ func (c *ACPConversation) Send(messageParts ...st.MessagePart) error {
 		return st.ErrMessageValidationChanging
 	}
 	c.messages = append(c.messages, st.ConversationMessage{
-		Id:      len(c.messages),
+		Id:      c.nextID,
 		Role:    st.ConversationRoleUser,
 		Message: message,
 		Time:    c.clock.Now(),
 	})
+	c.nextID++
 	// Add placeholder for streaming agent response
 	c.messages = append(c.messages, st.ConversationMessage{
-		Id:      len(c.messages),
+		Id:      c.nextID,
 		Role:    st.ConversationRoleAgent,
 		Message: "",
 		Time:    c.clock.Now(),
 	})
+	c.nextID++
 	c.streamingResponse.Reset()
 	c.prompting = true
 	status := c.statusLocked()

x/acpio/acp_conversation_test.go

@@ -511,6 +511,25 @@ func Test_ErrorRemovesPartialMessage(t *testing.T) {
 	require.Len(t, messages, 1, "partial agent message should be removed on error")
 	assert.Equal(t, screentracker.ConversationRoleUser, messages[0].Role)
 	assert.Equal(t, "test", messages[0].Message)
+
+	// First exchange allocated IDs 0 (user) and 1 (agent, now removed).
+	assert.Equal(t, 0, messages[0].Id)
+
+	// Send a second message — IDs must not reuse the removed agent message's ID (1).
+	mock.mu.Lock()
+	mock.writeErr = nil
+	mock.writeBlock = nil
+	mock.writeStarted = nil
+	mock.mu.Unlock()
+
+	err := conv.Send(screentracker.MessagePartText{Content: "retry"})
+	require.NoError(t, err)
+
+	messages = conv.Messages()
+	require.Len(t, messages, 3, "first user + second user + second agent")
+	assert.Equal(t, 0, messages[0].Id, "original user message keeps its ID")
+	assert.Equal(t, 2, messages[1].Id, "new user message skips removed ID 1")
+	assert.Equal(t, 3, messages[2].Id, "new agent message continues sequence")
 }
 
 func Test_LateChunkAfterError_DoesNotCorruptUserMessage(t *testing.T) {