refactor: extract message conversion logic to toMessages method in VertexAI client

- Extract message conversion into dedicated `toMessages` helper method
- Add proper role handling for system, user, and assistant messages
- Prepend system content to first user message per Anthropic format
- Enforce user/assistant message alternation with placeholder messages
- Skip empty messages during conversion processing
- Concatenate multiple text blocks in response output
- Add validation for empty message arrays before sending
- Handle edge case when only system content is provided

internal/plugins/ai/vertexai/vertexai.go

@@ -3,6 +3,7 @@ package vertexai
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	"github.com/anthropics/anthropic-sdk-go"
 	"github.com/anthropics/anthropic-sdk-go/vertex"
@@ -74,9 +75,9 @@ func (c *Client) Send(ctx context.Context, msgs []*chat.ChatCompletionMessage, o
 	}
 
 	// Convert chat messages to Anthropic format
-	anthropicMessages := make([]anthropic.MessageParam, len(msgs))
-	for i, msg := range msgs {
-		anthropicMessages[i] = anthropic.NewUserMessage(anthropic.NewTextBlock(msg.Content))
+	anthropicMessages := c.toMessages(msgs)
+	if len(anthropicMessages) == 0 {
+		return "", fmt.Errorf("no valid messages to send")
 	}
 
 	// Create the request
@@ -92,11 +93,18 @@ func (c *Client) Send(ctx context.Context, msgs []*chat.ChatCompletionMessage, o
 	}
 
 	// Extract text from response
-	if len(response.Content) > 0 {
-		return response.Content[0].Text, nil
+	var textParts []string
+	for _, block := range response.Content {
+		if block.Type == "text" && block.Text != "" {
+			textParts = append(textParts, block.Text)
+		}
 	}
 
-	return "", fmt.Errorf("no content in response")
+	if len(textParts) == 0 {
+		return "", fmt.Errorf("no content in response")
+	}
+
+	return strings.Join(textParts, ""), nil
 }
 
 func (c *Client) SendStream(msgs []*chat.ChatCompletionMessage, opts *domain.ChatOptions, channel chan string) error {
@@ -109,9 +117,9 @@ func (c *Client) SendStream(msgs []*chat.ChatCompletionMessage, opts *domain.Cha
 	ctx := context.Background()
 
 	// Convert chat messages to Anthropic format
-	anthropicMessages := make([]anthropic.MessageParam, len(msgs))
-	for i, msg := range msgs {
-		anthropicMessages[i] = anthropic.NewUserMessage(anthropic.NewTextBlock(msg.Content))
+	anthropicMessages := c.toMessages(msgs)
+	if len(anthropicMessages) == 0 {
+		return fmt.Errorf("no valid messages to send")
 	}
 
 	// Create streaming request
@@ -133,6 +141,70 @@ func (c *Client) SendStream(msgs []*chat.ChatCompletionMessage, opts *domain.Cha
 	return stream.Err()
 }
 
+func (c *Client) toMessages(msgs []*chat.ChatCompletionMessage) []anthropic.MessageParam {
+	// Convert messages to Anthropic format with proper role handling
+	// - System messages become part of the first user message
+	// - Messages must alternate user/assistant
+	// - Skip empty messages
+
+	var anthropicMessages []anthropic.MessageParam
+	var systemContent string
+
+	isFirstUserMessage := true
+	lastRoleWasUser := false
+
+	for _, msg := range msgs {
+		if strings.TrimSpace(msg.Content) == "" {
+			continue // Skip empty messages
+		}
+
+		switch msg.Role {
+		case chat.ChatMessageRoleSystem:
+			// Accumulate system content to prepend to first user message
+			if systemContent != "" {
+				systemContent += "\\n" + msg.Content
+			} else {
+				systemContent = msg.Content
+			}
+		case chat.ChatMessageRoleUser:
+			userContent := msg.Content
+			if isFirstUserMessage && systemContent != "" {
+				userContent = systemContent + "\\n\\n" + userContent
+				isFirstUserMessage = false
+			}
+			if lastRoleWasUser {
+				// Enforce alternation: add a minimal assistant message
+				anthropicMessages = append(anthropicMessages, anthropic.NewAssistantMessage(anthropic.NewTextBlock("Okay.")))
+			}
+			anthropicMessages = append(anthropicMessages, anthropic.NewUserMessage(anthropic.NewTextBlock(userContent)))
+			lastRoleWasUser = true
+		case chat.ChatMessageRoleAssistant:
+			// If first message is assistant and we have system content, prepend user message
+			if isFirstUserMessage && systemContent != "" {
+				anthropicMessages = append(anthropicMessages, anthropic.NewUserMessage(anthropic.NewTextBlock(systemContent)))
+				lastRoleWasUser = true
+				isFirstUserMessage = false
+			} else if !lastRoleWasUser && len(anthropicMessages) > 0 {
+				// Enforce alternation: add a minimal user message
+				anthropicMessages = append(anthropicMessages, anthropic.NewUserMessage(anthropic.NewTextBlock("Hi")))
+				lastRoleWasUser = true
+			}
+			anthropicMessages = append(anthropicMessages, anthropic.NewAssistantMessage(anthropic.NewTextBlock(msg.Content)))
+			lastRoleWasUser = false
+		default:
+			// Other roles are ignored for Anthropic's message structure
+			continue
+		}
+	}
+
+	// If only system content was provided, create a user message with it
+	if len(anthropicMessages) == 0 && systemContent != "" {
+		anthropicMessages = append(anthropicMessages, anthropic.NewUserMessage(anthropic.NewTextBlock(systemContent)))
+	}
+
+	return anthropicMessages
+}
+
 func (c *Client) NeedsRawMode(modelName string) bool {
 	return false
 }