feat(ai): add VertexAI provider for Claude models

Add support for Google Cloud Vertex AI as a provider to access Claude models
using Application Default Credentials (ADC). This allows users to route their
Fabric requests through Google Cloud Platform instead of directly to Anthropic,
enabling billing through GCP.

Features:
- Support for Claude models (Sonnet 4.5, Opus 4.5, Haiku 4.5, etc.) via Vertex AI
- Uses Google ADC for authentication (no API keys required)
- Configurable project ID and region (defaults to 'global' for cost optimization)
- Full support for streaming and non-streaming requests
- Implements complete ai.Vendor interface

Configuration:
- VERTEXAI_PROJECT_ID: GCP project ID (required)
- VERTEXAI_REGION: Vertex AI region (optional, defaults to 'global')

Closes #1570

internal/core/plugin_registry.go

@@ -23,6 +23,7 @@ import (
 	"github.com/danielmiessler/fabric/internal/plugins/ai/openai"
 	"github.com/danielmiessler/fabric/internal/plugins/ai/openai_compatible"
 	"github.com/danielmiessler/fabric/internal/plugins/ai/perplexity"
+	"github.com/danielmiessler/fabric/internal/plugins/ai/vertexai"
 	"github.com/danielmiessler/fabric/internal/plugins/strategy"
 
 	"github.com/samber/lo"
@@ -101,6 +102,7 @@ func NewPluginRegistry(db *fsdb.Db) (ret *PluginRegistry, err error) {
 		azure.NewClient(),
 		gemini.NewClient(),
 		anthropic.NewClient(),
+		vertexai.NewClient(),
 		lmstudio.NewClient(),
 		exolab.NewClient(),
 		perplexity.NewClient(), // Added Perplexity client

internal/plugins/ai/vertexai/vertexai.go

@@ -0,0 +1,138 @@
+package vertexai
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/anthropics/anthropic-sdk-go"
+	"github.com/anthropics/anthropic-sdk-go/vertex"
+	"github.com/danielmiessler/fabric/internal/chat"
+	"github.com/danielmiessler/fabric/internal/domain"
+	"github.com/danielmiessler/fabric/internal/plugins"
+)
+
+const (
+	cloudPlatformScope = "https://www.googleapis.com/auth/cloud-platform"
+	defaultRegion      = "global"
+	maxTokens          = 4096
+)
+
+// NewClient creates a new Vertex AI client for accessing Claude models via Google Cloud
+func NewClient() (ret *Client) {
+	vendorName := "VertexAI"
+	ret = &Client{}
+
+	ret.PluginBase = &plugins.PluginBase{
+		Name:            vendorName,
+		EnvNamePrefix:   plugins.BuildEnvVariablePrefix(vendorName),
+		ConfigureCustom: ret.configure,
+	}
+
+	ret.ProjectID = ret.AddSetupQuestion("Project ID", true)
+	ret.Region = ret.AddSetupQuestion("Region", false)
+	ret.Region.Value = defaultRegion
+
+	return
+}
+
+// Client implements the ai.Vendor interface for Google Cloud Vertex AI with Anthropic models
+type Client struct {
+	*plugins.PluginBase
+	ProjectID *plugins.SetupQuestion
+	Region    *plugins.SetupQuestion
+
+	client *anthropic.Client
+}
+
+func (c *Client) configure() error {
+	ctx := context.Background()
+	projectID := c.ProjectID.Value
+	region := c.Region.Value
+
+	// Initialize Anthropic client for Claude models via Vertex AI using Google ADC
+	vertexOpt := vertex.WithGoogleAuth(ctx, region, projectID, cloudPlatformScope)
+	client := anthropic.NewClient(vertexOpt)
+	c.client = &client
+
+	return nil
+}
+
+func (c *Client) ListModels() ([]string, error) {
+	// Return Claude models available on Vertex AI
+	return []string{
+		string(anthropic.ModelClaudeSonnet4_5),
+		string(anthropic.ModelClaudeOpus4_5),
+		string(anthropic.ModelClaudeHaiku4_5),
+		string(anthropic.ModelClaude3_7SonnetLatest),
+		string(anthropic.ModelClaude3_5HaikuLatest),
+	}, nil
+}
+
+func (c *Client) Send(ctx context.Context, msgs []*chat.ChatCompletionMessage, opts *domain.ChatOptions) (string, error) {
+	if c.client == nil {
+		return "", fmt.Errorf("VertexAI client not initialized")
+	}
+
+	// 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))
+	}
+
+	// Create the request
+	response, err := c.client.Messages.New(ctx, anthropic.MessageNewParams{
+		Model:       anthropic.Model(opts.Model),
+		MaxTokens:   int64(maxTokens),
+		Messages:    anthropicMessages,
+		Temperature: anthropic.Opt(opts.Temperature),
+	})
+
+	if err != nil {
+		return "", err
+	}
+
+	// Extract text from response
+	if len(response.Content) > 0 {
+		return response.Content[0].Text, nil
+	}
+
+	return "", fmt.Errorf("no content in response")
+}
+
+func (c *Client) SendStream(msgs []*chat.ChatCompletionMessage, opts *domain.ChatOptions, channel chan string) error {
+	if c.client == nil {
+		close(channel)
+		return fmt.Errorf("VertexAI client not initialized")
+	}
+
+	defer close(channel)
+	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))
+	}
+
+	// Create streaming request
+	stream := c.client.Messages.NewStreaming(ctx, anthropic.MessageNewParams{
+		Model:       anthropic.Model(opts.Model),
+		MaxTokens:   int64(maxTokens),
+		Messages:    anthropicMessages,
+		Temperature: anthropic.Opt(opts.Temperature),
+	})
+
+	// Process stream
+	for stream.Next() {
+		event := stream.Current()
+		if event.Delta.Text != "" {
+			channel <- event.Delta.Text
+		}
+	}
+
+	return stream.Err()
+}
+
+func (c *Client) NeedsRawMode(modelName string) bool {
+	return false
+}