feat: support VertexAI provider (#153)

* support: vertexai

fix

fix

set default for vertexai

added comment

fix

fix

* create schema

* fix README.md

* fix order

* added pupularity

* set tools if tools is exists

restore commentout

* fix comment

* set summarizer model

5 files changed, 91 insertions, 6 deletions

diff --git a/internal/llm/models/models.go b/internal/llm/models/models.go
index 16fd406c8..e4fe6603b 100644
--- a/internal/llm/models/models.go
+++ b/internal/llm/models/models.go
@@ -43,6 +43,7 @@ var ProviderPopularity = map[ModelProvider]int{
 	ProviderOpenRouter: 5,
 	ProviderBedrock:    6,
 	ProviderAzure:      7,
+	ProviderVertexAI:   8,
 }
 
 var SupportedModels = map[ModelID]Model{
@@ -95,4 +96,5 @@ func init() {
 	maps.Copy(SupportedModels, AzureModels)
 	maps.Copy(SupportedModels, OpenRouterModels)
 	maps.Copy(SupportedModels, XAIModels)
+	maps.Copy(SupportedModels, VertexAIGeminiModels)
 }
diff --git a/internal/llm/models/vertexai.go b/internal/llm/models/vertexai.go
new file mode 100644
index 000000000..d71dfc0be
--- /dev/null
+++ b/internal/llm/models/vertexai.go
@@ -0,0 +1,38 @@
+package models
+
+const (
+	ProviderVertexAI ModelProvider = "vertexai"
+
+	// Models
+	VertexAIGemini25Flash ModelID = "vertexai.gemini-2.5-flash"
+	VertexAIGemini25      ModelID = "vertexai.gemini-2.5"
+)
+
+var VertexAIGeminiModels = map[ModelID]Model{
+	VertexAIGemini25Flash: {
+		ID:                  VertexAIGemini25Flash,
+		Name:                "VertexAI: Gemini 2.5 Flash",
+		Provider:            ProviderVertexAI,
+		APIModel:            "gemini-2.5-flash-preview-04-17",
+		CostPer1MIn:         GeminiModels[Gemini25Flash].CostPer1MIn,
+		CostPer1MInCached:   GeminiModels[Gemini25Flash].CostPer1MInCached,
+		CostPer1MOut:        GeminiModels[Gemini25Flash].CostPer1MOut,
+		CostPer1MOutCached:  GeminiModels[Gemini25Flash].CostPer1MOutCached,
+		ContextWindow:       GeminiModels[Gemini25Flash].ContextWindow,
+		DefaultMaxTokens:    GeminiModels[Gemini25Flash].DefaultMaxTokens,
+		SupportsAttachments: true,
+	},
+	VertexAIGemini25: {
+		ID:                  VertexAIGemini25,
+		Name:                "VertexAI: Gemini 2.5 Pro",
+		Provider:            ProviderVertexAI,
+		APIModel:            "gemini-2.5-pro-preview-03-25",
+		CostPer1MIn:         GeminiModels[Gemini25].CostPer1MIn,
+		CostPer1MInCached:   GeminiModels[Gemini25].CostPer1MInCached,
+		CostPer1MOut:        GeminiModels[Gemini25].CostPer1MOut,
+		CostPer1MOutCached:  GeminiModels[Gemini25].CostPer1MOutCached,
+		ContextWindow:       GeminiModels[Gemini25].ContextWindow,
+		DefaultMaxTokens:    GeminiModels[Gemini25].DefaultMaxTokens,
+		SupportsAttachments: true,
+	},
+}
diff --git a/internal/llm/provider/gemini.go b/internal/llm/provider/gemini.go
index cc97463d4..8b8e33698 100644
--- a/internal/llm/provider/gemini.go
+++ b/internal/llm/provider/gemini.go
@@ -176,13 +176,16 @@ func (g *geminiClient) send(ctx context.Context, messages []message.Message, too
 
 	history := geminiMessages[:len(geminiMessages)-1] // All but last message
 	lastMsg := geminiMessages[len(geminiMessages)-1]
-	chat, _ := g.client.Chats.Create(ctx, g.providerOptions.model.APIModel, &genai.GenerateContentConfig{
+	config := &genai.GenerateContentConfig{
 		MaxOutputTokens: int32(g.providerOptions.maxTokens),
 		SystemInstruction: &genai.Content{
 			Parts: []*genai.Part{{Text: g.providerOptions.systemMessage}},
 		},
-		Tools: g.convertTools(tools),
-	}, history)
+	}
+	if len(tools) > 0 {
+		config.Tools = g.convertTools(tools)
+	}
+	chat, _ := g.client.Chats.Create(ctx, g.providerOptions.model.APIModel, config, history)
 
 	attempts := 0
 	for {
@@ -262,13 +265,16 @@ func (g *geminiClient) stream(ctx context.Context, messages []message.Message, t
 
 	history := geminiMessages[:len(geminiMessages)-1] // All but last message
 	lastMsg := geminiMessages[len(geminiMessages)-1]
-	chat, _ := g.client.Chats.Create(ctx, g.providerOptions.model.APIModel, &genai.GenerateContentConfig{
+	config := &genai.GenerateContentConfig{
 		MaxOutputTokens: int32(g.providerOptions.maxTokens),
 		SystemInstruction: &genai.Content{
 			Parts: []*genai.Part{{Text: g.providerOptions.systemMessage}},
 		},
-		Tools: g.convertTools(tools),
-	}, history)
+	}
+	if len(tools) > 0 {
+		config.Tools = g.convertTools(tools)
+	}
+	chat, _ := g.client.Chats.Create(ctx, g.providerOptions.model.APIModel, config, history)
 
 	attempts := 0
 	eventChan := make(chan ProviderEvent)
diff --git a/internal/llm/provider/provider.go b/internal/llm/provider/provider.go
index 6f2a20bd9..f21e051c2 100644
--- a/internal/llm/provider/provider.go
+++ b/internal/llm/provider/provider.go
@@ -123,6 +123,11 @@ func NewProvider(providerName models.ModelProvider, opts ...ProviderClientOption
 			options: clientOptions,
 			client:  newAzureClient(clientOptions),
 		}, nil
+	case models.ProviderVertexAI:
+		return &baseProvider[VertexAIClient]{
+			options: clientOptions,
+			client:  newVertexAIClient(clientOptions),
+		}, nil
 	case models.ProviderOpenRouter:
 		clientOptions.openaiOptions = append(clientOptions.openaiOptions,
 			WithOpenAIBaseURL("https://openrouter.ai/api/v1"),
diff --git a/internal/llm/provider/vertexai.go b/internal/llm/provider/vertexai.go
new file mode 100644
index 000000000..2a13a9572
--- /dev/null
+++ b/internal/llm/provider/vertexai.go
@@ -0,0 +1,34 @@
+package provider
+
+import (
+	"context"
+	"os"
+
+	"github.com/opencode-ai/opencode/internal/logging"
+	"google.golang.org/genai"
+)
+
+type VertexAIClient ProviderClient
+
+func newVertexAIClient(opts providerClientOptions) VertexAIClient {
+	geminiOpts := geminiOptions{}
+	for _, o := range opts.geminiOptions {
+		o(&geminiOpts)
+	}
+
+	client, err := genai.NewClient(context.Background(), &genai.ClientConfig{
+		Project:  os.Getenv("VERTEXAI_PROJECT"),
+		Location: os.Getenv("VERTEXAI_LOCATION"),
+		Backend:  genai.BackendVertexAI,
+	})
+	if err != nil {
+		logging.Error("Failed to create VertexAI client", "error", err)
+		return nil
+	}
+
+	return &geminiClient{
+		providerOptions: opts,
+		options:         geminiOpts,
+		client:          client,
+	}
+}