diff --git a/README.md b/README.md
index a60637a7c..dfca38bbe 100644
--- a/README.md
+++ b/README.md
@@ -383,6 +383,7 @@ PicoClaw supports 30+ LLM providers through the `model_list` configuration. Use
 | [Azure OpenAI](https://portal.azure.com/) | `azure/` | Required | Enterprise Azure deployment |
 | [GitHub Copilot](https://github.com/features/copilot) | `github-copilot/` | OAuth | Device code login |
 | [Antigravity](https://console.cloud.google.com/) | `antigravity/` | OAuth | Google Cloud AI |
+| [Google Vertex AI](https://cloud.google.com/vertex-ai) | `vertex/` | Required / OAuth | GCP Vertex AI |
 | [AWS Bedrock](https://console.aws.amazon.com/bedrock)* | `bedrock/` | AWS credentials | Claude, Llama, Mistral on AWS |
 
 > \* AWS Bedrock requires build tag: `go build -tags bedrock`. Set `api_base` to a region name (e.g., `us-east-1`) for automatic endpoint resolution across all AWS partitions (aws, aws-cn, aws-us-gov). When using a full endpoint URL instead, you must also configure `AWS_REGION` via environment variable or AWS config/profile.
@@ -416,6 +417,27 @@ PicoClaw supports 30+ LLM providers through the `model_list` configuration. Use
 }
 ```
 
+**Google AI Studio / Vertex AI (Custom Endpoints):**
+You can use the `vertex` provider to query Google AI Studio endpoints or custom Vertex AI URLs. This supports using an `api_key` as a query parameter (`?key=...`).
+```json
+{
+  "model_list": [
+    {
+      "model_name": "gemini-pro-studio",
+      "model": "vertex/gemini-1.5-pro",
+      "api_base": "https://generativelanguage.googleapis.com/v1beta/models",
+      "api_key": "YOUR_GEMINI_API_KEY"
+    },
+    {
+      "model_name": "gemini-pro-vertex",
+      "model": "vertex/gemini-3-pro-preview",
+      "api_base": "https://us-central1-aiplatform.googleapis.com/v1/publishers/google/models",
+      "api_key": "YOUR_VERTEX_API_KEY"
+    }
+  ]
+}
+```
+
 For full provider configuration details, see [Providers & Models](docs/providers.md).
 
 </details>
diff --git a/pkg/providers/vertex/provider.go b/pkg/providers/vertex/provider.go
index 45b67285a..59a3060b2 100644
--- a/pkg/providers/vertex/provider.go
+++ b/pkg/providers/vertex/provider.go
@@ -6,6 +6,7 @@
 package vertex
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"encoding/json"
@@ -73,19 +74,44 @@ func NewProvider(apiKey, apiBase, proxy, projectID, region string, opts ...Optio
 }
 
 // buildURL constructs the Vertex AI REST endpoint URL.
-func (p *Provider) buildURL(model string) string {
-	if p.apiBase != "" {
-		if strings.Contains(p.apiBase, "generateContent") {
-			return p.apiBase
-		}
-		return fmt.Sprintf("%s/models/%s:generateContent", p.apiBase, model)
+func (p *Provider) buildURL(model string, action string) string {
+	if action == "" {
+		action = "generateContent"
 	}
 
-	region := p.region
-	if region == "" {
-		region = "us-central1"
+	var baseURL string
+	if p.apiBase != "" {
+		if strings.Contains(p.apiBase, "generateContent") {
+			baseURL = p.apiBase
+		} else {
+			baseURL = fmt.Sprintf("%s/%s:%s", p.apiBase, model, action)
+		}
+	} else {
+		region := p.region
+		if region == "" {
+			region = "us-central1"
+		}
+		baseURL = fmt.Sprintf("https://%s-aiplatform.googleapis.com/v1/projects/%s/locations/%s/publishers/google/models/%s:%s", region, p.projectID, region, model, action)
 	}
-	return fmt.Sprintf("https://%s-aiplatform.googleapis.com/v1/projects/%s/locations/%s/publishers/google/models/%s:generateContent", region, p.projectID, region, model)
+
+	// Only append ?key= for custom apiBase endpoints
+	if p.apiBase != "" && p.apiKey != "" && !strings.Contains(baseURL, "key=") {
+		if strings.Contains(baseURL, "?") {
+			baseURL = fmt.Sprintf("%s&key=%s", baseURL, p.apiKey)
+		} else {
+			baseURL = fmt.Sprintf("%s?key=%s", baseURL, p.apiKey)
+		}
+	}
+
+	if action == "streamGenerateContent" && !strings.Contains(baseURL, "alt=sse") {
+		if strings.Contains(baseURL, "?") {
+			baseURL = fmt.Sprintf("%s&alt=sse", baseURL)
+		} else {
+			baseURL = fmt.Sprintf("%s?alt=sse", baseURL)
+		}
+	}
+
+	return baseURL
 }
 
 
@@ -287,7 +313,7 @@ func (p *Provider) Chat(
 		return nil, fmt.Errorf("failed to marshal request: %w", err)
 	}
 
-	requestURL := p.buildURL(model)
+	requestURL := p.buildURL(model, "generateContent")
 
 	req, err := http.NewRequestWithContext(ctx, "POST", requestURL, bytes.NewReader(jsonData))
 	if err != nil {
@@ -295,7 +321,7 @@ func (p *Provider) Chat(
 	}
 
 	req.Header.Set("Content-Type", "application/json")
-	if p.apiKey != "" {
+	if p.apiKey != "" && !strings.Contains(requestURL, "key=") {
 		req.Header.Set("Authorization", "Bearer "+p.apiKey)
 	}
 
@@ -317,6 +343,161 @@ func (p *Provider) Chat(
 	return p.parseResponse(bodyBytes)
 }
 
+func (p *Provider) ChatStream(
+	ctx context.Context,
+	messages []Message,
+	tools []ToolDefinition,
+	model string,
+	options map[string]any,
+	onChunk func(accumulated string),
+) (*LLMResponse, error) {
+	if p.apiBase == "" && p.projectID == "" {
+		return nil, fmt.Errorf("Vertex AI requires either an api_base or a project_id")
+	}
+
+	requestBody, err := p.buildRequestBody(messages, tools, options)
+	if err != nil {
+		return nil, fmt.Errorf("failed to build request body: %w", err)
+	}
+
+	jsonData, err := json.Marshal(requestBody)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal request: %w", err)
+	}
+
+	requestURL := p.buildURL(model, "streamGenerateContent")
+
+	req, err := http.NewRequestWithContext(ctx, "POST", requestURL, bytes.NewReader(jsonData))
+	if err != nil {
+		return nil, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+	if p.apiKey != "" && !strings.Contains(requestURL, "key=") {
+		req.Header.Set("Authorization", "Bearer "+p.apiKey)
+	}
+
+	resp, err := p.httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("failed to send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, common.HandleErrorResponse(resp, "vertex")
+	}
+
+	var accumulatedText string
+	var allToolCalls []ToolCall
+	var finalResponse *LLMResponse
+
+	scanner := bufio.NewScanner(resp.Body)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" {
+			continue
+		}
+
+		if strings.HasPrefix(line, "data: ") {
+			line = strings.TrimPrefix(line, "data: ")
+		} else if line == "[" || line == "]" || line == "," {
+			continue
+		}
+
+		var chunk struct {
+			Candidates []struct {
+				Content struct {
+					Parts []struct {
+						Text         string `json:"text"`
+						FunctionCall *struct {
+							Name string         `json:"name"`
+							Args map[string]any `json:"args"`
+						} `json:"functionCall,omitempty"`
+					} `json:"parts"`
+				} `json:"content"`
+				FinishReason string `json:"finishReason"`
+			} `json:"candidates"`
+			UsageMetadata *struct {
+				PromptTokenCount     int `json:"promptTokenCount"`
+				CandidatesTokenCount int `json:"candidatesTokenCount"`
+				TotalTokenCount      int `json:"totalTokenCount"`
+			} `json:"usageMetadata,omitempty"`
+		}
+
+		if err := json.Unmarshal([]byte(line), &chunk); err != nil {
+			continue
+		}
+
+		if len(chunk.Candidates) > 0 {
+			candidate := chunk.Candidates[0]
+			for _, part := range candidate.Content.Parts {
+				if part.Text != "" {
+					accumulatedText += part.Text
+					if onChunk != nil {
+						onChunk(accumulatedText)
+					}
+				}
+				if part.FunctionCall != nil {
+					argsJSON, _ := json.Marshal(part.FunctionCall.Args)
+					toolCall := ToolCall{
+						ID:        fmt.Sprintf("call_%s_%d", part.FunctionCall.Name, time.Now().UnixNano()),
+						Name:      part.FunctionCall.Name,
+						Arguments: part.FunctionCall.Args,
+						Function: &FunctionCall{
+							Name:      part.FunctionCall.Name,
+							Arguments: string(argsJSON),
+						},
+					}
+					allToolCalls = append(allToolCalls, toolCall)
+				}
+			}
+
+			if candidate.FinishReason != "" && finalResponse == nil {
+				finishReason := candidate.FinishReason
+				if finishReason == "STOP" {
+					finishReason = "stop"
+				} else if len(allToolCalls) > 0 {
+					finishReason = "tool_calls"
+				}
+
+				finalResponse = &LLMResponse{
+					Content:      accumulatedText,
+					ToolCalls:    allToolCalls,
+					FinishReason: finishReason,
+				}
+			}
+		}
+
+		if chunk.UsageMetadata != nil {
+			if finalResponse == nil {
+				finalResponse = &LLMResponse{
+					Content:   accumulatedText,
+					ToolCalls: allToolCalls,
+				}
+			}
+			finalResponse.Usage = &protocoltypes.UsageInfo{
+				PromptTokens:     chunk.UsageMetadata.PromptTokenCount,
+				CompletionTokens: chunk.UsageMetadata.CandidatesTokenCount,
+				TotalTokens:      chunk.UsageMetadata.TotalTokenCount,
+			}
+		}
+	}
+
+	if finalResponse == nil {
+		finishReason := "stop"
+		if len(allToolCalls) > 0 {
+			finishReason = "tool_calls"
+		}
+		finalResponse = &LLMResponse{
+			Content:      accumulatedText,
+			ToolCalls:    allToolCalls,
+			FinishReason: finishReason,
+		}
+	}
+
+	return finalResponse, nil
+}
+
 func (p *Provider) parseResponse(body []byte) (*LLMResponse, error) {
 	var vResp struct {
 		Candidates []struct {
diff --git a/pkg/providers/vertex/provider_test.go b/pkg/providers/vertex/provider_test.go
index 67a96d84b..65555083b 100644
--- a/pkg/providers/vertex/provider_test.go
+++ b/pkg/providers/vertex/provider_test.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"testing"
 
 
@@ -37,22 +38,22 @@ func TestProvider_buildURL(t *testing.T) {
 		},
 		{
 			name:      "Override with base URL without method",
-			apiBase:   "http://localhost:8080/v1",
+			apiBase:   "http://localhost:8080/v1/models",
 			model:     "gemini-1.0-pro",
-			expected:  "http://localhost:8080/v1/models/gemini-1.0-pro:generateContent",
+			expected:  "http://localhost:8080/v1/models/gemini-1.0-pro:generateContent?key=key",
 		},
 		{
 			name:      "Override with full endpoint URL",
 			apiBase:   "https://my-custom-proxy.com/my-endpoint:generateContent",
 			model:     "gemini-1.5-pro",
-			expected:  "https://my-custom-proxy.com/my-endpoint:generateContent",
+			expected:  "https://my-custom-proxy.com/my-endpoint:generateContent?key=key",
 		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			p := NewProvider("key", tt.apiBase, "", tt.projectID, tt.region)
-			actual := p.buildURL(tt.model)
+			actual := p.buildURL(tt.model, "generateContent")
 			assert.Equal(t, tt.expected, actual)
 		})
 	}
@@ -132,7 +133,9 @@ func TestProvider_Chat(t *testing.T) {
 	// Create a mock server
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		assert.Equal(t, "POST", r.Method)
-		assert.Equal(t, "Bearer test-key", r.Header.Get("Authorization"))
+		// Because we're using the query 'key=test-key' we no longer have Bearer authentication
+		// assert.Equal(t, "Bearer test-key", r.Header.Get("Authorization"))
+		assert.Contains(t, r.URL.String(), "key=test-key")
 
 		var reqBody map[string]any
 		err := json.NewDecoder(r.Body).Decode(&reqBody)
@@ -176,3 +179,112 @@ func TestProvider_Chat(t *testing.T) {
 	assert.Equal(t, 5, resp.Usage.CompletionTokens)
 	assert.Equal(t, 15, resp.Usage.TotalTokens)
 }
+
+func TestProvider_ChatStream(t *testing.T) {
+	// Create a mock server
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "POST", r.Method)
+		assert.Contains(t, r.URL.String(), "key=test-key")
+		assert.Contains(t, r.URL.String(), "alt=sse")
+
+		var reqBody map[string]any
+		err := json.NewDecoder(r.Body).Decode(&reqBody)
+		require.NoError(t, err)
+
+		w.Header().Set("Content-Type", "text/event-stream")
+		// Write mock chunks
+		w.Write([]byte(`data: {"candidates":[{"content":{"parts":[{"text":"Hello"}]}}]}` + "\n\n"))
+		w.Write([]byte(`data: {"candidates":[{"content":{"parts":[{"text":", world!"}]},"finishReason":"STOP"}],"usageMetadata":{"promptTokenCount":10,"candidatesTokenCount":5,"totalTokenCount":15}}` + "\n\n"))
+	}))
+	defer ts.Close()
+
+	p := NewProvider("test-key", ts.URL, "", "my-project", "us-central1")
+	opts := make(map[string]any)
+
+	var chunks []string
+	resp, err := p.ChatStream(
+		context.Background(),
+		[]protocoltypes.Message{{Role: "user", Content: "Say hello!"}},
+		nil,
+		"gemini-1.5-pro",
+		opts,
+		func(accumulated string) {
+			chunks = append(chunks, accumulated)
+		},
+	)
+
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+
+	assert.Equal(t, "Hello, world!", resp.Content)
+	assert.Equal(t, "stop", resp.FinishReason)
+	assert.NotNil(t, resp.Usage)
+	if resp.Usage != nil {
+		assert.Equal(t, 10, resp.Usage.PromptTokens)
+		assert.Equal(t, 5, resp.Usage.CompletionTokens)
+		assert.Equal(t, 15, resp.Usage.TotalTokens)
+	}
+
+	assert.Equal(t, []string{"Hello", "Hello, world!"}, chunks)
+}
+
+func TestProvider_Chat_Standard(t *testing.T) {
+	// Create a mock server
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "POST", r.Method)
+		// Standard Vertex without apiBase should use Bearer authentication
+		assert.Equal(t, "Bearer test-key", r.Header.Get("Authorization"))
+
+		var reqBody map[string]any
+		err := json.NewDecoder(r.Body).Decode(&reqBody)
+		require.NoError(t, err)
+
+		// Return a mock response
+		mockResp := `{
+			"candidates": [
+				{
+					"content": {
+						"parts": [
+							{"text": "Hello, world!"}
+						]
+					},
+					"finishReason": "STOP"
+				}
+			]
+		}`
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(mockResp))
+	}))
+	defer ts.Close()
+
+	// Use an empty apiBase so it builds standard Vertex URLs
+	p := NewProvider("test-key", "", "", "my-project", "us-central1")
+	// Since buildURL will use aiplatform.googleapis.com, we override the httpClient Transport
+	// to redirect requests to our mock server for this test by swapping the base URL out in a custom RoundTripper.
+	p.httpClient.Transport = roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+		req.URL, _ = url.Parse(ts.URL)
+		return http.DefaultTransport.RoundTrip(req)
+	})
+
+	opts := make(map[string]any)
+
+	resp, err := p.Chat(
+		context.Background(),
+		[]protocoltypes.Message{{Role: "user", Content: "Say hello!"}},
+		nil,
+		"gemini-1.5-pro",
+		opts,
+	)
+
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+
+	assert.Equal(t, "Hello, world!", resp.Content)
+	assert.Equal(t, "stop", resp.FinishReason)
+}
+
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}