diff --git a/CHANGELOG.md b/CHANGELOG.md
index f4e6190..b80a89f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Restriction on using `format: "data"` for screen captures to prevent JavaScript stack overflow errors
+  - Screen captures must use `format: "png"` or omit the format parameter
+  - Application window captures can still use `format: "data"`
+
 ## [1.0.0-beta.18] - 2025-06-08
 
 ### Added
diff --git a/README.md b/README.md
index 9934054..4330f48 100644
--- a/README.md
+++ b/README.md
@@ -289,15 +289,17 @@ Peekaboo provides three main tools for AI agents:
 
 Captures macOS screen content with automatic shadow/frame removal.
 
+**Important:** Screen captures cannot use `format: "data"` due to the large size of screen images causing JavaScript stack overflow errors. Always use `format: "png"` (or omit format) with a `path` for screen captures.
+
 **Examples:**
 ```javascript
-// Capture entire screen
+// Capture entire screen (must save to file)
 await use_mcp_tool("peekaboo", "image", {
   app_target: "screen:0",
   path: "~/Desktop/screenshot.png"
 });
 
-// Capture specific app window with analysis
+// Capture specific app window with analysis (can use format: "data")
 await use_mcp_tool("peekaboo", "image", {
   app_target: "Safari",
   question: "What website is currently open?",
diff --git a/peekaboo-cli/Sources/peekaboo/Version.swift b/peekaboo-cli/Sources/peekaboo/Version.swift
index d531b5e..f4521f9 100644
--- a/peekaboo-cli/Sources/peekaboo/Version.swift
+++ b/peekaboo-cli/Sources/peekaboo/Version.swift
@@ -1,4 +1,4 @@
 // This file is auto-generated by the build script. Do not edit manually.
 enum Version {
-    static let current = "1.0.0-beta.17"
+    static let current = "1.0.0-beta.18"
 }
diff --git a/src/tools/image.ts b/src/tools/image.ts
index a82baf5..38e3f09 100644
--- a/src/tools/image.ts
+++ b/src/tools/image.ts
@@ -28,6 +28,23 @@ export async function imageToolHandler(
   try {
     logger.debug({ input }, "Processing peekaboo.image tool call");
 
+    // Validate format restrictions for screen captures
+    const isScreenCapture = !input.app_target || input.app_target.startsWith("screen:");
+    if (isScreenCapture && input.format === "data") {
+      logger.warn("Screen capture with format 'data' is not allowed due to size constraints");
+      return {
+        content: [
+          {
+            type: "text",
+            text: "Screen captures cannot use format 'data' because they produce images too large for base64 encoding. " +
+                  "Please use format 'png' to save to a file instead.",
+          },
+        ],
+        isError: true,
+        _meta: { backend_error_code: "FORMAT_NOT_ALLOWED_FOR_SCREEN" },
+      };
+    }
+
     // Determine effective path and format for Swift CLI
     const swiftFormat = input.format === "data" ? "png" : (input.format || "png");
 
diff --git a/tests/unit/tools/image.test.ts b/tests/unit/tools/image.test.ts
index 2530a3c..949a0e2 100644
--- a/tests/unit/tools/image.test.ts
+++ b/tests/unit/tools/image.test.ts
@@ -109,14 +109,26 @@ describe("Image Tool", () => {
       expect(mockFsRm).not.toHaveBeenCalled();
     });
 
-    it("should capture screen with format: 'data'", async () => {
+    it("should reject screen capture with format: 'data'", async () => {
+      const result = await imageToolHandler(
+        { format: "data" },
+        mockContext,
+      );
+
+      expect(result.isError).toBe(true);
+      expect(result.content[0].text).toContain("Screen captures cannot use format 'data'");
+      expect(result._meta?.backend_error_code).toBe("FORMAT_NOT_ALLOWED_FOR_SCREEN");
+      expect(mockExecuteSwiftCli).not.toHaveBeenCalled();
+    });
+
+    it("should allow app capture with format: 'data'", async () => {
       // Mock resolveImagePath to return a temp directory for format: "data"
       mockResolveImagePath.mockResolvedValue({
         effectivePath: MOCK_TEMP_IMAGE_DIR,
         tempDirUsed: MOCK_TEMP_IMAGE_DIR,
       });
       
-      const mockResponse = mockSwiftCli.captureImage("screen", {
+      const mockResponse = mockSwiftCli.captureImage("Safari", {
         path: MOCK_SAVED_FILE_PATH,
         format: "png",
       });
@@ -124,7 +136,7 @@ describe("Image Tool", () => {
       mockReadImageAsBase64.mockResolvedValue("base64imagedata");
 
       const result = await imageToolHandler(
-        { format: "data" },
+        { app_target: "Safari", format: "data" },
         mockContext,
       );
 
@@ -140,7 +152,7 @@ describe("Image Tool", () => {
       expect(mockFsRm).not.toHaveBeenCalled();
     });
 
-    it("should save file and return base64 when format: 'data' with path", async () => {
+    it("should save file and return base64 when format: 'data' with path for app capture", async () => {
       const userPath = "/user/test.png";
       // Mock resolveImagePath to return the user path (no temp dir)
       mockResolveImagePath.mockResolvedValue({
@@ -151,7 +163,7 @@ describe("Image Tool", () => {
       const mockSavedFile: SavedFile = {
         path: userPath,
         mime_type: "image/png",
-        item_label: "Screen 1",
+        item_label: "Safari",
       };
       const mockResponse = {
         success: true,
@@ -162,7 +174,7 @@ describe("Image Tool", () => {
       mockReadImageAsBase64.mockResolvedValue("base64imagedata");
 
       const result = await imageToolHandler(
-        { format: "data", path: userPath },
+        { app_target: "Safari", format: "data", path: userPath },
         mockContext,
       );
 
@@ -644,7 +656,7 @@ describe("Image Tool", () => {
         tempDirUsed: MOCK_TEMP_IMAGE_DIR,
       });
       
-      const mockCliResponse = mockSwiftCli.captureImage("screen", {
+      const mockCliResponse = mockSwiftCli.captureImage("Safari", {
         path: MOCK_SAVED_FILE_PATH,
         format: "png",
       });
@@ -652,6 +664,7 @@ describe("Image Tool", () => {
 
       const result = await imageToolHandler(
         {
+          app_target: "Safari", // Use app capture to allow format: "data"
           question: MOCK_QUESTION,
           format: "data", // Even with format: "data"
         },