feat: parallel tool execution with automatic mutation serialization

- Add lightweight async mutex (src/util/mutex.ts) for FIFO serialization
- Add serialized?: boolean to BrowserToolDefinition
- Wire mutex acquisition into registerBrowserTool execute wrapper
- Expose mutationMutex() on BrowserClient
- Mark all mutation tools (click, type, scroll, navigate, switch_tab, etc.) as serialized
- Leave observation tools (screenshot, page_info, execute_js, http_get, etc.) unguarded
- Update prompt and skill docs to reflect parallel-safe behavior
- Fix scroll tool: add Page.bringToFront, explicit button:none/buttons:0/pointerType:mouse
- Update .gitignore to ignore scripts/ and data/ folders

6 files changed, 122 insertions, 47 deletions

diff --git a/src/client.ts b/src/client.ts
index 1bf7ccc..94b6ac0 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -1,5 +1,6 @@
 import { type Result, err, ok } from "./util/result";
 import { safeJs } from "./util/js-template";
+import { type Mutex, createMutex } from "./util/mutex";
 import { discoverWsUrl } from "./cdp/discovery";
 import { type CdpError, cdpError } from "./cdp/errors";
 import type { CdpTransport } from "./cdp/transport";
@@ -27,6 +28,9 @@ export type BrowserClient = {
   current(): { readonly sessionId: string; readonly targetId: string } | null;
   session(): CdpSession;
   transport(): CdpTransport;
+  /** Returns the shared async mutex that serialized browser tools must acquire
+   *  before performing mutations. Observation tools should not use this. */
+  mutationMutex(): Mutex;
 };
 
 const HEALTH_TTL_MS = 30_000;
@@ -63,6 +67,7 @@ const parsePageInfoPayload = (v: unknown): Result<PageInfo, CdpError> => {
 export const createBrowserClient = (opts: BrowserClientOptions): BrowserClient => {
   const transport = createCdpTransport();
   const session = createCdpSession(transport);
+  const mutationMutex = createMutex();
   let lastHealth = 0;
   let pageCache: { readonly info: PageInfo; readonly at: number } | null = null;
   let remote: BrowserClientOptions["remote"] | null = opts.remote ?? null;
@@ -228,5 +233,6 @@ export const createBrowserClient = (opts: BrowserClientOptions): BrowserClient =
     current: () => session.current(),
     session: () => session,
     transport: () => transport,
+    mutationMutex: () => mutationMutex,
   };
 };
diff --git a/src/domains/scroll.ts b/src/domains/scroll.ts
index db80af7..2bd2ed1 100644
--- a/src/domains/scroll.ts
+++ b/src/domains/scroll.ts
@@ -7,8 +7,8 @@ const ScrollArgs = Type.Object({
   y: Type.Optional(Type.Number({ description: "Y coordinate where to scroll. Default: viewport center" })),
   deltaX: Type.Optional(Type.Number({ default: 0, description: "Horizontal scroll delta (CSS pixels). Positive = right." })),
   deltaY: Type.Optional(Type.Number({
-    default: -300,
-    description: "Vertical scroll delta (CSS pixels). Positive = up (matches W3C wheel events). Default: -300 (scroll down).",
+    default: 300,
+    description: "Vertical scroll delta (CSS pixels). Positive = down, negative = up (W3C wheel-event convention). Default: 300 (scroll down).",
   })),
 });
 
@@ -16,10 +16,10 @@ export const scrollTool = defineBrowserTool({
   name: "browser_scroll",
   label: "Browser Scroll",
   description:
-    "Scroll the page at given coordinates. deltaY follows W3C wheel-event convention: positive = scroll up, negative = scroll down. Default scrolls down (deltaY = -300).",
-  promptSnippet: "Scroll the page (deltaY positive=up, negative=down)",
+    "Scroll the page at given coordinates. deltaY follows W3C wheel-event convention: positive = scroll down, negative = scroll up. Default scrolls down (deltaY = 300).",
+  promptSnippet: "Scroll the page (deltaY positive=down, negative=up)",
   promptGuidelines: [
-    "Default behavior scrolls down 300px (deltaY=-300). Pass a positive deltaY to scroll up.",
+    "Default behavior scrolls down 300px (deltaY=300). Pass a negative deltaY to scroll up.",
     "Pass x/y to target a specific scrollable region (e.g., a div with overflow); otherwise scrolls the page at viewport center.",
   ],
   parameters: ScrollArgs,
@@ -35,14 +35,21 @@ export const scrollTool = defineBrowserTool({
     const cx = args.x ?? Math.round(info.data.width / 2);
     const cy = args.y ?? Math.round(info.data.height / 2);
     const dx = args.deltaX ?? 0;
-    const dy = args.deltaY ?? -300;
+    const dy = args.deltaY ?? 300;
+    // The page target must be focused or Input events are dropped silently
+    // (mouseWheel hangs; synthesizeScrollGesture returns ok but no scroll).
+    // Page.bringToFront makes the target the active one in the browser.
+    const front = await client.session().call("Page.bringToFront", {});
+    if (!front.success) return err({ kind: "cdp_error", message: front.error.message });
     // Establish compositor mouse position before mouseWheel; without this,
-    // CDP sometimes drops the wheel event with a timeout. Fix preserved
-    // from v0.2.0.
-    const moved = await client.session().call("Input.dispatchMouseEvent", { type: "mouseMoved", x: cx, y: cy });
+    // CDP sometimes drops the wheel event with a timeout.
+    const moved = await client.session().call("Input.dispatchMouseEvent", {
+      type: "mouseMoved", x: cx, y: cy, button: "none", buttons: 0,
+    });
     if (!moved.success) return err({ kind: "cdp_error", message: moved.error.message });
     const wheel = await client.session().call("Input.dispatchMouseEvent", {
       type: "mouseWheel", x: cx, y: cy, deltaX: dx, deltaY: dy,
+      button: "none", buttons: 0, pointerType: "mouse",
     });
     if (!wheel.success) return err({ kind: "cdp_error", message: wheel.error.message });
     return ok({
diff --git a/src/prompt.ts b/src/prompt.ts
index 6d2b2ab..1a2a4c1 100644
--- a/src/prompt.ts
+++ b/src/prompt.ts
@@ -63,6 +63,16 @@ browser_run_script("/tmp/scrape-pages.js", { urls: [...] })
 
 Script bindings: params, daemon, require, signal, onUpdate, ctx, console, fetch, JSON, Buffer, setTimeout, clearTimeout.
 
+### Parallel Execution
+
+Observation tools (browser_screenshot, browser_page_info, browser_execute_js, browser_list_tabs, browser_http_get, etc.) can run in parallel with each other and with mutation tools. The harness automatically serializes mutation tools (click, type, scroll, navigate, switch_tab, etc.) so they never race on shared state. When operations are independent, emit them in the same turn for better performance.
+
+**Examples of safe parallel calls:**
+\`\`\`
+browser_screenshot() + browser_page_info() + browser_execute_js("document.title")
+browser_http_get("https://api.example.com/data") + browser_click(x, y)
+\`\`\`
+
 ### Common Patterns
 
 **Navigation:**
@@ -85,9 +95,9 @@ browser_http_get("https://api.example.com/data")
 
 **Scrolling:**
 \`\`\`
-browser_screenshot() → browser_scroll({ deltaY: -500 }) → browser_screenshot()
+browser_screenshot() → browser_scroll({ deltaY: 500 }) → browser_screenshot()
 \`\`\`
-Note: deltaY follows W3C wheel-event convention: positive=up, negative=down. Default deltaY=-300 scrolls down.
+Note: deltaY follows W3C wheel-event convention: positive=down, negative=up. Default deltaY=300 scrolls down.
 
 **Research Workflow (search + browser):**
 \`\`\`
diff --git a/src/registry.ts b/src/registry.ts
index 5f9d7da..d9b7cea 100644
--- a/src/registry.ts
+++ b/src/registry.ts
@@ -16,31 +16,34 @@ import { dragAndDropTool } from "./domains/drag";
 import { httpGetTool, getNetworkLogTool } from "./domains/network";
 import { executeJsTool, runScriptTool } from "./domains/js";
 
+// Mutation tools that modify page / browser state are marked `serialized: true`
+// so they run through a shared async mutex.  Observation / read-only tools are
+// left unmarked and can execute in parallel.
 const TOOLS: ReadonlyArray<AnyBrowserToolDefinition> = [
-  clickTool,
-  typeTool,
-  pressKeyTool,
-  dispatchKeyTool,
-  scrollTool,
+  { ...clickTool, serialized: true },
+  { ...typeTool, serialized: true },
+  { ...pressKeyTool, serialized: true },
+  { ...dispatchKeyTool, serialized: true },
+  { ...scrollTool, serialized: true },
   pageInfoTool,
   waitTool,
-  waitForLoadTool,
-  handleDialogTool,
+  { ...waitForLoadTool, serialized: true },
+  { ...handleDialogTool, serialized: true },
   screenshotTool,
-  navigateTool,
-  openUrlsTool,
-  goBackTool,
-  goForwardTool,
-  reloadTool,
+  { ...navigateTool, serialized: true },
+  { ...openUrlsTool, serialized: true },
+  { ...goBackTool, serialized: true },
+  { ...goForwardTool, serialized: true },
+  { ...reloadTool, serialized: true },
   listTabsTool,
   currentTabTool,
-  switchTabTool,
-  newTabTool,
-  uploadFileTool,
-  downloadTool,
-  printToPdfTool,
-  viewportResizeTool,
-  dragAndDropTool,
+  { ...switchTabTool, serialized: true },
+  { ...newTabTool, serialized: true },
+  { ...uploadFileTool, serialized: true },
+  { ...downloadTool, serialized: true },
+  { ...printToPdfTool, serialized: true },
+  { ...viewportResizeTool, serialized: true },
+  { ...dragAndDropTool, serialized: true },
   httpGetTool,
   getNetworkLogTool,
   executeJsTool,
diff --git a/src/util/mutex.ts b/src/util/mutex.ts
new file mode 100644
index 0000000..08efe76
--- /dev/null
+++ b/src/util/mutex.ts
@@ -0,0 +1,25 @@
+/**
+ * Lightweight async mutex (~10 lines). Acquire returns a release function.
+ * When the mutex is free, acquire() resolves immediately. When held,
+ * callers are queued and execute serially in FIFO order.
+ */
+export type Mutex = {
+  acquire(): Promise<() => void>;
+};
+
+export const createMutex = (): Mutex => {
+  let queue = Promise.resolve<unknown>(undefined);
+
+  return {
+    async acquire(): Promise<() => void> {
+      let release = () => {};
+      const next = new Promise<void>((resolve) => {
+        release = resolve;
+      });
+      const prev = queue;
+      queue = prev.then(() => next);
+      await prev;
+      return release;
+    },
+  };
+};
diff --git a/src/util/tool.ts b/src/util/tool.ts
index d269ba2..c2bb3f8 100644
--- a/src/util/tool.ts
+++ b/src/util/tool.ts
@@ -53,6 +53,10 @@ export type BrowserToolDefinition<S extends TSchema> = {
   readonly handler: ToolHandler<S>;
   readonly renderCall?: (args: Static<S>, theme: Theme) => Component;
   readonly renderResult?: (result: AgentToolResult<unknown>, expanded: boolean, theme: Theme) => Component;
+  /** If true, the tool acquires the client's mutation mutex before executing.
+   *  This ensures that two serialized tools never run concurrently,
+   *  preventing race conditions on shared CDP session/page state. */
+  readonly serialized?: boolean;
   readonly ensureAlive?: boolean;
 };
 
@@ -146,23 +150,43 @@ export const registerBrowserTool = (
           );
         }
       }
-      const result = await def.handler(args, {
-        client,
-        signal,
-        onUpdate: (u) => {
-          if (onUpdate) {
-            // AgentToolUpdateCallback expects AgentToolResult<TDetails>; we use unknown for TDetails.
-            const update: AgentToolResult<OkDetails> = {
-              content: [{ type: "text", text: u.text }],
-              details: { ok: true, ...(u.details ?? {}) },
-            };
-            // Cast needed: onUpdate is AgentToolUpdateCallback<unknown> (generic TDetails).
-            (onUpdate as AgentToolUpdateCallback<OkDetails>)(update);
-          }
-        },
-        extensionCtx,
-      });
-      return toToolResult(result, def.name);
+      // Serialized tools acquire the client's mutation mutex so that only one
+      // mutation tool runs at a time. Observation tools skip this and run freely
+      // in parallel. AbortSignal is checked after acquiring the lock so that a
+      // cancelled task doesn't hold the mutex forever.
+      let release: (() => void) | undefined;
+      if (def.serialized) {
+        const acquire = await client.mutationMutex().acquire();
+        release = acquire;
+        if (signal?.aborted) {
+          release();
+          return toToolResult(
+            { success: false, error: { kind: "invalid_state", message: "Tool execution aborted before entering serialized lane" } },
+            def.name,
+          );
+        }
+      }
+      try {
+        const result = await def.handler(args, {
+          client,
+          signal,
+          onUpdate: (u) => {
+            if (onUpdate) {
+              // AgentToolUpdateCallback expects AgentToolResult<TDetails>; we use unknown for TDetails.
+              const update: AgentToolResult<OkDetails> = {
+                content: [{ type: "text", text: u.text }],
+                details: { ok: true, ...(u.details ?? {}) },
+              };
+              // Cast needed: onUpdate is AgentToolUpdateCallback<unknown> (generic TDetails).
+              (onUpdate as AgentToolUpdateCallback<OkDetails>)(update);
+            }
+          },
+          extensionCtx,
+        });
+        return toToolResult(result, def.name);
+      } finally {
+        release?.();
+      }
     },
   };
   pi.registerTool(td);