[feat]: add support for WebMCP

[seanmcguire12]

why

this PR introduces first class WebMCP support to Stagehand. this means callers can:

• list tools exposed by a given page via page.listWebMCPTools()
• invoke the given tools via page.invokeWebMCPTool()

what changed

• added page.listWebMCPTools() to collect the current WebMCP tool snapshot from WebMCP.toolsAdded
• added page.invokeWebMCPTool() with frame disambiguation, result promises, cancel support, timeouts, & early response handling
• added WebMCP public types in packages/core/lib/v3/types/public/page.ts
• added StagehandUnsupportedBrowserFeatureError, which gets thrown when using a version of chrome that doesn't support WebMCP
• added flowlogger wrappers for PageListWebMCPTools & PageInvokeWebMCPTool
• also added a WebMCP example in packages/core/examples/webmcp.ts

behavioural notes:

• page.invokeWebMCPTool() starts the tool invocation & returns a WebMCPToolInvocation once chrome accepts the request. it does not wait for the tool itself to finish
• to wait for the actual tool execution, callers must do await invocation.result from the returned WebMCPToolInvocation
• this keeps the returned invocation usable for follow up actions like invocation.cancel() while the tool is still running
• this PR only implements WebMCP support for top level frames/pages. it does not currently handle WebMCP tools registered in OOPIFs

test plan

• added tests in packages/core/tests/unit/page-webmcp.test.ts which cover page.listWebMCPTools() snapshot collection, no stale tool reuse, unsupported browser errors, page.invokeWebMCPTool() frame resolution, duplicate tool names, result routing, early responses, cancel, timeout, & close cleanup
• updated packages/core/tests/unit/helpers/mockCDPSession.ts such that it now supports CDP event listeners so WebMCP events can be tested without a browser
• public API tests cover the new WebMCP page types & StagehandUnsupportedBrowserFeatureError

[changeset-bot]

🦋 Changeset detected

Latest commit: 45239ef

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@browserbasehq/stagehand	Minor
@browserbasehq/stagehand-evals	Patch
@browserbasehq/stagehand-server-v3	Patch

Not sure what this means? Click here to learn what changesets are.

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[cubic-dev-ai]

1 issue found across 10 files

Confidence score: 3/5

• There is a concrete stability risk in packages/core/lib/v3/understudy/page.ts: late responses from timed-out or abandoned WebMCP invocations are buffered indefinitely, which can grow memory usage over time on long-lived pages.
• Given the clear user-impacting behavior (potential memory leak) and relatively high confidence, this sits above minor-risk territory even though it appears localized to one area.
• Pay close attention to packages/core/lib/v3/understudy/page.ts - ensure buffered late responses are discarded or cleaned up to prevent unbounded memory growth.

Architecture diagram

sequenceDiagram
    participant Client as Client Code
    participant P as Page (Stagehand)
    participant MainS as Main CDP Session
    participant Chrome as Chrome Browser
    participant WebMCP as WebMCP CDP Domain

    Note over Client,WebMCP: WebMCP Tool Listing Flow

    Client->>P: page.listWebMCPTools({ timeoutMs })
    P->>P: ensureWebMCPEnabled() – register onWebMCPToolResponded listener
    P->>MainS: WebMCP.enable
    MainS->>Chrome: Send CDP "WebMCP.enable"
    Chrome->>WebMCP: Enable WebMCP domain
    WebMCP-->>Chrome: Emit "WebMCP.toolsAdded" events
    Chrome-->>MainS: CDP events
    MainS-->>P: onWebMCPToolResponded NOT triggered (wrong event type)
    alt Tools added within timeout window
        WebMCP->>WebMCP: Emit "WebMCP.toolsAdded" events with tool list
        WebMCP-->>Chrome: "WebMCP.toolsAdded" event
        Chrome-->>MainS: CDP event
        MainS->>P: onToolsAdded callback invoked
        P->>P: Strip debug fields (stackTrace), normalize tools
        P->>P: Wait for quiet window (no new tools for up to 100ms)
    else Timeout reached
        P->>P: Force resolve after timeoutMs
    end
    P->>P: Cleanup: remove onToolsAdded listener
    P-->>Client: Return WebMCPTool[] (current snapshot)

    Note over Client,WebMCP: WebMCP Tool Invocation Flow

    Client->>P: page.invokeWebMCPTool(toolName, input, { frameId?, timeoutMs? })
    P->>P: ensureWebMCPEnabled() – return cached promise if already enabled
    opt frameId not provided
        P->>MainS: WebMCP.enable (if not already enabled)
        MainS->>Chrome: Send CDP "WebMCP.enable"
        Chrome-->>MainS: "WebMCP.toolsAdded" events
        MainS-->>P: Collect tools to resolve frameId for toolName
        alt Tool not found or ambiguous across frames
            P-->>Client: Throw StagehandInvalidArgumentError
        end
    end
    P->>MainS: WebMCP.invokeTool({ frameId, toolName, input })
    MainS->>Chrome: Send CDP "WebMCP.invokeTool"
    Chrome->>WebMCP: Start tool execution
    WebMCP-->>Chrome: "WebMCP.invokeTool" response { invocationId }
    Chrome-->>MainS: CDP response { invocationId }
    MainS-->>P: Response { invocationId }
    P->>P: Create deferred promise for invocation.result
    P->>P: Start timeout timer
    P-->>Client: Return WebMCPToolInvocation { invocationId, result, cancel() }

    Note over Client,Chrome: Tool Execution & Result (async)

    alt Tool completes normally
        WebMCP-->>Chrome: "WebMCP.toolResponded" { invocationId, status: "Completed", output }
        Chrome-->>MainS: CDP event
        MainS->>P: onWebMCPToolResponded callback
        P->>P: Clear timeout, resolve deferred with WebMCPToolResult
    else Tool is canceled
        Client->>P: invocation.cancel()
        P->>MainS: WebMCP.cancelInvocation({ invocationId })
        MainS->>Chrome: Send CDP "WebMCP.cancelInvocation"
        Chrome-->>MainS: ACK
        MainS-->>P: Success
        WebMCP-->>Chrome: "WebMCP.toolResponded" { invocationId, status: "Canceled" }
        Chrome-->>MainS: CDP event
        MainS->>P: onWebMCPToolResponded callback
        P->>P: Resolve deferred with Canceled status
    else Tool errors
        WebMCP-->>Chrome: "WebMCP.toolResponded" { invocationId, status: "Error", errorText, exception }
        Chrome-->>MainS: CDP event
        MainS->>P: onWebMCPToolResponded callback
        P->>P: Resolve deferred with Error status
    else Timeout expires
        P->>P: Timer fires – reject deferred with timeout error
        P->>P: Cleanup pending invocation entry
    end

    Note over P,Chrome: Early Response Handling (response before invokeTool returns)

    alt Response arrives before invokeTool ACK
        WebMCP-->>Chrome: "WebMCP.toolResponded" event
        Chrome-->>MainS: CDP event
        MainS->>P: onWebMCPToolResponded callback
        P->>P: invocation not yet in pending map
        P->>P: Buffer result in bufferedWebMCPResults
        Chrome-->>MainS: "WebMCP.invokeTool" response { invocationId }
        MainS-->>P: Response with invocationId
        P->>P: Check buffer – find buffered result
        P->>P: Resolve deferred immediately (clear timeout)
    end

    Note over P: Page Close / Cleanup

    Client->>P: page.close()
    P->>P: teardownWebMCP()
    P->>P: Remove onWebMCPToolResponded listener
    loop For each pending invocation
        P->>P: Clear timeout, reject deferred with disposal error
    end
    P->>P: Clear buffered results and webMCPEnablePromise

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1 issue found across 10 files

Confidence score: 3/5

• There is a concrete regression risk in packages/core/lib/v3/understudy/page.ts: invokeWebMCPTool() appears to map all CDP invocation failures to unsupported-browser-feature errors, which can hide real argument or state problems from users and developers.
• Given the issue’s medium severity (6/10) and high confidence (9/10), this is more than a cosmetic concern and can impact debugging accuracy and runtime behavior, so merge risk is moderate.
• Pay close attention to packages/core/lib/v3/understudy/page.ts - error classification in invokeWebMCPTool() should preserve true CDP failure causes instead of collapsing them into a single unsupported-feature path.

Architecture diagram

sequenceDiagram
    participant C as Client App
    participant P as Stagehand Page
    participant CDP as CDP Session (Browser)
    participant CE as CDP Event Bus
    participant W as WebMCP Runtime (Browser)
    
    Note over C,P: Listing WebMCP Tools
    
    C->>P: page.listWebMCPTools({ timeoutMs })
    P->>CDP: WebMCP.enable
    CDP->>CE: Emit WebMCP.toolsAdded
    
    loop For each toolsAdded event
        CE->>P: onToolsAdded(event)
        P->>P: Collect tools in local map
        P->>P: Reset quiet timer on change
    end
    
    alt Quiet window expires (no new tools)
        P->>P: Resolve snapshot collection
    else Timeout reached
        P->>P: Force resolve snapshot
    end
    
    P->>CDP: WebMCP.disable
    P->>P: Remove toolsAdded listener
    P-->>C: Return WebMCPTool[]
    
    Note over C,P: Invoking a WebMCP Tool
    
    C->>P: page.invokeWebMCPTool(toolName, input, { frameId?, timeoutMs? })
    
    alt frameId not provided
        P->>P: Call listWebMCPTools (internal)
        alt Tool found in single frame
            P->>P: Use resolved frameId
        else Tool found in multiple frames
            P-->>C: Throw StagehandInvalidArgumentError
        else Tool not found
            P-->>C: Throw StagehandInvalidArgumentError
        end
    end
    
    P->>CDP: WebMCP.invokeTool({ frameId, toolName, input })
    CDP-->>P: Return { invocationId }
    
    P->>P: Create deferred result promise
    P->>P: Set timeout timer
    
    Note over P: Invocation returned to client<br/>while tool executes
    
    P-->>C: Return WebMCPToolInvocation { invocationId, result, cancel() }
    
    alt Normal completion
        CE->>P: WebMCP.toolResponded({ invocationId, status, output })
        P->>P: Clear timeout, resolve deferred
        C->>P: await invocation.result
        P-->>C: Return WebMCPToolResult
    else Early response (before invoke response)
        CE->>P: WebMCP.toolResponded (arrives early)
        P->>P: Buffer result in pendingWebMCPInvokeToolResponses window
        CDP-->>P: Return { invocationId }
        P->>P: Retrieve buffered result, resolve deferred
        C->>P: await invocation.result
        P-->>C: Return WebMCPToolResult
    else Timeout
        P->>P: Timer fires, reject deferred
        C->>P: await invocation.result
        P-->>C: Reject with timeout error
    else Client calls cancel()
        C->>P: invocation.cancel()
        P->>CDP: WebMCP.cancelInvocation({ invocationId })
        alt Tool responds before cancel
            CE->>P: WebMCP.toolResponded (status: Canceled)
            P->>P: Resolve deferred normally
        end
        P-->>C: void
        C->>P: await invocation.result
        P-->>C: Return WebMCPToolResult (status: Canceled)
    end
    
    Note over P: Cleanup & Browser Support
    
    alt Unsupported browser (WebMCP.enable fails)
        P->>P: Catch error, create StagehandUnsupportedBrowserFeatureError
        P-->>C: Throw error with guidance message
    end
    
    opt Page close with pending invocations
        C->>P: page.close()
        P->>P: Reject all pending deferred promises
        P->>P: Clear all timers and buffers
        P->>CDP: Remove WebMCP.toolResponded listener
    end

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1 issue found across 10 files

Confidence score: 3/5

• There is a concrete error-handling risk in packages/core/lib/v3/understudy/page.ts: listWebMCPTools currently maps all failures to an unsupported-browser error, which can hide actual runtime or CDP failures.
• Because this can mislead users and operators about the real root cause, it introduces moderate regression/debuggability risk even though it is localized to one function.
• Pay close attention to packages/core/lib/v3/understudy/page.ts - error classification should preserve real failure types instead of collapsing them into unsupported-browser responses.

Architecture diagram

sequenceDiagram
    participant Client as External Caller
    participant Page as Stagehand Page
    participant MainSession as Main CDP Session
    participant Chrome as Chrome Browser (WebMCP)

    Note over Client,Chrome: WebMCP API Flow

    Client->>Page: page.listWebMCPTools({timeoutMs})

    Page->>MainSession: ensureWebMCPEnabled() - check if already enabled
    MainSession->>MainSession: Register listener for WebMCP.toolResponded

    Page->>MainSession: WebMCP.enable (triggers browser snapshot)
    MainSession-->>Page: WebMCP.toolsAdded (event with all tools)
    MainSession->>MainSession: Listen for WebMCP.toolsAdded events

    loop Collect tools during quiet window
        MainSession-->>Page: WebMCP.toolsAdded (new/changed tools)
        Page->>Page: Accumulate tools, update quiet timer
    end

    Page->>Page: Return collected WebMCPTool array (no stale cache)

    Note over Client,Chrome: Tool Invocation

    Client->>Page: page.invokeWebMCPTool(toolName, input, {frameId?, timeoutMs?})

    alt FrameId not provided
        Page->>Page: listWebMCPTools() to resolve frameId
        alt Multiple frames have same tool name
            Page-->>Client: StagehandInvalidArgumentError (ambiguous)
        end
    end

    Page->>MainSession: WebMCP.invokeTool({frameId, toolName, input})
    alt Unsupported browser
        MainSession-->>Page: Error (code -32601 or "method not found")
        Page-->>Client: StagehandUnsupportedBrowserFeatureError
    end

    MainSession-->>Page: {invocationId}
    Page-->>Client: WebMCPToolInvocation (immediate, not waiting for tool result)

    Note over Client,Chrome: Caller controls result/cancel lifecycle

    alt Early response arrives before invokeTool returns
        MainSession-->>Page: WebMCP.toolResponded (event arrives during send)
        Page->>Page: Buffer result in bufferedWebMCPResults (pending response > 0)
    end

    par Caller waits for result
        Client->>Page: await invocation.result
        alt Result already buffered
            Page-->>Client: Resolve from bufferedWebMCPResults
        else Normal response
            MainSession-->>Page: WebMCP.toolResponded (status, output, error)
            Page->>Page: Clear timer, delete pending, resolve deferred
            Page-->>Client: WebMCPToolResult
        else Timeout reached
            Page->>Page: Reject deferred with timeout error
            Page-->>Client: StagehandInvalidArgumentError (timeout)
        end
    and Caller cancels
        Client->>Page: await invocation.cancel()
        Page->>MainSession: WebMCP.cancelInvocation({invocationId})
        MainSession->>MainSession: WebMCP.toolResponded with Canceled status
        Page->>Page: Clear timer, delete pending, resolve/reject deferred
    end

    alt Page closes
        Page->>Page: teardownWebMCP()
        Page->>MainSession: Remove WebMCP.toolResponded listener
        loop All pending invocations
            Page->>Page: Reject deferred with disposal error
        end
        Page->>Page: Clear pending maps and buffer
    end

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1 issue found across 12 files

Confidence score: 3/5

• There is a concrete API-contract risk in packages/core/lib/v3/understudy/page.ts: success statuses from WebMCP may be returned as "Success" instead of the documented "Completed" value.
• I’m scoring this as moderate merge risk because the issue is medium severity (6/10) with high confidence (8/10) and can directly affect client-side status handling.
• Pay close attention to packages/core/lib/v3/understudy/page.ts - normalize WebMCP success states to the public API contract before or immediately after merge.

Architecture diagram

sequenceDiagram
    participant Client as Caller Code
    participant Page as Page (understudy/page.ts)
    participant CDP as CDPSession
    participant Browser as Chrome Browser (WebMCP)
    participant Tools as Page WebMCP Tools
    participant Timer as Timeout Timer

    Note over Client,Browser: WebMCP Tool Listing Flow

    Client->>Page: listWebMCPTools({ timeoutMs })
    Page->>CDP: on("WebMCP.toolsAdded")
    Page->>CDP: on("WebMCP.toolsRemoved")
    Page->>CDP: send("WebMCP.enable")
    CDP->>Browser: Enable WebMCP
    Browser-->>CDP: OK

    Browser-->>CDP: event "WebMCP.toolsAdded"
    CDP-->>Page: onToolsAdded(event)
    Page->>Page: Store tools in local Map

    Browser-->>CDP: event "WebMCP.toolsRemoved"
    CDP-->>Page: onToolsRemoved(event)
    Page->>Page: Remove tools from local Map

    Note over Page: Wait for quiet window or timeout

    alt Tools settled within timeout
        Page->>Page: Resolve with collected tools
    else Timeout reached
        Page->>Page: Resolve with current tools snapshot
    end

    Page->>CDP: off("WebMCP.toolsAdded")
    Page->>CDP: off("WebMCP.toolsRemoved")
    Page-->>Client: WebMCPTool[]

    Note over Client,Browser: WebMCP Tool Invocation Flow

    Client->>Page: invokeWebMCPTool(name, input, { frameId, timeoutMs })
    alt frameId not provided
        Page->>Page: Collect tool snapshot to resolve frame
        alt Tool found in single frame
            Page->>Page: Use that frameId
        else Tool ambiguous across frames
            Page-->>Client: Throw StagehandInvalidArgumentError
        end
    end

    Page->>Page: Create deferred result promise
    Page->>CDP: send("WebMCP.invokeTool", { toolName, input, frameId })
    CDP->>Browser: Invoke tool
    alt Browser does not support WebMCP
        Browser-->>CDP: Error: method not found
        CDP-->>Page: Error
        Page->>Page: Check isWebMCPUnsupportedBrowserError
        Page-->>Client: Throw StagehandUnsupportedBrowserFeatureError
    else Browser accepts
        Browser-->>CDP: { invocationId }
        CDP-->>Page: { invocationId }
        Page->>Page: Setup timeout timer
        Page->>Page: Check buffered results for this invocationId
        alt Response already buffered
            Page->>Page: Resolve deferred with buffered result
        end
        Page-->>Client: WebMCPToolInvocation { invocationId, result, cancel }

        alt Tool completes before timeout
            Browser-->>CDP: event "WebMCP.toolResponded"
            CDP-->>Page: onWebMCPToolResponded(event)
            Page->>Timer: Clear timeout
            Page->>Page: Resolve deferred with result
        else Tool responds quickly before invokeTool returns
            Browser-->>CDP: event "WebMCP.toolResponded"
            CDP-->>Page: Buffer result temporarily
            Page->>Page: On invokeTool response, consume buffer
        else Timeout expires
            Timer-->>Page: Timeout callback
            Page->>Page: Reject deferred with timeout error
            Page-->>Client: invocation.result rejects
        end
    end

    Note over Client,Page: Cancel Flow (from returned invocation)

    Client->>Page: invocation.cancel()
    Page->>CDP: send("WebMCP.cancelInvocation", { invocationId })
    CDP->>Browser: Cancel tool
    Browser-->>CDP: OK
    CDP-->>Page: OK
    Page->>Page: Cleanup pending invocation state
    Page-->>Client: void

    Note over Client,Browser: Error Handling: Unsupported Browser

    alt listWebMCPTools with unsupported browser
        Client->>Page: listWebMCPTools()
        Page->>CDP: send("WebMCP.enable")
        CDP->>Browser: Enable WebMCP
        Browser-->>CDP: Error: Method not found (code -32601)
        CDP-->>Page: Error
        Page->>Page: isWebMCPUnsupportedBrowserError()
        Page-->>Client: Throw StagehandUnsupportedBrowserFeatureError
    end

    Note over Page: Cleanup on page close

    Client->>Page: close()
    Page->>Page: teardownWebMCP()
    Page->>CDP: off("WebMCP.toolResponded")
    Page->>Page: Reject all pending deferreds
    Page->>Page: Clear pending map

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No issues found across 12 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant Client as Caller Code
    participant Page as Page (understudy/page.ts)
    participant CDP as CDPSession
    participant Browser as Chrome Browser (WebMCP)
    participant Tools as Page WebMCP Tools
    participant Timer as Timeout Timer

    Note over Client,Browser: WebMCP Tool Listing Flow

    Client->>Page: listWebMCPTools({ timeoutMs })
    Page->>CDP: on("WebMCP.toolsAdded")
    Page->>CDP: on("WebMCP.toolsRemoved")
    Page->>CDP: send("WebMCP.enable")
    CDP->>Browser: Enable WebMCP
    Browser-->>CDP: OK

    Browser-->>CDP: event "WebMCP.toolsAdded"
    CDP-->>Page: onToolsAdded(event)
    Page->>Page: Store tools in local Map

    Browser-->>CDP: event "WebMCP.toolsRemoved"
    CDP-->>Page: onToolsRemoved(event)
    Page->>Page: Remove tools from local Map

    Note over Page: Wait for quiet window or timeout

    alt Tools settled within timeout
        Page->>Page: Resolve with collected tools
    else Timeout reached
        Page->>Page: Resolve with current tools snapshot
    end

    Page->>CDP: off("WebMCP.toolsAdded")
    Page->>CDP: off("WebMCP.toolsRemoved")
    Page-->>Client: WebMCPTool[]

    Note over Client,Browser: WebMCP Tool Invocation Flow

    Client->>Page: invokeWebMCPTool(name, input, { frameId, timeoutMs })
    alt frameId not provided
        Page->>Page: Collect tool snapshot to resolve frame
        alt Tool found in single frame
            Page->>Page: Use that frameId
        else Tool ambiguous across frames
            Page-->>Client: Throw StagehandInvalidArgumentError
        end
    end

    Page->>Page: Create deferred result promise
    Page->>CDP: send("WebMCP.invokeTool", { toolName, input, frameId })
    CDP->>Browser: Invoke tool
    alt Browser does not support WebMCP
        Browser-->>CDP: Error: method not found
        CDP-->>Page: Error
        Page->>Page: Check isWebMCPUnsupportedBrowserError
        Page-->>Client: Throw StagehandUnsupportedBrowserFeatureError
    else Browser accepts
        Browser-->>CDP: { invocationId }
        CDP-->>Page: { invocationId }
        Page->>Page: Setup timeout timer
        Page->>Page: Check buffered results for this invocationId
        alt Response already buffered
            Page->>Page: Resolve deferred with buffered result
        end
        Page-->>Client: WebMCPToolInvocation { invocationId, result, cancel }

        alt Tool completes before timeout
            Browser-->>CDP: event "WebMCP.toolResponded"
            CDP-->>Page: onWebMCPToolResponded(event)
            Page->>Timer: Clear timeout
            Page->>Page: Resolve deferred with result
        else Tool responds quickly before invokeTool returns
            Browser-->>CDP: event "WebMCP.toolResponded"
            CDP-->>Page: Buffer result temporarily
            Page->>Page: On invokeTool response, consume buffer
        else Timeout expires
            Timer-->>Page: Timeout callback
            Page->>Page: Reject deferred with timeout error
            Page-->>Client: invocation.result rejects
        end
    end

    Note over Client,Page: Cancel Flow (from returned invocation)

    Client->>Page: invocation.cancel()
    Page->>CDP: send("WebMCP.cancelInvocation", { invocationId })
    CDP->>Browser: Cancel tool
    Browser-->>CDP: OK
    CDP-->>Page: OK
    Page->>Page: Cleanup pending invocation state
    Page-->>Client: void

    Note over Client,Browser: Error Handling: Unsupported Browser

    alt listWebMCPTools with unsupported browser
        Client->>Page: listWebMCPTools()
        Page->>CDP: send("WebMCP.enable")
        CDP->>Browser: Enable WebMCP
        Browser-->>CDP: Error: Method not found (code -32601)
        CDP-->>Page: Error
        Page->>Page: isWebMCPUnsupportedBrowserError()
        Page-->>Client: Throw StagehandUnsupportedBrowserFeatureError
    end

    Note over Page: Cleanup on page close

    Client->>Page: close()
    Page->>Page: teardownWebMCP()
    Page->>CDP: off("WebMCP.toolResponded")
    Page->>Page: Reject all pending deferreds
    Page->>Page: Clear pending map

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