feat(integrations): fx harness example (MCP config)

[miguelg719]

Summary

• add packages/integrations/fx — config-only integration for fx (Vercel Labs' coding agent, v0.0.3)
• ship the ~/.fx/mcp.json template, a project .fx.json sized for snapshots and MCP discovery, the headless permission allowlist, and a stagehand-facade skill generated from FACADE_AGENT_INSTRUCTIONS
• bound the sdk-parity notification test at 30s (third test to trip the 5s default on cold runners)

Why

fx only consumes MCP from user-global config: it has no plugin API and libfx hard-disables MCP mounting, so the native-tools and SDK-embedded shapes don't apply. The skill is load-bearing — fx v0.0.3's mcp_search_tools returns no results for the server, so the skill teaches the exact mcp_stagehand_* names; without it, runs stall and fall back to run_command exploration (which can dump env credentials into the transcript — README recommends denying it for headless browser work). The template omits environment because fx replaces the child env wholesale when one is set.

Testing

• fx ask --json smoke with the pinned v0.0.3 binary, sandboxed HOME, Browserbase: skill → exact-name select → run → snapshot → "Example Domain" [0-19], 5 steps, reproduced twice
• allowlist rule-key shape (mcp_stagehand_run) and no-environment inheritance verified empirically
• pnpm run fmt:check; vitest run rules/ast-grep/sdk-parity.test.ts (14/14)

Docs PR fast-follows as a stack. ACP-client route (fx acp per-session mounts) tracked as follow-up for evals/embedding; ACP rejects image blocks.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 71d1045

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[cubic-dev-ai]

2 issues found across 4 files

Confidence score: 2/5

• packages/integrations/fx/mcp.json has no environment allowlist, so agent-provider credentials from the shell can be passed to the MCP child and consumed by the facade, weakening the intended credential boundary — explicitly allowlist only the required variables.
• packages/integrations/fx/README.md documents a no-environment setup that forwards all shell variables, including OpenAI, Anthropic, and Google provider keys, which could encourage unintended credential exposure — update the example to use the same restricted environment configuration.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="packages/integrations/fx/README.md">

<violation number="1" location="packages/integrations/fx/README.md:106">
P2: With the documented no-`environment` setup, fx passes every shell variable to the facade, and `stagehandFacadeConfigFromEnv` reads provider keys such as `OPENAI_API_KEY`, `ANTHROPIC_API_KEY`, and Google keys. Correct this security claim and direct users to configure the MCP child with `STAGEHAND_MODEL_NAME`/`STAGEHAND_MODEL_API_KEY`, separate from fx's `AI_GATEWAY_API_KEY`, so unrelated host credentials are not silently consumed.</violation>
</file>

<file name="packages/integrations/fx/mcp.json">

<violation number="1" location="packages/integrations/fx/mcp.json:3">
P2: When the shell contains agent-provider credentials, this entry passes them into the MCP child because it has no `environment` allowlist. The facade can consume those credentials, contradicting the MCP credential boundary; provide an explicit Stagehand/Browserbase allowlist while preserving `PATH` and `HOME`.

(Based on your team's feedback about explicit credentials for MCP children.)</violation>
</file>

Architecture diagram

sequenceDiagram
    participant User as User / Terminal
    participant FX as fx CLI (v0.0.3)
    participant Skill as stagehand-facade Skill
    participant MCPCfg as ~/.fx/mcp.json
    participant FXConfig as .fx.json
    participant Permission as ~/.fx/settings.json
    participant MCP as MCP Server (stdio-server.mjs)
    participant Facade as Stagehand Facade
    participant Browser as Browserbase Browser

    Note over User,Browser: fx + Stagehand MCP Integration Flow

    User->>FX: cd packages/integrations/fx
    User->>FX: fx ask --json "browser instructions"

    Note over FX: Loads project config
    FX->>FXConfig: Read max_agent_steps: 60, max_tool_result_bytes: 262144

    Note over FX: Loads skill guidance
    FX->>Skill: Parse SKILL.md

    alt Tool discovery phase
        FX->>MCPCfg: Read MCP server config
        MCPCfg-->>FX: stagehand: stdio server definition

        FX->>MCP: Spawn node /path/to/stdio-server.mjs
        Note over FX,MCP: 10s startup timeout enforced by fx

        FX->>MCP: mcp_search_tools()
        Note over FX: v0.0.3 bug: returns empty for this server

        alt Skill provides exact tool names
            FX->>Skill: mcp_select_tool("mcp_stagehand_run")
            Skill-->>FX: Tool selected
            FX->>Skill: mcp_select_tool("mcp_stagehand_snapshot")
            Skill-->>FX: Tool selected
            FX->>Skill: mcp_select_tool("mcp_stagehand_screenshot")
            Skill-->>FX: Tool selected
        end
    end

    alt Permission check
        FX->>Permission: Check rule shape for mcp_stagehand_run
        alt Pre-allowed in settings.json
            Permission-->>FX: allow
        else No pre-allow
            FX->>User: Permission prompt (headless fails)
            alt --auto flag
                FX->>FX: Model-based adjudication
            end
        end
    end

    Note over FX,Browser: Tool execution (happy path)

    FX->>MCP: mcp_stagehand_run({ code: "..." })
    MCP->>Facade: Execute browser action
    Facade->>Browser: Launch browser (lazy)
    Browser-->>Facade: Page ready
    Facade->>Browser: Execute JavaScript
    Browser-->>Facade: Result
    Facade-->>MCP: Tool response
    MCP-->>FX: Result

    FX->>MCP: mcp_stagehand_snapshot()
    MCP->>Facade: Snapshot page
    Facade->>Browser: Capture DOM state
    Browser-->>Facade: Snapshot data
    Facade-->>MCP: Snapshot response
    MCP-->>FX: Snapshot

    FX-->>User: Final result

    Note over FX,Facade: Unhappy path: fallback to shell
    alt Skill not loaded (wrong directory)
        FX->>User: run_command exploration
        Note over User: Leaks host env into transcript
        alt run_command denied in settings
            FX->>Permission: Check run_command: deny
            Permission-->>FX: Blocked
            FX-->>User: Error: tool not available
        end
    end

    Note over FX,MCP: fx discards MCP server stderr
    Note over MCP,Facade: Debug standalone if issues

Loading

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

1 issue found across 2 files (changes from recent commits).

Confidence score: 3/5

• The allowlisting example in packages/integrations/fx/README.md omits Stagehand model credentials, so act, extract, and observe calls can fail while browser-only operations continue to work; update the example to include the required credentials or clearly document the limitation.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="packages/integrations/fx/README.md">

<violation number="1" location="packages/integrations/fx/README.md:116">
P2: When allowlisting the MCP child for `run` code that calls `act`, `extract`, or `observe`, this example passes no Stagehand model credentials, so those calls fail even though browser-only page operations work. State that these calls require separate `STAGEHAND_MODEL_NAME` and `STAGEHAND_MODEL_API_KEY` values, and include them in the allowlist example or explicitly scope the example to browser-only code.

(Based on your team's feedback about documenting separate Stagehand model credentials for MCP children.) .</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[Kylejeong2]

approved, fix merge conflicts