Fix browser download directory to automatically use /shared-downloads in Docker

[devin-ai-integration]

Fix browser download directory to automatically use /shared-downloads in Docker

Summary

This PR fixes a critical issue where browser downloads in Docker environments were failing to save properly to the /shared-downloads directory. The root cause was that Chrome was downloading files to temporary locations, and when save_as() was called to copy them to /shared-downloads, the files ended up empty.

The fix automatically configures Chrome's download directory to /shared-downloads in Docker environments by:

1. Adding a model validator to BrowserProfile that sets downloads_path = '/shared-downloads' when IN_DOCKER=True and no explicit path is configured
2. Adding the --download-default-directory Chrome launch argument when downloads_path is set

This ensures downloads go directly to the shared directory without requiring manual save_as() calls that were causing empty files.

Review & Testing Checklist for Human

• Test actual downloads in Docker container: Run the user's original test script in a Docker environment to verify downloads work correctly and files have proper content
• Verify Chrome flag compatibility: Ensure --download-default-directory works correctly across different Chrome versions and doesn't cause browser launch issues
• Test backward compatibility: Verify existing non-Docker workflows and explicit downloads_path configurations still work as expected
• Validate IN_DOCKER detection: Confirm the IN_DOCKER flag is reliable in your Docker environments and activates the feature when needed

Recommended test plan: Set up a Docker container with /shared-downloads mounted, run the original failing script, and verify downloaded files contain expected content instead of being empty.

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TB
    subgraph "Browser Profile System"
        Profile["browser_use/browser/profile.py"]:::major-edit
        Session["browser_use/browser/session.py"]:::context
        Context["browser_use/browser/context.py"]:::context
    end
    
    subgraph "Docker Environment"
        Docker["IN_DOCKER flag"]:::context
        SharedDir["/shared-downloads"]:::context
    end
    
    subgraph "Chrome Launch"
        ChromeArgs["Chrome Arguments"]:::minor-edit
        DownloadDir["--download-default-directory"]:::major-edit
    end
    
    Profile --> ChromeArgs
    Profile --> DownloadDir
    Docker --> Profile
    SharedDir --> DownloadDir
    Session --> Profile
    Context --> Profile
    
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#F5F5F5

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Notes

• This fix addresses the specific issue reported where save_as() was creating empty files due to copying from temporary Chrome download locations
• The implementation is conservative - it only auto-configures the download path in Docker environments when no explicit path is already set
• All existing tests pass (118/118) confirming no regressions in core functionality
• Link to Devin run: https://app.devin.ai/sessions/635dc9ca07984292b55d2696de0f7af9
• Requested by: jerry@browserless.io

[devin-ai-integration]

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