Teams 1:1 Chat with External Guest: Drag-and-drop file sharing fails with "Access Denied" unless permissions are manually applied

The behavior described is consistent with how Teams, OneDrive, and SharePoint enforce sharing and access control, and with known limitations around external/guest access and link types.

From the provided information, there are two relevant mechanics:

1. Limited-access user permission lockdown mode
   • When this site collection feature is enabled, it restricts how users (especially external/anonymous) can access content via application pages and links. It is explicitly called out as a cause of access issues for files shared through Teams and OneDrive.
   • For Teams/OneDrive scenarios, this feature is not recommended because it can block access to files shared via links, including those generated in Teams chats.
   • The documented fix for similar “You don't have access to this file” errors in Teams is to deactivate this feature on the OneDrive site:
     1. Sign into the OneDrive site.
     2. Go to Settings → OneDrive Settings → More Settings.
     3. Under Features and storage, select Site Collection Features.
     4. Deactivate Limited-access user permission lockdown mode.
2. Link type and large/complex sharing scenarios
   • Teams file sharing in chat uses SharePoint/OneDrive link types such as Specific people (which corresponds to “People currently in this chat”).
   • Documentation notes that when Specific people links are used in larger chats (more than 100 participants), some users may not automatically receive permissions and must request access. This illustrates that automatic permission assignment for certain link types is not guaranteed to succeed for all participants in all scenarios.

In the scenario described:

• When the file is dragged and dropped, Teams automatically creates a link and attempts to assign permissions in the background.
• When the sharing control is explicitly opened and Apply is clicked before sending, the link and permissions are explicitly re-evaluated and applied, which succeeds for the guest.

This aligns with the documented behavior where:

• Access issues can occur when site-level features (such as Limited-access user permission lockdown mode) interfere with link-based access.
• Automatic link/permission assignment for certain link types and participant combinations (including guests) can fail, requiring manual adjustment.

The documentation provided does not identify this as a specific “bug” in Teams, nor does it reference a Purview/DLP setting as the cause. Instead, it attributes similar access-denied issues to:

• The Limited-access user permission lockdown mode feature being enabled on OneDrive/SharePoint.
• The way Specific people links behave in larger or more complex sharing scenarios.

Given the context, the recommended actions are:

1. Confirm again that Limited-access user permission lockdown mode is deactivated on the specific OneDrive site that stores the chat files (not just at a generic level).
2. Ensure that external sharing settings remain configured as documented in the SharePoint and Microsoft 365 guest sharing references.
3. Continue using the explicit Apply step as a reliable workaround when sharing with guests, as this forces the correct link and permission assignment.

The available documentation does not expose any additional hidden setting (such as a Purview/DLP toggle) that specifically controls this drag-and-drop background permission assignment for B2B guests.

References:

• Error when opening a file in Teams: You don't have access to this file
• Can't share files in a large group chat or meeting chat
• Sharing options are greyed out when sharing from SharePoint Online or OneDrive
• "Your organization's policies do not allow you to share with these users" error when sharing an item or site in SharePoint Online or OneDrive for Business
• Access Denied or You need permission errors in SharePoint Online and OneDrive