Describe the bug

Python's FoundryToolbox never sends the Foundry-Features header, and ignores the
platform-injected FOUNDRY_AGENT_TOOLSET_FEATURES environment variable. The .NET implementation
does both, so the two SDKs disagree on what a toolbox request looks like.

The Foundry toolbox reference documents the header as mandatory:

Header: Always include Foundry-Features: Toolboxes=V1Preview.

Python — packages/foundry_hosting/agent_framework_foundry_hosting/_toolbox.py, the only place
request headers are set (_ToolboxAuth._apply_headers):

def _apply_headers(self, request: httpx.Request, token: AccessToken) -> None:
    request.headers["Authorization"] = f"Bearer {token.token}"
    for key, value in get_request_context().platform_headers().items():
        request.headers[key] = value

grep -rn "Foundry-Features" python/packages returns nothing, and no code reads
FOUNDRY_AGENT_TOOLSET_FEATURES.

.NET — dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/FoundryToolboxService.cs reads the
features variable at startup and merges it into the header it sends on every request:

this._featuresHeader = Environment.GetEnvironmentVariable("FOUNDRY_AGENT_TOOLSET_FEATURES");

Impact

Requests currently succeed without the header against today's service, so this is latent rather
than breaking. The risk is that it is documented as required and is the mechanism by which the
service gates preview toolbox behaviour — so a future service change, or any feature flag the
platform passes via FOUNDRY_AGENT_TOOLSET_FEATURES, would silently not apply to Python callers.

To Reproduce

1. Build a FoundryToolbox and connect it to any toolbox MCP endpoint.
2. Capture the outbound request headers.
3. Observe Authorization and x-agent-foundry-call-id, but no Foundry-Features.

Expected behavior

FoundryToolbox should send Foundry-Features: Toolboxes=V1Preview by default and merge any
values from FOUNDRY_AGENT_TOOLSET_FEATURES, matching FoundryToolboxService in .NET.

Platform

• Language: Python
• Package: agent-framework-foundry-hosting