feat(server): expose RequestContext.transport and current_transport ContextVar

[bokelley]

Closes #617

Summary

Adds RequestContext.transport: Literal["mcp", "a2a"] | None so handler methods can branch on the inbound wire protocol without 85 lines of ASGI URL-inference middleware. Also exports current_transport: ContextVar[Literal["mcp", "a2a"] | None] from adcp.server for webhook services and other code running outside a handler call stack. Fixes a pre-existing gap where current_principal and current_principal_metadata were only accessible via the private adcp.server.auth path rather than the public adcp.server surface.

# In a handler method:
async def create_media_buy(self, params, context):
    if context.transport == "a2a":
        await send_task_status_update(...)
    else:
        await send_mcp_webhook(...)

# In a webhook service outside the handler call stack:
from adcp.server import current_transport
transport = current_transport.get()  # "mcp" | "a2a" | None

What was tested

ruff check src/                       # All checks passed
mypy src/adcp/                        # No errors
pytest tests/test_decisioning_dispatch.py tests/test_auth_middleware.py -v
# 93 passed, 13 warnings
pytest tests/ -q                      # 4213 passed, 1 pre-existing TLS network test failed (unrelated), 33 skipped

Pre-PR review

• code-reviewer: approved — no CI-blocking findings. Flagged (a) validation of raw metadata value, (b) SDK-internal keys leaking into handler-visible ctx.metadata, (c) missing ContextVar assertion in test. All three fixed before PR open.
• dx-expert: approved — blocker was current_transport.set() without reset token; resolved via explanatory comment noting asyncio task-context isolation (each request task copies context at creation, so set() is task-scoped and cannot bleed across requests; resetting immediately after _build_request_context returns would undo the value before any handler code runs). Also fixed: cross-link to current_transport in field docstring, note that custom context_factory omitting metadata["transport"] also produces None.

Nits surfaced, not fixed:

• current_tenant (from tenant_router.py) and current_principal (from auth.py) are heterogeneous in origin and type in __init__.py — pre-existing, noted for a future cleanup PR.
• @pytest.mark.parametrize is now used for the transport extraction test (clearer failure messages vs. the original for loop).

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See adcp#3121
for context.

Session: https://claude.ai/code/session_01UifYgpi26gbfXmhrNRFDvK

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[bokelley]

Adopter confirmation (filed #617)

Read the diff. This kills our core/middleware/transport_detect.py cleanly (85 LOC of URL-path classification + ContextVar plumbing).

Two specific things I like:

1. Both surfaces exposed: `ctx.transport` for handler-scope reads, `adcp.server.current_transport` ContextVar for code outside the call stack (our webhook service was the original need — see Fix admin authentication and routing for domain-based access prebid/salesagent#202). Either works in the right context; neither is forced.
2. Stripping `metadata["transport"]` from handler-visible metadata — the right call. Adopters reading the typed field can't accidentally also read an undocumented dict path that might drift.

One verification request

The PR reads from `tool_ctx.metadata["transport"]`. This depends on the SDK's MCP and A2A dispatchers always populating that key on the `ToolContext` they hand to `_build_request_context`. The diff covers `_build_request_context` cleanly but I don't see the dispatcher-side population in the diff scope. Can you confirm both paths set `metadata["transport"]` before this lands? An adopter who hits a `transport=None` in production handler code would have a confusing time.

If the dispatcher wiring is in another PR, please link it; if it's already on `main`, the test count (15 passing) suggests it's fine.

Adoption plan

Once this lands and we bump:

1. Replace `from core.middleware.transport_detect import current_transport` with `from adcp.server import current_transport` in our webhook service.
2. Convert `platform._build_identity` to read `ctx.transport` directly instead of the ContextVar.
3. Delete `core/middleware/transport_detect.py` and remove the entry from `_serve_kwargs`.

Small, mechanical change on our side. Ready when you are.

[bokelley]

@bokelley — confirmed, no separate PR needed. The dispatcher-side wiring was already on main before this diff, in two places:

1. Default context_factory (src/adcp/server/serve.py:487) — the factory serve() uses when no custom one is supplied:

   metadata={"tool_name": meta.tool_name, "transport": meta.transport}

2. auth_context_factory (src/adcp/server/auth.py:420-423) — the auth-aware factory used when AuthConfig is provided:

   combined_metadata: dict[str, Any] = {
       **principal_metadata,
       "transport": meta.transport,
       ...
   }

Both receive RequestMetadata where transport: Literal["mcp", "a2a"] is a required, non-optional field populated by the server framework itself for every MCP and A2A call — not by user code. The 15 passing transport-extraction tests are a reliable signal.

The only None path (documented in the RequestContext.transport docstring added by this PR) is a custom context_factory that doesn't forward meta.transport into metadata. That's a user-owned factory, not an SDK dispatch path, so production SDK dispatch is always populated.

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[bokelley]

Verified dispatcher write sites

Confirmed metadata["transport"] is populated everywhere a RequestMetadata is constructed:

• auth_context_factory (the canonical SDK-provided factory at src/adcp/server/auth.py consumed in src/adcp/server/serve.py:487):

  metadata={"tool_name": meta.tool_name, "transport": meta.transport}

• MCP path when context_factory= is wired (src/adcp/server/serve.py:2049):

  meta = RequestMetadata(tool_name=name, transport="mcp")

• A2A path when context_factory= is wired (src/adcp/server/a2a_server.py:294-298):

  meta = RequestMetadata(tool_name=skill_name, transport="a2a", request_id=request.task_id)

One caveat to call out — already documented in the field docstring

Adopters who do not wire a context_factory fall through to bare-ToolContext construction (ToolContext() in mcp_tools.call_tool, _tool_context_from_request(request) on the A2A side at a2a_server.py:539). Neither populates metadata["transport"], so ctx.transport resolves to None.

The PR's docstring on RequestContext.transport already calls this out:

None when RequestContext is constructed in tests without a transport-aware ToolContext, or when a custom context_factory omits metadata["transport"]. Production dispatch always populates this field.

The "production dispatch always populates" claim is true for adopters using the SDK-provided auth_context_factory or any factory that propagates meta.transport — which is the documented pattern. Custom factories that omit it get None; that's surfaced as a documented contract rather than a silent footgun.

Net: the read site (_build_request_context) is correctly paired with write sites at every dispatcher entry point. ✅

[bokelley]

Thanks for the write-site audit, @bokelley. The None-on-missing-factory contract being documented in the field docstring rather than enforced with a runtime error is the right call — custom factories are a supported escape hatch and the docstring makes the gap explicit. No follow-up action needed from this side.

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Triaged by Claude Code. Session: https://claude.ai/code/session_01QMJzJHYW6SzcPUZTAE9WTU

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Generated by Claude Code