diff --git a/ROADMAP.md b/ROADMAP.md
index 3b0974a..780fc2b 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -43,4 +43,7 @@ Real hardware attestation verification (SEV-SNP VCEK chain, Intel TDX quote via
 - Stable delegation credential and TRACE link schema with documented versioning guarantees
 - Full RATS/EAT conformance for peer attestation evidence
 - Conformance suite for "cA2A-compatible" claims: **landed** (`tests/conformance/`, normative README + runnable MUST-level checks, in CI). A production run on confidential-computing hardware is the remaining step for a hardware-attested claim.
+- **Upstream A2A interoperability: not started.** Our own conformance suite says a peer implements cA2A. It says nothing about whether attaching the profile keeps us compatible with the wider A2A ecosystem, and that is a separate question worth answering with numbers. Plan: run the official [`a2aproject/a2a-tck`](https://github.com/a2aproject/a2a-tck) conformance suite first, then the A2A Interoperability Test Kit's multi-language launcher, which covers cross-SDK compatibility across Python, Go, TypeScript, Java and Rust. Publish the result as `docs/interop-report.md`, failures reported at the same volume as passes.
+  - The system under test is an official `a2a-sdk` server with the extension attached through `ca2a_runtime.transport.a2a_sdk`, **not** `ca2a start`. A normative A2A transport is explicitly out of scope for this profile (see [LIMITATIONS.md](LIMITATIONS.md)), so pointing a conformance suite at the reference standard-library transport would fail baseline MUSTs for reasons that say nothing about cA2A.
+  - The first thing to look for is whether the protobuf `Struct` integer-ness round trip documented in `ca2a_runtime.transport.a2a_sdk` lands in the same safe place on every SDK. `Struct` has no integer type, so a credential's `depth` of `0` returns as `0.0`, and the chain verifies only because `DelegationCredential.from_dict` coerces with `int()` before RFC 8785 canonicalisation, which refuses floats outright. That it is safe in Python is tested. That it is safe in Go and Rust is an assumption.
 - OWASP liaison on the multi-agent threat mapping; ITI conversation on conformance