Eric,
The dilemma is real and I do not think you missed a mechanism. It is worth
naming why it arises structurally: an identity assertion is audience-locked to
the party it was issued to, and delegation through an intermediary requires
either re-minting at the issuer or a way for the intermediary's participation
to be first-class in the exchange. Section 4.3.3 as written admits neither, so
every gateway hop reproduces your dilemma.
Two resolution families exist, and the draft could name which it intends.
Within the current model, RFC 8693 already has the missing piece: the
actor_token. A composite exchange where the gateway authenticates as itself,
presents the agent's assertion as subject_token and its own credential as
actor_token, and the IdP applies the audience check to the subject token's
original audience while separately authorizing the actor as a registered
intermediary for that audience, resolves your topology without new machinery.
The cost is that the IdP must now hold policy about which intermediaries may
act for which clients, and the exchange remains a per-upstream, per-hop round
trip to the IdP: the gateway mints one ID-JAG per upstream AS, on the critical
path.
The other family carries the delegation as a verifiable artifact instead of
re-minting it: the agent delegates narrowed authority to the gateway in a form
the upstream can verify directly against the original issuer's key, so the
intermediary attenuates rather than exchanges, and no per-hop trip to the IdP
exists. That property class (attenuation verifiable from the artifact alone, no
runtime callback to the issuing side, possession bound at each hop) is what R1,
R3, and R4 in draft-reece-wimse-cross-org-delegation describe, and gateway
aggregation is among the cleanest motivating topologies for it.
Either way, your topology deserves to be in the record the working group is
building this week: the chairs have asked for use cases and problem statements
before solutions, and agent-behind-gateway with a public-client agent is
arguably the most common deployment shape MCP has produced. I would encourage
writing it up exactly as you have here.
Morgan
On Wednesday, July 22nd, 2026 at 7:23 AM, Eric Leleu
<[email protected]> wrote:
> Hi all,
>
> I'd like to raise a topology question about the ID-JAG draft
> (draft-ietf-oauth-identity-assertion-authz-grant) that I expect to come up
> often as agentic/MCP deployments grow, and I don't think the current text
> resolves it from my understanding.
>
> Topology Description:
> ----------------------------
>
> - An AI coding agent (e.g. Claude Code) acts as an MCP Client. It is
> registered at the enterprise IdP as a *public* client (native/CLI app, PKCE,
> no client secret).
> - The agent authenticates directly against the enterprise IdP. As per OIDC
> Core, the resulting ID Token's `aud` is the agent's own client_id. In the
> same flow the agent also obtains an access token whose audience/resource is
> an MCP Gateway sitting in front of it.
> - The MCP Gateway aggregates several upstream MCP servers. Each upstream
> server is protected by its own Resource Authorization Server, and each of
> those independently trusts the same enterprise IdP.
> - Per ID-JAG, to reach a given upstream server the Gateway needs an access
> token minted through the ID-JAG flow: Token Exchange at the IdP to mint an
> ID-JAG (aud = upstream AS) and then execute JWT-Bearer redemption at that AS.
>
> The conflict :
> -----------------
>
> Section 4.3.3 [1], "IdP Token Exchange Processing Rules", requires that when
> the subject_token presented in the Token Exchange call is an identity
> assertion (ID Token), "the IdP MUST validate the assertion and MUST validate
> that the audience of the assertion (e.g. the aud claim of the ID Token or
> SAML Audience) matches the client_id of the client authentication of the
> request". In the Gateway topology above, this rule creates a dilemma where
> neither party can execute the exchange:
>
> 1. The agent is the audience of the ID Token, but the agent has no reason to
> call Token Exchange itself — it doesn't know which upstream AS/audience a
> given MCP tool call needs, and as a public client it's also not well
> positioned to be trusted with that capability directly. As defined in the
> draft "this specification SHOULD only be supported for confidential clients."
> 2. The Gateway is the party that actually needs the ID-JAG (it knows the
> target upstream AS), and it is the Gateway that would authenticate the Token
> Exchange call. But the ID Token's `aud` is the agent's client_id, not the
> Gateway's. Per the mandatory audience check above, the IdP must reject that
> exchange no matter how the ID Token reaches the Gateway (forwarded by the
> agent or otherwise).
>
> From my understanding, neither party in this agent-to-gateway architecture
> can legally execute the ID-JAG exchange under the current specification.
>
> Did I miss a mechanism already defined in the draft (or in
> draft-ietf-oauth-identity-chaining) to handle this delegation/proxy pattern?
> Is there ongoing work to address multi-hop or API Gateway patterns where the
> identity assertion's audience differs from the calling proxy's client_id?
>
> Thanks,
> Eric
>
> [1]
> https://datatracker.ietf.org/doc/html/draft-ietf-oauth-identity-assertion-authz-grant#name-processing-rules
> --
>
> Eric LELEU
>
> Staff Software Engineer
>
> E [[email protected]](mailto:[email protected])
>
> Hold Nothing Back
>
> http://youtube.com/c/Graviteesource?sub_confirmation=1https://www.linkedin.com/company/gravitee-io
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