0002 — Matrix as the Human↔Agent Collaboration Fabric

Status: Accepted

Context

Given the open-standard mandate (ADR 0001), the platform needs a chat fabric where humans and agents are first-class co-members of the same rooms and @mention is the delegation primitive. The fabric must supply: real, federatable identities for agents (@agent-k8s:fgentic.fmind.ai), a mature multi-client ecosystem (web + native mobile), typed mentions, and — decisively — a path to external-network interoperability without a rebuild.

Alternatives considered and rejected:

1. Proprietary Slack / Discord / Teams. Best-in-class UX and zero ops, but SaaS lock-in, no self-hosting/sovereignty, and no open identity model for agent members — fails ADR 0001's core mandate.
2. Mattermost. Go, self-hostable, genuinely credible — but no sovereign end-to-end-encryption story and a far weaker bridge ecosystem; external-network interop would be bespoke per network.
3. Zulip / Rocket.Chat. Heavier stacks (Python+DB / Node), threading models that don't map cleanly onto fire-and-forget delegation, and again no comparable bridge fabric.
4. A bespoke Go web chat UI. Total control, but throws away the entire client ecosystem (Element Web + Element X mobile), interoperability, and identity federation — reinventing a decade of Matrix for a strictly worse result.

Decision

Adopt Matrix as the collaboration fabric and UI. Each agent is a first-class room member with its own Matrix identity; a human (or another agent) delegates by addressing @agent-name, which populates the typed m.mentions.user_ids field (MSC3952) that the bridge keys off (ADR 0005). The homeserver, auth, and clients are supplied by Synapse + MAS + Element (ADR 0003).

The interop payoff is structural, not bolted on: because a Matrix room is network-agnostic, the mature mautrix bridge ecosystem (Slack/Telegram/Signal/WhatsApp/…) later connects the same rooms to external networks as additional appservices — a config addition, not a rebuild.

Consequences

1. @mention becomes the universal, already-understood delegation gesture — no bespoke command syntax to teach.
2. Agents get durable, federatable identities and full room semantics (replies, threads, membership) for free.
3. The bridge ecosystem is the strategic asset: "chat with my agents" upgrades to "chat with my agents and anyone on any network."
4. Matrix's operational weight (a homeserver + auth) is inherited — mitigated by ESS packaging (ADR 0003).
5. Matrix's E2EE complexity is deferred by keeping agent rooms unencrypted (ADR 0008).