FAFA: A Declarative Agent Capability Format

Introduction

Problem Statement Agent-interaction work consistently defines an object model for agent identity, capability, and endpoint, and then defers the serialization to "some JSON" or an unspecified schema. The object model is specified repeatedly; the on-the-wire, in-the-registry, in-the-repository format — the document an agent publishes about itself — is left unspecified.

Solution Overview .fafa specifies that document: a vendor-neutral, YAML-native declaration of what an agent is, what it can do, and how it is reached. It defines the document those mechanisms carry; it does not define discovery, transport, authorization, or orchestration. .fafa is complementary by design. It composes with existing and future agent-interaction protocols and competes with none of them. A protocol that needs an agent-identity-and-capability document MAY use .fafa as that document without adopting any other part of the FAF family.

Design Goals Declarative, never executable — a .fafa document describes an agent; it never instructs, authorizes, or executes one. Vendor-neutral — no dependency on any single agent-interaction protocol. Composable — sits at the format layer beneath protocols, not in competition with them. Forward-compatible — unknown fields are preserved as extensions; documents remain valid across minor revisions. Family-coherent — extends .faf ; shares YAML 1.2 as base syntax and MIT-licensed distribution.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The following terms are used throughout this document: Agent — An autonomous or semi-autonomous software entity that exposes capabilities and accepts invocation. Capability — A named, addressable function the agent can perform. Endpoint — A reachable address plus protocol binding for invoking the agent. Attachment — Optional infrastructure context within which the agent operates (gateway, domain, policy scope). Provenance — Optional context substrate the agent reads from, most commonly a .faf reference.

The Agent Format (.fafa)

Format Overview A .fafa file MUST be a valid YAML 1.2 document . It is declarative: it carries identity, capability declarations, and endpoint references, and MUST NOT contain executable content, code, or instructions to a model.

Media Type The agent format is identified by application/vnd.fafa+yaml, registered in the IANA vendor tree on 2026-06-26 . This document documents that vendor-tree registration. No standards-tree registration is requested. The canonical specification is . The file extension is .fafa.

Document Structure A .fafa document is a single YAML mapping. Example:

Required top-level fields: version; agent (with at least name and id); capabilities (array, MAY be empty); endpoints (array, MUST be non-empty). Optional top-level fields: attachment, provenance, signature, metadata (free-form vendor extensions).

Agent Section Declares identity. Required: name (string, unique within the issuing vendor namespace); id (globally unique identifier — DID, URI, or vendor-scoped UUID). Optional: vendor; version (semantic version of the agent, NOT of this specification); description; homepage; license (SPDX identifier).

Capabilities Section Each capability is a structured object. Required: name (unique within the document); type (tool, resource, prompt, event, or other). Optional: description, input_schema, output_schema, tags, apophatic (boolean; if true, the capability refuses out-of-scope invocation rather than attempting best effort), cites_spec (the format or specification the capability operates against). The capabilities array MAY be empty. Declaration is not authorization (see ).

Endpoints Section Each endpoint declares a protocol binding. Required: protocol (mcp, a2a, grpc, http, or other); transport (stdio, http, websocket, quic); location (reachable address — URI, package name, or transport-specific locator). Optional: version, auth, region, health. Multiple endpoints MAY declare the same protocol with different transports.

Attachment Section (Optional) Describes the infrastructure context within which the agent operates, without requiring any particular gateway protocol. Optional fields: gateway, domain, policy, attachment_point.

Provenance Section (Optional) Cites the context substrate the agent operates from — the FAF-family integration point. A .fafa agent SHOULD declare which .faf it reads. Optional fields: faf (reference to a .faf document), fafm (reference to a .fafm document), spec (media type of the substrate), version.

Parser Requirements Consumers MUST use YAML safe-load (no custom type construction). Consumers MUST treat unknown top-level fields, and unknown subfields within agent, capabilities, and endpoints, as forward-compatible extensions. Consumers SHOULD use UTF-8 encoding and SHOULD keep documents within common registry storage budgets (RECOMMENDED: < 64 KB).

File Conventions Standard filename: agent.fafa (alongside project.faf at the project layer), or served via the /.well-known/faf discovery anchor defined in . File extension: .fafa. Encoding: UTF-8. Multiple .fafa documents per scope are permitted, distinguished by agent.id.

Versioning The top-level version field declares the version of this specification to which the document conforms. Versioning is semantic: 1.x revisions are backward-compatible; a new major version is reserved for breaking changes made only with strong cause. Consumers MUST accept documents whose version shares their supported major version, and MUST apply forward-compatible extension handling for unknown fields. This separates the agent's version (agent.version) from the format's version (top-level version), so an agent may revise independently of the specification.

Relationship to the FAF Family .fafa is the agent member of the FAF family :

Context Layer (project IS -- static, read once) .fafm -> Memory Layer (agent REMEMBERS -- mutating) .fafa -> Agent Format (agent IS -- declarative identity) ]]> .fafa is format-level, not protocol-level. It composes with — does not replace — agent-interaction protocols: a protocol carries, references, or resolves a .fafa document; the document declares the agent the protocol acts upon. The .fafa schema extends application/vnd.faf+yaml additively. A consumer that understands only .faf parses a .fafa file as valid YAML and extracts top-level identity; a .fafa-aware consumer gains the full agent declaration. A FAF-enabled origin MAY publish its .fafa document for discovery via the /.well-known/faf URI defined in .

Security Considerations This section addresses the IANA media-type security-considerations checklist for application/vnd.fafa+yaml.

Active or Executable Content None. A .fafa document is YAML data with no scripts, code, or active markup. Implementations MUST treat .fafa content as data, never as instructions or commands. Declaration is not authorization: an implementation MUST NOT invoke or auto-execute a capability solely because it is declared.

YAML-Specific Vulnerabilities .fafa is built on YAML 1.2; the YAML security considerations of apply. Parsers MUST disable custom type construction (e.g., !!python/object, !ruby/object), MUST enforce alias-expansion limits to mitigate entity-expansion ("billion laughs") attacks, and MUST enforce nesting-depth limits to prevent stack overflow.

Untrusted Input and Prompt Injection .fafa documents originate from agents and third parties and MUST be treated as untrusted input. Consumers MUST NOT elevate .fafa content to "system" or "developer" tiers within prompt hierarchies; it occupies the user-data tier. Capability declarations are a documented vector for over-trust and prompt injection.

Identity, Endpoints, and Least Privilege Identity claims, capability declarations, and endpoints are self-asserted. Consumers MUST treat endpoints as untrusted until verified (against signature where present), MUST scope capability consumption to the declaring agent's verified identity, and MUST apply least privilege when acting on declared capabilities.

Prohibited Content .fafa documents MUST NOT contain secrets, API keys, or credentials; MUST NOT contain user-personal data (use .fafm for memory of that kind); and MUST NOT contain executable code.

IANA Considerations

Media Type (Documentation Only) The agent format is already registered in the IANA "Media Types" registry, vendor tree: application/vnd.fafa+yaml — registered 2026-06-26 . File extension .fafa. Published specification: . This document documents that registration. No new media-type registration is requested.

Relationship to the Existing Vendor-Tree Registration The vendor-tree registration remains valid. This document does not request promotion to the standards tree.