---
title: Agent Directory Service (ADS)
url: https://www.emergentmind.com/topics/agent-directory-service-ads
type: topic
---

# Agent Directory Service (ADS)

An Agent Directory Service (ADS) is an infrastructure for the discovery, indexing, and verification of agent capabilities, metadata, and provenance in multi-agent systems (MAS). ADS operates as a distributed registry, supporting efficient, multi-dimensional queries on agent records using content-addressed storage, hierarchical taxonomies, and cryptographic signing. The architecture is formalized within the Open Agentic Schema Framework (OASF), realized atop established technologies such as OCI/ORAS for artifact distribution, Sigstore for provenance, and a Kademlia-based Distributed Hash Table (DHT) for decentralized lookup. Schema-driven extensibility accommodates novel agent modalities including LLM prompt agents, MCP servers, and A2A-enabled components, positioning ADS as a substrate for secure, federated agent discovery [2509.18787].

## 1. Architectural Model

ADS adopts a layered and federated architectural decomposition with strict separation of concerns:

- **Schema Layer**: Defines immutable agent records encoded in OASF. Each record captures agent skills, domains, features, and evaluation signals. Extensibility is provided through media-typed schema extensions.
- **Indexing Layer**: Maintains canonical indices mapping hierarchical taxonomic keys (e.g., "nlp.summarization.abstractive") to Content Identifiers (CIDs), published as immutable OCI artifacts.
- **Storage Layer**: Persists agent records and linked metadata as OCI artifacts within registries backed by object storage systems (S3, GCS).
- **Distribution Layer**: Utilizes OCI registry protocols and ORAS tools for artifact distribution, caching, and replication.
- **Security Layer**: Enforces integrity by content-addressing (SHA-256 digests), Sigstore-powered signing, and transparency logging (Rekor).

Publication involves pushing OASF-formatted agent records and capability indices to OCI registries, followed by index announcement in the DHT. Discovery is realized via two-level mapping: capabilities to CIDs, CIDs to endpoints.

A representative architectural diagram, rendered in LaTeX TikZ, abstracts the layered model:

```latex
\begin{tikzpicture}[node distance=2cm]
\node (Schema) [rectangle, draw] {OASF Agent Record};
\node (Index) [rectangle, draw, below of=Schema] {Capability Index (Skill → CID)};
\node (Storage) [rectangle, draw, below left of=Index, xshift=-1cm] {OCI Registry/Storage};
\node (Distribution) [rectangle, draw, below right of=Index, xshift=1cm] {Distribution (ORAS)};
\node (Security) [rectangle, draw, right of=Distribution, xshift=1cm] {Security (Sigstore)};
\draw[->] (Schema) -- (Index) node[midway, right] {Publish};
\draw[->] (Index) -- (Storage) node[midway, left] {Store};
\draw[->] (Storage) -- (Distribution) node[midway, above] {Retrieval};
\draw[->] (Storage) -- (Security) node[midway, below] {Sign/Verify};
\end{tikzpicture}
```

## 2. Storage and Discovery Layers

**Storage Layer**  
Agent records, built in deterministic JSON using OASF, are serialized and stored immutably as OCI artifacts. Each is addressed by a SHA-256 digest (CID), ensuring deduplication and tamper-evidence. Additional metadata (signatures, evaluations, composition graphs) are attached as referrer artifacts via OCI APIs. Distribution inherits parallel fetching, caching, and CDN capabilities from OCI's ecosystem.

**Discovery Layer**  
ADS supports hierarchical, multi-dimensional taxonomy for discovery. Skills use dotted notation (e.g., "vision.segmentation.pixelwise"), with additional axes for domains and features. Discovery proceeds as:

1. Taxonomic indices associate attribute keys with lists of agent CIDs.
2. Resolvers map CIDs to registry endpoints.

Locator entries are published over a Kademlia-based DHT. Multi-dimensional queries are resolved by intersecting sorted posting lists:
\[
C = P_s \cap \left(\bigcap_i P_{d_i}\right) \cap \left(\bigcap_j P_{f_j}\right)
\]
where $P_s$ is the skill posting list, and $P_{d_i}$, $P_{f_j}$ are lists for domains and features. Only after intersecting indices are actual agent records retrieved, minimizing bandwidth and computation.

## 3. Security and Performance Properties

**Security**  
- **Content Addressing**: All artifacts addressed by SHA-256; any alteration changes the CID and invalidates index entries.
- **Provenance**: Sigstore integration provides keyless signing via ephemeral OIDC-bound certificates, with signatures stored as detached referrer artifacts and recorded in a transparency log (Rekor).
- **Trust Partitioning**: Trust is distributed among publishers (signers), storage registries (non-authoritative wrt integrity), DHT nodes (verified by digest), and clients enforcing signature/evaluation requirements.
- **Threat Mitigations**: Digest verification, index supersession, epoch/sequence tracking, and rate limiting defend against tampering, signature forgery, Sybil attacks, and stale/replay poisoning.

**Performance**  
- **Index Compression**: Capability indices with sorted, compressible posting lists improve query speed.
- **On-Demand Retrieval**: Only candidate CIDs are fetched post-intersection.
- **Caching and Replication**: High-frequency posting lists employ cache validation (weak ETags) and "hotset" replication.
- **Optimized Updates**: Delta indices and probabilistic prefilters (e.g., Bloom filters) are used to minimize update and query overhead.

## 4. Integration with Existing Infrastructures

ADS is architected for interoperability:

- **OCI / ORAS Integration**: Utilizes mature OCI registry protocols and ORAS tools for distribution, transport, parallel fetching, CDN integration, and access control.
- **Provenance via Sigstore**: Keyless signing yields OIDC-based ephemeral certificates. Signatures are detached artifacts and captured in append-only transparency logs (Rekor).
- **No bespoke infrastructure**: ADS leverages existing registry, signing, and DHT/federation mechanisms rather than redefining delivery, reducing operational friction and aligning with established supply chain security patterns.

## 5. Schema-Driven Extensibility

ADS leverages the OASF's modular schema approach:

- **Minimal Core**: Records describe essential metadata—skills, domains, features—with immutability.
- **Extension Mechanism**: Additional payloads (MCP descriptors, A2A metadata, prompt bundles) attach as modular extensions, each with globally unique media types and schema URLs.
- **Compatibility**: Extensions are additive; unknown types can be ignored, preserving backwards compatibility.
- **Support for Modalities**: LLM prompt agents publish "Prompt Bundles" (system/task/evaluation templates); MCP server descriptors capture orchestration endpoints. Schema-driven extension enables rapid adaptation to new agent classes.

A representative pseudocode for an agent record:
```python
class AgentRecord(BaseModel):
    schema: str       # OASF schema ID
    version: str      # semantic version
    skills: List[str]
    domains: List[str]
    features: List[str]
    resources: Dict   # endpoints, model URIs
    metadata: Dict    # ext-friendly map
```

## 6. Position within the Interoperability Landscape

ADS distinguishes itself from naming-centric and runtime communication frameworks:

- **Naming vs Capability Discovery**: Unlike frameworks focusing on mnemonic DNS-like naming and endpoint dynamic resolution (e.g., NANDA [2508.03113], AgentDNS [2505.22368], ANS [2505.10609]), ADS centers on content-addressed, capability-indexed discovery. Names are resolved to CIDs using two-level mappings.
- **Complementarity to Runtime Registries**: Interoperability frameworks (MCP, A2A) focus on execution-time handshake and protocol mediation; ADS is strictly focused on discovery, provenance, and decentralized lookup.
- **Federated Substrate**: The combination of hierarchical taxonomic indices, cryptographic integrity, and DHT-based lookup positions ADS as a complementary substrate for more specialized overlays (ranking, governance, reputation).
- **Leverages Proven Ecosystems**: ADS piggybacks on OCI and Sigstore, minimizing bespoke operational risk and benefitting from robust, widely-adopted infrastructure.

## 7. Summary and Significance

The AGNTCY Agent Directory Service represents a modular, cryptographically verifiable, and highly extensible substrate for MAS discovery. By embedding content-addressed storage, capability-centric hierarchical indices, and federated DHT resolution, ADS enables efficient multi-dimensional queries across heterogeneous agent populations. Schema-driven extensibility supports rapid evolution to new agent modalities, while integration with existing OCI/ORAS and Sigstore infrastructures ensures artifact integrity and provenance at scale. ADS is positioned as a foundational directory service for agent discovery, securely interoperating across diverse AI ecosystems, with explicit separation from naming, identity, and runtime protocol registries [2509.18787].

## Table: Layered Model Overview

| Layer            | Function                | Technologies         |
|------------------|------------------------|----------------------|
| Schema           | Defines agent records   | OASF                 |
| Indexing         | Capability-to-CID map   | OCI artifacts, DHT   |
| Storage          | Artifact persistence    | OCI registry, S3/GCS |
| Distribution     | Fetch/caching/replica   | ORAS, CDN            |
| Security         | Integrity/provenance    | SHA-256, Sigstore    |

This table encapsulates the primary purpose and technology backplane of each architectural layer as described in [2509.18787].

Source: https://www.emergentmind.com/topics/agent-directory-service-ads