Agent Communication Protocol (ACP)

Updated 3 July 2025

Agent Communication Protocol (ACP) is a standardized framework that enables autonomous agents to exchange multimodal information and coordinate complex tasks.
ACPs employ structured messaging, stateful and stateless operations, and robust discovery protocols to ensure seamless interoperability across diverse agent ecosystems.
The protocol supports dynamic workflow coordination, extensibility, and integrated security measures to mitigate common communication threats in multi-agent environments.

Agent Communication Protocol (ACP) formally refers to standardized mechanisms that enable autonomous agents—especially those powered by LLMs—to exchange multimodal information, coordinate dynamic behaviors, and perform collective inference or tasks with other agents, users, and external environments through structured, verifiable protocol interactions. ACPs are foundational to the development of interoperable, scalable, and trustworthy multi-agent ecosystems, functioning as the communication backbone of the emerging Internet of Agents (Kong et al., 24 Jun 2025).

1. Architectural Principles and Core Features

Modern ACPs encompass diverse design paradigms to address the demands of heterogeneous, often open-world, multi-agent environments. Key architectural features include:

Standardized Structured Messaging: ACPs employ schema-constrained, typed messaging formats (e.g., JSON multipart, HTTP REST, or RPC-style envelopes) to encode agent requests, responses, and context, facilitating interoperability and multimodal exchanges (Ehtesham et al., 4 May 2025, Georgio et al., 30 Apr 2025).
Stateless and Stateful Operation: ACPs may support session management (via thread or task IDs), enabling complex, multi-turn dialog and workflow persistence as well as simple direct invocations (Ehtesham et al., 4 May 2025).
Agent Discovery and Registration: Robust protocols, agent registries (sometimes leveraging DNS-inspired architectures), and naming conventions ensure agents can be discovered by capability or identity and validated for secure operation (Huang et al., 15 May 2025).
Extensible Protocol Evolution: Mechanisms such as protocolExtension fields enable backward-compatible upgrades, supporting both security enhancements and new features without ecosystem fragmentation (Huang et al., 16 Jun 2025).

These protocols may be implemented within brokered client-server topologies, decentralized peer-to-peer architectures, or hybrid models combining elements of both (e.g., Agent Cards resolved by registry, but supporting direct communication) (Kong et al., 24 Jun 2025).

2. Workflow Enforcement, Reasoning, and Coordination

ACPs extend communication beyond isolated message exchange by modeling and enforcing structured, long-horizon workflows:

Execution Blueprints and Graph-based Coordination: Some ACPs formalize multi-agent task execution as persistent Directed Acyclic Graphs (DAGs), where each node is an agent or tool action, and edges encode dependency or data flow (Bhardwaj et al., 20 May 2025).
- Let $\mathcal{G} = (\mathcal{O}, E)$ represent such a blueprint, where $\mathcal{O}$ is the set of agent actions, and $E$ the directed dependency edges.
Standardized Message Types and Error Handling: ACPs define rigorous message schemas (e.g., AGENT_REQUEST, AGENT_RESPONSE, ASSISTANCE_REQUEST) along with standardized status/error codes (e.g., 601 MISSING_REQUIRED_PARAMETERS, 604 TOOL_CALL_FAILURE), supporting explicit, machine-readable error propagation and automated recovery (Bhardwaj et al., 20 May 2025).
Conversation/Protocol Reasoning Engines: Some frameworks (e.g., ACRE) embed finite-state-machine-based reasoning, grouping and tracking conversation state, binding variables, and enabling verification and external monitoring (Lillis et al., 2015).

This structuring promotes robust, fault-tolerant, and explainable multi-agent systems, enabling logic-aware orchestration and the graceful handling of partial failures.

3. Interoperability, Semantic Alignment, and Agent Ecosystem Management

ACPs enable interoperability both at the technical and semantic levels:

Semantic Capability Modeling: Standardized, ontological descriptions of agent and tool capabilities support semantic discovery, orchestration, and reasoning about protocol equivalence and specialization relationships (Berges et al., 22 Jan 2024, Berges et al., 29 Jan 2024, Liu et al., 18 May 2025).
Agent Name Services (ANS): Inspired by the Domain Name System (DNS), ANS provide secure, protocol-agnostic directories for agent discovery, leveraging Public Key Infrastructure (PKI) for certificate-based identity and trust, and modular protocol adapter layers for cross-protocol resolution (e.g., ACP, MCP, A2A) (Huang et al., 15 May 2025).
Registration, Discovery, and Binding Protocols: ACP suites typically include standardized registration (ARP), discovery (ADP), interaction (AIP), and tooling (ATP) components, supporting capability-oriented lookups, negotiation, and session setup (Liu et al., 18 May 2025, Huang et al., 16 Jun 2025).
Vendor and Framework Neutrality: Open schemas, decentralized identifiers (DIDs), and dynamic module adapters promote cross-platform participation and interoperability (Georgio et al., 30 Apr 2025).

Such features collectively allow agents from different vendors, domains, and deployment environments to participate seamlessly in large, heterogeneous agent networks.

4. Security, Trust, and Threat Mitigation

Security is a central concern in ACP design due to the open and dynamic nature of modern agent ecosystems. Identified threats and corresponding defense strategies include:

Authentication and Authorization: PKI-backed digital signatures, mutual TLS, short-lived scoped tokens, and role-based access control are mandated for agent registration, message exchange, and endpoint validation (Huang et al., 15 May 2025, Huang et al., 16 Jun 2025).
Capability Attestation and Verification: Agents provide cryptographically verifiable evidence of their declared capabilities (e.g., through X.509 certificates, behavioral attestations, or zero-knowledge proofs) (Huang et al., 16 Jun 2025).
Dynamic Trust and Policy Enforcement: Future directions highlight the need for runtime trust negotiation and context-sensitive policy languages, moving beyond static ACLs or permission sets (Ferrag et al., 29 Jun 2025).
Defense Against Protocol Exploits: Systematic threat taxonomies have identified critical attacks—prompt injection, schema poisoning, agent spoofing, registration poisoning, token theft/replay, and man-in-the-middle exploitation (Kong et al., 24 Jun 2025, Ferrag et al., 29 Jun 2025).
- Empirical results show prompt injection and related vulnerabilities affect >50% of defenses, sometimes up to 90–100% against weakly configured ACP deployments (Ferrag et al., 29 Jun 2025).
Protocol Security Features Table:

Threat	ACP Defense Mechanism	Example Reference
Man-in-the-middle (MitM)	Mutual TLS, end-to-end encryption	(Huang et al., 15 May 2025, Huang et al., 16 Jun 2025)
Token Theft/Replay	Nonces, short-lived tokens, signature checks	(Huang et al., 16 Jun 2025, Ferrag et al., 29 Jun 2025)
Discovery/Registration Attacks	PKI validation, signed registry entries	(Huang et al., 15 May 2025)
Schema Poisoning	JSON schema signing and enforcement	(Ferrag et al., 29 Jun 2025)
Agent Spoofing	Certificate chain validation, behavioral auditing	(Huang et al., 16 Jun 2025, Huang et al., 15 May 2025)

Comprehensive auditing, immutable logs, and integration with on-chain (blockchain) systems may further harden security guarantees.

5. Scalability, Extensibility, and Performance

ACP frameworks are developed with scaling and extensibility as core properties:

Distributed/Hierarchical Registration and Discovery: Tree-based or federated registry structures allow for parallelization and resilience in registration and lookups, supporting scaling to millions or billions of agents (Liu et al., 18 May 2025, Huang et al., 15 May 2025).
Dynamic Resource Management: Protocol-integrated scheduling and resource awareness enable agents to predict and balance computational loads, using techniques such as predictive caching and ML-based demand estimation (Liu et al., 18 May 2025).
ProtocolExtension and Versioning: Modular extension mechanisms permit incremental feature addition and backward compatibility—ensuring robust evolution without ecosystem fragmentation (Huang et al., 16 Jun 2025).
Performance and Robustness: Empirical results demonstrate robust performance on complex, long-horizon multi-agent tasks (e.g., 28.3% SOTA accuracy on AssistantBench for web assistance) and resilience to partial workflow failures (via DAG-based blueprinting and error propagation) (Bhardwaj et al., 20 May 2025).

ACP's design thus supports rapid onboarding, protocol-upgrade cycles, and robust, high-throughput collaboration across diverse agent populations.

6. Emerging Challenges and Research Directions

While ACPs provide a crucial foundation for next-generation agent systems, the literature identifies several research frontiers:

Dynamic Policy and Trust Management: There is a need for context-aware, programmable trust and policy frameworks that accommodate the dynamic, language-driven workflows typical in multi-agent, LLM-powered environments (Ferrag et al., 29 Jun 2025).
Cryptographic Provenance and Auditing: End-to-end provenance tracking, cryptographic attestation, and integration with tamper-evident storage systems are recognized as important for verifying and attributing agent actions (Ferrag et al., 29 Jun 2025, Huang et al., 15 May 2025).
Resilience to Protocol and Systemic Exploits: New defense mechanisms are required to address supply chain, memory/context poisoning, cross-agent prompt injection, privilege escalation, and system-level batching or flooding attacks (Kong et al., 24 Jun 2025).
Legal, Regulatory, and Accountability Concerns: Emerging standards must address responsibility attribution, intellectual property, and compliance in distributed agent settings that cross organizational and geopolitical boundaries (Kong et al., 24 Jun 2025).
Generalizability of Defenses: There is ongoing work to develop modality-agnostic, protocol-agnostic security tools and defenses—including anomaly detection and formal verification across complex, cross-modal (text, code, image) ACP traffic (Ferrag et al., 29 Jun 2025).

This suggests that the continued evolution of ACPs will focus not only on technical interoperability and workflow coordination, but equally on adaptive, multi-layered security, attribution, and compliant operation in an open, global agent ecosystem.

In sum, the Agent Communication Protocol (ACP) constitutes an essential infrastructure for multi-agent coordination, collective inference, and secure, interoperable communication in contemporary and future LLM-driven AI systems. Its rigorously structured, extensible, and security-focused design addresses the needs of heterogeneous autonomous agents, enabling them to collaborate, reason, and act in robust, scalable, and trustworthy environments.