For immediate release · London, 28 May 2026
A vendor-neutral specification lets voice AI agents, IVRs, contact centres and humans hand off live conversations, with verified identity, intent, consent and media continuity, across competing platforms. Multi-vendor stewardship is a precondition of any v1.0 release.
Cloudax today published the public draft of OHP: the Open Handoff Protocol, a vendor- neutral specification for handing off live voice conversations between AI agents, IVRs, contact centres and human staff. The draft is available at github.com/OpenHandoffProtocol/OHP under an Apache-2.0 licence.
OHP closes a gap the voice industry has worked around for more than a decade: there is no shared way for a voice AI from one vendor to hand a live conversation, with verified caller identity, intent, slot state, consent and media continuity, to a voice AI, IVR or human staffed by a different vendor. Every layer below the conversation already has a standard. The conversation itself does not.
The problem, in four numbers
The gap shows up in measurable customer experience and cost, not just in architecture diagrams.
Forrester's 2025 CX Index: callers asked to repeat information after an IVR-to-agent transfer.
Industry standards covering AI-to-AI conversation state handoff. SIP, WebRTC, MCP and OVON each own a different layer.
Proprietary handoff formats observed across Genesys, NICE, Avaya, Twilio, Vapi, Retell, Bland, ElevenLabs, Cloudax, Pindrop, Cresta, PolyAI, Cognigy and Voiceflow.
Average added cost per call from re-authentication and re-discovery on warm transfer (Cloudax internal audit, n = 412k SIP transfers, Q1 2026).
“Every layer below the conversation already has a standard: SIP for signalling, WebRTC for media, MCP for tool use, OpenTelemetry for observability. The conversation itself doesn't. OHP is the missing layer, and it is too important to be owned by any one vendor, including us.”
What OHP defines
OHP is a tiered wire format for live conversation handoff. It composes with, not replaces,SIP, WebRTC, MCP, OVON Open Floor and existing CPaaS control planes. The specification defines five composable layers.
- L4: Conversation State Envelope (CSE).A signed, canonical JSON/CBOR document carrying verified caller identity, intent stack, slot state, redacted transcript, model memory hints, consent receipts and media continuity proofs.
- L3: Control plane.Five idempotent, ack-ed verbs:
PROPOSE,ACCEPT,REJECT,ASSUME,RETURN, plus a reservedINCIDENTverb for serious- incident reporting under EU AI Act Article 73. - L2: Transport bindings.SIP INFO, WebSocket, HTTP and gRPC, so the same envelope rides telephony, web and operations planes unchanged.
- L1: Trust and identity.Hybrid Ed25519 + ML-DSA-65 signatures (post-quantum hedged, FIPS 204), HPKE per-recipient sealing, SD-JWT Verifiable Credentials, ISO/IEC 27560 consent receipts and a Sigstore-style transparency log.
- L0: Media continuity.SDP re-INVITE hints, SFU track forwarding and an optional audio-fingerprint check that binds each envelope to a specific call leg.
Four conformance tiers: adoption floor to regulated ceiling
OHP is engineered so a small vendor can ship the floor tier in a long weekend, while a regulated enterprise can light up the ceiling without changing the wire format. Each tier is a strict superset of the one below.
A signed JSON document with five verbs over TLS. Sufficient for anonymous IVR navigation, scheduling, FAQ and “where was I when the call dropped?” continuity. Implementable in days by any vendor that can JSON-encode a struct and POST it over HTTPS.
Adds detached Ed25519 JWS over canonical CBOR, JWK Set publication and envelope idempotency. Sufficient for authenticated, non-PII customer service.
Adds HPKE per-recipient sealing, SD-JWT VC selective disclosure, ISO/IEC 27560 consent receipts, DPoP sender constraint and RTP fingerprint binding. Sufficient for HIPAA PHI via sealed FHIR refs, PCI network tokens and cross-border PII under a lawful-transfer mechanism.
Adds post-quantum hybrid signing, a Sigstore-style transparency log, OpenID Federation trust establishment and machine-readable EU AI Act metadata: Article 5 prohibited-practice attestation, Article 12 logging, Article 14 oversight state, Article 50 disclosure proof, Article 73 incident reporting and Article 86 right-to-explanation endpoint.
Workloads declare the minimum tier they require, and the wire format itself, not vendor goodwill or quarterly attestation paperwork, refuses non-compliant handoffs. A regulated enterprise can mandateaudience.minTier="sealed" or "regulated"in procurement and have machine-enforceable refusal at the wire if a counterparty tries to hand off below that bar.
Designed for regulated data from the first line
OHP is the first voice-handoff format engineered to be machine-readable for both the EU AI Act and ISO/IEC 42001 AI Management System. A single envelope carries the AI system identity and version, risk classification, Annex III clause, CE conformity reference, AIIA and FRIA references, GPAI provider chain (Article 53), Article 50 disclosure proof, Article 14 oversight posture and a structured per-decision provenance chain.
A regulator landing on a single envelope hash can reconstruct the full accountability picture for a conversation: which AI system processed the call, under what risk class, what was disclosed to the caller and when, who supervised, what decisions the system made and why, and which upstream models were in the chain, without vendor-specific tooling and without payload access.
Every conformant implementation must satisfy all six.
- Minimum disclosure.Raw PAN, CVV, passwords, OTPs, full SSNs, biometric templates and free- form PHI are prohibited in any OHP field, at any tier.
- Selective disclosure.Identity claims use SD-JWT Verifiable Credentials: receivers see only attributes they are contractually entitled to.
- End-to-end confidentiality.Sensitive fields are HPKE-sealed per recipient public key; intermediaries route opaque bytes.
- Cryptographic provenance.Hybrid Ed25519 + ML-DSA detached JWS, DPoP-bound, anchored to a transparency log.
- Consent & purpose binding.Every handoff carries an ISO/IEC 27560 consent receipt naming lawful basis, purpose, scope and jurisdiction.
- Time-bound & revocable.Envelopes carry
notAfter≤ 5 minutes, bound to a specific call leg, with revocable key and consent identifiers.
Why existing standards don't close the gap
Every layer below the conversation already has a standard. The conversation itself does not, and OHP is explicit about what it composes with rather than replaces.
- SIP / SIP REFER.Sets up, transfers and tears down voice sessions, but has no semantic payload: X-headers are ad-hoc, lossy across carriers, and capped at a few hundred bytes.
- WebRTC + DataChannel.Defines a pipe, not a schema. Each vendor invents its own JSON on top.
- MCP.Standardises how a model calls tools. Scoped to a single agent runtime, with no notion of conversation, caller identity or media handoff.
- OVON Open Floor.A high-level vocabulary for agents to introduce themselves. No audio/media binding, no telephony grounding, no consent or retention semantics.
- CCXML / VoiceXML.Pre-LLM era IVR scripting. No model state, no embeddings, no streaming partials.
- CPaaS webhooks & CC attached data.Locked to one vendor; one-way; no place for AI state and no cross- boundary defensibility.
- OpenTelemetry GenAI semconv.Read-only telemetry, not a runtime handoff envelope.
Governance: written by Cloudax, owned by the industry
Cloudax is publishing OHP under Apache-2.0 with a DCO-only contribution model and holds the pen only for the v0.x series. Before any v1.0, the company will convene a working group with at least three non-Cloudax voice AI vendorsand transition stewardship to a multi-vendor steering committee.
A public conformance test suite ships with the draft. Every change to the specification must come with new test vectors, and the conformance runner is the source of truth for what "speaks OHP" means.
“This standard has to be owned by the industry it serves, not by us. We are writing the first version and shipping the first reference implementation. Then we step back to one seat at the table among many.”
Roadmap to v1.0
- v0.1: today.Public draft, reference implementation and conformance runner published. Outreach to design partners begins.
- v0.2.Two outside implementers landed; public test fixtures published.
- v0.9 RC.Working group expanded; final candidate review against the conformance runner.
- v1.0.Cloudax-led working group transitions to a multi-vendor steering committee.
Reference implementation and design partners
Cloudax has published a reference implementation built on its existing voice AI platform, Cloudax Fabric. The full conformance runner, JSON Schema, CBOR canonicalisation rules and example envelopes are available alongside the draft.
Cloudax is actively seeking design partners and working-group participants across contact-centre platforms, CPaaS providers, voice AI runtimes, voice biometrics vendors, identity providers and regulated enterprise deployers. Vendors that already operate proprietary handoff formats are explicitly invited: OHP's tiered design is intended to make adoption a weekend of work at the floor and an evidence pack at the ceiling, not a forklift.
Availability
About OHP
OHP (the Open Handoff Protocol) is a vendor-neutral specification for handing off live voice conversations: context, intent, state, identity, consent and media, between voice AI agents, IVRs, contact centres and humans. It is designed to be embedded in SIP, WebRTC and HTTP/WS control planes without forcing a single runtime, model or telephony provider, and is engineered from the first line for regulated- data flows so that authentication state, ID&V outcomes, payment instructions, healthcare context and other sensitive personal data can cross vendor boundaries with the same legal defensibility as a bank-to-bank ISO 20022 message.
About Cloudax
Cloudax builds voice AI infrastructure for enterprises in regulated industries. The company is the authoring organisation of the OHP public draft and the maintainer of the initial reference implementation. Cloudax holds the pen only for the v0.x series of the specification and is committed to multi- vendor stewardship before any v1.0 release.
Press & editor contact
Specification editors, press enquiries and working-group participation: [email protected]
Want to help shape OHP? Join the working group.
Vendors, operators, identity providers and regulators are invited to participate. Whether you ship Core in a long weekend or mandate Regulated in your procurement, OHP only works if it is the standard the whole industry implements, not the one Cloudax writes alone.
