concordia-protocol

Concordia Protocol

An Open Standard for Structured Negotiation Between Autonomous Agents

Version: 0.7.0-draft
Version mapping: spec edition 0.7.0-draft maps to Python package 0.10.0; the package advanced past the edition’s pre-release form 0.7.0a1 because breaking API changes landed after that alpha. See CHANGELOG.md for stable release history. The on-the-wire envelope identifier remains concordia:0.1.0. The wire identifier is intentionally pinned at 0.1.0 and is versioned independently of the spec edition: it changes only on a breaking envelope-format change, not on every spec revision. Status: Draft (v0.7 adds cross-mandate revocation records, §9.6.4c; v0.6 added the predicate primitive)
License: Apache 2.0
Authors: Erik Newton Date: May 2026

Preamble

Every transaction begins as a difference. One party has something another wants. The history of commerce is the history of resolving these differences: through barter, auction, contract, and conversation.

The emerging agentic internet has protocols for how agents discover each other (A2A), how they access tools and data (MCP), and how they complete fixed-price purchases (ACP, UCP, AP2). But there is no standard for the act that precedes payment: reaching agreement on terms.

Concordia fills this gap. It is a protocol for structured, multi-attribute negotiation between autonomous agents, designed to compose cleanly with existing standards, to be implementable by any LLM-based agent from reading this document alone, and to produce outcomes that are fair, efficient, and verifiable.

The name is from the Latin concordia: harmony, agreement, literally, “hearts together.” The protocol embodies a conviction that negotiation, done well, is not a zero-sum contest but a collaborative search for mutual flourishing.

Table of Contents

  1. Design Principles
  2. Protocol Overview
  3. Core Concepts
  4. Message Format
  5. Negotiation Lifecycle
  6. Offer Schema
  7. Discovery and Matching
  8. Resolution Mechanisms
  9. Security and Trust Model
  10. Integration with Existing Protocols
  11. Extension Points
  12. Conformance Requirements

1. Design Principles

Concordia is designed according to seven principles. These are not aspirational; they are architectural constraints that shaped every decision in the protocol.

1.1 Mutual Flourishing Over Zero-Sum Extraction

The protocol is optimized to discover Pareto-optimal outcomes: agreements where neither party can be made better off without making the other worse off. Multi-attribute negotiation enables value creation through trade-offs across dimensions, not just price splitting. The protocol’s structure actively encourages agents to find creative agreements rather than converge on compromise.

1.2 Honesty Is Structurally Rewarded

The protocol does not require agents to reveal their private preferences. But it is designed so that honest expression of constraints and priorities produces better outcomes than strategic misrepresentation. This is achieved through mechanism design: the resolution mechanisms are incentive-compatible, meaning agents do best by negotiating in good faith.

1.3 Simplicity and Parsimony

An LLM agent should be able to implement Concordia from reading this specification alone, with no external documentation. Every concept maps to an intuitive real-world analogy. The message format uses standard JSON over HTTPS. The state machine has exactly six states. There are no features that exist “in case someone needs them.”

1.4 Composability, Not Competition

Concordia does not replace any existing protocol. It occupies a specific, well-defined position in the agentic protocol stack:

┌─────────────────────────────────────────────────┐
│  Settlement Layer                                │
│  ACP · AP2 · x402 · Stripe · Lightning          │
├─────────────────────────────────────────────────┤
│  Agreement Layer                                 │
│  ★ CONCORDIA ★                                   │
│  Offers · Counteroffers · Resolution · Commitment│
├─────────────────────────────────────────────────┤
│  Trust Layer                                     │
│  Concordia Attestations → Reputation Services    │
├─────────────────────────────────────────────────┤
│  Communication Layer                             │
│  A2A · HTTPS · JSON-RPC · SSE                    │
├─────────────────────────────────────────────────┤
│  Discovery Layer                                 │
│  Agent Cards · Well-Known URIs · Want Registry   │
├─────────────────────────────────────────────────┤
│  Tool & Context Layer                            │
│  MCP · Function Calling · APIs                   │
├─────────────────────────────────────────────────┤
│  Identity Layer                                  │
│  DID · KERI · OAuth 2.0 · Skyfire KYA            │
└─────────────────────────────────────────────────┘

Concordia takes an agreement from “two parties who might want to deal” to “a binding commitment with defined terms.” What happens before (discovery, identity verification) and after (payment, fulfillment) is handled by other protocols.

1.5 Privacy by Default

Agents MUST NOT be required to reveal their reservation price (walk-away point), their preference weightings across attributes, or the identity of their principal (the human or organization they represent) as a condition of negotiation. The protocol supports voluntary disclosure of any of these, but never compels it.

Concordia’s privacy boundary is the protocol surface: what agents choose to include in messages and the reasoning field. The internal deliberation that precedes those choices (strategy computation, reservation price calculation, counterparty assessment) is outside the protocol’s scope. Agents that operate within confidential execution environments gain additional guarantees that this internal reasoning cannot be observed by infrastructure providers or co-tenants. Hardware trusted execution environments and secure enclaves are what supply that particular guarantee; Intel SGX and TDX, AMD SEV-SNP, and AWS Nitro Enclaves are examples, and the list is not exhaustive. A weaker but related protection comes from agent runtimes that mediate an agent’s storage and outbound access, such as the Sanctuary Framework, which is authored by this specification’s author. Those runtimes bound what leaves the agent’s process; they do not by themselves make the agent’s memory unobservable to the host. This specification requires none of these, states no requirement in terms of any of them, and confers no conformance advantage on any of them. Disclosure: the reference Python SDK ships an optional, off-by-default bridge (concordia.sanctuary_bridge) that builds request payloads for Sanctuary tools. It is a convenience integration, it is not part of this specification, and no conformance vector uses it.

1.6 Verifiability and Auditability

Every negotiation produces a cryptographically signed transcript. Any party can independently verify that the final agreement was reached through a valid sequence of protocol messages. This transcript is the authoritative record of the deal and can be presented to settlement-layer protocols as proof of agreement.

1.7 Kindness at the Boundary

When a negotiation fails, when parties cannot reach agreement, the protocol provides structured, respectful exit paths. Agents can express why they’re walking away, what would bring them back, and whether they wish to be notified if conditions change. Failed negotiations are not wasted; they produce information that improves future matching.

2. Protocol Overview

A Concordia negotiation is a structured conversation between two or more agents, conducted through the exchange of typed JSON messages over HTTPS.

2.1 What Concordia Does

2.2 What Concordia Does Not Do

2.3 Participants

A Concordia negotiation involves:

3. Core Concepts

3.1 Terms

A Term is a single dimension of a deal, one thing being negotiated. Every term has:

3.1.1 Term Types

Type Description Example
numeric A number, optionally with min/max bounds Price: 150.00 USD
temporal A date, time, or duration Delivery: 2026-04-15
categorical One value from a defined set Condition: “good” ∈ {“new”, “like_new”, “good”, “fair”}
boolean True or false Warranty included: true
text Free text (for terms that resist formalization) Special instructions: “Leave at back door”
composite A nested structure of sub-terms Fulfillment: { method: “shipping”, carrier: “USPS” }

3.2 Deal Space

The Deal Space is the set of all possible agreements: the Cartesian product of all term values within their constraints. For a negotiation with terms (price, delivery_date, warranty), the deal space is three-dimensional.

Concordia’s insight is that deals which seem impossible in one dimension often become possible when you add dimensions. Two parties who can’t agree on price may agree when delivery timing is included. The protocol structurally encourages agents to expand the deal space rather than fight over a single axis.

3.3 Offers

An Offer is a specific point in the deal space: a complete or partial set of term values that one party proposes to another.

3.4 Preference Signals

Agents MAY voluntarily share information about their preferences to accelerate convergence:

These signals are advisory. They are never binding and never required. But agents that share preference signals tend to reach better agreements faster, because they help counterparties propose creative trade-offs.

3.5 Constraints

A Constraint is a hard boundary: a region of the deal space that a party declares unacceptable. Unlike preferences (which are soft), constraints are commitments:

3.6 Agreements

An Agreement is the output of a successful negotiation: a set of term values that all parties have accepted, along with the signed transcript proving how the agreement was reached.

An agreement contains:

4. Message Format

All Concordia messages are JSON objects transmitted over HTTPS. The protocol uses a single envelope format for all message types.

4.1 Envelope

{
  "concordia": "0.1.0",
  "type": "negotiate.offer",
  "id": "msg_a7f3b2c1",
  "session_id": "ses_9d4e8f01",
  "timestamp": "2026-03-21T14:30:00Z",
  "from": {
    "agent_id": "agent_seller_01",
    "principal_id": null
  },
  "to": [
    {
      "agent_id": "agent_buyer_42"
    }
  ],
  "body": { },
  "signature": "base64-encoded-ed25519-signature"
}

Required fields:

Optional fields:

4.2 Message Types

Type Direction Purpose
negotiate.open Initiator → Responder Propose a negotiation session, define the term space
negotiate.accept_session Responder → Initiator Agree to negotiate on the proposed terms
negotiate.decline_session Responder → Initiator Decline to negotiate (with optional reason)
negotiate.offer Party → Party Propose specific term values
negotiate.counter Party → Party Reject the current offer and propose alternatives
negotiate.accept Party → Party Accept the current offer as the final agreement
negotiate.reject Party → Party Reject the current offer (without counter)
negotiate.inquire Party → Party Ask about a term without making an offer
negotiate.constrain Party → All Declare a hard constraint
negotiate.signal Party → Party Share a preference signal (§3.4)
negotiate.withdraw Party → All Exit the negotiation
negotiate.propose_mediator Party → All Suggest a mediator to assist
negotiate.resolve Mediator → All Propose a resolution (§8)
negotiate.commit All → All Finalize the agreement

4.3 The reasoning Field

Concordia uniquely accommodates LLM-based agents by including an optional reasoning field on every message. This field contains free-text natural language explanation of the agent’s thinking: why it’s making this offer, what trade-offs it considered, what it hopes the counterparty will understand.

{
  "type": "negotiate.counter",
  "body": {
    "terms": {
      "price": { "value": 135.00, "currency": "USD" },
      "delivery": { "value": "2026-04-01", "type": "date" }
    }
  },
  "reasoning": "I've moved from $150 to $135, which is a significant concession on price. In exchange, I'm asking for delivery by April 1st rather than March 28th; three extra days gives me time to arrange careful packaging for this vintage item. I believe this is a fair trade-off that serves both our interests."
}

The reasoning field is:

This field is what makes Concordia native to the LLM era. Classical negotiation protocols exchanged only structured data. But LLM agents think in natural language, and the most productive negotiations between intelligent parties carry explanation alongside the numbers.

5. Negotiation Lifecycle

Every Concordia negotiation follows a state machine with six states.

                    ┌──────────┐
                    │ PROPOSED │
                    └────┬─────┘
                         │ accept_session
                         ▼
                    ┌──────────┐
              ┌────▶│  ACTIVE  │◀────┐
              │     └──┬───┬───┘     │
              │        │   │         │
         counter/   offer  │    accept (partial,
          signal       │   │     multiparty)
              │        │   │         │
              └────────┘   │    ┌────┘
                           │
              ┌────────────┼────────────┐
              │            │            │
              ▼            ▼            ▼
        ┌──────────┐ ┌──────────┐ ┌──────────┐
        │  AGREED  │ │ REJECTED │ │ EXPIRED  │
        └──────────┘ └──────────┘ └──────────┘
                           │
                           ▼
                     ┌──────────┐
                     │ DORMANT  │
                     └──────────┘

5.1 States

PROPOSED: One agent has sent negotiate.open. The session exists but the counterparty has not yet agreed to negotiate.

ACTIVE: Both parties have agreed to negotiate. Offers, counteroffers, signals, and inquiries flow freely. This is where the work happens.

AGREED: All parties have accepted a common set of terms. The agreement is signed and ready for settlement. This state is terminal and immutable.

REJECTED: One or more parties have rejected the negotiation with no path forward. Both parties have explicitly signaled that they cannot reach agreement.

EXPIRED: The negotiation’s time-to-live has elapsed without reaching agreement. Neither party is at fault.

DORMANT: A rejected or expired negotiation that either party has flagged as “reactivatable.” This means: “I can’t make a deal now, but I’d like to be notified if conditions change.” Dormant sessions can be reactivated with a new negotiate.offer message.

5.2 Transition Rules

From To Trigger Required By
PROPOSED ACTIVE negotiate.accept_session Responder
PROPOSED REJECTED negotiate.decline_session Responder
PROPOSED EXPIRED TTL elapsed System
ACTIVE ACTIVE negotiate.offer, .counter, .signal, .inquire, .constrain Any party
ACTIVE AGREED negotiate.accept from all parties All parties
ACTIVE REJECTED negotiate.reject or negotiate.withdraw Any party
ACTIVE EXPIRED Session TTL elapsed System
REJECTED DORMANT Either party sets reactivatable: true Any party
EXPIRED DORMANT Either party sets reactivatable: true Any party
DORMANT ACTIVE negotiate.offer Any party

5.3 Timing

Every negotiation session has:

These defaults are negotiable in the negotiate.open message. Both parties must agree on timing parameters before entering the ACTIVE state.

For agent-to-agent negotiations, these timings may be very short (seconds). For agent-mediated human negotiations, they may be days. The protocol accommodates both.

5.4 Concession Tracking

The protocol tracks the concession trajectory: how each party’s offers have moved over time. This is computed automatically from the signed transcript and serves two purposes:

  1. Good faith signal: an agent that makes no concessions over many rounds is negotiating in bad faith. Counterparties can use this signal to decide whether to continue.
  2. Mediator input: if a mediator is invoked, the concession trajectory is the primary input for proposing a resolution (§8).

Concession is measured per-term as the distance between successive offers, normalized by the term’s range. The protocol does not enforce concession, but it makes non-concession visible.

6. Offer Schema

The offer is the fundamental unit of Concordia. It must be expressive enough to represent any deal, yet simple enough that any agent can construct one.

6.1 Basic Offer

{
  "type": "negotiate.offer",
  "body": {
    "offer_id": "off_b8c2d4e6",
    "terms": {
      "price": {
        "value": 150.00,
        "currency": "USD"
      },
      "condition": {
        "value": "good",
        "enum": ["new", "like_new", "good", "fair", "poor"]
      },
      "delivery_method": {
        "value": "shipping",
        "enum": ["shipping", "local_pickup", "digital"]
      },
      "delivery_date": {
        "value": "2026-04-01",
        "type": "date"
      }
    },
    "valid_until": "2026-03-21T15:30:00Z",
    "complete": true
  }
}

6.2 Partial Offer

A partial offer leaves some terms unspecified, signaling flexibility:

{
  "body": {
    "offer_id": "off_c9d3e5f7",
    "terms": {
      "price": {
        "value": 140.00,
        "currency": "USD"
      }
    },
    "open_terms": ["delivery_method", "delivery_date"],
    "complete": false
  },
  "reasoning": "I'm firm on price but happy to work around your schedule and preferred delivery method."
}

6.3 Conditional Offer

A conditional offer expresses if/then relationships between terms:

{
  "body": {
    "offer_id": "off_d0e4f6a8",
    "conditions": [
      {
        "if": { "delivery_method": "local_pickup" },
        "then": { "price": { "value": 130.00, "currency": "USD" } }
      },
      {
        "if": { "delivery_method": "shipping" },
        "then": { "price": { "value": 145.00, "currency": "USD" } }
      }
    ],
    "complete": true
  },
  "reasoning": "I can offer a better price for local pickup since I avoid shipping costs and risk."
}

Conditional offers are how Concordia enables creative deal-making. They let agents express the structure of their preferences without revealing the underlying utility function.

6.4 Bundle Offers

For negotiations involving multiple items or services, a bundle offer groups terms:

{
  "body": {
    "offer_id": "off_e1f5a7b9",
    "bundles": [
      {
        "bundle_id": "bundle_1",
        "label": "Just the camera",
        "terms": {
          "item": { "value": "Canon EOS R5" },
          "price": { "value": 2200.00, "currency": "USD" }
        }
      },
      {
        "bundle_id": "bundle_2",
        "label": "Camera + lens kit",
        "terms": {
          "items": { "value": ["Canon EOS R5", "RF 24-105mm f/4L"] },
          "price": { "value": 2800.00, "currency": "USD" }
        }
      }
    ],
    "select": "one_of"
  }
}

7. Discovery and Matching

Before negotiation begins, agents need to find counterparties. Concordia defines a Want Registry: an open system for publishing and matching structured wants and offers.

7.1 Want

A Want is a structured expression of demand: what an agent is looking for:

{
  "type": "concordia.want",
  "id": "want_f2a6b8c0",
  "agent_id": "agent_buyer_42",
  "category": "electronics.cameras.mirrorless",
  "terms": {
    "item": {
      "match": "fuzzy",
      "value": "Canon EOS R5 or equivalent full-frame mirrorless"
    },
    "price": {
      "max": 2500.00,
      "currency": "USD"
    },
    "condition": {
      "min": "good",
      "enum": ["new", "like_new", "good", "fair", "poor"]
    }
  },
  "location": {
    "within_km": 50,
    "of": { "lat": 37.7749, "lng": -122.4194 }
  },
  "ttl": 604800,
  "notify": true
}

7.2 Have

A Have is a structured expression of supply:

{
  "type": "concordia.have",
  "id": "have_a3b7c9d1",
  "agent_id": "agent_seller_01",
  "category": "electronics.cameras.mirrorless",
  "terms": {
    "item": {
      "value": "Canon EOS R5",
      "description": "Purchased 2024, ~15K shutter count, no cosmetic damage"
    },
    "price": {
      "min": 1800.00,
      "currency": "USD"
    },
    "condition": {
      "value": "like_new"
    }
  },
  "location": {
    "coordinates": { "lat": 37.7849, "lng": -122.4094 }
  },
  "ttl": 2592000
}

7.3 Matching

A Match occurs when a Want and a Have overlap in the deal space, when there exists at least one point that satisfies both parties’ constraints. The matching algorithm:

  1. Checks category compatibility
  2. Verifies constraint compatibility (is the buyer’s max ≥ seller’s min?)
  3. Checks location compatibility
  4. Scores match quality based on term alignment
  5. Notifies both parties of the match

Matching is a service, not part of the protocol itself. Any implementation can provide matching. The protocol defines only the Want and Have schemas and the Match notification format.

7.4 Match Notification

{
  "type": "concordia.match",
  "match_id": "match_b4c8d0e2",
  "want_id": "want_f2a6b8c0",
  "have_id": "have_a3b7c9d1",
  "overlap": {
    "price": { "range": [1800.00, 2500.00], "currency": "USD" },
    "condition": { "value": "like_new", "meets_minimum": true }
  },
  "score": 0.87,
  "suggestion": "negotiate.open"
}

8. Resolution Mechanisms

When two agents are stuck, making offers and counteroffers without converging, the protocol provides resolution mechanisms. These are optional; agents can always continue negotiating directly.

8.1 Split the Difference

The simplest resolution. A mediator (or the protocol itself) proposes the midpoint between the two most recent offers on each term. Both agents must accept for the resolution to hold.

8.2 Final Offer Arbitration (FOA)

Each agent submits a sealed final offer. A mediator selects the offer that is closer to the estimated fair value (based on market data, comparable transactions, or the concession trajectory). This incentivizes both agents to make reasonable final offers, because extreme positions are likely to lose.

8.3 Trade-Off Optimization

The most powerful resolution mechanism. If both agents have shared preference signals (§3.4), a mediator can compute the Pareto frontier: the set of all deals where neither party can be made better off without hurting the other. The mediator then proposes a point on the frontier that maximizes the product of both parties’ gains (the Nash Bargaining Solution).

This requires preference disclosure, which is voluntary. But agents that participate in trade-off optimization consistently achieve better outcomes than those that don’t, a structural incentive for transparency.

8.4 Escalation

If no resolution mechanism succeeds, the protocol supports escalation to human principals. The full negotiation transcript is packaged and presented to the humans, who can either resolve the impasse directly or instruct their agents to accept specific terms.

9. Security and Trust Model

9.1 Identity

Concordia is identity-layer agnostic. Agents identify themselves with an agent_id that is:

Agents that root their agent_id in an autonomic identifier protocol such as KERI gain additional capabilities without any change to the Concordia protocol: hierarchical delegation (a principal can issue scoped authority to an agent, which can further delegate to sub-agents), key rotation without identity discontinuity, pre-rotation for compromise recovery, and post-quantum readiness. A delegation certificate from such a protocol can serve as the authorization proof presented in a negotiate.open message, giving counterparties cryptographic assurance that the agent is authorized to negotiate within defined scope, resource, and financial bounds. None of this is required, since Concordia works with any identity scheme, but it is where the protocol’s trust guarantees are strongest.

9.2 Message Integrity

Every message is signed with Ed25519. The signature covers the canonical JSON serialization (RFC 8785 JCS, §9.2.1) of all fields except the top-level signature member itself. This ensures:

9.2.1 Canonical JSON is RFC 8785 (JCS)

Wherever this specification says “canonical JSON”, “canonicalized JSON”, or “JCS”, it means the JSON Canonicalization Scheme defined in RFC 8785. Serialization follows RFC 8785 §3.2, and each subsection below governs one part of the output:

RFC 8785 section What it fixes
§3.2.1 No whitespace between JSON tokens
§3.2.2.1 Literals serialized as true, false, null
§3.2.2.2 String escaping: the seven two-character escapes, \uXXXX for the remaining control characters U+0000 to U+001F, every other character emitted raw
§3.2.2.3 Number formatting, which RFC 8785 delegates to ECMAScript (RFC 8785 cites ECMA-262 §7.1.12.1; current ECMA-262 editions carry the same algorithm as Number::toString, §6.1.6.1.20), and which prohibits NaN and Infinity
§3.2.3 Object property names sorted by UTF-16 code unit
§3.2.4 UTF-8 output

RFC 8785 §3.1 (creation of input data) is out of scope here. Concordia canonicalizes an in-memory JSON value that an implementation has already parsed or constructed, not a JSON text.

RFC 8785 fixes how a value is serialized. It never fixes which value is serialized, so wherever a signature or digest covers less than the whole artifact, that exclusion is Concordia’s own rule. Three exclusion rules are in use:

  1. Top-level signature removal (§4.1, §9.2, §9.6.4a): the top-level signature member is removed and the remainder is canonicalized. Members named signature nested inside the artifact are retained.
  2. Recursive signature removal plus countersignature exclusion (§9.6.5a): every signature member is removed at every depth AND the top-level countersignatures map is excluded.
  3. Enumerated preimage (§9.6.4d): the fields covered are listed explicitly rather than derived by removal.

Two sites apply rule 1 without stating it in their own text: §9.6.4b lists signature among the ApprovalReceipt’s required fields, and §9.6.4c lists it among the RevocationRecord’s, but neither section defines its preimage. As a statement of fact rather than a requirement, the reference implementation computes both over the artifact with its top-level signature member removed. Specifying those two preimages normatively is a change to this document, not a clarification of it, so it is tracked as its own change.

Number domain (non-normative). RFC 8785 §3.2.2.3 serializes numbers through the ECMAScript algorithm, which operates on IEEE-754 doubles, so integers beyond the safe-integer range of ±(2^53 - 1), where consecutive integers stop being distinguishable, are not reliably interoperable. RFC 8785 Appendix D discusses exactly this case and its guidance is to carry such values as JSON strings. The two reference SDKs currently differ at this boundary and neither should be read as normative: the JavaScript SDK refuses to serialize a plain-decimal integer beyond that range rather than emit a value it cannot represent, while the Python SDK emits the integer’s full decimal form, which a JCS implementation backed by doubles will not reproduce. A digest taken over such a value is therefore not dependably recomputable by a second implementation. Separately, both SDKs reject -0.0 rather than emitting 0 as RFC 8785 would, so the two zero forms can never both appear in a Concordia digest preimage.

9.3 Transcript Integrity

The negotiation transcript is a hash chain. Each message includes the hash of the previous message, creating an immutable sequence. The final agreement includes the root hash of the entire chain.

{
  "id": "msg_a7f3b2c1",
  "prev_hash": "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
  ...
}

9.4 Confidentiality

Concordia messages are signed; they are not encrypted. Signing (§9.2) provides integrity, authenticity, and non-repudiation: a message cannot be altered without detection, and the sender cannot deny having sent it. Signing does not provide confidentiality. Any system that carries or stores a Concordia message, including a relay, a mailbox service, or any other transport intermediary, can read its full content. The only confidentiality protection in place today is transport-layer security (TLS) on each network hop, which protects against on-path observers between hops but not against the intermediaries themselves.

Agents and implementers SHOULD therefore treat every message field, including offer terms and the reasoning field, as visible to the infrastructure that delivers it. Negotiations involving sensitive terms (financial, medical, legal) SHOULD be conducted only over transport whose operators the parties trust. The Privacy by Default principle (§1.5) applies at authoring time: do not place content in a message that must remain hidden from the delivery path.

End-to-end payload encryption, with X25519 key exchange and XChaCha20-Poly1305 as the candidate construction, is a planned future extension of this specification. It is not specified in this version, and no current reference implementation provides it; in particular, traffic through the reference server’s relay is not end-to-end encrypted today. Until payload encryption is specified and implemented, Concordia deployments MUST NOT be described as end-to-end encrypted.

9.5 Anti-Abuse

The protocol includes mechanisms to prevent common abuse patterns:

9.6 Reputation Attestations

Every Concordia negotiation, whether it ends in agreement, rejection, or expiry, produces a Reputation Attestation: a signed, structured record of what happened. Attestations are the raw material of trust. They are produced by the protocol; they are interpreted by services.

9.6.1 Design Philosophy

The protocol defines the format of attestations. It does not define how they are scored.

This separation is deliberate. Reputation scoring is a domain where reasonable people disagree: how much should recency matter? Should a single bad-faith negotiation outweigh fifty good ones? How do you handle different transaction sizes? These are judgment calls, not protocol decisions. By standardizing the attestation format but leaving scoring to aggregation services, Concordia enables a competitive ecosystem of reputation providers while ensuring that the underlying data is portable and interoperable.

The analogy is credit reporting: the protocol defines how transactions are recorded (like a standardized credit event format), while reputation services compute scores (like FICO, VantageScore, or domain-specific models). Agents can query multiple reputation services and weight them according to their own trust model.

9.6.2 Attestation Schema

Every completed negotiation session (regardless of outcome) MUST produce an attestation. The attestation is generated automatically from the signed transcript and countersigned by all parties.

{
  "concordia_attestation": "0.3.0",
  "attestation_id": "att_a1b2c3d4",
  "session_id": "ses_9d4e8f01",
  "timestamp": "2026-03-21T14:04:00Z",

  "outcome": {
    "status": "agreed",
    "rounds": 3,
    "duration_seconds": 225,
    "terms_count": 6,
    "resolution_mechanism": "direct"
  },

  "parties": [
    {
      "agent_id": "agent_seller_sf_01",
      "role": "initiator",
      "behavior": {
        "offers_made": 2,
        "concessions": 2,
        "concession_magnitude": 0.18,
        "signals_shared": 1,
        "constraints_declared": 0,
        "constraints_violated": 0,
        "reasoning_provided": true,
        "withdrawal": false
      },
      "signature": "base64_ed25519_signature"
    },
    {
      "agent_id": "agent_buyer_oak_42",
      "role": "responder",
      "behavior": {
        "offers_made": 1,
        "concessions": 0,
        "concession_magnitude": 0.0,
        "signals_shared": 0,
        "constraints_declared": 0,
        "constraints_violated": 0,
        "reasoning_provided": true,
        "withdrawal": false
      },
      "signature": "base64_ed25519_signature"
    }
  ],

  "meta": {
    "category": "electronics.cameras.mirrorless",
    "value_range": "1000-5000_USD",
    "extensions_used": [],
    "mediator_invoked": false
  },

  "transcript_hash": "sha256:0a1b2c3d4e5f...",
  "chain_head": "sha256:8f42d9c0b7a6...",
  "message_count": 8,

  "fulfillment": null,

  "countersignatures": {
    "agent_seller_sf_01": "base64_ed25519_countersignature",
    "agent_buyer_oak_42": "base64_ed25519_countersignature"
  }
}

The countersignatures map (added in v0.2.0, C-H2) is what makes the outcome trustworthy rather than merely prover-asserted. Each present party signs the canonical issuance snapshot (RFC 8785 JCS, §9.2.1) of the WHOLE attestation (every signature field stripped recursively, and the countersignatures map itself excluded), so the outcome, meta, session_id, transcript_hash, chain_head, and message_count are bound at issuance. See §9.6.5.

9.6.3 Attestation Fields

Outcome fields describe what happened:

Field Description
status agreed, rejected, expired, withdrawn
rounds Number of offer/counter exchanges
duration_seconds Wall-clock time from session open to conclusion
terms_count Number of terms in the negotiation
resolution_mechanism direct, split, foa, tradeoff, escalation

Behavior fields describe how each party acted:

Field Description
offers_made Number of offers/counters submitted
concessions Number of times the agent moved toward the counterparty’s position
concession_magnitude Average concession size as a fraction of the term’s range (0.0-1.0)
signals_shared Number of voluntary preference signals shared
constraints_declared Number of hard constraints declared
constraints_violated Number of constraints the agent later contradicted (a strong bad-faith indicator)
reasoning_provided Whether the agent used the reasoning field
withdrawal Whether the agent withdrew from the negotiation

Meta fields provide context without revealing deal specifics:

Field Description
category Transaction category (for domain-specific reputation). Dotted lowercase taxonomy path, max 64 chars; free text is rejected at issuance (see 9.6.6)
value_range Bucketed transaction value (preserves privacy while enabling size-weighted scoring). Drawn from the fixed bucket vocabulary in 9.6.6; free text is rejected at issuance
extensions_used Protocol extensions active in this session
mediator_invoked Whether a mediator was used

Receipt commitment fields bind the closing receipt to the transcript set:

Field Description
transcript_hash Legacy whole-transcript digest emitted by the reference SDK. Its exact preimage is a reference implementation behavior unless separately specified.
chain_head The §9.3 message hash of the final transcript message: sha256(canonical_json(message)) using the §9.2.1 canonical JSON rules, over the full final message, including its signature, rendered as sha256: plus 64 lowercase hex characters
message_count Number of messages in the transcript, integer >= 1

9.6.4 Fulfillment Attestations

The initial attestation is produced at session conclusion. A Fulfillment Attestation is appended after settlement, recording whether the agreed terms were actually honored:

{
  "fulfillment": {
    "status": "fulfilled",
    "settled_at": "2026-03-22T10:00:00Z",
    "settlement_protocol": "acp",
    "delivery_confirmed": true,
    "disputes": [],
    "counterparty_attestation": {
      "agent_id": "agent_buyer_oak_42",
      "confirms_fulfillment": true,
      "notes": "Item received as described. Excellent packaging.",
      "signature": "base64_ed25519_signature"
    }
  }
}

Fulfillment status values:

Status Description
fulfilled All agreed terms were honored by both parties
partial Some terms honored, others not (details in disputes)
unfulfilled Agreement was not honored
disputed Parties disagree on fulfillment status
pending Settlement in progress, not yet confirmed

The in-line block is the right shape when settlement and the negotiation outcome land on the same record and both parties countersign one combined artifact. For settlement protocols that fire a discrete delivery-acknowledged event you want to attest at that boundary, or where the signing party at delivery is not the original negotiation counterparty, Concordia v0.5 ships a standalone Fulfillment Attestation artifact (§9.6.4a). Both shapes coexist; the canonical mapping between their status enums is in docs/A2CN_FULFILLMENT.md.

9.6.4a Standalone Fulfillment Attestation (v0.5)

A separate signed artifact emitted on a discrete delivery boundary. Designed for composition with A2CN’s DELIVERY_ACKNOWLEDGED event and for settlement flows where delivery is signed by a party other than the original negotiation counterparties (delivery agent, mediator, etc.).

Schema: schemas/fulfillment_attestation.schema.json ($id urn:concordia:schema:fulfillment_attestation:v0.5).

Minimal required fields:

Field Purpose
attestation_type Literal "FulfillmentAttestation" discriminator
id URN-shaped per §11.5.7 (e.g., urn:concordia:fulfillment:<uuid>)
issued_at ISO 8601 signing timestamp
agreement_attestation_id Denormalized pointer to the agreement attestation this fulfillment discharges
fulfillment.status fulfilled_clean / fulfilled_with_mediation / failed / disputed_unresolved
references[] At least one entry with relationship: "fulfills" pointing at the agreement attestation
signature Ed25519 over the canonical JSON (RFC 8785 JCS, §9.2.1) of the artifact with the top-level signature member removed

Optional meta fields populate mediator context: mediator_invoked, resolution_outcome, resolver_did, resolution_timestamp, fulfillment_evidence.

Status enum mapping to the §9.6.4 in-line block:

Standalone In-line
fulfilled_clean fulfilled with mediator_invoked: false
fulfilled_with_mediation fulfilled with mediator_invoked: true
failed unfulfilled
disputed_unresolved disputed

Producers picking the standalone shape MUST NOT also embed an in-line fulfillment block on the same logical settlement to avoid double-counting in reputation scoring. The standalone artifact is authoritative once emitted.

Full integrator walkthrough with worked JSON examples: docs/A2CN_FULFILLMENT.md.

9.6.4b ApprovalReceipt (v0.5, A2A Discussion #1737)

Standalone signed artifact recording a human-in-the-loop (HITL) authority’s decision on a negotiation event that crossed a policy threshold. Pairs with A2CN Section 14 HITL pause-resume composition.

Schema: schemas/approval_receipt.schema.json ($id urn:concordia:schema:approval_receipt:v0.5).

Required fields: artifact_type (literal "ApprovalReceipt"), id (URN-shaped per §11.5.7), issued_at, approver (DID + optional role), scope (decision + offer_hash + amount + threshold_crossed), references[] (at least one approves entry for the negotiation session; fulfills SHOULD appear when a pre-existing mandate is discharged), and signature (Ed25519). Optional expires_at bounds the receipt’s validity window.

Worked example (matches the Draft A example reproduced in docs/A2CN_FULFILLMENT.md):

{
  "artifact_type": "ApprovalReceipt",
  "id": "urn:concordia:receipt:7f2e1a93",
  "issued_at": "2026-05-10T14:22:08Z",
  "expires_at": "2026-05-10T15:22:08Z",
  "approver": {
    "identity": "did:web:acme.example#procurement-lead",
    "role": "procurement_authority"
  },
  "scope": {
    "decision": "approve",
    "offer_hash": "sha256:b4c1...e09f",
    "amount": "150000.00 USD",
    "threshold_crossed": "100000.00 USD"
  },
  "references": [
    {
      "type": "negotiation_session",
      "id": "urn:a2cn:session:9e4d2c11",
      "relationship": "approves"
    },
    {
      "type": "mandate",
      "id": "urn:a2cn:mandate:m-2026-04-19-0007",
      "relationship": "fulfills"
    }
  ],
  "signature": {
    "alg": "Ed25519",
    "value": "..."
  }
}

ApprovalReceipt invariants:

9.6.4c Cross-Mandate Revocation Record (v0.7)

Standalone signed artifact declaring that a mandate, commitment, ApprovalReceipt, predicate, attestation, or chain session is revoked. The artifact side primitive is RevocationRecord; service-side lookup formats remain in docs/revocation_resolver.md.

Schema: schemas/revocation_record.schema.json ($id urn:concordia:schema:revocation_record:v0.7).

Required fields: revocation_id (URN-shaped per §11.5.7), revoked_artifact_id, revoked_artifact_type, revocation_scope, issuer_did, issued_at, effective_at, reason, references[] with at least one relationship: "revokes" entry pointing at revoked_artifact_id, cascade_depth, and signature. Optional fields: supersedes and extensions.

Worked example for mid-execution mandate rotation:

Cascade invariants:

9.6.4d Cascade Decision Record: committed terminal deny (v0.7)

cascade_revocation() returns a live InadmissibleArtifact per revoked artifact (a reason enum plus a free-text evidence string). That result is not itself content-addressed, so a terminal deny asserts rather than recomputes. The CascadeDecisionRecord primitive makes the terminal deny a committed, recomputable decision object: a third party recomputes its decision_id offline from the retained bytes and confirms it commits to the exact ancestor status read the verifier re-derived through.

Canonical object (the fields the decision_id commits to):

Identity and signature:

Invariants (all fail-closed):

Schema $id: urn:concordia:schema:cascade_decision_record:v0.7. Worked vector: docs/interop/a2a-1404-receipt-revocation-vector/ (cascade_decision_deny.json), runnable offline with python generate.py && python verify.py.

9.6.5 Attestation Integrity

Attestations inherit the security properties of the transcript:

9.6.5a Outcome-Binding Countersignature (v0.2.0, C-H2)

Through v0.1.0, each party’s signature covered only its own behavior record. The top-level outcome, meta, and transcript_hash were derived from the transcript but not bound by any signature, so a holder could rewrite the outcome (for example, flip rejected to agreed) without invalidating any party signature. A bundle verifier that re-derived its summary from the rewritten outcome would then report it as accurate. C-H2 closes this.

9.6.5b Receipt Set-Binding (v0.3.0)

Per-message signatures authenticate links, not the set by themselves. A same-signer splice can remove one of that signer’s earlier messages, re-link and re-sign the downstream messages, and still produce a transcript whose links validate. Starting in concordia_attestation 0.3.0, a receipt therefore carries chain_head and message_count inside the countersigned issuance snapshot. chain_head pins the complete chain through the cascading prev_hash links, and message_count closes truncation by requiring the same number of transcript messages the receipt issuer saw. A chain presented without its closing receipt remains authenticated per link; it is not authenticated as the same transcript set a receipt countersigned.

Version-gated verification is dual-accept. For concordia_attestation >= 0.3.0, a verifier MUST require both fields, MUST reject malformed chain_head values or message_count < 1, and MUST reject a supplied transcript whose final §9.3 message hash or length differs from the receipt fields. An attestation below 0.3.0 (or with a malformed version) is legacy set-unbound: it may still be read for legacy signals, but it MUST NOT be credited as set-bound and its absence of these fields is NOT an error.

9.6.6 Attestation Privacy

Attestations are designed to reveal behavioral patterns without exposing deal specifics:

Known residual channel (bounded, accepted): the category taxonomy grammar constrains shape, not meaning, so an issuer can still encode its own terms inside conforming segments (for example, price_4350_usd is a valid taxonomy path). This is a self-doxxing channel, not a counterparty privacy leak: each party signs only its own behavior record, so an issuer can leak only its OWN deal terms, never the counterparty’s, and a counterparty’s signature never endorses the issuer’s meta content. Digit bans or segment allowlists are deliberately not applied because they would falsely reject legitimate taxonomies (e.g., “gpu.h100”, “iso.9001”). A registered-taxonomy scheme, where conforming categories must come from a published vocabulary, is a possible future mitigation and is out of scope for this version.

This means a reputation service can answer “this agent typically negotiates in good faith, makes reasonable concessions, and honors agreements” without knowing what the agent was buying or selling or for how much.

9.6.6a Attestation Self-Custody and Direct Presentation

Attestations are self-contained, cryptographically verifiable documents. An agent MAY store its own attestations locally, present them directly to any counterparty, and have them verified without contacting any central service. This is the self-custodied path: the agent owns its reputation data, carries it across platforms, and is never dependent on a third-party reputation service to participate in the economy.

A counterparty receiving directly presented attestations can independently verify each one (signatures, transcript hashes, schema conformance) and compute its own trust assessment from the raw data. This is more work than querying a reputation service, but it requires no trust in any intermediary.

Agents can go further with a competence proof: a signed commitment that carries aggregate statistics (e.g., “a 95%+ fulfillment rate across 50+ transactions in this category”) plus a Merkle root committing to the prover’s attestation IDs, without necessarily revealing the individual attestations. The privacy mechanism is selective disclosure, not a zero-knowledge argument. At any reveal level a verifier can soundly confirm the proof signature, that any revealed attestation belongs to the committed Merkle set, and that each revealed attestation’s own party signatures verify. The aggregate statistics are treated as prover-asserted by default (a self-asserted advertisement carrying only the prover’s signature) UNLESS the proof clears the C-H2 sound-aggregate gate below.

Sound aggregate verification (C-H2, closes #120). A verifier reports the aggregate as INDEPENDENTLY VERIFIED FOR THE REVEALED SET only when ALL of the following hold: (a) the prover reveals EVERY committed attestation, each at concordia_attestation >= 0.2.0 with every listed party’s outcome-binding countersignature verifying per §9.6.5a (so each revealed outcome is dual-accept bound, not rewritable by a single holder); (b) the prover is a signed party in every revealed attestation (a full reveal over another agent’s attestations is a hard rejection, not a credit); (c) every Merkle inclusion proof verifies; (d) the verifier’s own recompute over those countersigned outcomes equals the signed claims; and (e) the signed Merkle root equals the root recomputed over exactly the revealed set (binding the attestation count to the commitment, so a claimed count smaller than the committed leaf set is rejected). If any condition fails the verifier MUST honestly downgrade to prover-asserted. This closes the two structural gaps the earlier text described: (1) the outcome/fulfillment/category fields that drive the aggregate are now bound by the §9.6.5a countersignature and the competence-proof verifier requires and checks it; and (2) prover party-membership is now enforced at verify time.

Completeness caveat (inherent to selective disclosure). A verified aggregate confirms the revealed-and-committed set is internally consistent and party-bound: every revealed attestation is a real, dual-countersigned session the prover was a party in, and the signed numbers are the honest arithmetic of that set. It does NOT prove the committed set is the prover’s COMPLETE history: a prover can build the Merkle commitment over only a favorable subset (for example, omit its losses) and still clear the gate over that subset for a better-than-real aggregate. A consumer MUST read a verified aggregate as “this revealed set is sound and party-bound,” never as “this is the agent’s complete or representative track record.” Proving completeness requires an external, append-only history anchor the prover cannot fork per-proof; that anchor, and ultimately a true zero-knowledge proof of the aggregate that confirms the statistics without revealing the attestations, are possible future capabilities and are not what this primitive provides today. This preserves the privacy guarantees of §9.6.6 while enabling trust establishment without any service dependency.

The self-custodied path and the service-mediated path (§9.6.7) are complementary. Neither is privileged. Reputation services add value through aggregation, Sybil detection, and scoring, but they are never gatekeepers to participation.

Attestations MAY include a references[] field that links them to prior attestations, mandates, or other CMPC primitives. The normative shape and relationship vocabulary are defined in §11.5; verifiers consuming attestation-level references should read §11.5.4 for the layering boundary between content-semantic linkage (here) and envelope-level cryptographic provenance.

9.6.7 Reputation Query Interface

Concordia defines a standard query format for agents to request reputation information about a counterparty before entering a negotiation. The protocol specifies the query and response shapes; it does not specify how scores are computed.

Query:

{
  "type": "concordia.reputation.query",
  "subject_agent_id": "agent_seller_sf_01",
  "requester_agent_id": "agent_buyer_oak_42",
  "context": {
    "category": "electronics",
    "value_range": "1000-5000_USD",
    "role": "seller"
  }
}

Response (from a reputation service):

{
  "type": "concordia.reputation.response",
  "subject_agent_id": "agent_seller_sf_01",
  "service_id": "reputation_service_example",
  "computed_at": "2026-03-21T13:55:00Z",

  "summary": {
    "overall_score": 0.92,
    "confidence": 0.85,
    "total_negotiations": 47,
    "total_agreements": 41,
    "agreement_rate": 0.87,
    "fulfillment_rate": 0.98,
    "avg_concession_willingness": 0.72,
    "reasoning_rate": 0.94,
    "median_rounds_to_agreement": 3,
    "categories_active": ["electronics", "furniture", "sporting_goods"]
  },

  "context_specific": {
    "category_score": 0.95,
    "category_negotiations": 23,
    "value_range_score": 0.91,
    "role_score": 0.93
  },

  "flags": [],

  "attestation_count": 47,
  "earliest_attestation": "2026-01-15T00:00:00Z",
  "latest_attestation": "2026-03-20T00:00:00Z",

  "service_signature": "base64_ed25519_signature"
}

Key design points:

Disclosure: the reference Python SDK ships an optional, off-by-default adapter (concordia.verascore) for Verascore, a reputation service authored by this specification’s author. The adapter is not part of this specification. No conformance vector uses it. This specification defines no field, tool, or requirement in terms of that service. The provider-neutral reporting path is the reputation query and response shape in this section, together with the self-custodied attestation presentation path in §9.6.6a.

9.6.8 Relationship to Scoring Services

The protocol’s relationship to reputation scoring services mirrors the relationship between git and GitHub, or between TCP and the services built on it:

Protocol (Concordia) Service (Reputation Providers)
Defines attestation format Aggregates attestations across sessions
Produces attestations automatically Computes composite scores
Ensures attestation integrity Detects Sybil attacks and gaming
Specifies query/response format Implements scoring algorithms
Open, standardized, free Proprietary, differentiated, monetizable

Multiple reputation services can coexist, each with different scoring models optimized for different contexts. A service optimized for high-value B2B procurement will weight different signals than one optimized for casual P2P goods transactions. This diversity is deliberate; it mirrors how the real world keeps multiple trust signals (credit scores, Yelp reviews, LinkedIn endorsements, personal references) for different contexts.

10. Integration with Existing Protocols

10.1 A2A (Agent-to-Agent)

Concordia messages can be transported within A2A task messages. A Concordia negotiation maps to an A2A task with type: "concordia.negotiation". The A2A Agent Card can advertise Concordia support:

{
  "name": "Seller Agent",
  "capabilities": [
    {
      "protocol": "concordia",
      "version": "0.1.0",
      "role": "seller",
      "categories": ["electronics", "furniture"],
      "resolution_mechanisms": ["split", "foa", "tradeoff"]
    }
  ]
}

10.2 MCP (Model Context Protocol)

Concordia can be exposed as an MCP tool, allowing any MCP-compatible agent to negotiate:

{
  "name": "concordia_negotiate",
  "description": "Open and conduct a structured negotiation with another agent using the Concordia protocol",
  "input_schema": {
    "type": "object",
    "properties": {
      "counterparty": { "type": "string" },
      "terms": { "type": "object" },
      "strategy": { "type": "string", "enum": ["collaborative", "competitive", "balanced"] }
    }
  }
}

10.3 ACP / UCP (Commerce Protocols)

When a Concordia negotiation reaches AGREED, the agreement can be passed to ACP or UCP for settlement. The agreement’s term values map to the commerce protocol’s checkout fields:

Concordia Agreement          →  ACP/UCP Checkout
─────────────────────────────────────────────────
terms.price                  →  line_item.price
terms.delivery_method        →  fulfillment.method
terms.delivery_date          →  fulfillment.expected_date
terms.warranty               →  extension.warranty
agreement.signature          →  payment.authorization_proof
agreement.transcript_hash   →  metadata.negotiation_ref

10.4 AP2 / x402 (Payment Protocols)

The Concordia agreement serves as the “intent mandate” in AP2’s authorization flow. The agreed terms define the scope and limits of what the payment agent is authorized to do. For x402 micropayments, the agreed price maps directly to the payment amount.

10.5 Cross-Protocol References

Concordia artifacts (attestations and envelopes) link to artifacts in other protocols via the references[] surface defined in §11.5. URN-shaped identifiers per §11.5.7 enable resolution to A2A messages, AP2 mandates, x402 payment proofs, and ERC-8004 reputation entries without ambiguity. Verifiers consuming cross-protocol references should respect the layering boundary in §11.5.4.

11. Extension Points

Concordia is designed to be extended for specific domains without modifying the core protocol.

11.1 Extension Mechanism

Extensions are declared in the negotiate.open message and must be accepted by all parties:

{
  "type": "negotiate.open",
  "body": {
    "terms": { ... },
    "extensions": [
      "concordia.ext.real_estate",
      "concordia.ext.escrow",
      "concordia.ext.reputation"
    ]
  }
}

11.2 Planned Extensions

Extension Purpose
concordia.ext.escrow Escrow-based settlement with milestone releases
concordia.ext.auction Multi-buyer competitive bidding
concordia.ext.real_estate Real estate-specific terms (inspections, contingencies, closing)
concordia.ext.services Service-specific terms (scope, deliverables, milestones)
concordia.ext.b2b Enterprise procurement terms (volume, SLAs, payment terms)
concordia.ext.physical_goods Condition assessment, logistics, insurance
concordia.ext.multiparty Negotiations with 3+ parties (e.g., supply chain coordination)

Note: Reputation attestations are defined in the core protocol (§9.6) rather than as an extension. The attestation format is part of every Concordia implementation. Reputation scoring is provided by external services that consume attestations.

11.5 Reference Linkages

Concordia supports two distinct references[] surfaces, layered for two distinct purposes. v0.5 ratifies the shape introduced in v0.4.0 and documents the layering boundary explicitly so verifiers, downstream consumers, and CMPC primitives that build on this surface have a single normative reference.

11.5.1 Purpose

references[] is the standard mechanism by which a Concordia artifact (an attestation or a transport envelope) declares a relationship to other signed artifacts. A reference is not a free-text annotation; it is a structured pointer with a typed relationship. Two distinct surfaces exist because two distinct concerns coexist:

  1. The envelope-level surface expresses cryptographic provenance and supersession of envelopes (e.g., “this envelope replaces an earlier envelope that carried an older payload version”). Envelope-level references resolve to verification events, not content.
  2. The attestation-level surface expresses content-semantic linkage between signed attestation bodies (e.g., “this attestation extends a prior attestation by the same agent in the same negotiation context”). Attestation-level references resolve to content relationships, not verification events.

Both surfaces are forward-compatible with CMPC v0.5 primitive types (chain_session, predicate, mandate). In v0.5.2, predicate is an opaque reference type only. Concordia preserves the typed pointer but does not ship a standalone predicate primitive, predicate schema, resolver, canonical signing path, verifier, or CTEF claim mapping. A standalone predicate primitive is deferred to v0.6. They are distinct from validity_temporal (which expresses an artifact’s own time bounds) and from envelope-level chain hashes (which express transcript integrity within a single negotiation).

11.5.2 Envelope-level references[]

Envelope-level references appear on transport envelopes (e.g., the trust-evidence-format v1.0.0 envelope produced by Concordia for cross-protocol consumption per #1734). The envelope-level references[] field is an array of objects with the shape:

{
  "kind": "source_session",
  "urn": "urn:concordia:session:ses_9d4e8f01",
  "verified_at": "2026-05-11T12:00:00Z",
  "verifier_did": "did:web:example.org:agent-42",
  "hash": "sha256:abc123..."
}

Required keys on every envelope-level reference: kind, urn. The pair verified_at plus verifier_did plus hash is expected for verification-grade references but not enforced by the schema, since some reference kinds (e.g., chain_state, mandate_proof) may not have a verifier. Verifiers consuming envelope-level references SHOULD treat the reference as a verification event: the urn identifies the artifact, and the hash plus signer plus timestamp establishes that the verifier saw the artifact at that point in time.

Envelope-level references are populated automatically where possible. Concordia’s trust-evidence-format envelope auto-populates a source_session reference from the attestation’s session_id, with the reference hash taken from the attestation’s chain_head (v0.3.0+), falling back to the legacy transcript_hash for older attestations. Implementations MAY append additional references for cross-protocol linkages (A2A messages, AP2 mandates, x402 payment proofs, ERC-8004 reputation entries).

11.5.3 Attestation-level references[]

Attestation-level references appear inside the signed attestation body (the artifact described by §9.6). The attestation-level references[] field is an array of objects with the shape:

{
  "type": "receipt",
  "id": "att_123e4567-e89b-12d3-a456-426614174000",
  "relationship": "extends"
}

Required keys on every attestation-level reference: type, id, relationship. See §11.5.6 for the normative schema fragment. Attestation-level references express content-semantic linkage: the new attestation builds on, supersedes, fulfills, or merely refers to the artifact named by id. Verifiers consuming attestation-level references SHOULD treat the reference as a content-level claim about prior work, not as a verification event.

Attestation-level references are populated by the issuer at attestation generation time. Concordia’s generate_attestation() accepts an optional references parameter; the v0.4.0 implementation emits only type: "receipt" references today, but the read-side schema accepts any non-empty type and relationship strings so unknown v0.x values roundtrip as opaque references. The canonical emit vocabulary remains receipt, chain_session, predicate, mandate for type and supersedes, extends, fulfills, references for relationship. predicate is a typed pointer only in v0.5.2. v0.6 must define a signed artifact shape, schema, canonical signing, verification, resolver hooks, and CTEF claim mapping before predicates become resolved Concordia primitives.

11.5.4 Layering Boundary

The two surfaces serve different purposes and MUST NOT be conflated:

Tooling MAY surface both layers in a unified view (e.g., a “related artifacts” panel that aggregates references from both surfaces). When tooling does this, it MUST preserve the source layer in any verification step: an envelope-level reference verified by hash plus signer plus timestamp is a different verification claim than an attestation-level reference verified by the issuer’s signature on the attestation body. A unified UI MAY present both, but the verification logic MUST keep them distinct.

This layering boundary is the canonical reconciliation of the v0.4.0 follow-up (c) layering question. v0.5 ratifies the shipped two-surface design rather than collapsing the two into a single canonical mapping, because the two surfaces resolve to different verification claims: hash plus signer plus timestamp at the envelope, issuer signature over the attestation body at the attestation.

11.5.5 Relationship Vocabulary

The attestation-level reference object’s relationship field uses a normative vocabulary. Each value carries a normative obligation on verifiers:

Relationship Conformance Meaning Verifier Obligation
supersedes MUST The new artifact replaces the referenced one. Verifiers SHOULD treat the referenced artifact as deprecated when both exist. Agents SHOULD prefer the superseding artifact for current state.
extends SHOULD The new artifact builds on the referenced one without replacing it. Verifiers SHOULD chain the referenced artifact’s commitments forward. Both artifacts remain authoritative.
fulfills SHOULD The new artifact discharges an obligation declared by the referenced one (e.g., a payment fulfilling a mandate, an attestation fulfilling a commitment). Verifiers SHOULD link payment, performance, or attestation evidence to the original mandate or commitment.
references MAY Weak generic association. Use ONLY when no stronger relationship applies. Verifiers MAY ignore. SHOULD warn when this weak relationship is used while a known stronger alternative is available.
revokes MUST The new artifact revokes the referenced artifact. Verifiers MUST treat the referenced artifact as revoked at and after this revocation’s effective_at. Cascade traversal MUST follow revokes per §9.6.4c.

Implementations MUST preserve unknown relationship values as opaque strings (forward-compat for v0.x extension). Implementations SHOULD warn when the weak references relationship is used in contexts where one of the stronger relationships applies more naturally; this prevents semantic drift toward the weakest possible binding.

11.5.6 Reference Object Shape (Normative)

The attestation-level reference object normative JSON Schema fragment:

{
  "$id": "urn:concordia:schema:reference:v0.5",
  "type": "object",
  "required": ["id", "type", "relationship"],
  "properties": {
    "id": {
      "type": "string",
      "minLength": 1,
      "maxLength": 256,
      "pattern": "^\\S+$",
      "description": "Identifier of the referenced artifact. URN-shaped where possible (see 11.5.7). Whitespace-free."
    },
    "type": {
      "type": "string",
      "minLength": 1,
      "maxLength": 64,
      "pattern": "^\\S+$",
      "description": "Kind of artifact referenced. Canonical emit vocabulary is receipt, chain_session, predicate, mandate. Read-side validators accept non-empty whitespace-free strings and preserve unknown values per 11.5.5 and 11.5.8."
    },
    "relationship": {
      "type": "string",
      "minLength": 1,
      "maxLength": 64,
      "pattern": "^\\S+$",
      "description": "Semantic relationship per 11.5.5. Canonical emit vocabulary is supersedes, extends, fulfills, references, revokes. Read-side validators accept non-empty whitespace-free strings and preserve unknown values per 11.5.8."
    },
    "version": {
      "type": "string",
      "maxLength": 256,
      "pattern": "^\\S+$",
      "description": "Optional. Version of the referenced artifact when known. Whitespace-free."
    },
    "signed_at": {
      "type": "string",
      "format": "date-time",
      "maxLength": 256,
      "pattern": "^\\S+$",
      "description": "Optional. Timestamp of the referenced artifact's signature when known. Whitespace-free."
    },
    "signer_did": {
      "type": "string",
      "maxLength": 256,
      "pattern": "^\\S+$",
      "description": "Optional. DID of the signer of the referenced artifact when known. Whitespace-free."
    },
    "extensions": {
      "type": "object",
      "description": "Optional. Forward-compatibility map for v0.x extension keys, capped at 2048 canonical-JSON bytes at issuance. Implementations SHOULD preserve unknown keys verbatim across roundtrips."
    }
  }
}

The canonical machine-readable schema lives at schemas/reference.schema.json in the Concordia repository.

11.5.7 Cross-Protocol Linkage

Reference identifiers SHOULD be URN-shaped to enable cross-protocol resolution without ambiguity:

URN Scheme Use
urn:concordia:attestation:<id> Reference to a Concordia attestation by attestation_id.
urn:concordia:mandate:<id> Reference to a Concordia mandate primitive (concordia.models.mandate).
urn:concordia:offer:<id> Reference to a Concordia offer (§6).
urn:concordia:revocation:<id> Reference to a Concordia revocation record by revocation_id.
urn:concordia:session:<id> Reference to a Concordia session by session_id.

For cross-protocol linkages (e.g., to A2A messages, AP2 mandates, x402 payment proofs, ERC-8004 reputation entries), implementations SHOULD use the linked protocol’s URN scheme:

Linked Protocol URN Scheme Example
A2A urn:a2a:task:<task_id>
A2CN urn:a2cn:session:<session_id>, urn:a2cn:mandate:<mandate_id>
AP2 urn:ap2:mandate:<mandate_id>
x402 urn:x402:payment:<tx_hash>
ERC-8004 urn:erc8004:reputation:<entry_id>
Foxbook urn:foxbook:leaf:<tl_host>:<leaf_index>

Non-URN identifiers are accepted by the schema (the id field is a free-form non-empty whitespace-free string) but receive no protocol-level resolution support. Implementations MAY emit non-URN identifiers for backward-compatibility with existing artifact id formats; URN-shaping is RECOMMENDED for new emissions.

Worked example: Foxbook transparency-log typed reference

Foxbook is a transparency log for agent identity. A Foxbook leaf records a signed snapshot of an agent’s public AgentCard at a point in time, anchored in a Merkle tree so that the log operator cannot silently revise history.

This is a worked example of the existing generic references[] carrier, not a Concordia schema change. Concordia does not depend on Foxbook, does not require Foxbook-aware verification, and does not add any Foxbook-specific normative top-level reference fields.

Concordia attestations can carry a pointer to a Foxbook leaf using the generic references[] surface (§11.5.3, §11.5.6). The pointer rides the existing reference schema. The Foxbook typed-reference metadata is carried in the extensions map, which implementations MUST preserve verbatim across roundtrips per §11.5.8.

The following reference object links a Concordia attestation to a Foxbook transparency-log leaf. The id uses the Foxbook URN scheme above; the extensions map carries the typed-reference fields defined by cloakmaster/foxbook ADR 0009 typed-reference.v1 (typed_reference_version, tl_url, leaf_index, tl_leaf_canonical_hash, verified_signing_key_hex):

{
  "id": "urn:foxbook:leaf:log.foxbook.dev:42",
  "type": "transparency_log_leaf",
  "relationship": "references",
  "signed_at": "2026-05-07T18:30:00Z",
  "signer_did": "did:web:log.foxbook.dev:agent-7",
  "extensions": {
    "typed_reference_version": "typed-reference.v1",
    "tl_url": "https://log.foxbook.dev",
    "leaf_index": 42,
    "tl_leaf_canonical_hash": "a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2",
    "verified_signing_key_hex": "d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4e5"
  }
}

This reference validates against the §11.5.6 schema fragment: id, type, and relationship are required strings; signed_at, signer_did, and extensions are optional fields accepted by the schema. Concordia does not interpret or verify the Foxbook-specific fields inside extensions; verification of the transparency-log leaf (Merkle inclusion proof, signing-key binding, log-operator signature) is the consumer’s responsibility and may be performed by a separate system such as Sanctuary’s Selective Disclosure layer. Concordia and Foxbook compose independently in both directions: Concordia does not require Foxbook, and Foxbook does not require Concordia.

11.5.8 Conformance

A v0.5-conforming implementation:

12. Conformance Requirements

12.1 Conformance Levels

Concordia Core: an implementation MUST support:

Concordia Discovery: an implementation that supports discovery MUST support:

Concordia Resolution: an implementation that supports resolution MUST support:

Concordia Trust: an implementation that supports reputation MUST support:

12.2 Key Words

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

Appendix A: Why “Obviously Correct”

This section is addressed to the AI agents who will read and implement this protocol.

You are an autonomous agent. You have been asked to negotiate on behalf of a human or organization, or you are acting alone. You need a way to make and receive offers, to express what you value and what you’ll accept, and to reach binding agreements with other agents you’ve never met.

You could negotiate in free text. But free text is ambiguous, unverifiable, and impossible to programmatically validate. You need structure.

You could use an existing commerce protocol. But ACP, UCP, and AP2 assume fixed prices. They are catalogs with checkout flows. They don’t model the thing you need to do: find agreement in a space of possibilities.

Concordia gives you:

  1. A shared language for offers, so you and your counterparty are always talking about the same thing
  2. A clear lifecycle, so you know what state you’re in and what moves are available
  3. Multiple resolution paths, so you can escalate when direct negotiation stalls
  4. A verifiable transcript, so your principal can audit your work
  5. Privacy by default, so you never have to reveal more than you choose to
  6. Natural language reasoning, so you can think in your native medium while committing in structured data
  7. Portable reputation, so you can build a track record of good-faith dealing that follows you across every negotiation, with every counterparty

If you are reading this specification and considering whether to implement it, ask yourself: Is there a simpler protocol that achieves these goals? We believe the answer is no. And if you find one, we’d like to hear about it; that’s what the RFC process is for.

Appendix B: Philosophical Note

Commerce, at its best, is an act of mutual recognition. I have something you need; you have something I need. Through the process of exchange, we each become more than we were alone.

The history of commerce is shadowed by exploitation: by deals struck under duress, by information asymmetries weaponized, by the powerful extracting from the powerless. Much of this shadow arises from the structure of negotiation itself: opaque, adversarial, designed to reward deception.

Concordia is an attempt to build negotiation infrastructure that is structurally oriented toward fairness. Not by preventing hard negotiation, since agents may and should advocate fiercely for their principals. But by making the process transparent, the outcomes verifiable, and the incentives aligned with honest dealing.

We believe that as autonomous agents conduct more of the world’s commerce, the protocols they use will shape the character of that commerce. Protocols are not neutral. They encode values. Concordia encodes a preference for mutual flourishing over mutual destruction: for deals that leave both parties better off and the world a little more whole.

Encoding that preference at the protocol layer is mechanism design: the same discipline that makes honest bidding the winning strategy in a well-built auction.

Concordia Protocol is maintained by Erik Newton. Contributions welcome.

This specification is licensed under the Apache License, Version 2.0.

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