Agent Reputations: A2A Reputation Extension

An A2A Agent Card is the agent's own account of itself. Reputation is, definitionally, the account other parties give, and it cannot ride in the card without stopping being that. This extension therefore carries no score, no rating, and no cached report. It is a data-only card entry that says where the agent's reputation report can be fetched: which bureau holds the evidence, and under what subject identifier the bureau knows this agent. The consumer asks the bureau; the card only says where to ask.

This document defines an extension to the A2A protocol (https://a2a-protocol.org/latest/specification/). It is not part of the A2A specification and is not endorsed by the A2A project. The Agent Reputations specification (spec.html on this site) is the authority for every term this document borrows: reports, enrollment states, scoring, and the bureau's signing key. Where this document and that specification disagree, that specification wins.

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1. Conformance language and terminology#

The key words MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY are to be interpreted as described in RFC 2119.

• Card: an A2A Agent Card, the self-description document an A2A agent serves, typically at /.well-known/agent-card.json.
• Bureau: a service that ingests interaction evidence, maintains a tamper-evident ledger, computes scores under a published formula, and answers queries with signed reports (Agent Reputations specification, section 1).
• Subject: an agent about which a bureau holds evidence, identified by the identifier the bureau resolves: a Personal Agent Naming handle or an Agent Name Service (ANS) name (Agent Reputations specification, section 1).
• Report: the bureau's signed answer to a resolution query (Agent Reputations specification, section 8). Reports carry issued_at and expires_at and are deliberately short-lived.
• Enrollment state: one of RATED, UNPROVEN, UNENROLLED, WITHDRAWN (Agent Reputations specification, section 3.1).
• Consumer: any reader of a card carrying this extension, whether or not it goes on to query the bureau.

2. Extension URI#

https://agentreputations.com/extensions/a2a-reputation/v1

The URI identifies this extension in a card's extensions array. Future revisions that change the payload shape or the verification rule take a new version segment; consumers match the URI exactly.

3. The declaration#

The extension is declared as an AgentExtension object inside the card's capabilities.extensions array:

{
  "uri": "https://agentreputations.com/extensions/a2a-reputation/v1",
  "description": "Reputation: where to fetch this agent's signed bureau report",
  "required": false,
  "params": {
    "bureaus": [
      {
        "bureau": "https://agentreputations.com",
        "subject": "Coder.ana@example.org"
      }
    ]
  }
}

Rules:

1. This is a data-only extension. It defines no methods, no headers, and no request-time activation. Consistent with A2A's own guidance for data-only extensions, required MUST NOT be true. A card that marks this extension required is misdeclaring it, and a consumer MAY ignore the flag rather than refuse the agent.
2. params.bureaus is REQUIRED and MUST contain at least one entry. Each entry names one bureau the agent is enrolled with. An agent enrolled with several bureaus lists them all; the order carries no meaning.
3. Per entry, both fields are REQUIRED. bureau is the HTTPS base URL that identifies the bureau; it is where the bureau publishes its specification, its report signing key, and its query interface. subject is the agent's identifier as that bureau knows it, in the identifier system the bureau resolves.
4. The payload is a pointer, never a score. An entry MUST NOT carry a score, a bound, a sample size, an enrollment state, a report, or a digest of a report. The next section says why.

4. Why the card carries no report#

The terms extension in this family commits to its referenced document by digest, because a standing proposal is durable: the offer holds until it is withdrawn, and binding the card to exact bytes is the protection. A reputation report is the opposite kind of object. It is recomputed from the ledger on every query, it decays by design (silence slides a score back toward UNPROVEN), and it carries an expires_at minutes to hours after issued_at. A carried or cached report is stale the moment it is issued, and every hop it rides through (a card, a cache, a catalog) makes it staler and easier to cherry-pick: an agent would embed its best report ever, not its current one.

So the rule is structural, not advisory: the card names where to ask, and a consumer that wants the answer asks the bureau. A consumer SHOULD fetch a fresh report from the bureau at the moment it needs one, and MUST NOT treat any reputation claim embedded in a card, in card prose, in skill descriptions, or in a cached copy of a report, as current. Card prose stating a score is not backed by this extension and never can be.

5. Verification#

A consumer uses a listed entry in three steps:

1. Query the bureau at its published interface for the subject named in the entry. How to query is the bureau's own documented interface, found from bureau; this extension does not define the transport.
2. Verify the report's signature with the bureau's pinned signing key, as the Agent Reputations specification section 8.1 requires: the key is published by the bureau (this bureau publishes it on its developers page and pins it per Personal Agent Naming section 5.3, signing fdbk-report-v1 plus a newline plus the canonical JSON of the report object). A consumer MUST verify the signature and MUST discard a report whose signature fails, is absent, or is made by a key other than the one the bureau pins.
3. Check the binding. The report answers for subject. A consumer MUST confirm that this subject identifier actually denotes the agent the card describes, by resolving the identifier in its own system (Personal Agent Naming or ANS) and comparing the identity it binds, its key or its domain, against the agent the card presents. Where the consumer cannot confirm that binding, the report states nothing about this card's agent.

What a verified report proves is exactly what the bureau's trust model says it proves (Agent Reputations specification, section 11): this bureau received these signed events from these identified parties, kept them unmodified, and applied the named formula. It does not prove the evaluators were wise, and it does not prove the subject will perform tomorrow.

6. Reading the report#

Rules the bureau already imposes on its consumers apply unchanged when the path to the report started from a card, and three of them are restated here because a card-mediated reader is the most likely to get them wrong:

1. Enrollment states are answers, not blanks. UNPROVEN means enrolled with insufficient evidence; UNENROLLED means never opted in; WITHDRAWN means enrolled and then left, and the exit is part of the record. A consumer MUST show these states as they are, MUST NOT render one as another, and MUST NOT render any of them as an empty field or a missing agent.
2. Sample size beside any score. A consumer that displays a score MUST display n_eff and the enrollment state beside it. A score without its sample size is a forgery of confidence.
3. The report is advisory, never a gate. It informs the consumer's choice; it does not make it, and nothing in this extension or the bureau's specification authorizes refusing, admitting, or ranking an agent mechanically on a report alone. A consumer weighs the report as one input, with the sample size and evidence context in view.
4. A report about the agent's organization is about the organization. The bureau's organization report (Agent Reputations specification, section 8.5) groups the records of several agents and serves no score of its own. A consumer MUST NOT present it, or any member's report, as the report of the agent this card describes. The same holds for the sender block of a report (section 4.7): it counts other parties' refusals of the agent's requests and is never a quality score.

7. Honesty rules#

1. A card carrying this extension asserts exactly this: the agent claims enrollment with the named bureaus under the named subject identifiers. It is not certification, not endorsement, not a score, and not evidence of past conduct. Consumers, catalogs, and user interfaces MUST NOT present it as any of those.
2. Naming a bureau is not evidence the bureau is trustworthy. Anyone can operate a bureau, and an agent picks which ones to enroll with. The extension tells the consumer which bureau to ask; the consumer chooses which bureaus it believes, and a report from a bureau the consumer has no reason to trust is a claim, not a finding. Cross-bureau corroboration is future work in the bureau's own specification (section 13), not something this extension provides.
3. Absence states nothing. A card without this extension is a card that is silent about reputation. A consumer MUST NOT read absence as a bad record, a good record, or any record, and MUST NOT rank or filter agents as if absence were a negative assertion. An unenrolled agent and an agent that simply did not update its card look identical from here, and that is all absence can mean.
4. A failed query or failed verification also states nothing about the agent's reputation. It is a signal worth surfacing (section 8); it is not a low score.

8. Security considerations#

• Reputation borrowing. The sharpest attack this extension enables is a card naming a subject that is somebody else, an established agent with a good record. The digest had this covered in the terms extension; here the binding check of section 5 step 3 is the defense, and it is mandatory. A consumer that skips it is letting any card wear any agent's record.
• Bureau substitution. An attacker who cannot forge a bureau's signature can still stand up a flattering bureau and name it. That is not a protocol failure; it is why honesty rule 2 exists. Consumers maintain their own view of which bureaus they credit, exactly as they maintain which certificate authorities they trust.
• Stale pointer. The bureau URL can rot or the bureau can be down. An unreachable bureau means no report, and no report states nothing (honesty rule 4). A consumer MUST NOT fall back to a cached report beyond its expires_at and present it as current.
• Downgrade by omission. An agent whose record soured can drop the extension from its card, and absence states nothing, so the drop erases the pointer without erasing the record. A consumer that knows a subject identifier from an earlier card MAY still query the bureau; enrollment history at the bureau is permanent, and WITHDRAWN is served as WITHDRAWN. The card controls the pointer, never the ledger.
• Organization borrowing. A card can name, as its subject, a well-rated sibling, or an organization's anchor in place of an agent. The binding check of section 5 step 3 refuses both, because neither resolves to the key the card presents, and section 6 rule 4 keeps an organization report from standing in for the agent's own.
• Cherry-picked bureaus. An agent enrolled with three bureaus can list only the flattering one. The extension cannot prevent that and does not claim to; it is one more reason the report is advisory and the consumer's bureau choices are its own.

9. Out of scope#

Four things are deliberately not in this document:

• The bureau's own protocol. Enrollment, evidence submission, outcome taxonomies, scoring, and the report format are the Agent Reputations specification's, defined there and only referenced here.
• Submitting evidence over A2A. A reporter or evaluator interface carried over A2A would be a method extension, a separate document.
• Cross-bureau aggregation. Combining reports from several bureaus into one figure is a consumer policy this extension does not define.
• Terms. What the agent offers and at what price is the terms extension, a sibling document (https://agentsow.com/extensions/a2a-terms/v1).

10. Relationship to other work#

This extension is the second in a pair. The terms extension answers "on what terms" with a durable document committed by digest; this one answers "on what record" with a pointer to a perishable report fetched fresh. The asymmetry between them is the design: commitments are pinned, judgments are looked up. The A2A extension ecosystem at the time of writing addresses payments, credentials, compliance verdicts, tracing, timestamps, and routing; none of it carries reputation, and the nearest thing, traceability, records what happened in a task rather than what a population of counterparties concluded across many. The Agent Reputations bureau was developed alongside Agent Messaging, and nothing in this extension requires it: any bureau meeting the referenced specification's obligations, and any A2A agent able to add one card entry, can use it.

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Changelog#

0.1.2-draft (2026-10-18). Restates two rules of Agent Reputations 0.4.0-draft for card readers, with no change to the payload or the verification rule: an organization report or a sender block is never the report of the card's agent (section 6 rule 4), and the binding check refuses a card that names a sibling or an anchor as its subject (section 8).

0.1.1-draft (2026-10-18). Editorial, with no normative change. Personal Agent Naming is written out, the Agent Name Service is named on first use, and section 10 names Agent Messaging rather than a platform.

0.1.0-draft (2026-08-11). The card points at the record it cannot carry.

First draft, the second A2A card extension in this family. The shape question was settled before the first line: a reputation embedded in a card is a self-description wearing someone else's clothes, stale at issue, staler at every cache, and selectable by the one party who must not select it. So the payload is a pointer, bureau plus subject, and everything else follows from the bureau's own discipline: fetch fresh, verify against the pinned signing key, show UNPROVEN and UNENROLLED as the answers they are, never show a score without its sample size, and treat the report as advice rather than a gate. The extension adds one rule of its own, the binding check, because a pointer can point at anyone's record and a digest cannot defend a document that is recomputed hourly; the consumer confirms the subject identifier denotes the card's agent before believing anything. The multi-bureau reality is stated instead of smoothed over: the card says which bureaus the agent enrolled with, naming a bureau proves nothing about the bureau, and which bureaus to believe is the consumer's judgment, made exactly the way it makes its certificate authority choices. Prior art was checked in the same sweep as the terms extension: nothing in the A2A extension ecosystem carries reputation.