Tenuo A2A Integration

Overview

Tenuo A2A adds warrant-based authorization to agent-to-agent communication. When Agent A delegates a task to Agent B, the warrant specifies exactly what Agent B is allowed to do.

┌─────────────┐                     ┌─────────────┐
│   Agent A   │   Task + Warrant    │   Agent B   │
│ (Orchestrator)│──────────────────▶│  (Worker)   │
│             │                     │             │
│             │◀──────────────────  │             │
│             │      Result         │             │
└─────────────┘                     └─────────────┘

Warrant says: "Agent B can only search arxiv.org for this task"

Use cases:

  • Multi-agent systems where agents delegate tasks
  • Orchestrators that dispatch work to specialized workers
  • Agent networks with least-privilege access control

Not for: Single-agent tool enforcement (use tenuo.openai or tenuo.langchain instead)

Installation

uv pip install "tenuo[a2a]"

Quick Start (Minimal Example)

Server (Worker Agent):

from tenuo.a2a import A2AServerBuilder

# Build server with fluent API
server = (A2AServerBuilder()
    .name("Worker")
    .url("https://worker.example.com")
    .key(my_signing_key)                    # Your identity
    .accept_warrants_from(orchestrator_key) # Who can give you tasks
    .build())

@server.skill("echo")
async def echo(msg: str) -> str:
    return f"Echo: {msg}"

# uvicorn server:server.app --port 8000

Or use the direct constructor:

from tenuo.a2a import A2AServer

server = A2AServer(
    name="Worker",
    url="https://worker.example.com",
    public_key=my_public_key,
    trusted_issuers=[orchestrator_public_key],
)

Client (Orchestrator):

from tenuo.a2a import A2AClientBuilder
from tenuo import Warrant

# Create warrant for this task
task_warrant = (Warrant.mint_builder()
    .capability("echo")
    .holder(worker_public_key)
    .ttl(300)
    .mint(orchestrator_key))

# Build client with default warrant
client = (A2AClientBuilder()
    .url("http://localhost:8000")
    .warrant(task_warrant, orchestrator_key)  # Pre-configure for repeated use
    .build())

# Send task (warrant already configured)
result = await client.send_task(
    "hello",
    skill="echo",
    arguments={"msg": "hello"},
)
print(result.output)  # "Echo: hello"

Or use the direct constructor:

from tenuo.a2a import A2AClient

client = A2AClient("http://localhost:8000")
result = await client.send_task(
    "hello",
    skill="echo",
    arguments={"msg": "hello"},
    warrant=task_warrant,
    signing_key=orchestrator_key,
)

That’s it. The warrant proves the orchestrator authorized this specific task.

Full Example (With Constraints)

Server (Worker)

from tenuo.a2a import A2AServerBuilder
from tenuo.constraints import Subpath, UrlSafe

server = (A2AServerBuilder()
    .name("Research Agent")
    .url("https://research-agent.example.com")
    .key(my_signing_key)
    .accept_warrants_from(orchestrator_public_key)
    .build())

# Register skills with constraint bindings
@server.skill("search_papers", constraints={"sources": UrlSafe()})
async def search_papers(query: str, sources: list[str]) -> list[dict]:
    return await do_search(query, sources)

@server.skill("read_file", constraints={"path": Subpath("/data")})
async def read_file(path: str) -> str:
    with open(path) as f:
        return f.read()

# uvicorn server:server.app --host 0.0.0.0 --port 8000

Automated Registration (CSR Handshake)

A2A supports an automated handshake for agent registration, eliminating the need for out-of-band key sharing. This follows the Certificate Signing Request (CSR) pattern.

The connecting agent dynamically generates a self-signed challenge token to cryptographically prove key ownership. The server verifies this signature and uses a registered handler to decide what capabilities to grant, minting a fresh delegation warrant on the fly.

Server (Control Plane / Parent Agent):

from tenuo.a2a.types import VerifiedWarrantRequest

# The handler decides whether to grant the requested capabilities
async def registration_handler(req: VerifiedWarrantRequest, issue):
    if req.verified_key_hex not in ALLOWLIST:
        raise RegistrationDeniedError("Agent not approved")
    
    # Issue a new warrant bound to the requested capabilities
    await issue(capabilities=req.capabilities, ttl=86400) # 24 hrs

server = (A2AServerBuilder()
    .name("Control Plane")
    .url("https://control.example.com")
    .key(server_signing_key) # MUST be a SigningKey to issue warrants
    .trust(server_signing_key.public_key)
    .registration_handler(registration_handler) # Enable handshake
    .build())

Client (Child Agent):

from tenuo.a2a import A2AClient
from tenuo import SigningKey

client = A2AClient("https://control.example.com")
worker_key = SigningKey.generate() 

# Request a warrant with specific capabilities
# The client automatically generates the self-signed challenge token
warrant = await client.request_warrant(
    signing_key=worker_key,
    capabilities={"search_papers": {}}
)

# You can now immediately use this warrant (and key) for tasks
result = await client.send_task(
    "Search for AI Agents papers",
    skill="search_papers",
    arguments={"query": "AI Agents"},
    warrant=warrant,
    signing_key=worker_key,
)

Note: Extension data (like AWS Nitro Enclaves or SGX TEE quotes) can be attached to the request via the extensions parameter in request_warrant() and inspected in the server handler via req.extensions.

Client (Orchestrator)

from tenuo.a2a import A2AClient
from tenuo.constraints import UrlSafe

# Discover agent capabilities
client = A2AClient("https://research-agent.example.com")
card = await client.discover()

# Attenuate warrant for this delegation
task_warrant = (my_warrant
    .grant_builder()
    .capability("search_papers", sources=UrlSafe(allow_domains=["arxiv.org"]))
    .holder(card.public_key)
    .ttl(300)
    .build(my_signing_key))

# Send task with warrant
result = await client.send_task(
    message="Find papers on capability-based security",
    warrant=task_warrant,
    skill="search_papers",
    arguments={"query": "capability-based security", "sources": ["https://arxiv.org"]},
)

Streaming Tasks

For long-running tasks, use streaming to receive incremental updates:

# Stream results as they arrive
async for update in client.send_task_streaming(
    message="Analyze these papers",
    warrant=task_warrant,
    skill="analyze_papers",
    arguments={"paper_ids": ["arxiv:2401.12345"]},
):
    if update.type.value == "status":
        print(f"Status: {update.data.get('status')}")
    elif update.type.value == "message":
        print(f"Chunk: {update.data.get('content')}")
    elif update.type.value == "complete":
        print(f"Done: {update.data.get('output')}")

The server emits SSE events for status updates, intermediate messages, and final completion.

Stream timeout (DoS protection):

# Default timeout is 300 seconds (5 minutes)
async for update in client.send_task_streaming(
    ...,
    stream_timeout=600.0,  # 10 minute timeout
):
    ...

If the stream exceeds stream_timeout, a TimeoutError is raised. This prevents slow-drip DoS attacks where a malicious server holds connections indefinitely.

Proof-of-Possession (PoP)

Proof-of-Possession adds an additional security layer by requiring the client to prove they control the private key associated with the warrant’s holder.

When to Use PoP

Require PoP when:

  • Agents communicate over untrusted networks (Internet, shared infrastructure)
  • Compliance requires cryptographic proof of authorization
  • Protection against warrant theft is critical
  • Multi-hop delegation across organizational boundaries

PoP is optional when:

  • All agents run on trusted infrastructure (same data center, VPC)
  • Network isolation provides security (private network, mTLS)
  • Performance is critical and risk is low (every extra signature operation matters)

Never skip PoP when:

  • Agents are on the public Internet
  • Warrants have long TTLs (hours/days)
  • Untrusted intermediaries exist in the call chain

How PoP Works

PoP signatures prove that the caller possesses the private key corresponding to the warrant’s sub (holder) field:

┌──────────────────────────────────────────────────────┐
│ Warrant (JWT):                                       │
│   sub: "z6Mk..."  ← Orchestrator's public key       │
│   grants: ["search"]                                 │
│   exp: 1234567890                                    │
│   Signature: <signed by control plane>              │
└──────────────────────────────────────────────────────┘
                       +
┌──────────────────────────────────────────────────────┐
│ PoP Signature:                                       │
│   sign(orchestrator_private_key, "search", args, ts) │
│   → Proves orchestrator controls the private key     │
└──────────────────────────────────────────────────────┘
                       =
               Authorization Proof

What PoP Prevents:

  • Warrant Theft: If an attacker intercepts a warrant, they can’t use it without the private key
  • Replay Attacks: Each PoP signature includes a timestamp and is checked once
  • Man-in-the-Middle: Modified arguments invalidate the PoP signature

Client Usage

Enable PoP by passing signing_key to send_task():

from tenuo.a2a import A2AClient

client = A2AClient("https://worker.example.com")

# Without PoP (only warrant validation)
result = await client.send_task(
    "search for papers",
    warrant=my_warrant,
    skill="search",
    arguments={"query": "papers"},
)

# With PoP (warrant + signature proof)
result = await client.send_task(
    "search for papers",
    warrant=my_warrant,
    skill="search",
    arguments={"query": "papers"},
    signing_key=orchestrator_key,  # ← Proves possession
)

Or configure PoP by default using the builder:

from tenuo.a2a import A2AClientBuilder

client = (A2AClientBuilder()
    .url("https://worker.example.com")
    .warrant(my_warrant, orchestrator_key)  # ← Pre-configure PoP
    .build())

# All requests automatically include PoP
result = await client.send_task(
    "search for papers",
    skill="search",
    arguments={"query": "papers"},
)

Server Configuration

Control PoP requirements on the server:

server = A2AServer(
    name="Worker",
    url="https://worker.example.com",
    public_key=worker_public_key,
    trusted_issuers=[control_plane_key],  # Required — warrants must be signed by these keys

    # PoP configuration
    require_pop=True,          # Reject requests without PoP (default: True)
)

Important: Always configure trusted_issuers. Without it, the builder raises ValueError. This ensures only warrants signed by your control plane are accepted — self-signed warrants from attackers are rejected.

Security Defaults:

  • trusted_issuers is required (fail-closed)
  • require_pop=True by default (fail-safe)
  • Can be disabled via TENUO_A2A_REQUIRE_POP=false environment variable
  • If require_pop=True but client doesn’t provide PoP → PopRequiredError

Performance Impact

Enabling PoP adds two extra Ed25519 signature operations per request on top of warrant verification:

  • Without PoP: warrant verification only.
  • With PoP: warrant verification + client-side PoP signing + server-side PoP verification.

All three operations are local and offline. See Performance Benchmarks for measured timings.

Recommendation: Always use PoP in production unless you have network-level security (mTLS + VPC).

Error Handling

from tenuo.a2a import PopRequiredError, PopVerificationError

try:
    result = await client.send_task("search", warrant=warrant, skill="search", arguments={})
except PopRequiredError:
    print("Server requires PoP signature - add signing_key parameter")
except PopVerificationError as e:
    print(f"PoP signature invalid: {e}")
    # Possible causes:
    #   - Wrong signing key (not matching warrant.sub)
    #   - Arguments modified after signing
    #   - Clock skew between client/server

Debugging PoP Issues

Issue: PopVerificationError: Signature verification failed

Causes:

  1. Wrong signing key: Key doesn’t match warrant’s sub field
  2. Modified arguments: Arguments changed after PoP computation
  3. Clock skew: Client/server clocks differ significantly

Debug:

# Verify signing key matches warrant holder
assert warrant.sub == str(signing_key.public_key)

# Log PoP computation
import logging
logging.getLogger("tenuo.a2a.client").setLevel(logging.DEBUG)
# Shows: "Generated PoP signature for skill 'search'"

Server Configuration

server = A2AServer(
    # Required
    name="Agent Name",                    # Display name
    url="https://agent.example.com",      # Public URL (for audience validation)
    public_key=my_public_key,             # This agent's public key
    trusted_issuers=[...],                # List of trusted issuer public keys
    
    # Optional (shown with defaults)
    trust_delegated=True,                 # Accept warrants delegated from trusted issuers
    require_warrant=True,                 # Reject tasks without warrants
    require_audience=True,                # Require warrant audience matches our URL
    check_replay=True,                    # Enforce jti uniqueness
    replay_window=3600,                   # Seconds to remember jti values
    max_chain_depth=10,                   # Maximum delegation chain length
    
    # Audit
    audit_log=sys.stderr,                 # Destination (file, callable, or stderr)
    audit_format="json",                  # "json" or "text"
)

Trust Model

The server trusts warrants based on trusted_issuers:

  1. Direct Trust: Warrant signed by a trusted issuer → accepted
  2. Delegated Trust (if trust_delegated=True): Warrant with valid chain back to trusted issuer → accepted
┌─────────────────────┐
│    Trusted Root     │  ← In trusted_issuers
│   (Control Plane)   │
└──────────┬──────────┘
           │ delegates
           ▼
┌─────────────────────┐
│   Orchestrator A    │  ← Warrant signed by root
└──────────┬──────────┘
           │ delegates
           ▼
┌─────────────────────┐
│     Worker B        │  ← Warrant with chain [root → A → B]
└─────────────────────┘

Skill Constraints

Constraints bind warrant parameters to skill parameters:

@server.skill("read_file", constraints={"path": Subpath("/data")})
async def read_file(path: str) -> str:
    # "path" constraint checked against warrant's path constraint
    # Blocked if: warrant allows Subpath("/data") but arg is "/etc/passwd"
    ...

Constraint binding validation happens at startup:

# This raises ConstraintBindingError at startup:
@server.skill("read_file", constraints={"file_path": Subpath("/data")})  # "file_path" not a param
async def read_file(path: str) -> str:  # param is "path"
    ...

Client Configuration

client = A2AClient(
    url="https://agent.example.com",
    
    # Optional
    pin_key="z6Mk...",    # Expected public key (raises KeyMismatchError if different)
    timeout=30.0,          # Request timeout in seconds
)

Key Pinning

Pin the expected public key to prevent TOFU (Trust On First Use) attacks:

# If agent returns different key, raises KeyMismatchError
client = A2AClient(
    "https://research-agent.example.com",
    pin_key="z6MkResearchAgentKey123"  # From your config/secrets
)

card = await client.discover()  # Fails if key doesn't match

Key Format Compatibility

A2A accepts public keys in multiple formats:

# All of these work:
server = (A2AServerBuilder()
    .key(signing_key)  # PublicKey object
    .accept_warrants_from("a1b2c3...")  # Hex (64 chars)
    .accept_warrants_from("z6MkpT...")  # Multibase (base58btc)
    .accept_warrants_from("did:key:z6MkpT...")  # W3C DID
    .build())

All formats are automatically normalized for comparison. Multibase and DID support requires uv pip install base58.

Agent Card (Discovery)

Agents expose their capabilities via /.well-known/agent.json:

{
  "name": "Research Agent",
  "url": "https://research-agent.example.com",
  "skills": [
    {
      "id": "search_papers",
      "name": "Search Papers",
      "x-tenuo-constraints": {
        "sources": {"type": "UrlSafe", "required": true}
      }
    }
  ],
  "x-tenuo": {
    "version": "0.1.0",
    "required": true,
    "public_key": "z6Mk..."
  }
}

Delegation Chains

When delegating through multiple agents, the full chain is transmitted as a single header.

The server validates:

  1. Root warrant is from a trusted issuer
  2. Each link: child issuer = parent holder
  3. Skills narrow monotonically (no privilege escalation)
  4. Chain depth ≤ max_chain_depth

WarrantStack Transport

The current implementation packs the entire delegation chain into a single X-Tenuo-Warrant header using WarrantStack encoding, rather than the legacy two-header approach (X-Tenuo-Warrant + X-Tenuo-Warrant-Chain).

The client encodes the chain with encode_warrant_stack and the server decodes it with decode_warrant_stack_base64:

from tenuo import encode_warrant_stack

# Client sends delegation chain as a single header
chain = [root_warrant, child_warrant]
stack_b64 = encode_warrant_stack(chain)
# stack_b64 goes in X-Tenuo-Warrant header
# Server automatically detects and unpacks WarrantStack

This simplifies proxy and load-balancer configurations (one header to forward instead of two) and avoids ordering ambiguities in multi-hop chains.

Human Approval

Define gates and approvers on the warrant. On retry, attach SignedApproval objects via header or param. See Human Approvals for minting and signing.

from tenuo.a2a import A2AClient, ApprovalRequiredError, InsufficientApprovalsError
from tenuo.approval import sign_approval

try:
    result = await client.send_task(skill="transfer", arguments={"amount": 5000})
except ApprovalRequiredError as e:
    # -32019 — gate fired; e.data has request_hash, min_approvals, skill
    signed = sign_approval(approval_request, approver_key)  # from your UI flow
    result = await client.send_task(
        skill="transfer",
        arguments={"amount": 5000},
        approvals=[signed],  # encoded into X-Tenuo-Approvals
    )
except InsufficientApprovalsError as e:
    # -32020 — collect additional signatures; check e.data["required"] vs ["received"]
    ...

Wire encoding: X-Tenuo-Approvals = base64(JSON(["base64(CBOR SignedApproval)", ...])). Same outer wrapper as FastAPI.

A2A JSON-RPC Wire code When Key data fields
-32019 1707 Gate fired, no approvals request_hash, min_approvals, skill
-32020 1700 Partial multi-sig required, received
-32021 1701 Invalid / malformed approval reason

Note: A2A -32002 is invalid signature (1100), not approval. Do not reuse MCP’s -32002 semantics on A2A.

Error Handling

All A2A errors inherit from A2AError and map to JSON-RPC error codes with canonical wire codes:

from tenuo.a2a import (
    A2AError,
    MissingWarrantError,      # -32001: Warrant required but not provided
    InvalidSignatureError,     # -32002: Signature verification failed
    UntrustedIssuerError,      # -32003: Issuer not in trusted_issuers
    WarrantExpiredError,       # -32004: Warrant has expired
    AudienceMismatchError,     # -32005: Audience doesn't match server URL
    ReplayDetectedError,       # -32006: jti already used
    SkillNotGrantedError,      # -32007: Skill not in warrant grants
    ConstraintViolationError,  # -32008: Argument violates constraint
    ChainInvalidError,         # -32010: Delegation chain validation failed
    KeyMismatchError,          # -32012: Public key doesn't match pinned key
    ApprovalRequiredError,     # -32019: Approval gate fired
    InsufficientApprovalsError,  # -32020: Partial multi-sig
    InvalidApprovalError,      # -32021: Bad signed approval
)

try:
    result = await client.send_task(...)
except SkillNotGrantedError as e:
    print(f"Skill {e.data['skill']} not in granted: {e.data['granted_skills']}")
except A2AError as e:
    print(f"A2A error {e.code}: {e.message}")

Wire Code Support

A2A error responses now include canonical Tenuo wire codes (1000-2199) for cross-protocol compatibility:

{
  "jsonrpc": "2.0",
  "error": {
    "code": -32008,
    "message": "Constraint violation",
    "data": {
      "tenuo_code": 1501,
      "field": "amount",
      "reason": "Value exceeds maximum"
    }
  },
  "id": "task_123"
}

This enables:

  • Cross-protocol debugging: Same wire codes used in HTTP, gRPC, and JSON-RPC
  • Precise error mapping: JSON-RPC code -32008 maps to canonical code 1501
  • Machine-readable errors: Clients can programmatically handle specific error types
A2A JSON-RPC Code Canonical Wire Code Name
-32001 — (A2A-specific) Missing warrant
-32002 1100 Invalid signature
-32003 1406 Untrusted issuer
-32004 1300 Warrant expired
-32007 1500 Tool not authorized
-32008 1501 Constraint violation
-32010 1405 Chain invalid
-32019 1707 Approval required (gate fired)
-32020 1700 Insufficient approvals (partial multi-sig)
-32021 1701 Invalid approval

See wire format specification for the complete list.

Accessing the Warrant

Inside a skill, access the current warrant via context:

from tenuo.a2a import current_task_warrant

@server.skill("my_skill")
async def my_skill(query: str) -> str:
    warrant = current_task_warrant.get()
    if warrant:
        print(f"Warrant issuer: {warrant.iss}")
        print(f"Warrant subject: {warrant.sub}")
    return "done"

Audit Logging

Server emits structured audit events:

# JSON format (default)
{"timestamp": "...", "event": "warrant_validated", "skill": "search", "outcome": "allowed", ...}

# Text format
[WARRANT_VALIDATED] search: allowed

Custom audit handler:

async def my_audit_handler(event: AuditEvent):
    await send_to_siem(event.to_dict())

server = A2AServer(..., audit_log=my_audit_handler)

Example: Full Multi-Agent System

# control_plane.py
from tenuo import SigningKey, Warrant

control_key = SigningKey.from_env("CONTROL_PLANE_KEY")

def issue_orchestrator_warrant(orchestrator_pubkey):
    return (Warrant.mint_builder()
        .capability("search_papers", {})
        .capability("read_file", {"path": Subpath("/data")})
        .holder(orchestrator_pubkey)
        .ttl(86400)  # 24 hours
        .mint(control_key))

# orchestrator.py
from tenuo.a2a import A2AClient

async def delegate_research(topic: str, my_warrant, my_key, target_pubkey):
    client = A2AClient("https://research-agent.example.com")
    
    # Attenuate warrant for this specific task
    task_warrant = (my_warrant
        .grant_builder()
        .capability("search_papers", sources=UrlSafe(allow_domains=["arxiv.org"]))
        .holder(target_pubkey)
        .ttl(300)
        .build(my_key))
    
    return await client.send_task(
        message=f"Research: {topic}",
        warrant=task_warrant,
        skill="search_papers",
        arguments={"query": topic, "sources": ["https://arxiv.org"]},
    )

# research_agent.py
from tenuo.a2a import A2AServer
from tenuo.constraints import UrlSafe

server = A2AServer(
    name="Research Agent",
    url="https://research-agent.example.com",
    public_key=my_public_key,
    trusted_issuers=[control_plane_public_key],
)

@server.skill("search_papers", constraints={"sources": UrlSafe()})
async def search_papers(query: str, sources: list[str]) -> list[dict]:
    # Only allowed URLs pass through
    return await search_arxiv(query, sources)

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(server.app, host="0.0.0.0", port=8000)

API Reference

See API Reference for complete type signatures.

Protocol Specification

For the wire format and protocol details, see the Protocol Spec and Wire Format.