crestapps-core-a2a

CrestApps.Core A2A

Use A2A when the remote system is an agent that reasons on its own (its own model, tools and context). Use MCP (crestapps-core-mcp) when the remote system exposes tools or resources. CrestApps.Core.AI.A2A ships the client; hosting is application code built on the A2A and A2A.AspNetCore 1.0 SDK packages, using A2AHostOptions from the framework.

Client

builder.Services.AddCrestAppsCore(crestApps => crestApps
    .AddAISuite(ai => ai
        .AddOpenAI()
        .AddA2AClient(a2a => a2a
            .AddEntityCoreStores()))   // or .AddYesSqlStores(); registers ICatalog<A2AConnection>
    .AddEntityCoreSqliteDataStore("Data Source=app.db"));

AddCoreAIA2AClient() registers a resilient named HttpClient, IMemoryCache, A2AAICompletionContextBuilderHandler, A2AToolRegistryProvider, IA2AAgentCardCacheService (15-minute cache), IA2AConnectionAuthService, and the system tools listAvailableAgents, findAgentForTask and findToolsForTask.

Connections

using CrestApps.Core;
using CrestApps.Core.AI.A2A;
using CrestApps.Core.AI.A2A.Models;
using CrestApps.Core.AI.Models;

var protector = dataProtectionProvider.CreateProtector(A2AConstants.DataProtectionPurpose); // "A2AClientConnection"

var connection = new A2AConnection
{
    DisplayText = "Legal Review Agent",
    Endpoint = "https://agents.example.com/",
};

connection.Put(new A2AConnectionMetadata
{
    AuthenticationType = ClientAuthenticationType.ApiKey,
    ApiKeyHeaderName = "Authorization",
    ApiKeyPrefix = "Bearer",
    ApiKey = protector.Protect(apiKey),
});

await connectionCatalog.CreateAsync(connection);

profile.Put(new AIProfileA2AMetadata { ConnectionIds = [connection.ItemId] });

• Authentication types are Anonymous, ApiKey, Basic, OAuth2ClientCredentials, OAuth2PrivateKeyJwt, OAuth2Mtls and CustomHeaders. Protect every secret with the A2AClientConnection purpose.
• The card is fetched with A2ACardResolver from connection.Endpoint (the SDK reads /.well-known/agent-card.json under it) and each skill becomes a tool a2a:{connectionId}:{skillId} whose name keeps only letters, digits and _.
• A2AAgentProxyTool sends a 1.0 SendMessageRequest with a Message (Role.User, Part.FromText, ContextId, metadata agentName) to connection.Endpoint, not to the URL in the card's supportedInterfaces. Set Endpoint to the address that accepts JSON-RPC messages and also serves the card.
• A2ACardResolver returns a single AgentCard. A host that publishes an array of cards (one per agent) is best consumed in skill mode (ExposeAgentsAsSkill = true).
• Replace IA2AAgentCardCacheService for a distributed cache, or IA2AConnectionAuthService for a custom auth scheme.