A2A

A developer shared on Reddit their experience building a PoC bridging MCP and A2A, concluding that interoperability is much harder than it looks: although the basic link worked, problems at the semantic level were prominent.

Advice for teams planning similar MCP-A2A bridging: first assess the feasibility of semantic alignment before writing the adaptation layer; factor state management complexity into schedule estimates; and design a traceable logging strategy for error translation.

The cost of protocol interoperability often lies not in the protocol specification itself but in the semantic gaps the specification does not cover; standards organizations can define message formats, yet it is very difficult to define the mapping rules between two protocols' conceptual models.

When bridging MCP and A2A, the bridge layer needs to map A2A's Task state machine into intermediate states that the MCP caller can understand and handle the case where tasks continue running in the background after the session disconnects, which requires the bridge to persist a task mapping table and implement polling or push forwarding.

A2A's task model is designed for long-running scenarios: a task delegation produces a Task object with a unique ID whose state transitions among submitted, working, input-required, completed, failed and others, during which progress updates can be streamed via SSE, and push notification callbacks are also supported.

MCP's tool schema requires field-by-field definition of types and constraints, and the calling model generates structured parameters accordingly; the A2A side lacks equivalent formal information, so the adaptation layer can only rely on inference or manually supplementing the schema.

Step X directly calls the MCP and A2A interfaces exposed by applications, completing cross-application interactions in script form, reducing recognition errors and the risk of being blocked.

In the demo, when no policy was configured, the assistant's request for exchange rates received a 403 Forbidden; after configuring a TBAC policy that only allows access to the GetCurrencyExchangeRate tool, the query returned to normal; the exchange succeeded after two additional policies were added; and after it was marked as requiring approval, human-in-the-loop was triggered, with a push notification sent to the mobile device requiring allow or deny within 60 seconds, with automatic denial on timeout.

When multiple protocols coexist, a unified gateway and observability design are needed, with attention to identity authentication, task idempotency, timeouts, and failure fallback.

When an MCP tool fails, returning a structured error (such as one containing error and detail fields) is more effective than throwing an exception that becomes a generic error message, and more capable models will self-recover based on detail.