[Enhancement] Add voice agent middleware pipeline for custom interceptors and hooks

[deepgram-robot]

Summary

Add a middleware/interceptor pipeline for the Voice Agent client that lets developers inject custom logic at key points in the conversation lifecycle — before/after STT results, before/after LLM processing, before/after TTS output, and around function calls — without modifying the SDK internals.

Problem it solves

Developers building production voice agents need to add cross-cutting concerns — logging, PII detection, compliance checks, metric collection, response filtering, A/B testing — but the current event-based API requires writing this logic inline in event handlers. A middleware pipeline provides a composable, reusable pattern for layering concerns, similar to ASGI/WSGI middleware in web frameworks that Python developers already understand.

Proposed API

from deepgram import DeepgramClient, AgentMiddleware

class PIIRedactionMiddleware(AgentMiddleware):
    async def on_transcript(self, transcript, next):
        redacted = self.redact_pii(transcript)
        return await next(redacted)

    async def on_agent_response(self, response, next):
        if self.contains_pii(response):
            response = self.redact_pii(response)
        return await next(response)

class MetricsMiddleware(AgentMiddleware):
    async def on_transcript(self, transcript, next):
        start = time.monotonic()
        result = await next(transcript)
        self.record_latency("stt", time.monotonic() - start)
        return result

client = DeepgramClient()
agent = client.agent.v1.websocket(
    middleware=[PIIRedactionMiddleware(), MetricsMiddleware()]
)

Acceptance criteria

• Middleware base class with hooks for each conversation lifecycle stage
• Middleware chain executes in order, each calling next to proceed
• Middleware can modify, filter, or reject messages
• Zero overhead when no middleware is configured
• Documented with usage example
• Compatible with existing event-based API (middleware runs before event handlers)

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Raised by the DX intelligence system.