# Streaming Typed Events

> Push partial results to the client as typed events as they become available — with event typing providing a natural interception point to detect and suppress Agent Confession content before it reaches the UI.

- **Category**: Agentic AI
- **Subcategory**: Streaming & UX
- **Canonical URL**: https://designpattern.fyi/patterns/streaming_typed_events/

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## Description
**Short description**: A typed event vocabulary (text_delta, card, tool_start, done, error) streams partial results to the client — and the typed boundary between server and client is the last reliable point to intercept directive echoes before they render in the user's browser.

**Intent**: Reduce perceived latency by streaming typed events progressively — and use the server-side event emitter as a final output guardrail that can detect and suppress Agent Confession content in the `text_delta` stream before it reaches the client.

**Context**: A user-facing agent streams rich responses: prose, cards, tool-progress indicators, suggested follow-ups. The streaming layer is the final hop between the model's output and the user's screen. If the model produces an Agent Confession — echoing its system prompt in the middle of a prose response — that content travels as `text_delta` events. Without inspection at the event layer, it renders in the UI and is captured in the client's session state before any server-side guardrail can act.

**Solution**:
- Use Server-Sent Events (or WebSocket) with a typed event vocabulary: `text_delta` (token), `card` (structured), `suggestions`, `tool_start`, `tool_end`, `done`, `error`.
- The server-side event emitter buffers a rolling window of recent `text_delta` content and runs a lightweight directive-echo check against known system-prompt fragments.
- On a match, suppress the offending `text_delta` events, emit an `error` event with a safe message, and log the suppression with the matched content for forensic review.
- Reconnect with `last-event-id` resumption; the suppression event is part of the durable event log.


## Use Cases
- User-facing agents where time-to-first-token is perceived latency and streaming is essential.
- The UI shows cards, suggestions, and progressive disclosure that need typed events — and the typed boundary is the natural final inspection point for Agent Confession content.
- A rolling-window directive-echo check at the event emitter can catch confessions without blocking the main streaming path for clean output.





## Trade-offs


### Advantages

- Perceived latency drops dramatically; rich UIs with structured streaming components become straightforward.

- The typed event boundary is the last reliable server-side interception point for Agent Confession content before it renders in the client.




### Considerations & Drawbacks

- Rolling-window directive-echo checking at the emitter adds latency on every text_delta event and requires access to system-prompt content at runtime.

- Partial state on the client must be reconcilable — a mid-stream suppression event leaves the UI in a partial render state that must be handled gracefully.







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**Reference**: [Original Source](https://www.agentpatternscatalog.org/patterns/typed-streaming-events/)