---
title: "Background injection"
sidebarTitle: "Background injection"
description: "Inject context from background work into the agent's conversation — self-review, RAG augmentation, or any async analysis."
---

## Overview

`chat.inject()` queues model messages for injection into the conversation. Messages are picked up at the start of the next turn or at the next `prepareStep` boundary (between tool-call steps).

This is the backend counterpart to [pending messages](/ai-chat/pending-messages) — pending messages come from the user via the frontend, while `chat.inject()` comes from your task code.

## Basic usage

```ts
import { chat } from "@trigger.dev/sdk/ai";

// Queue a system message for injection
chat.inject([
  {
    role: "system",
    content: "The user's account was just upgraded to Pro.",
  },
]);
```

Messages are appended to the model messages before the next LLM inference call. The LLM sees them as part of the conversation context.

## Common pattern: defer + inject

The most powerful pattern combines `chat.defer()` (background work) with `chat.inject()` (inject results). Background work runs in parallel with the idle wait between turns, and results are injected before the next response.

```ts
export const myChat = chat.agent({
  id: "my-chat",
  onTurnComplete: async ({ messages }) => {
    // Kick off background analysis — doesn't block the turn
    chat.defer(
      (async () => {
        const analysis = await analyzeConversation(messages);
        chat.inject([
          {
            role: "system",
            content: `[Analysis of conversation so far]\n\n${analysis}`,
          },
        ]);
      })()
    );
  },
  run: async ({ messages, signal }) => {
    return streamText({
      ...chat.toStreamTextOptions({ registry }),
      messages,
      abortSignal: signal,
      stopWhen: stepCountIs(15),
    });
  },
});
```

### Timing

1. Turn completes, `onTurnComplete` fires
2. `chat.defer()` registers the background work
3. The run immediately starts waiting for the next message (no blocking)
4. Background work completes, `chat.inject()` queues the messages
5. User sends next message, turn starts
6. Injected messages are appended before `run()` executes
7. The LLM sees the injected context alongside the new user message

If the background work finishes *during* a tool-call loop (not between turns), the messages are picked up at the next `prepareStep` boundary instead.

## Example: self-review

A cheap model reviews the agent's response after each turn and injects coaching for the next one. Uses [Prompts](/ai/prompts) for the review prompt and `generateObject` for structured output.

```ts
import { chat } from "@trigger.dev/sdk/ai";
import { prompts } from "@trigger.dev/sdk";
import { streamText, generateObject, createProviderRegistry, stepCountIs } from "ai";
import { anthropic } from "@ai-sdk/anthropic";
import { z } from "zod";

const registry = createProviderRegistry({ anthropic });

const selfReviewPrompt = prompts.define({
  id: "self-review",
  model: "anthropic:claude-haiku-4-5",
  content: `You are a conversation quality reviewer. Analyze the assistant's most recent response.

Focus on:
- Whether the response answered the user's question
- Missed opportunities to use tools or provide more detail
- Tone mismatches

Be concise. Only flag issues worth fixing.`,
});

export const myChat = chat.agent({
  id: "my-chat",
  onTurnComplete: async ({ messages }) => {
    chat.defer(
      (async () => {
        const resolved = await selfReviewPrompt.resolve({});

        const review = await generateObject({
          model: registry.languageModel(resolved.model ?? "anthropic:claude-haiku-4-5"),
          ...resolved.toAISDKTelemetry(),
          system: resolved.text,
          prompt: messages
            .filter((m) => m.role === "user" || m.role === "assistant")
            .map((m) => {
              const text =
                typeof m.content === "string"
                  ? m.content
                  : Array.isArray(m.content)
                    ? m.content
                        .filter((p: any) => p.type === "text")
                        .map((p: any) => p.text)
                        .join("")
                    : "";
              return `${m.role}: ${text}`;
            })
            .join("\n\n"),
          schema: z.object({
            needsImprovement: z.boolean(),
            suggestions: z.array(z.string()),
          }),
        });

        if (review.object.needsImprovement) {
          chat.inject([
            {
              role: "system",
              content: `[Self-review]\n\n${review.object.suggestions.map((s) => `- ${s}`).join("\n")}\n\nApply these naturally.`,
            },
          ]);
        }
      })()
    );
  },
  run: async ({ messages, signal }) => {
    return streamText({
      ...chat.toStreamTextOptions({ registry }),
      messages,
      abortSignal: signal,
      stopWhen: stepCountIs(15),
    });
  },
});
```

The self-review runs on `claude-haiku-4-5` (fast, cheap) in the background. If the user sends another message before it completes, the coaching is still injected — `chat.inject()` persists across the idle wait.

## Other use cases

- **RAG augmentation**: After each turn, fetch relevant documents and inject them as context for the next response
- **Safety checks**: Run a moderation model on the response, inject warnings if issues are detected
- **Fact-checking**: Verify claims in the response using search tools, inject corrections
- **Context enrichment**: Look up user/account data based on what was discussed, inject it as system context

## `chat.defer` standalone

`chat.defer()` is also useful on its own, without `chat.inject()`. Any work whose timing has no resume implication — analytics, audit logs, search-index writes, cache warming — can run in parallel with streaming instead of in the critical path. All deferred promises are awaited (with a 5s timeout) before `onTurnComplete` fires.

```ts
export const myChat = chat.agent({
  id: "my-chat",
  onTurnStart: async ({ chatId, runId }) => {
    // Analytics — fire-and-forget, irrelevant to resume.
    chat.defer(analytics.track("turn_started", { chatId, runId }));
  },
  run: async ({ messages, signal }) => {
    return streamText({ model: anthropic("claude-sonnet-4-5"), messages, abortSignal: signal });
  },
});
```

`chat.defer()` can be called from anywhere during a turn — hooks, `run()`, or nested helpers. All deferred promises are collected and awaited together before `onTurnComplete`.

<Warning>
**Don't use `chat.defer()` for the message-history write in `onTurnStart`.** That write must land *before* the model starts streaming, otherwise a mid-stream page refresh will read `[]` from your DB and lose the user's message from the rendered conversation. See [Database persistence — `onTurnStart`](/ai-chat/patterns/database-persistence#onturnstart). Reserve `chat.defer` for writes whose timing has no resume implication.
</Warning>

## How it differs from pending messages

| | `chat.inject()` | [Pending messages](/ai-chat/pending-messages) |
|---|---|---|
| **Source** | Backend task code | Frontend user input |
| **Triggered by** | Your code (e.g. `onTurnComplete` + `chat.defer()`) | User sending a message during streaming |
| **Injection point** | Start of next turn, or next `prepareStep` boundary | Next `prepareStep` boundary only |
| **Message role** | Any (`system`, `user`, `assistant`) | Typically `user` |
| **Frontend visibility** | Not visible unless you write custom `data-*` chunks | Visible via `usePendingMessages` hook |

## API reference

### chat.inject()

```ts
chat.inject(messages: ModelMessage[]): void
```

Queue model messages for injection at the next opportunity. Messages persist across the idle wait between turns — they are not reset when a new turn starts.

**Parameters:**

| Parameter | Type | Description |
|-----------|------|-------------|
| `messages` | `ModelMessage[]` | Model messages to inject (from the `ai` package) |

Messages are drained (consumed) when:
1. A new turn starts — before `run()` executes
2. A `prepareStep` boundary is reached — between tool-call steps during streaming

<Note>
  `chat.inject()` writes to an in-memory queue in the current process. It works from any code running in the same task — lifecycle hooks, deferred work, tool execute functions, etc. It does not work from subtasks or other runs.
</Note>