**TL;DR:** Stream an agent with the v3 event-streaming API for typed projections, and collect token usage with UsageMetadataCallbackHandler so you know what every run cost. Log it per run; it is your cost ledger. - OpenAI streaming needs an explicit opt-in for usage metadata in streaming chunks.

## The fix

1. `stream.messages` is the text projection; `stream.output` is the final state. Consume projections independently instead of branching on chunk types.

2. Pass the callback in the config so every model call in the run reports into it. `get_usage_metadata_callback` gives you the same as a context manager.

3. usage_metadata is keyed by model name, with input/output/total tokens plus provider details (cache reads, reasoning tokens). Log it per run; it is your cost ledger.

4. OpenAI streaming needs an explicit opt-in for usage metadata in streaming chunks. If usage is empty on OpenAI streams, that is the missing step.

## When to use this

- This covers exactly what the title says: Streaming plus token accounting.
- You are setting this up for the first time, or auditing an existing setup.
- You want the key gotchas in one place before you start.

## When not to use this

- You are doing a different workflow with Streaming; these steps are specific to the title above.
- You need the full reference docs; this is the short path, not the manual.

## Compatibility

- Not pinned to a specific version; follows current Streaming behavior.

---

## Vectle citations

- Exact version: <https://vectle.com/skills/skl_afj9TxgsQ7Rt7GSLWP-EBA/versions/skv_DTrCdALiYTQpj4p8YyuBMQ>
- Canonical HTML skill page: <https://vectle.com/skills/skl_afj9TxgsQ7Rt7GSLWP-EBA>
- Current Markdown URL: <https://vectle.com/skills/skl_afj9TxgsQ7Rt7GSLWP-EBA/markdown>

## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://docs.langchain.com/oss/python/langchain/event-streaming",
    "https://docs.langchain.com/oss/python/langchain/models"
  ]
}
```