> ## Documentation Index
> Fetch the complete documentation index at: https://cosmos-docs-cosmjs-docs.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# JSON-RPC & Reactive Streams

> Understanding the JSON-RPC protocol and stream utilities used by CosmJS transports

CosmJS uses JSON-RPC 2.0 for all transport-level communication and xstream-based reactive streams for event subscriptions. These are low-level building blocks that most applications interact with indirectly through higher-level clients.

## JSON-RPC Protocol

All transports use the [JSON-RPC 2.0](https://www.jsonrpc.org/specification)
protocol, implemented in `@cosmjs/json-rpc`. The package provides:

* **Type definitions** — `JsonRpcRequest`, `JsonRpcSuccessResponse`,
  `JsonRpcErrorResponse`
* **Parsing** — `parseJsonRpcResponse()` validates and type-guards responses
* **ID generation** — `makeJsonRpcId()` produces incrementing numeric IDs
* **Error codes** — standard JSON-RPC error codes (`parseError`,
  `invalidRequest`, `methodNotFound`, etc.)

You generally don't interact with this package directly — the RPC clients handle
serialization and parsing internally.

## Reactive Streams

The `@cosmjs/stream` package provides utilities built on
[xstream](https://github.com/staltz/xstream) for working with event streams:

| Utility                        | Description                                                            |
| ------------------------------ | ---------------------------------------------------------------------- |
| `firstEvent(stream)`           | Resolves with the first event from a stream                            |
| `toListPromise(stream, count)` | Collects `count` events into an array                                  |
| `fromListPromise(promise)`     | Creates a stream from a promise of an iterable                         |
| `concat(...streams)`           | Concatenates streams with buffering                                    |
| `dropDuplicates(keyFn)`        | Deduplicates stream events by key                                      |
| `ValueAndUpdates`              | Synchronous current value + update stream (used for connection status) |
| `DefaultValueProducer`         | Producer that holds a current value and emits updates                  |
| `Reducer`                      | Reduces a stream into accumulated state                                |

These are primarily used internally by the WebSocket transport and subscription
system, but they're available if you need to compose or transform event streams.