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

# tools/call

> Invoke a Computer Use tool by name with its JSON arguments.

## Request

```bash theme={null}
curl -s -X POST "http://localhost:8081/mcp" \
  -H "Authorization: Bearer $MCP_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $SESSION_ID" \
  -H "X-Chat-Id: my-session" \
  -d '{
    "jsonrpc": "2.0",
    "id": 3,
    "method": "tools/call",
    "params": {
      "name": "bash_tool",
      "arguments": {"command": "echo Hello from sandbox", "description": "greet"}
    }
  }'
```

## Response shape

```json theme={null}
{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "content": [
      {"type": "text", "text": "Hello from sandbox\n"}
    ],
    "isError": false
  }
}
```

`content` is an array of MCP content blocks. Today the server returns only `text` blocks. Image returns are planned but blocked on Open WebUI's client-side MCP renderer (see [known bug #5](/reference/known-bugs#5-mcp-tools-cannot-return-images-to-the-model-open-webui-limitation)).

## Streaming

Tool calls stream progress as SSE events on the same response. Clients that want partial output must set `Accept: application/json, text/event-stream` and parse `data:` lines. The `mcp` Python SDK and Open WebUI both do this automatically.

Heartbeats are emitted every 15 seconds for long-running commands so proxies don't close the connection.

## Common examples

### Create a document with the docx skill

```json theme={null}
{
  "jsonrpc": "2.0", "id": 4, "method": "tools/call",
  "params": {
    "name": "bash_tool",
    "arguments": {
      "command": "python3 /mnt/skills/public/docx/scripts/create_report.py --out /mnt/user-data/outputs/report.docx",
      "description": "generate report"
    }
  }
}
```

### Read a file with line numbers

```json theme={null}
{
  "jsonrpc": "2.0", "id": 5, "method": "tools/call",
  "params": {
    "name": "view",
    "arguments": {"path": "/home/assistant/README.md", "view_range": [1, 40]}
  }
}
```

### Delegate to Claude Code

```json theme={null}
{
  "jsonrpc": "2.0", "id": 6, "method": "tools/call",
  "params": {
    "name": "sub_agent",
    "arguments": {
      "prompt": "Clone https://github.com/org/repo, run the tests, and fix any failures. Return a summary.",
      "model": "sonnet"
    }
  }
}
```

To pass MCP servers into Claude Code, set `X-MCP-Servers: github,jira` on the HTTP request — see [Sub-agents](/features/sub-agents).

## Errors

| `isError: true` cause | What it means                                                    |
| --------------------- | ---------------------------------------------------------------- |
| Command non-zero exit | The command failed; stderr is in `content[0].text`               |
| Timeout               | Exceeded `COMMAND_TIMEOUT` (or per-call `timeout`)               |
| Path outside sandbox  | `view` / `create_file` rejected a path outside the allowed roots |
| `old_str` not unique  | `str_replace` found zero or multiple matches                     |

Transport-level errors (`401`, `400`, `500`) come back as HTTP error responses, not JSON-RPC errors.

## Related

* [tools/list](/api-reference/mcp/tools-list) — tool signatures
* [resources/list](/api-reference/mcp/resources) — browse uploaded files