# Bash tool (`bash_20250124`)

**Status:** DOCUMENTED · LIVE_VERIFIED 2026-09-18 (h7 on haiku 4.5 → `tool_use{name:"bash", input:{"command":"echo OK"}}`; j4 `count_tokens` 751). Nothing was executed.
**Sources:** [Bash tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/bash-tool) · [Tool reference](https://platform.claude.com/docs/en/agents-and-tools/tool-use/tool-reference) · [Release notes 2025-02-24](https://platform.claude.com/docs/en/release-notes/api).
**Last verified:** 2026-09-18.

## Definition and versions

```json
{"type": "bash_20250124", "name": "bash"}
```
Schema-less client tool; `name` must be `bash`. Generic properties allowed: `cache_control`, `defer_loading`, `strict`, `input_examples`, `allowed_callers`.

| Version | Header | Models |
|---|---|---|
| `bash_20250124` | none | every model since Claude Sonnet 3.7, incl. all current models (live haiku 4.5) |
| `bash_20241022` | `computer-use-2024-10-22` | Claude Sonnet 3.5 (Oct 2024) only — retired; SDK beta namespace only |

## Input Claude sends

| Field | Required | Meaning |
|---|---|---|
| `command` | yes unless `restart` | the bash command to run |
| `restart` | no | `true` → kill and restart the session (state lost); answer with a confirmation |

Your application owns a **persistent** bash process (cwd, env vars, files persist across calls), runs `command`, returns stdout+stderr as the `tool_result` content, `is_error:true` on failure/timeout. Several `tool_use` blocks in one turn → run in order in the same session, return all results in one user message. The API does not truncate tool results (oversized request rejected) — truncate yourself.

## Limits and security
No interactive commands (vim, less, prompts), no GUI, no output streaming. Run in an isolated container/VM as least-privileged user, allowlist commands (a tokenizer-based allowlist is a tripwire, not a boundary), `ulimit`, audit log, redact secrets. When the server-side code execution tool is also present, tell Claude the two environments do not share state.

## Pricing
Definition adds ≈325 input tokens (Opus 5 / 4.8 / 4.7) or ≈244 (Opus 4.6, Sonnet 4.6 and earlier) on top of the tool-use system prompt. Live `count_tokens` haiku 4.5: 751 tokens for a one-line prompt + bash tool (11 without tools).

Examples: `examples/anthropic/tools/bash/basic.{sh,py,ts}`; test `test_anthropic_schema_client_tools_emit_tool_use[bash]`.