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Capsule Bash Server

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MCP de Bash Capsule

Servidor MCP

Un servidor MCP que proporciona a tu agente de IA la capacidad de ejecutar comandos bash en un entorno seguro, persistente y aislado (sandbox).

Cómo funciona

Cada sesión se ejecuta dentro de un entorno aislado de WebAssembly. El entorno proporciona:

  • Estado persistente: el directorio de trabajo (cwd), las variables de entorno y los cambios en el sistema de archivos persisten entre comandos dentro de una sesión.

  • Diferencias en el sistema de archivos: cada respuesta de run incluye una diferencia (diff) de lo que cambió en el disco.

  • Memoria aislada: cada sesión obtiene su propio espacio de direcciones, sin fugas entre sesiones.

  • Sin acceso al host: el entorno aislado no puede acceder a tu sistema de archivos ni a la red del host.

Obtén más información sobre Capsule Bash.

Related MCP server: execkit-mcp

Herramientas

Herramienta

Descripción

run

Ejecuta un comando bash en una sesión aislada. Devuelve stdout, stderr, código de salida, diferencias del sistema de archivos y estado actual (cwd + env).

reset

Restablece el sistema de archivos y el estado (cwd, variables de entorno) de una sesión a sus valores iniciales.

sessions

Lista todas las sesiones activas.

Sesiones

Los comandos dentro del mismo session_id comparten el directorio de trabajo, las variables de entorno y el estado del sistema de archivos entre llamadas.

Ejemplo

Pídele a tu agente de IA:

"Escribe un script de Python que calcule el promedio de una lista de números."

El agente llama a run secuencialmente:

{ "command": "mkdir -p /data && cd /data", "session_id": "custom_session" }
{ "command": "echo 'nums = [x for x in [1, 2, 3, []] if isinstance(x, int)]\nprint(sum(nums) / len(nums))' > avg.py", "session_id": "custom_session" }
{ "command": "python3 avg.py", "session_id": "custom_session" }

Cada llamada devuelve stdout, stderr, exitCode, un diff del sistema de archivos y el state actualizado para enriquecer el contexto y mantener el seguimiento en el historial de la conversación.

Configuración

Añádelo a la configuración de tu cliente MCP (por ejemplo, Claude Desktop, Cursor):

{
  "mcpServers": {
    "bash": {
      "command": "npx",
      "args": ["-y", "@capsule-run/bash-mcp"]
    }
  }
}

Limitaciones

  • No todos los comandos y opciones de bash están implementados. No dudes en abrir una incidencia si falta un comando o se comporta de forma inesperada.

Available Tools

3 tools
resetA

Reset a session's filesystem and shell state (cwd, env vars) to their initial values. Useful to start fresh without creating a new session.

ParametersJSON Schema
NameRequiredDescriptionDefault
session_idNoThe session to reset. Defaults to 'default'.default

TDQS

A4.3/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full burden. It clearly explains that the tool resets filesystem and shell state to initial values, which is specific and transparent about the behavior. No side effects or additional information is provided, but the action is straightforward.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description consists of two sentences, both front-loaded with essential information: the action and scope in the first sentence, usage context in the second. No wasted words.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's simplicity (one parameter, no output schema), the description provides all necessary information: what it does, what it affects, and when to use it. No gaps remain.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has 100% description coverage, and the description adds no additional meaning beyond what the schema already provides for the single parameter 'session_id'. Baseline of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the verb 'reset' and the resource 'session's filesystem and shell state', specifying what is reset (cwd, env vars) and distinguishing from siblings 'run' and 'sessions' which have different purposes.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explicitly says 'useful to start fresh without creating a new session', giving a clear usage scenario and implying an alternative (creating a new session). However, it does not explicitly state when not to use it or list other alternatives.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

runB

Execute a Bash command in the sandboxed environment

ParametersJSON Schema
NameRequiredDescriptionDefault
commandYesThe bash command to execute.
session_idNoIdentifier for the shell session. Commands within the same session share cwd, env, and filesystem state. Defaults to 'default'.default

TDQS

B3.3/5.0
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description bears full burden for behavioral disclosure. It only notes the sandboxed environment but fails to describe side effects (e.g., file system changes, output handling, error behavior) or any permissions needed.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, focused sentence with no wasted words. It is appropriately sized for a simple tool and front-loades key information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness3/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's simplicity (2 parameters, no output schema, no annotations), the description is adequate but not thorough. It lacks information about return values (e.g., stdout/stderr) and potential side effects, which could help an agent understand the tool's full behavior.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, with clear explanations for both parameters (command and session_id). The tool description adds no extra parameter information. Baseline of 3 is appropriate as the schema already documents parameter semantics.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description 'Execute a Bash command in the sandboxed environment' clearly states the action (execute) and the resource (Bash command in sandbox). It distinguishes from siblings 'reset' and 'sessions' by implying this tool is for running commands, not managing sessions.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

No guidance is provided on when to use this tool versus alternatives like 'reset' or 'sessions'. The description does not mention prerequisites, scenarios, or exclusions, leaving the agent without context for tool selection.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

sessionsA

List all active session IDs and their current state.

ParametersJSON Schema
NameRequiredDescriptionDefault

No parameters

TDQS

A4.1/5.0
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Lacking annotations, the description states the tool lists active sessions, conveying a non-destructive, read-only behavior. But it omits details like whether state includes sub-attributes or if sessions are global.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

A single, front-loaded sentence with no wasted words. Efficiently captures the core functionality.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness3/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given no output schema, the description could be more complete by hinting at return structure or example use. It covers the basic purpose but lacks actionable output details.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

No parameters; schema coverage is 100% by default. The description does not need to add param info. Baseline 4 for 0 parameters.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

Describes a specific action (list) and resource (active session IDs and state). Clearly distinguishes from siblings 'reset' and 'run' which imply modification actions.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies a read-only listing use case, and with siblings being action-oriented, the context is clear. However, no explicit when-not or alternative guidance is provided.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

Tool Schema Changelog

Recent tool additions, removals, and schema changes observed during successful MCP inspections.

  1. 3 tool updatesv0.1.0
    • First observedreset
    • First observedrun
    • First observedsessions
Deploy Server
One endpoint for every MCP toolRayrun runs every MCP server behind one URL and holds the credentials. Each call is decided per client, tool, and argument, then recorded.ray.runAd

TDQS

A4/5.0

Scored across 3 tools

Disambiguation5/5

Each tool has a unique, clear purpose: reset reverts state, run executes commands, and sessions lists active sessions. No ambiguity or overlap exists.

Naming Consistency5/5

All tool names are single, imperative verbs (reset, run, sessions) following a consistent, simple pattern.

Tool Count5/5

Three tools is well-scoped for a bash sandbox server, covering essential operations without superfluous or missing tools.

Completeness4/5

The set covers core actions (run, reset, list sessions) but lacks explicit session creation; likely sessions are created implicitly, leaving a minor gap.

Maintenance

ActivityInactive
ResponsivenessUnresponsive

Resources

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