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mcp-nexus

mcp-nexus

by fyrlabs
OverviewSchemaSourceRelated ServersDiscussionsAdmin
TypeScript
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Execute capability

execute_capability

Run any downstream MCP capability by ID with matching arguments. Starts the target server on demand and forwards inputs verbatim, enabling execution across connected servers without loading schemas into context.

Instructions

Execute a downstream MCP capability by ID. The underlying server is started on demand and the arguments are forwarded verbatim.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "capabilityId": "<capabilityId>"
    },
    "name": "execute_capability"
  }
}

Input Schema

NameRequiredDescriptionDefault
argumentsNoArguments matching the tool's input schema (see describe_capabilities)
capabilityIdYesCapability ID to execute

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.10.0

TDQS

A3.9/5.0
Behavior3/5

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

With no annotations, the description carries the transparency burden. It discloses two genuine behaviors: the backing server is 'started on demand' (affecting latency/resource use) and arguments are 'forwarded verbatim' (no transformation). It does not mention output shape, error behavior, or potential side effects of the downstream capability, so coverage is partial.

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?

Two sentences, each adding distinct information: the core action and two behavioral caveats. No redundant or filler wording, and the most important information is front-loaded.

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?

The definition is largely sufficient for a simple pass-through invocation, and the arguments schema points to describe_capabilities for the input contract. However, there is no output schema or description of the return value/error behavior, so an agent does not know what to expect after invocation. This is a moderate gap given no annotations.

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 schema already documents both parameters completely (100% coverage), so the baseline is 3. The description's 'arguments are forwarded verbatim' reinforces the arguments parameter's semantics, but adds no new parameter-specific detail beyond the schema, which already tells the agent to see describe_capabilities for the expected argument shape.

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?

States a specific action – 'Execute a downstream MCP capability by ID' – with a clear verb and resource. The 'by ID' scoping and the 'downstream MCP' qualifier distinguish it from siblings like describe_capabilities and search_capabilities, which are informational rather than invocational.

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 usage: you invoke this when you have a capability ID and want to run it, and the 'forwarded verbatim' phrasing indicates arguments should already conform to the target schema. It does not explicitly list exclusions or alternatives, but the sibling names make the distinction clear. This is adequate context without explicit when-not-to-use guidance.

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

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