mcp-broker

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broker_call_tool

Invokes a broker-managed upstream MCP tool using its qualified name and exact argument object, with optional projection for trimmed responses.

Instructions

Call one profile-visible upstream MCP tool through the broker using its broker-qualified name and exact argument object. The broker's mutating-tool policy remains gated by the active profile's allowlist, so call broker_describe_tool first and pass only arguments accepted by the described upstream schema.

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": {
      "name": "<name>",
      "arguments": "<arguments>"
    },
    "name": "broker_call_tool"
  }
}

Input Schema

Name	Required	Description
`name`	Yes	Full broker-qualified tool name to invoke. Use a name returned by broker_search_tools or broker_describe_tool, not an unqualified upstream name.
`arguments`	Yes	Exact JSON object accepted by the described upstream tool schema. Use an empty object only when broker_describe_tool shows that the upstream tool accepts no parameters.
`projection`	No	Optional server-side projection applied to the upstream response before it reaches the client, so a verbose result is trimmed to only the fields you need and large arrays are capped. This cuts context tokens and latency. Omit it to receive the full, unmodified upstream response.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv2.0.0Jul 4, 2026

addedInput schema / properties / projection

Added value: +{
+  "additionalProperties": false,
+  "description": "Optional server-side projection applied to the upstream response before it reaches the client, so a verbose result is trimmed to only the fields you need and large arrays are capped. This cuts context tokens and latency. Omit it to receive the full, unmodified upstream response.",
+  "properties": {
+    "max_array_items": {
+      "description": "Truncate every array in the projected response to at most this many items, so a long list returns only its first entries.",
+      "minimum": 0,
+      "type": "integer"
+    },
+    "paths": {
+      "description": "Dotted field paths to keep, such as data.items.id. A path that reaches a list is applied to every element. Omit to keep all fields and only cap arrays with max_array_items.",
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    }
+  },
+  "type": "object"
+}

Changed8 schema fields changedv1.1.2May 28, 2026

addedInput schema / additionalProperties
```
Added value: +false
```

addedInput schema / description

Added value: +"Invocation request for a broker-managed upstream tool. The broker resolves the namespaced name, enforces profile exposure and mutating-tool policy, serializes calls when configured, and returns the upstream tool result."

addedInput schema / properties / arguments / additionalProperties
```
Added value: +true
```
addedInput schema / properties / arguments / default
```
Added value: +{}
```

addedInput schema / properties / arguments / description

Added value: +"Exact JSON object accepted by the described upstream tool schema. Use an empty object only when broker_describe_tool shows that the upstream tool accepts no parameters."

addedInput schema / properties / name / description

Added value: +"Full broker-qualified tool name to invoke. Use a name returned by broker_search_tools or broker_describe_tool, not an unqualified upstream name."

addedInput schema / properties / name / examples

Added value: +[
+  "github.get_file_contents",
+  "filesystem.read_file"
+]

addedInput schema / properties / name / minLength
```
Added value: +1
```

First observedv1.1.1May 28, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Discloses the broker's mutating-tool policy is gated by the profile's allowlist, and hints at server-side projection to cut tokens and latency. Missing details on error handling or response format, but acceptable given no annotations.

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 with no wasted words. The first sentence states purpose, the second provides a critical usage guideline. Highly efficient.

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

Completeness4/5

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

Covers invocation, policy, and prerequisite steps. Lacks explicit statement about return format (dynamic), but the projection parameter description partially compensates. Enough for a proxy tool with variable output.

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 coverage is 100%, so baseline is 3. The description adds context that names must come from broker_search_tools or broker_describe_tool, but does not significantly enhance understanding beyond the schema descriptions.

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 tool invokes an upstream MCP tool via the broker using a qualified name and arguments. It distinguishes from sibling tools (broker_describe_tool, broker_search_tools, broker_status) by focusing on execution.

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

Usage Guidelines5/5

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

Explicitly advises calling broker_describe_tool first and passing only accepted arguments, providing clear when-to-use guidance and key steps for correct invocation.

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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