Mcp.ToolsCall

Examples below assume mcp-client is imported as McpClient.

Dispatches the tools/call MCP RPC through a referenced Mcp.Client (Mcp.HttpClient, Mcp.StdioClient, or any future transport). Has zero transport knowledge — JSON-RPC framing, session handling, content-type negotiation, and re-handshake-on-invalidation all live inside the client.

Schema

kind: McpClient.ToolsCall
metadata: { name: <Name> }
client: <ClientName>                # any resource extending Mcp.Client

Inputs

inputs:
  name: <tool_name>                 # required
  arguments: { ... }                # optional, validated server-side

Output

result:
  content: [ <content block>, ... ] # required
  structuredContent: { ... }        # optional, open shape in v1

content[] is a closed union keyed by type:

type	Required fields
text	text
image	data (base64), mimeType
audio	data (base64), mimeType
resource_link	uri; optional name, description, mimeType
resource	resource.uri; optional resource.mimeType, resource.text, resource.blob (base64)

In v1 the chain validator catches accesses to undeclared properties on any content variant (e.g. result.content[0].bogus). Cross-variant access (result.content[0].data against a text item) is still accepted today and becomes a static error once the analyzer's oneOf-discriminator narrowing ships.

structuredContent is intentionally open in v1: the MCP spec ties its shape to a per-tool outputSchema declared by the server. A typed Mcp.Tool + Mcp.ToolsCallTyped variant will land when per-tool schemas are first-class.

Soft failures

isError: true on the server response is converted to an ERR_MCP_TOOL_ERROR throw — the success path of Mcp.ToolsCall never observes the isError field. Map this throw in a surrounding Run.Sequence try/catch (or HTTP route catches: block) when you want to render the LLM-readable failure message as user-facing content.

steps:
  - name: Wrap
    try:
      - name: Call
        inputs: { name: get_weather, arguments: { city: Atlantis } }
        invoke: { kind: McpClient.ToolsCall, name: CallGetWeather }
    catch:
      - name: HandleToolError
        inputs:
          content: "${{ error.data.content }}"
        invoke: ...

Errors

Code	Source
ERR_MCP_TOOL_ERROR	Originated by Mcp.ToolsCall from isError: true server responses.
ERR_MCP_JSON_RPC_ERROR	Originated by the client when the server returned a JSON-RPC error envelope.
ERR_MCP_TRANSPORT	Originated by the client on network failure / unexpected Content-Type / non-2xx HTTP responses.
ERR_MCP_PROTOCOL	Originated by the client on malformed JSON-RPC envelopes.
ERR_MCP_SESSION_INVALID	HTTP only. Originated by the client after exhausted internal retry.