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mcp

Since v1.24.0+ · Top-level declaration

Grammar​

# The current `mcp` parser is the generic-declaration form
# (parser.rs::parse_generic_declaration). Permissive grammar:
mcp <Name> [(<args>)] [{ ... body ... }]

mcp declares an outbound Model Context Protocol binding — the inverse of running an MCP server. Where the ℰMCP runtime serves tools and resources to coding agents, an mcp declaration consumes an external MCP server's surface from within an AXON program.

Use mcp when your flow needs to call out to another MCP- compliant tool catalogue (a specialised retrieval server, a domain-specific knowledge base, another agent's tool surface) and have its primitives appear as if declared locally.

Surface​

mcp is a top-level declaration. It is not nested inside another primitive. The current parser uses the permissive generic-declaration form (parser.rs::parse_generic_declaration) — the syntax accepts a name, optional parens, and an optional braced block; field constraints are deferred to the v2.0.0 D11 deploy-time validation.

mcp ClinicalKB {
    server:  "clinical-mcp.internal"
    uri:     "kb://guidelines/2025"
    tools:   [search, get_guideline]
}

Anatomy​

Header — mcp <Name>​

A PascalCase identifier unique within the module. The runtime resolves the name against its MCP-client registry at deploy time.

Body — { ... }​

The body is currently open at the parser level (the permissive parse_generic_declaration shape). Conventional fields (validated at deploy time, not at parse time) include:

FieldTypeMeaning
server:stringMCP server slug or hostname.
uri:stringResource URI prefix the binding scopes to.
tools:bracketed identifiersTool subset to import (default: all).
resources:bracketed identifiersResource subset to import (default: all).
prompts:bracketed identifiersPrompt subset to import (default: all).
transport:identifierstdio (subprocess) or sse (network).

A future cycle will tighten the grammar to a typed MCPDefinition AST node; until then, the runtime does the validation.

Runtime behaviour​

At deploy time, the runtime opens an MCP client connection to the named server, fetches its tools/list, resources/list, and prompts/list, and exposes them under the binding's namespace. Inside a flow, an imported MCP tool is invoked the same way as any local tool — the bridge is transparent.

Audit rows: every outbound MCP call emits mcp:<binding>:<method>:<outcome> carrying (remote_server, remote_uri, latency, cost_credits_estimated). The audit chain treats outbound MCP calls as external dependencies — they appear in the dependency graph the v2.0.0 supply-chain check walks.

What this primitive is NOT​

  • Not the ℰMCP server itself. This is the client side — consuming another MCP server's surface. The server side of the protocol (what axon-emcp itself implements) is the AXON runtime's built-in.
  • Not a generic HTTP binding. MCP is a typed JSON-RPC 2.0 protocol with initialize handshake + capability declaration. For arbitrary HTTP egress, declare a tool with provider: http.
  • Not free of compliance gating. Outbound MCP calls cross a trust boundary. A mcp binding inside a HIPAA-tagged manifest requires the remote server to attest the same framework; the v2.0.0 deploy-time check enforces this.

See also​

  • axon://primitives/tool — the in-flow surface that imported MCP tools appear as.
  • axon://primitives/corpus — the corpus … from mcp(...) shorthand for RAG-style MCP bindings.
  • axon://primitives/manifest — outbound MCP bindings appear in the manifest's external-dependency graph.
  • axon://compliance/hipaa — example of cross-server compliance attestation.
  • The ℰMCP server side: src/README.md.