MCP as substrate protocol

@@ -5,7 +5,7 @@ slug: mcp-substrate-protocol
category: substrate
type: topic
quality: complete
short_description: "Every Foundry Ring 1 and Ring 2 service exposes a Model Context Protocol server interface as its primary external contract, with the Doorman as the MCP gateway."
short_description: "Every Ring 1 and Ring 2 service exposes a Model Context Protocol server interface as its primary external contract, with the Doorman as the MCP gateway."
status: active
bcsc_class: public-disclosure-safe
last_edited: 2026-05-15
@@ -18,17 +18,17 @@ references:
paired_with: mcp-substrate-protocol.es.md
---

**MCP-as-Substrate-Protocol** designates the Model Context Protocol (MCP) as the wire contract for all inter-service composition in the Foundry platform. Every Ring 1 and Ring 2 service exposes an MCP server interface as its primary external contract. The Doorman (`service-slm`) is the MCP gateway. Customer extensions plug in as additional MCP servers. This is not a preference or an implementation convenience — it is a structural decision.
**MCP-as-Substrate-Protocol** designates the Model Context Protocol (MCP) as the wire contract for all inter-service composition in the platform. Every Ring 1 and Ring 2 service exposes an MCP server interface as its primary external contract. The Doorman (`service-slm`) is the MCP gateway. Customer extensions plug in as additional MCP servers. This is not a preference or an implementation convenience — it is a structural decision.

## Why MCP is substrate-level

The Model Context Protocol has become the industry standard for AI-native application composition. It defines a stable, machine-readable interface between clients, servers, and host processes. [^1] Foundry adopts it at the substrate level because the alternative — bespoke per-service wire formats — accumulates versioning debt, per-pair contract testing costs, and custom client implementations in every consumer. MCP is the coordination point that prevents that drift.
The Model Context Protocol has become the industry standard for AI-native application composition. It defines a stable, machine-readable interface between clients, servers, and host processes. [^1] The platform adopts it at the substrate level because the alternative — bespoke per-service wire formats — accumulates versioning debt, per-pair contract testing costs, and custom client implementations in every consumer. MCP is the coordination point that prevents that drift.

The practical result: a customer-built agent, an IDE extension, and the Foundry operator TUI all interact with the same service interfaces using the same protocol. There is no "developer API" distinct from the "user API." The wire contract is unified.
The practical result: a customer-built agent, an IDE extension, and the platform operator TUI all interact with the same service interfaces using the same protocol. There is no "developer API" distinct from the "user API." The wire contract is unified.

## How the three-ring architecture maps to MCP roles

MCP defines three roles. Foundry's architecture maps onto them directly:
MCP defines three roles. The platform's architecture maps onto them directly:

**MCP Server** — each Ring 1 and Ring 2 service: the file system ledger, the knowledge runtime, the extraction service, the input service, the people service, the email service, and the egress service. When Phase 5 delivers the marketplace and settlement services, those are MCP servers too.

@@ -40,9 +40,9 @@ The Doorman is both MCP Client (calling `service-content` for graph grounding) a

## Tool semantics

Each Foundry service exposes a small set of named MCP tools. `service-content` exposes graph query, graph mutation, vector search, and temporal query. The file ledger service exposes ledger append, ledger query, and checkpoint. The extraction service exposes entity extraction and classification. The people service exposes person lookup, upsert, and relationship query.
Each service exposes a small set of named MCP tools. `service-content` exposes graph query, graph mutation, vector search, and temporal query. The file ledger service exposes ledger append, ledger query, and checkpoint. The extraction service exposes entity extraction and classification. The people service exposes person lookup, upsert, and relationship query.

The marketplace service (planned Phase 5) is intended to expose listing creation, listing query, and transaction initiation as MCP tools. Customers extending Foundry add new MCP servers that expose their own vertical-specific tools. The Doorman discovers new tools at session start via MCP's describe mechanism; no core code changes are required to accommodate a new extension.
The marketplace service (planned Phase 5) is intended to expose listing creation, listing query, and transaction initiation as MCP tools. Customers extending the platform add new MCP servers that expose their own vertical-specific tools. The Doorman discovers new tools at session start via MCP's describe mechanism; no core code changes are required to accommodate a new extension.

## The Doorman as MCP gateway

@@ -54,7 +54,7 @@ A customer with a vertical-specific data source writes an MCP server that expose

## Composition with Code-for-Machines First

MCP is the structural realization of the [[code-for-machines-first]] principle. Every service contract is machine-readable and introspectable at runtime. Human-facing surfaces — the TUI, web interfaces — are clients of the same MCP interfaces that any other agent would use. There is no human-only data surface in Foundry.
MCP is the structural realization of the [[code-for-machines-first]] principle. Every service contract is machine-readable and introspectable at runtime. Human-facing surfaces — the TUI, web interfaces — are clients of the same MCP interfaces that any other agent would use. There is no human-only data surface in the platform.

## OpenAI-compatible HTTP boundary preserved