Merge pull request modelcontextprotocol#61 from modelcontextprotocol/justin/design-principles

Capture our design principles

Author: jspahrsummers

docs/specification/architecture/_index.md

@@ -69,6 +69,36 @@ Servers provide specialized context and capabilities:
 - Must respect security constraints
 - Can be local processes or remote services
 
+## Design Principles
+
+MCP is built on several key design principles that inform its architecture and implementation:
+
+1. **Servers should be extremely easy to build**
+    - Clients handle complex orchestration responsibilities
+    - Servers focus on specific, well-defined capabilities
+    - Simple interfaces minimize implementation overhead
+    - Clear separation enables maintainable code
+
+2. **Servers should be highly composable**
+    - Each server provides focused functionality in isolation
+    - Multiple servers can be combined seamlessly
+    - Shared protocol enables interoperability
+    - Modular design supports extensibility
+
+3. **Servers should not be able to read the whole conversation, nor "see into" other servers**
+    - Servers receive only necessary contextual information
+    - Full conversation history stays with client/host
+    - Each server connection maintains isolation
+    - Cross-server interactions are client-controlled
+    - Host process enforces security boundaries
+
+4. **Features can be added to servers and clients progressively**
+    - Core protocol provides minimal required functionality
+    - Additional capabilities can be negotiated as needed
+    - Servers and clients evolve independently
+    - Protocol designed for future extensibility
+    - Backwards compatibility is maintained
+
 ## Message Types
 MCP defines three core message types based on [JSON-RPC 2.0](https://www.jsonrpc.org/specification):