feat: Add template variable support for remote transport URLs

Enable URL templating for StreamableHttpTransport and SseTransport by adding
'arguments' and 'environmentVariables' arrays. This allows remote servers
to support multi-tenant deployments with configurable endpoints.

Key changes:
- Add optional 'arguments' array to both transport types for valueHint-based templating
- Add optional 'environmentVariables' array for environment-based templating
- Update URL descriptions to clarify template variable resolution behavior
- Add comprehensive multi-tenant example demonstrating the new functionality
- Maintain backwards compatibility - existing configs continue to work

This addresses the common use case where remote MCP servers are deployed
across multiple tenants/regions, each requiring different endpoint URLs.
Variables in {curly_braces} now properly resolve from the defined arrays.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: tadasant

docs/reference/server-json/generic-server-json.md

@@ -670,6 +670,103 @@ For MCP servers that follow a custom installation path or are embedded in applic
 }
 ```
 
+### Multi-Tenant Remote Server with URL Templating
+
+This example demonstrates how to use URL templating for remote servers deployed across multiple tenants or regions, addressing scenarios where each deployment has its own endpoint:
+
+```json
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-09-16/server.schema.json",
+  "name": "io.github.example/multi-tenant-analytics",
+  "description": "Multi-tenant analytics MCP server with region-specific endpoints",
+  "repository": {
+    "url": "https://github.com/example/multi-tenant-analytics",
+    "source": "github",
+    "id": "tenant-abc123-def456-789012-345678-901234567890"
+  },
+  "version": "2.1.0",
+  "remotes": [
+    {
+      "type": "streamable-http",
+      "url": "https://{tenant_host}/api/ai/v1/mcp/",
+      "headers": [
+        {
+          "name": "X-API-Key",
+          "description": "API key for authentication",
+          "isRequired": true,
+          "isSecret": true
+        },
+        {
+          "name": "X-Region",
+          "value": "{deployment_region}",
+          "description": "Deployment region identifier"
+        }
+      ],
+      "environmentVariables": [
+        {
+          "name": "TENANT_HOST",
+          "description": "Tenant-specific hostname (e.g., 'us-cell1.example.com', 'emea-cell1.example.com')",
+          "isRequired": true
+        },
+        {
+          "name": "DEPLOYMENT_REGION",
+          "description": "Deployment region code",
+          "default": "us-east-1",
+          "choices": [
+            "us-east-1",
+            "us-west-2",
+            "eu-west-1",
+            "ap-southeast-1"
+          ]
+        }
+      ],
+      "arguments": [
+        {
+          "type": "positional",
+          "valueHint": "tenant_host",
+          "description": "Override tenant host from environment",
+          "isRequired": false
+        },
+        {
+          "type": "positional",
+          "valueHint": "deployment_region",
+          "description": "Override deployment region from environment",
+          "isRequired": false
+        }
+      ]
+    },
+    {
+      "type": "sse",
+      "url": "https://{tenant_host}/api/ai/v1/mcp/events",
+      "environmentVariables": [
+        {
+          "name": "TENANT_HOST",
+          "description": "Tenant-specific hostname",
+          "isRequired": true
+        }
+      ]
+    }
+  ],
+  "_meta": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
+      "tool": "multi-tenant-deployer",
+      "version": "1.5.0",
+      "build_info": {
+        "timestamp": "2023-12-15T10:45:00Z",
+        "supported_tenants": ["us-cell1", "us-cell2", "emea-cell1", "apac-cell1"],
+        "deployment_strategy": "multi-region"
+      }
+    }
+  }
+}
+```
+
+This configuration allows clients to:
+- Set `TENANT_HOST=us-cell1.example.com` to connect to US Cell 1
+- Set `TENANT_HOST=emea-cell1.example.com` to connect to EMEA Cell 1
+- Specify the deployment region for proper routing
+- Use either streamable-http or SSE transports with the same tenant-specific endpoints
+
 ### Deprecated Server Example
 
 ```json

docs/reference/server-json/server.schema.json

@@ -377,7 +377,7 @@
         },
         "url": {
           "type": "string",
-          "description": "URL template for the streamable-http transport. Variables in {curly_braces} reference argument valueHints, argument names, or environment variable names. After variable substitution, this should produce a valid URI.",
+          "description": "URL template for the streamable-http transport. Variables in {curly_braces} reference argument valueHints from 'arguments' array or names from 'environmentVariables' array. If arguments/environmentVariables are not provided, {curly_braces} should be treated as literal text. After variable substitution, this should produce a valid URI.",
           "example": "https://api.example.com/mcp"
         },
         "headers": {
@@ -386,6 +386,20 @@
           "items": {
             "$ref": "#/definitions/KeyValueInput"
           }
+        },
+        "arguments": {
+          "type": "array",
+          "description": "Arguments that can be referenced in URL template {curly_braces} via their valueHint property",
+          "items": {
+            "$ref": "#/definitions/Argument"
+          }
+        },
+        "environmentVariables": {
+          "type": "array",
+          "description": "Environment variables that can be referenced in URL template {curly_braces} via their name property",
+          "items": {
+            "$ref": "#/definitions/KeyValueInput"
+          }
         }
       }
     },
@@ -407,7 +421,7 @@
         "url": {
           "type": "string",
           "format": "uri",
-          "description": "Server-Sent Events endpoint URL",
+          "description": "Server-Sent Events endpoint URL template. Variables in {curly_braces} reference argument valueHints from 'arguments' array or names from 'environmentVariables' array. If arguments/environmentVariables are not provided, {curly_braces} should be treated as literal text. After variable substitution, this should produce a valid URI.",
           "example": "https://mcp-fs.example.com/sse"
         },
         "headers": {
@@ -416,6 +430,20 @@
           "items": {
             "$ref": "#/definitions/KeyValueInput"
           }
+        },
+        "arguments": {
+          "type": "array",
+          "description": "Arguments that can be referenced in URL template {curly_braces} via their valueHint property",
+          "items": {
+            "$ref": "#/definitions/Argument"
+          }
+        },
+        "environmentVariables": {
+          "type": "array",
+          "description": "Environment variables that can be referenced in URL template {curly_braces} via their name property",
+          "items": {
+            "$ref": "#/definitions/KeyValueInput"
+          }
         }
       }
     },