feat: Add LocalTransport/RemoteTransport with URL template variables

docs/reference/api/openapi.yaml

@@ -315,10 +315,7 @@ components:
           description: A hint to help clients determine the appropriate runtime for the package. This field should be provided when `runtimeArguments` are present.
           examples: [npx, uvx, docker, dnx]
         transport:
-          anyOf:
-            - $ref: '#/components/schemas/StdioTransport'
-            - $ref: '#/components/schemas/StreamableHttpTransport'
-            - $ref: '#/components/schemas/SseTransport'
+          $ref: '#/components/schemas/LocalTransport'
           description: Transport protocol configuration for the package
         runtimeArguments:
           type: array
@@ -474,8 +471,9 @@ components:
           example: "streamable-http"
         url:
           type: string
-          description: URL template for the streamable-http transport. Variables in {curly_braces} reference argument valueHints, argument names, or environment variable names. After variable substitution, this should produce a valid URI.
+          description: "URL template for the streamable-http transport. Variables in {curly_braces} are resolved based on context: In Package context, they reference argument valueHints, argument names, or environment variable names from the parent Package. In Remote context, they reference variables from the transport's 'variables' object. After variable substitution, this should produce a valid URI."
           example: "https://api.example.com/mcp"
+          pattern: "^https?://[^\\s]+$"
         headers:
           type: array
           description: HTTP headers to include
@@ -495,15 +493,36 @@ components:
           example: "sse"
         url:
           type: string
-          format: uri
-          description: Server-Sent Events endpoint URL
+          description: "Server-Sent Events endpoint URL template. Variables in {curly_braces} are resolved based on context: In Package context, they reference argument valueHints, argument names, or environment variable names from the parent Package. In Remote context, they reference variables from the transport's 'variables' object. After variable substitution, this should produce a valid URI."
           example: "https://mcp-fs.example.com/sse"
+          pattern: "^https?://[^\\s]+$"
         headers:
           type: array
           description: HTTP headers to include
           items:
             $ref: '#/components/schemas/KeyValueInput'
 
+    LocalTransport:
+      anyOf:
+        - $ref: '#/components/schemas/StdioTransport'
+        - $ref: '#/components/schemas/StreamableHttpTransport'
+        - $ref: '#/components/schemas/SseTransport'
+      description: Transport protocol configuration for local/package context
+
+    RemoteTransport:
+      allOf:
+        - anyOf:
+            - $ref: '#/components/schemas/StreamableHttpTransport'
+            - $ref: '#/components/schemas/SseTransport'
+        - type: object
+          properties:
+            variables:
+              type: object
+              description: "Configuration variables that can be referenced in URL template {curly_braces}. The key is the variable name, and the value defines the variable properties."
+              additionalProperties:
+                $ref: '#/components/schemas/Input'
+      description: Transport protocol configuration for remote context - extends StreamableHttpTransport or SseTransport with variables
+
     Icon:
       type: object
       description: An optionally-sized icon that can be displayed in a user interface.
@@ -592,9 +611,7 @@ components:
         remotes:
           type: array
           items:
-            anyOf:
-              - $ref: '#/components/schemas/StreamableHttpTransport'
-              - $ref: '#/components/schemas/SseTransport'
+            $ref: '#/components/schemas/RemoteTransport'
         _meta:
           type: object
           description: "Extension metadata using reverse DNS namespacing for vendor-specific data"

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

@@ -672,3 +672,90 @@ For MCP servers that follow a custom installation path or are embedded in applic
 }
 ```
 
+
+### Remote Server with URL Templating
+
+This example demonstrates URL templating for remote servers, useful for multi-tenant deployments where each instance has its own endpoint. Unlike Package transports (which reference parent arguments/environment variables), Remote transports define their own variables:
+
+```json
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json",
+  "name": "io.modelcontextprotocol.anonymous/multi-tenant-server",
+  "description": "MCP server with configurable remote endpoint",
+  "title": "Multi-Tenant Server",
+  "version": "1.0.0",
+  "remotes": [
+    {
+      "type": "streamable-http",
+      "url": "https://anonymous.modelcontextprotocol.io/mcp/{tenant_id}",
+      "variables": {
+        "tenant_id": {
+          "description": "Tenant identifier (e.g., 'us-cell1', 'emea-cell1')",
+          "isRequired": true
+        }
+      }
+    }
+  ]
+}
+```
+
+Clients configure the tenant identifier, and the `{tenant_id}` variable in the URL gets replaced with the provided variable value to connect to the appropriate tenant endpoint (e.g., `https://anonymous.modelcontextprotocol.io/mcp/us-cell1` or `https://anonymous.modelcontextprotocol.io/mcp/emea-cell1`).
+
+The same URL templating works with SSE transport:
+
+```json
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json",
+  "name": "io.modelcontextprotocol.anonymous/events-server",
+  "description": "MCP server using SSE with tenant-specific endpoints",
+  "version": "1.0.0",
+  "remotes": [
+    {
+      "type": "sse",
+      "url": "https://events.anonymous.modelcontextprotocol.io/sse/{tenant_id}",
+      "variables": {
+        "tenant_id": {
+          "description": "Tenant identifier",
+          "isRequired": true
+        }
+      }
+    }
+  ]
+}
+```
+
+### Local Server with URL Templating
+
+This example demonstrates URL templating for local/package servers, where variables reference parent Package arguments or environment variables:
+
+```json
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json",
+  "name": "io.github.example/configurable-server",
+  "description": "Local MCP server with configurable port",
+  "title": "Configurable Server",
+  "version": "1.0.0",
+  "packages": [
+    {
+      "registryType": "npm",
+      "registryBaseUrl": "https://registry.npmjs.org",
+      "identifier": "@example/mcp-server",
+      "version": "1.0.0",
+      "transport": {
+        "type": "streamable-http",
+        "url": "http://localhost:{--port}/mcp"
+      },
+      "packageArguments": [
+        {
+          "type": "named",
+          "name": "--port",
+          "description": "Port for the server to listen on",
+          "default": "3000"
+        }
+      ]
+    }
+  ]
+}
+```
+
+The `{--port}` variable in the URL references the `--port` argument `name` from packageArguments. For positional arguments, an argument with the `valueHint` of `port` could similarly be referened as `{port}`. When the package runs with `--port 8080`, the URL becomes `http://localhost:8080/mcp`.

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

@@ -156,6 +156,20 @@
         }
       ]
     },
+    "LocalTransport": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/StdioTransport"
+        },
+        {
+          "$ref": "#/definitions/StreamableHttpTransport"
+        },
+        {
+          "$ref": "#/definitions/SseTransport"
+        }
+      ],
+      "description": "Transport protocol configuration for local/package context"
+    },
     "NamedArgument": {
       "allOf": [
         {
@@ -262,17 +276,7 @@
           "type": "string"
         },
         "transport": {
-          "anyOf": [
-            {
-              "$ref": "#/definitions/StdioTransport"
-            },
-            {
-              "$ref": "#/definitions/StreamableHttpTransport"
-            },
-            {
-              "$ref": "#/definitions/SseTransport"
-            }
-          ],
+          "$ref": "#/definitions/LocalTransport",
           "description": "Transport protocol configuration for the package"
         },
         "version": {
@@ -337,6 +341,33 @@
       ],
       "description": "A positional input is a value inserted verbatim into the command line."
     },
+    "RemoteTransport": {
+      "allOf": [
+        {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/StreamableHttpTransport"
+            },
+            {
+              "$ref": "#/definitions/SseTransport"
+            }
+          ]
+        },
+        {
+          "properties": {
+            "variables": {
+              "additionalProperties": {
+                "$ref": "#/definitions/Input"
+              },
+              "description": "Configuration variables that can be referenced in URL template {curly_braces}. The key is the variable name, and the value defines the variable properties.",
+              "type": "object"
+            }
+          },
+          "type": "object"
+        }
+      ],
+      "description": "Transport protocol configuration for remote context - extends StreamableHttpTransport or SseTransport with variables"
+    },
     "Repository": {
       "description": "Repository metadata for the MCP server source code. Enables users and security experts to inspect the code, improving transparency.",
       "properties": {
@@ -427,14 +458,7 @@
         },
         "remotes": {
           "items": {
-            "anyOf": [
-              {
-                "$ref": "#/definitions/StreamableHttpTransport"
-              },
-              {
-                "$ref": "#/definitions/SseTransport"
-              }
-            ]
+            "$ref": "#/definitions/RemoteTransport"
           },
           "type": "array"
         },
@@ -487,9 +511,9 @@
           "type": "string"
         },
         "url": {
-          "description": "Server-Sent Events endpoint URL",
+          "description": "Server-Sent Events endpoint URL template. Variables in {curly_braces} are resolved based on context: In Package context, they reference argument valueHints, argument names, or environment variable names from the parent Package. In Remote context, they reference variables from the transport's 'variables' object. After variable substitution, this should produce a valid URI.",
           "example": "https://mcp-fs.example.com/sse",
-          "format": "uri",
+          "pattern": "^https?://[^\\s]+$",
           "type": "string"
         }
       },
@@ -533,8 +557,9 @@
           "type": "string"
         },
         "url": {
-          "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} are resolved based on context: In Package context, they reference argument valueHints, argument names, or environment variable names from the parent Package. In Remote context, they reference variables from the transport's 'variables' object. After variable substitution, this should produce a valid URI.",
           "example": "https://api.example.com/mcp",
+          "pattern": "^https?://[^\\s]+$",
           "type": "string"
         }
       },

internal/validators/constants.go

@@ -13,8 +13,9 @@ var (
 	ErrReservedVersionString = errors.New("version string 'latest' is reserved and cannot be used")
 	ErrVersionLooksLikeRange = errors.New("version must be a specific version, not a range")
 
-	// Remote validation errors
-	ErrInvalidRemoteURL = errors.New("invalid remote URL")
+	// Transport validation errors
+	ErrInvalidPackageTransportURL = errors.New("invalid package transport URL")
+	ErrInvalidRemoteURL           = errors.New("invalid remote URL")
 
 	// Registry validation errors
 	ErrUnsupportedRegistryBaseURL   = errors.New("unsupported registry base URL")

internal/validators/utils.go

@@ -61,7 +61,13 @@ func replaceTemplateVariables(rawURL string) string {
 		result = strings.ReplaceAll(result, placeholder, replacement)
 	}
 
-	// Handle any remaining {variable} patterns with generic placeholder
+	// Handle any remaining {variable} patterns with context-appropriate placeholders
+	// If the variable is in a port position (after a colon in the host), use a numeric placeholder
+	// Pattern: :/{variable} or :{variable}/ or :{variable} at end
+	portRe := regexp.MustCompile(`:(\{[^}]+\})(/|$)`)
+	result = portRe.ReplaceAllString(result, ":8080$2")
+
+	// Replace any other remaining {variable} patterns with generic placeholder
 	re := regexp.MustCompile(`\{[^}]+\}`)
 	result = re.ReplaceAllString(result, "placeholder")
 
@@ -132,8 +138,11 @@ func IsValidRemoteURL(rawURL string) bool {
 		return false
 	}
 
+	// Replace template variables with placeholders before parsing for localhost check
+	testURL := replaceTemplateVariables(rawURL)
+
 	// Parse the URL to check for localhost restriction
-	u, err := url.Parse(rawURL)
+	u, err := url.Parse(testURL)
 	if err != nil {
 		return false
 	}
@@ -149,8 +158,8 @@ func IsValidRemoteURL(rawURL string) bool {
 
 // IsValidTemplatedURL validates a URL with template variables against available variables
 // For packages: validates that template variables reference package arguments or environment variables
-// For remotes: disallows template variables entirely
-func IsValidTemplatedURL(rawURL string, availableVariables []string, allowTemplates bool) bool {
+// For remotes: validates that template variables reference the transport's variables map
+func IsValidTemplatedURL(rawURL string, availableVariables []string) bool {
 	// First check basic URL structure
 	if !IsValidURL(rawURL) {
 		return false
@@ -164,11 +173,6 @@ func IsValidTemplatedURL(rawURL string, availableVariables []string, allowTempla
 		return true
 	}
 
-	// If templates are not allowed (e.g., for remotes), reject URLs with templates
-	if !allowTemplates {
-		return false
-	}
-
 	// Validate that all template variables are available
 	availableSet := make(map[string]bool)
 	for _, v := range availableVariables {