Add arbitrary field support to MCP server configuration (#161)

* Initial plan

* Add support for arbitrary fields in MCP server configuration

Co-authored-by: alexec <1142830+alexec@users.noreply.github.com>

* Add example demonstrating arbitrary fields in MCP server config

Co-authored-by: alexec <1142830+alexec@users.noreply.github.com>

* Add defensive initialization of Content map in UnmarshalJSON

Co-authored-by: alexec <1142830+alexec@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexec <1142830+alexec@users.noreply.github.com>

Author: Copilot

examples/agents/tasks/example-mcp-arbitrary-fields.md

@@ -0,0 +1,93 @@
+---
+task_name: example-mcp-arbitrary-fields
+agent: cursor
+mcp_servers:
+  # Example with standard fields only
+  filesystem:
+    type: stdio
+    command: filesystem
+  
+  # Example with standard fields plus arbitrary custom fields
+  custom-database:
+    type: stdio
+    command: database-mcp
+    args: ["--verbose"]
+    # Arbitrary fields below
+    cache_enabled: true
+    max_cache_size: 1000
+    connection_pool_size: 10
+    
+  # Example HTTP server with custom metadata
+  api-server:
+    type: http
+    url: https://api.example.com
+    headers:
+      Authorization: Bearer token123
+    # Arbitrary fields below
+    api_version: v2
+    rate_limit: 100
+    timeout_seconds: 30
+    retry_policy: exponential
+    region: us-west-2
+    
+  # Example with nested custom configuration
+  advanced-server:
+    type: stdio
+    command: python
+    args: ["-m", "server"]
+    env:
+      PYTHON_PATH: /usr/bin/python3
+    # Arbitrary nested fields below
+    custom_config:
+      host: localhost
+      port: 5432
+      ssl: true
+      pool:
+        min: 2
+        max: 10
+    monitoring:
+      enabled: true
+      metrics_port: 9090
+---
+
+# Example Task with Arbitrary MCP Server Fields
+
+This task demonstrates the ability to add arbitrary fields to MCP server configurations, just like we can with FrontMatter.
+
+## Why Arbitrary Fields?
+
+Different MCP servers may need different configuration options beyond the standard fields (`type`, `command`, `args`, `env`, `url`, `headers`). Arbitrary fields allow you to:
+
+1. **Add custom metadata**: Version info, regions, endpoints, etc.
+2. **Configure behavior**: Caching, retry policies, timeouts, rate limits
+3. **Include nested config**: Complex configuration objects specific to your server
+4. **Future-proof**: Add new fields without changing the schema
+
+## How It Works
+
+The `MCPServerConfig` struct now includes a `Content` field (similar to `BaseFrontMatter`) that captures all fields from YAML/JSON:
+
+```go
+type MCPServerConfig struct {
+    // Standard fields
+    Type    TransportType
+    Command string
+    Args    []string
+    Env     map[string]string
+    URL     string
+    Headers map[string]string
+    
+    // Arbitrary fields via inline map
+    Content map[string]any `yaml:",inline"`
+}
+```
+
+## Example Usage
+
+The examples above show:
+- **Simple custom fields**: `cache_enabled`, `max_cache_size`
+- **API configuration**: `api_version`, `rate_limit`, `timeout_seconds`
+- **Nested objects**: `custom_config` with sub-fields like `host`, `port`, `ssl`
+- **Multiple custom sections**: `custom_config` and `monitoring` as separate objects
+
+All these fields are preserved when the configuration is parsed and can be accessed via the `Content` map.

pkg/codingcontext/mcp_server_config.go

@@ -1,28 +1,63 @@
 package codingcontext
 
+import (
+	"encoding/json"
+)
+
 // MCPServerConfig defines the common configuration fields supported by both platforms.
+// It also supports arbitrary additional fields via the Content map.
 type MCPServerConfig struct {
 	// Type specifies the connection protocol.
 	// Values: "stdio", "sse", "http".
-	Type TransportType `json:"type,omitempty"`
+	Type TransportType `json:"type,omitempty" yaml:"type,omitempty"`
 
 	// Command is the executable to run (e.g. "npx", "docker").
 	// Required for "stdio" type.
-	Command string `json:"command,omitempty"`
+	Command string `json:"command,omitempty" yaml:"command,omitempty"`
 
 	// Args is an array of arguments for the command.
-	Args []string `json:"args,omitempty"`
+	Args []string `json:"args,omitempty" yaml:"args,omitempty"`
 
 	// Env defines environment variables for the server process.
-	Env map[string]string `json:"env,omitempty"`
+	Env map[string]string `json:"env,omitempty" yaml:"env,omitempty"`
 
 	// URL is the endpoint for "http" or "sse" types.
 	// Required for remote connections.
-	URL string `json:"url,omitempty"`
+	URL string `json:"url,omitempty" yaml:"url,omitempty"`
 
 	// Headers contains custom HTTP headers (e.g. {"Authorization": "Bearer ..."}).
 	// Used for "http" and "sse" types.
-	Headers map[string]string `json:"headers,omitempty"`
+	Headers map[string]string `json:"headers,omitempty" yaml:"headers,omitempty"`
+
+	// Content holds arbitrary additional fields from YAML/JSON that aren't in the struct
+	Content map[string]any `json:"-" yaml:",inline"`
+}
+
+// UnmarshalJSON custom unmarshaler that populates both typed fields and Content map
+func (m *MCPServerConfig) UnmarshalJSON(data []byte) error {
+	// First unmarshal into a temporary type to avoid infinite recursion
+	type Alias MCPServerConfig
+	aux := &struct {
+		*Alias
+	}{
+		Alias: (*Alias)(m),
+	}
+
+	if err := json.Unmarshal(data, aux); err != nil {
+		return err
+	}
+
+	// Initialize Content map if needed
+	if m.Content == nil {
+		m.Content = make(map[string]any)
+	}
+
+	// Also unmarshal into Content map
+	if err := json.Unmarshal(data, &m.Content); err != nil {
+		return err
+	}
+
+	return nil
 }
 
 // MCPServerConfigs maps server names to their configurations.