Completions capability flag

docs/specification/draft/server/utilities/completion.md

@@ -21,6 +21,18 @@ However, implementations are free to expose completion through any interface pat
 suits their needs—the protocol itself does not mandate any specific user
 interaction model.
 
+## Capabilities
+
+Servers that support completions **MUST** declare the `completions` capability:
+
+```json
+{
+  "capabilities": {
+    "completions": {}
+  }
+}
+```
+
 ## Protocol Messages
 
 ### Requesting Completions
@@ -113,6 +125,15 @@ sequenceDiagram
   - `total`: Optional total matches
   - `hasMore`: Additional results flag
 
+## Error Handling
+
+Servers **SHOULD** return standard JSON-RPC errors for common failure cases:
+
+- Method not found: `-32601` (Capability not supported)
+- Invalid prompt name: `-32602` (Invalid params)
+- Missing required arguments: `-32602` (Invalid params)
+- Internal errors: `-32603` (Internal error)
+
 ## Implementation Considerations
 
 1. Servers **SHOULD**:

schema/draft/schema.json

@@ -1788,6 +1788,12 @@
         "ServerCapabilities": {
             "description": "Capabilities that a server may support. Known capabilities are defined here, in this schema, but this is not a closed set: any server can define its own, additional capabilities.",
             "properties": {
+                "completions": {
+                    "additionalProperties": true,
+                    "description": "Present if the server supports argument autocompletion suggestions.",
+                    "properties": {},
+                    "type": "object"
+                },
                 "experimental": {
                     "additionalProperties": {
                         "additionalProperties": true,

schema/draft/schema.ts

@@ -217,6 +217,10 @@ export interface ServerCapabilities {
    * Present if the server supports sending log messages to the client.
    */
   logging?: object;
+  /**
+   * Present if the server supports argument autocompletion suggestions.
+   */
+  completions?: object;
   /**
    * Present if the server offers any prompt templates.
    */