fix: add enum type safety for Mode properties in C# RPC types

The mode property now uses SessionModeGetResultMode enum with
Interactive, Plan, and Autopilot values for compile-time validation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/Generated/Rpc.cs

@@ -220,7 +220,7 @@ public class SessionModeGetResult
 {
     /// <summary>The current agent mode.</summary>
     [JsonPropertyName("mode")]
-    public string Mode { get; set; } = string.Empty;
+    public SessionModeGetResultMode Mode { get; set; }
 }
 
 internal class GetRequest
@@ -233,7 +233,7 @@ public class SessionModeSetResult
 {
     /// <summary>The agent mode after switching.</summary>
     [JsonPropertyName("mode")]
-    public string Mode { get; set; } = string.Empty;
+    public SessionModeGetResultMode Mode { get; set; }
 }
 
 internal class SetRequest
@@ -242,7 +242,7 @@ internal class SetRequest
     public string SessionId { get; set; } = string.Empty;
 
     [JsonPropertyName("mode")]
-    public string Mode { get; set; } = string.Empty;
+    public SessionModeGetResultMode Mode { get; set; }
 }
 
 public class SessionPlanReadResult
@@ -346,6 +346,18 @@ internal class StartRequest
     public string? Prompt { get; set; }
 }
 
+[JsonConverter(typeof(JsonStringEnumConverter<SessionModeGetResultMode>))]
+public enum SessionModeGetResultMode
+{
+    [JsonStringEnumMemberName("interactive")]
+    Interactive,
+    [JsonStringEnumMemberName("plan")]
+    Plan,
+    [JsonStringEnumMemberName("autopilot")]
+    Autopilot,
+}
+
+
 /// <summary>Typed server-scoped RPC methods (no session required).</summary>
 public class ServerRpc
 {

scripts/codegen/csharp.ts

@@ -459,13 +459,19 @@ export async function generateSessionEvents(schemaPath?: string): Promise<void>
 
 let emittedRpcClasses = new Set<string>();
 let rpcKnownTypes = new Map<string, string>();
+let rpcEnumOutput: string[] = [];
 
 function singularPascal(s: string): string {
     const p = toPascalCase(s);
     return p.endsWith("s") ? p.slice(0, -1) : p;
 }
 
 function resolveRpcType(schema: JSONSchema7, isRequired: boolean, parentClassName: string, propName: string, classes: string[]): string {
+    // Handle enums (string unions like "interactive" | "plan" | "autopilot")
+    if (schema.enum && Array.isArray(schema.enum)) {
+        const enumName = getOrCreateEnum(parentClassName, propName, schema.enum as string[], rpcEnumOutput);
+        return isRequired ? enumName : `${enumName}?`;
+    }
     if (schema.type === "object" && schema.properties) {
         const className = `${parentClassName}${propName}`;
         classes.push(emitRpcClass(className, schema, "public", classes));
@@ -708,6 +714,8 @@ function emitSessionApiClass(className: string, node: Record<string, unknown>, c
 function generateRpcCode(schema: ApiSchema): string {
     emittedRpcClasses.clear();
     rpcKnownTypes.clear();
+    rpcEnumOutput = [];
+    generatedEnums.clear(); // Clear shared enum deduplication map
     const classes: string[] = [];
 
     let serverRpcParts: string[] = [];
@@ -730,6 +738,7 @@ namespace GitHub.Copilot.SDK.Rpc;
 `);
 
     for (const cls of classes) if (cls) lines.push(cls, "");
+    for (const enumCode of rpcEnumOutput) lines.push(enumCode, "");
     for (const part of serverRpcParts) lines.push(part, "");
     for (const part of sessionRpcParts) lines.push(part, "");