Pass structured tool results via RPC instead of stringifying

In both the Node and Go SDKs, _executeToolAndRespond / executeToolAndRespond
was converting structured ToolResultObject values into plain JSON strings
before sending them over RPC. This destroyed the object shape, causing
toolTelemetry (and other structured fields like resultType) to be silently
lost on the server side.

Node SDK: detect ToolResultObject by checking for textResultForLlm +
resultType properties and pass it directly to handlePendingToolCall, which
already accepts the union type (string | object) in its RPC schema.

Go SDK: send a ResultUnion with ResultResult populated (preserving
TextResultForLlm, ResultType, Error, and ToolTelemetry) instead of
extracting only the text and sending ResultUnion with String.

Fixes the SDK side of github/copilot-agent-runtime#5574.

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

Author: Morabbin

go/session.go

@@ -620,13 +620,21 @@ func (s *Session) executeToolAndRespond(requestID, toolName, toolCallID string,
 		return
 	}
 
-	resultStr := result.TextResultForLLM
-	if resultStr == "" {
-		resultStr = fmt.Sprintf("%v", result)
+	rpcResult := rpc.ResultUnion{
+		ResultResult: &rpc.ResultResult{
+			TextResultForLlm: result.TextResultForLLM,
+			ToolTelemetry:    result.ToolTelemetry,
+		},
+	}
+	if result.ResultType != "" {
+		rpcResult.ResultResult.ResultType = &result.ResultType
+	}
+	if result.Error != "" {
+		rpcResult.ResultResult.Error = &result.Error
 	}
 	s.RPC.Tools.HandlePendingToolCall(ctx, &rpc.SessionToolsHandlePendingToolCallParams{
 		RequestID: requestID,
-		Result:    &rpc.ResultUnion{String: &resultStr},
+		Result:    &rpcResult,
 	})
 }
 

nodejs/src/session.ts

@@ -33,6 +33,8 @@ import type {
     SessionUiApi,
     Tool,
     ToolHandler,
+    ToolResult,
+    ToolResultObject,
     TraceContextProvider,
     TypedSessionEventHandler,
     UserInputHandler,
@@ -459,11 +461,13 @@ export class CopilotSession {
                 traceparent,
                 tracestate,
             });
-            let result: string;
+            let result: ToolResult;
             if (rawResult == null) {
                 result = "";
             } else if (typeof rawResult === "string") {
                 result = rawResult;
+            } else if (isToolResultObject(rawResult)) {
+                result = rawResult;
             } else {
                 result = JSON.stringify(rawResult);
             }
@@ -1043,3 +1047,18 @@ export class CopilotSession {
         await this.rpc.log({ message, ...options });
     }
 }
+
+/**
+ * Type guard that checks whether a value is a {@link ToolResultObject}.
+ * A valid object must have a string `textResultForLlm` and a string `resultType`.
+ */
+function isToolResultObject(value: unknown): value is ToolResultObject {
+    return (
+        typeof value === "object" &&
+        value !== null &&
+        "textResultForLlm" in value &&
+        typeof (value as ToolResultObject).textResultForLlm === "string" &&
+        "resultType" in value &&
+        typeof (value as ToolResultObject).resultType === "string"
+    );
+}