@W-21474595 - add MCP tool-level response token usage (responseCharCount)

packages/mcp/src/sf-mcp-server.ts

@@ -141,6 +141,14 @@ export class SfMcpServer extends McpServer implements ToolMethodSignatures {
       this.logger.debug(`Tool ${name} completed in ${runtimeMs}ms`);
       if (result.isError) this.logger.debug(`Tool ${name} errored`);
 
+      // Calculate response character count for token usage (never let telemetry instrumentation fail a tool call)
+      let responseCharCount = 0;
+      try {
+        responseCharCount = this.calculateResponseCharCount(result);
+      } catch (err) {
+        // never let telemetry instrumentation fail a tool call
+      }
+
       this.telemetry?.sendEvent('TOOL_CALLED', {
         name,
         runtimeMs,
@@ -151,6 +159,7 @@ export class SfMcpServer extends McpServer implements ToolMethodSignatures {
         //
         // https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult
         isError: result.isError ?? false,
+        responseCharCount: responseCharCount.toString(),
       });
 
       this.telemetry?.sendPdpEvent({
@@ -165,4 +174,37 @@ export class SfMcpServer extends McpServer implements ToolMethodSignatures {
     const tool = super.registerTool(name, config, wrappedCb as ToolCallback<InputArgs>);
     return tool;
   }
+
+  /**
+   * Calculates the total character count from tool result content and structured output.
+   * Used for token usage. Accounts for both:
+   * - content: text (and other) content items
+   * - structuredContent: structured tool output when the tool defines an outputSchema
+   *
+   * @see https://modelcontextprotocol.io/specification/2025-11-25/server/tools#output-schema
+   * @param result - The CallToolResult from tool execution
+   * @returns Total character count across text content and structured content
+   */
+  private calculateResponseCharCount(result: CallToolResult): number {
+    let total = 0;
+
+    // Plain text (and other) content items
+    if (result.content && Array.isArray(result.content)) {
+      total += result.content
+        .filter((item): item is { type: 'text'; text: string } => item.type === 'text')
+        .reduce((sum, item) => sum + item.text.length, 0);
+    }
+
+    // Structured content (JSON object per outputSchema)
+    const structured = (result as CallToolResult & { structuredContent?: unknown }).structuredContent;
+    if (structured !== undefined && structured !== null && typeof structured === 'object') {
+      try {
+        total += JSON.stringify(structured).length;
+      } catch {
+        // ignore serialization errors
+      }
+    }
+
+    return total;
+  }
 }