Add support for output-format stream-jsonflag for headless mode (#10883)

packages/core/src/output/stream-json-formatter.ts

@@ -0,0 +1,62 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { JsonStreamEvent, StreamStats } from './types.js';
+import type { SessionMetrics } from '../telemetry/uiTelemetry.js';
+
+/**
+ * Formatter for streaming JSON output.
+ * Emits newline-delimited JSON (JSONL) events to stdout in real-time.
+ */
+export class StreamJsonFormatter {
+  /**
+   * Formats a single event as a JSON string with newline (JSONL format).
+   * @param event - The stream event to format
+   * @returns JSON string with trailing newline
+   */
+  formatEvent(event: JsonStreamEvent): string {
+    return JSON.stringify(event) + '\n';
+  }
+
+  /**
+   * Emits an event directly to stdout in JSONL format.
+   * @param event - The stream event to emit
+   */
+  emitEvent(event: JsonStreamEvent): void {
+    process.stdout.write(this.formatEvent(event));
+  }
+
+  /**
+   * Converts SessionMetrics to simplified StreamStats format.
+   * Aggregates token counts across all models.
+   * @param metrics - The session metrics from telemetry
+   * @param durationMs - The session duration in milliseconds
+   * @returns Simplified stats for streaming output
+   */
+  convertToStreamStats(
+    metrics: SessionMetrics,
+    durationMs: number,
+  ): StreamStats {
+    let totalTokens = 0;
+    let inputTokens = 0;
+    let outputTokens = 0;
+
+    // Aggregate token counts across all models
+    for (const modelMetrics of Object.values(metrics.models)) {
+      totalTokens += modelMetrics.tokens.total;
+      inputTokens += modelMetrics.tokens.prompt;
+      outputTokens += modelMetrics.tokens.candidates;
+    }
+
+    return {
+      total_tokens: totalTokens,
+      input_tokens: inputTokens,
+      output_tokens: outputTokens,
+      duration_ms: durationMs,
+      tool_calls: metrics.tools.totalCalls,
+    };
+  }
+}