Agent Debug: Add configurable max session log size setting for debug file logging (#4890)

* Add configurable max session log size setting for debug file logging

* update

Author: vijayupadya

package.json

@@ -4650,6 +4650,17 @@
 							"onExp"
 						]
 					},
+					"github.copilot.chat.agentDebugLog.fileLogging.maxSessionLogSizeMB": {
+						"type": "number",
+						"default": 100,
+						"minimum": 1,
+						"markdownDescription": "%github.copilot.config.chat.agentDebugLog.fileLogging.maxSessionLogSizeMB%",
+						"tags": [
+							"advanced",
+							"experimental",
+							"onExp"
+						]
+					},
 					"github.copilot.chat.otel.enabled": {
 						"type": "boolean",
 						"default": false,

package.nls.json

@@ -379,6 +379,7 @@
 	"github.copilot.config.chat.agentDebugLog.fileLogging.enabled": "Enable writing chat debug events to JSONL files on disk for diagnostics. When disabled, the built-in `troubleshoot` skill is also disabled. Requires window reload to take effect.",
 	"github.copilot.config.chat.agentDebugLog.fileLogging.flushIntervalMs": "How often (in milliseconds) buffered debug log entries are flushed to disk. Lower values provide more up-to-date logs at the cost of more frequent disk writes.",
 	"github.copilot.config.chat.agentDebugLog.fileLogging.maxRetainedSessionLogs": "Maximum number of chat debug session log directories to retain on disk. Each chat session produces one directory. Older session logs are automatically deleted when this limit is exceeded.",
+	"github.copilot.config.chat.agentDebugLog.fileLogging.maxSessionLogSizeMB": "Maximum size in megabytes for a single chat debug session log file. When the log exceeds this size, older entries are truncated to retain the most recent data. Defaults to 100 MB.",
 	"github.copilot.config.inlineEdits.diagnosticsContextProvider.enabled": "Enable diagnostics context provider for next edit suggestions.",
 	"github.copilot.config.inlineEdits.chatSessionContextProvider.enabled": "Enable chat session context provider for next edit suggestions.",
 	"github.copilot.config.codesearch.agent.enabled": "Enable code search capabilities in agent mode.",

src/extension/chat/vscode-node/chatDebugFileLoggerService.ts

@@ -25,8 +25,8 @@ const DEFAULT_FLUSH_INTERVAL_MS = 4_000;
 const MIN_FLUSH_INTERVAL_MS = 2_000;
 const MAX_ATTR_VALUE_LENGTH = 5_000;
 const MAX_PENDING_CORE_EVENTS = 100;
-const MAX_SESSION_LOG_BYTES = 100 * 1024 * 1024; // 100MB
-const TRUNCATION_RETAIN_BYTES = 60 * 1024 * 1024; // 60 MB
+const DEFAULT_MAX_SESSION_LOG_MB = 100;
+const TRUNCATION_RETAIN_RATIO = 0.6; // retain 60% of max on truncation
 const MAX_SPAN_SESSION_INDEX = 10_000;
 
 
@@ -87,6 +87,7 @@ export class ChatDebugFileLoggerService extends Disposable implements IChatDebug
 	private _debugLogsDirUri: URI | undefined;
 	private _autoFlushTimer: ReturnType<typeof setInterval> | undefined;
 	private _autoFlushIntervalMs: number;
+	private _maxSessionLogBytes: number;
 	private _totalBytesWritten = 0;
 	private _totalSessionCount = 0;
 
@@ -111,17 +112,22 @@ export class ChatDebugFileLoggerService extends Disposable implements IChatDebug
 			*/
 			this._telemetryService.sendMSFTTelemetryEvent('chatDebugFileLogger.disabled');
 			this._autoFlushIntervalMs = DEFAULT_FLUSH_INTERVAL_MS;
+			this._maxSessionLogBytes = DEFAULT_MAX_SESSION_LOG_MB * 1024 * 1024;
 			return;
 		}
 
 		this._autoFlushIntervalMs = Math.max(MIN_FLUSH_INTERVAL_MS, this._configurationService.getConfig(ConfigKey.Advanced.ChatDebugFileLoggingFlushInterval) ?? DEFAULT_FLUSH_INTERVAL_MS);
+		this._maxSessionLogBytes = this._resolveMaxSessionLogBytes();
 
-		// React to flush interval changes at runtime
+		// React to changes at runtime
 		this._register(this._configurationService.onDidChangeConfiguration(e => {
 			if (e.affectsConfiguration(ConfigKey.Advanced.ChatDebugFileLoggingFlushInterval.fullyQualifiedId)) {
 				this._autoFlushIntervalMs = Math.max(MIN_FLUSH_INTERVAL_MS, this._configurationService.getConfig(ConfigKey.Advanced.ChatDebugFileLoggingFlushInterval) ?? DEFAULT_FLUSH_INTERVAL_MS);
 				this._restartFlushTimer();
 			}
+			if (e.affectsConfiguration(ConfigKey.Advanced.ChatDebugFileLoggingMaxSessionLogSizeMB.fullyQualifiedId)) {
+				this._maxSessionLogBytes = this._resolveMaxSessionLogBytes();
+			}
 		}));
 
 		// Subscribe to OTel span completions
@@ -142,6 +148,12 @@ export class ChatDebugFileLoggerService extends Disposable implements IChatDebug
 		}
 	}
 
+	private _resolveMaxSessionLogBytes(): number {
+		const raw = this._configurationService.getExperimentBasedConfig(ConfigKey.Advanced.ChatDebugFileLoggingMaxSessionLogSizeMB, this._experimentationService);
+		const mb = typeof raw === 'number' && Number.isFinite(raw) ? raw : DEFAULT_MAX_SESSION_LOG_MB;
+		return Math.max(1, Math.floor(mb)) * 1024 * 1024;
+	}
+
 	override dispose(): void {
 		if (this._autoFlushTimer) {
 			clearInterval(this._autoFlushTimer);
@@ -764,7 +776,7 @@ export class ChatDebugFileLoggerService extends Disposable implements IChatDebug
 			}
 			await fs.promises.appendFile(session.uri.fsPath, content, 'utf-8');
 			session.bytesWritten += Buffer.byteLength(content, 'utf-8');
-			if (session.bytesWritten > MAX_SESSION_LOG_BYTES) {
+			if (session.bytesWritten > this._maxSessionLogBytes) {
 				await this._truncateLogFile(session);
 			}
 		} catch (err) {
@@ -773,18 +785,20 @@ export class ChatDebugFileLoggerService extends Disposable implements IChatDebug
 	}
 
 	/**
-	 * Truncate a log file to retain the newest ~80MB using a streaming
-	 * approach via a temp file to avoid loading the entire tail into memory.
+	 * Truncate a log file to retain the newest portion (TRUNCATION_RETAIN_RATIO
+	 * of the configured max size) using a streaming approach via a temp file
+	 * to avoid loading the entire tail into memory.
 	 */
 	private async _truncateLogFile(session: IActiveLogSession): Promise<void> {
 		try {
 			const filePath = session.uri.fsPath;
 			const stat = await fs.promises.stat(filePath);
-			if (stat.size <= MAX_SESSION_LOG_BYTES) {
+			if (stat.size <= this._maxSessionLogBytes) {
 				return;
 			}
 
-			const skipBytes = stat.size - TRUNCATION_RETAIN_BYTES;
+			const retainBytes = Math.floor(this._maxSessionLogBytes * TRUNCATION_RETAIN_RATIO);
+			const skipBytes = stat.size - retainBytes;
 			const fd = await fs.promises.open(filePath, 'r');
 			try {
 				// Read a small probe around the cut point to find the next newline

src/platform/configuration/common/configurationService.ts

@@ -692,6 +692,7 @@ export namespace ConfigKey {
 		export const ChatDebugFileLogging = defineAndMigrateExpSetting<boolean>('chat.chatDebug.fileLogging.enabled', 'chat.agentDebugLog.fileLogging.enabled', false);
 		export const ChatDebugFileLoggingFlushInterval = defineAndMigrateSetting<number>('chat.chatDebug.fileLogging.flushIntervalMs', 'chat.agentDebugLog.fileLogging.flushIntervalMs', 4000);
 		export const ChatDebugFileLoggingMaxRetainedSessionLogs = defineSetting<number>('chat.agentDebugLog.fileLogging.maxRetainedSessionLogs', ConfigType.ExperimentBased, 50);
+		export const ChatDebugFileLoggingMaxSessionLogSizeMB = defineSetting<number>('chat.agentDebugLog.fileLogging.maxSessionLogSizeMB', ConfigType.ExperimentBased, 100);
 
 		// OTel settings
 		export const OTelEnabled = defineSetting<boolean>('chat.otel.enabled', ConfigType.Simple, false);