Add transcript line count to compaction summary (#4811)

* Add transcript line count to compaction summary

Expose getLineCount() on ISessionTranscriptService so the
summarization prompt can tell the model how many lines the transcript
had before compaction. This helps the model target the right region
when reading the transcript file for context recovery.

- Track lineCount on IActiveSession, increment in _bufferEntry
- On session resume, count existing lines from the file on disk
- Pass transcriptLineCount through SummarizedConversationHistory
  props to SummaryMessageElement
- Include 'The transcript had N lines before compaction' in the hint

* Update src/extension/prompts/node/agent/summarizedConversationHistory.tsx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: bhavyaus

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

@@ -38,6 +38,8 @@ interface IActiveSession {
 	readonly buffer: string[];
 	/** Chain of flush operations to serialize writes. */
 	flushPromise: Promise<void>;
+	/** Running count of lines in the transcript (flushed + buffered). */
+	lineCount: number;
 }
 
 export class SessionTranscriptService implements ISessionTranscriptService {
@@ -89,6 +91,7 @@ export class SessionTranscriptService implements ISessionTranscriptService {
 			lastEntryId: null,
 			buffer: [],
 			flushPromise: Promise.resolve(),
+			lineCount: 0,
 		};
 		this._activeSessions.set(sessionId, session);
 
@@ -102,7 +105,12 @@ export class SessionTranscriptService implements ISessionTranscriptService {
 		}
 
 		if (fileAlreadyExists) {
-			// Session file exists — we're resuming; no need to replay history
+			// Session file exists — we're resuming; count existing lines so getLineCount stays accurate
+			try {
+				const content = await fs.promises.readFile(fileUri.fsPath, 'utf-8');
+				session.lineCount = content.split('\n').filter(l => l.length > 0).length;
+			} catch {
+			}
 			return;
 		}
 
@@ -216,6 +224,10 @@ export class SessionTranscriptService implements ISessionTranscriptService {
 		return this._activeSessions.get(sessionId)?.uri;
 	}
 
+	getLineCount(sessionId: string): number | undefined {
+		return this._activeSessions.get(sessionId)?.lineCount;
+	}
+
 	isTranscriptUri(uri: URI): boolean {
 		const dir = this._getTranscriptsDir();
 		if (!dir) {
@@ -352,6 +364,7 @@ export class SessionTranscriptService implements ISessionTranscriptService {
 		} as TranscriptEntry;
 
 		session.lastEntryId = id;
+		session.lineCount++;
 		session.buffer.push(JSON.stringify(fullEntry) + '\n');
 	}
 

src/extension/prompts/node/agent/summarizedConversationHistory.tsx

@@ -248,7 +248,7 @@ class ConversationHistory extends PromptElement<SummarizedAgentHistoryProps> {
 		}
 
 		if (summaryForCurrentTurn) {
-			history.push(<SummaryMessageElement endpoint={this.props.endpoint} summaryText={summaryForCurrentTurn} transcriptPath={this.props.transcriptPath} />);
+			history.push(<SummaryMessageElement endpoint={this.props.endpoint} summaryText={summaryForCurrentTurn} transcriptPath={this.props.transcriptPath} transcriptLineCount={this.props.transcriptLineCount} />);
 
 			return (<PrioritizedList priority={this.props.priority} descending={false} passPriority={true}>
 				{history.reverse()}
@@ -308,7 +308,7 @@ class ConversationHistory extends PromptElement<SummarizedAgentHistoryProps> {
 
 			if (summaryForTurn) {
 				// We have a summary for a tool call round that was part of this turn
-				turnComponents.push(<SummaryMessageElement endpoint={this.props.endpoint} summaryText={summaryForTurn.text} transcriptPath={this.props.transcriptPath} />);
+				turnComponents.push(<SummaryMessageElement endpoint={this.props.endpoint} summaryText={summaryForTurn.text} transcriptPath={this.props.transcriptPath} transcriptLineCount={this.props.transcriptLineCount} />);
 			} else if (!turn.isContinuation) {
 				turnComponents.push(<AgentUserMessage flexGrow={1} {...getUserMessagePropsFromTurn(turn, this.props.endpoint, {
 					userQueryTagName: this.props.userQueryTagName,
@@ -410,6 +410,8 @@ export interface SummarizedAgentHistoryProps extends BasePromptElementProps, Age
 	readonly summarizationSource?: 'background' | 'foreground';
 	/** Path to the conversation transcript JSONL file, used to inform the model after summarization */
 	readonly transcriptPath?: string;
+	/** Number of lines in the transcript at the time of compaction */
+	readonly transcriptLineCount?: number;
 }
 
 /**
@@ -458,13 +460,15 @@ export class SummarizedConversationHistory extends PromptElement<SummarizedAgent
 
 		// Resolve transcript path and flush to disk so the model can read the up-to-date file
 		let transcriptPath: string | undefined;
+		let transcriptLineCount: number | undefined;
 		if (transcriptLookupEnabled) {
 			const sessionId = this.props.promptContext.conversation?.sessionId;
 			if (sessionId) {
 				const transcriptUri = this.sessionTranscriptService.getTranscriptPath(sessionId);
 				if (transcriptUri) {
 					await this.sessionTranscriptService.flush(sessionId);
 					transcriptPath = transcriptUri.fsPath;
+					transcriptLineCount = this.sessionTranscriptService.getLineCount(sessionId);
 				}
 			}
 		}
@@ -475,6 +479,7 @@ export class SummarizedConversationHistory extends PromptElement<SummarizedAgent
 				{...this.props}
 				promptContext={promptContext}
 				transcriptPath={transcriptPath}
+				transcriptLineCount={transcriptLineCount}
 				enableCacheBreakpoints={this.props.enableCacheBreakpoints} />
 		</>;
 	}
@@ -1002,6 +1007,7 @@ interface SummaryMessageProps extends BasePromptElementProps {
 	readonly summaryText: string;
 	readonly endpoint: IChatEndpoint;
 	readonly transcriptPath?: string;
+	readonly transcriptLineCount?: number;
 }
 
 class SummaryMessageElement extends PromptElement<SummaryMessageProps> {
@@ -1010,7 +1016,7 @@ class SummaryMessageElement extends PromptElement<SummaryMessageProps> {
 			<Tag name='conversation-summary'>
 				{this.props.summaryText}
 			</Tag>
-			{this.props.transcriptPath && <><br />If you need specific details from before compaction (such as exact code snippets, error messages, tool results, or content you previously generated), use the {ToolName.ReadFile} tool to look up the full uncompacted conversation transcript at: {this.props.transcriptPath}</>}
+			{this.props.transcriptPath && <><br />If you need specific details from before compaction (such as exact code snippets, error messages, tool results, or content you previously generated), use the {ToolName.ReadFile} tool to look up the full uncompacted conversation transcript at: "{this.props.transcriptPath}"{this.props.transcriptLineCount !== undefined && <><br />At the time of this request, the transcript has {this.props.transcriptLineCount} lines.</>}<br />Example usage: {ToolName.ReadFile}(filePath: "{this.props.transcriptPath}")</>}
 			{this.props.endpoint.family === 'gpt-4.1' && <Tag name='reminderInstructions'>
 				<DefaultOpenAIKeepGoingReminder />
 			</Tag>}

src/platform/chat/common/sessionTranscriptService.ts

@@ -237,6 +237,12 @@ export interface ISessionTranscriptService {
 	 */
 	getTranscriptPath(sessionId: string): URI | undefined;
 
+	/**
+	 * Get the current number of lines in the transcript for a session.
+	 * Returns `undefined` if the session is not active.
+	 */
+	getLineCount(sessionId: string): number | undefined;
+
 	/**
 	 * Remove transcript files for sessions that are no longer active,
 	 * keeping at most `maxRetained` most-recent ended sessions.
@@ -263,6 +269,7 @@ export class NullSessionTranscriptService implements ISessionTranscriptService {
 	async flush(): Promise<void> { }
 	async endSession(): Promise<void> { }
 	getTranscriptPath(): URI | undefined { return undefined; }
+	getLineCount(): number | undefined { return undefined; }
 	async cleanupOldTranscripts(): Promise<void> { }
 	isTranscriptUri(): boolean { return false; }
 }