diff --git a/src/vs/platform/agentHost/node/agentService.ts b/src/vs/platform/agentHost/node/agentService.ts
index 134e2f1f93e..df34f8e5399 100644
--- a/src/vs/platform/agentHost/node/agentService.ts
+++ b/src/vs/platform/agentHost/node/agentService.ts
@@ -179,10 +179,7 @@ export class AgentService extends Disposable implements IAgentService {
 			const sourceState = this._stateManager.getSessionState(config.fork.session.toString());
 			let sourceTurns: ITurn[] = [];
 			if (sourceState) {
-				const forkIdx = sourceState.turns.findIndex(t => t.id === config.fork!.turnId);
-				if (forkIdx >= 0) {
-					sourceTurns = sourceState.turns.slice(0, forkIdx + 1);
-				}
+				sourceTurns = sourceState.turns.slice(0, config.fork.turnIndex + 1);
 			}
 
 			const summary: ISessionSummary = {
diff --git a/src/vs/platform/agentHost/node/copilot/copilotAgentForking.ts b/src/vs/platform/agentHost/node/copilot/copilotAgentForking.ts
index b8e662adc0c..76387322157 100644
--- a/src/vs/platform/agentHost/node/copilot/copilotAgentForking.ts
+++ b/src/vs/platform/agentHost/node/copilot/copilotAgentForking.ts
@@ -310,24 +310,41 @@ export async function forkSessionInDb(
 			[newSessionId, sourceSessionId, forkTurnIndex],
 		);
 
-		// Copy search index entries for kept turns
-		await dbRun(db,
-			`INSERT INTO search_index (content, session_id, source_type, source_id)
-			SELECT content, ?, source_type,
-			REPLACE(source_id, ?, ?)
+		// Copy search index entries for kept turns only.
+		// source_id format is "<session_id>:turn:<turn_index>"; filter by
+		// parsing the turn index so we don't leak content from later turns.
+		await dbAll(db,
+			`SELECT content, source_type, source_id
 			FROM search_index
-			WHERE session_id = ?
-			AND source_type = 'turn'`,
-			[newSessionId, sourceSessionId, newSessionId, sourceSessionId],
-		);
+			WHERE session_id = ? AND source_type = 'turn'`,
+			[sourceSessionId],
+		).then(async rows => {
+			const prefix = `${sourceSessionId}:turn:`;
+			for (const row of rows) {
+				const sourceId = row.source_id as string;
+				if (sourceId.startsWith(prefix)) {
+					const turnIdx = parseInt(sourceId.substring(prefix.length), 10);
+					if (!isNaN(turnIdx) && turnIdx <= forkTurnIndex) {
+						const newSourceId = sourceId.replace(sourceSessionId, newSessionId);
+						await dbRun(db,
+							`INSERT INTO search_index (content, session_id, source_type, source_id)
+							VALUES (?, ?, ?, ?)`,
+							[row.content, newSessionId, row.source_type, newSourceId],
+						);
+					}
+				}
+			}
+		});
 
-		// Copy checkpoints at or before the fork point
+		// Copy checkpoints at or before the fork point.
+		// checkpoint_number is 1-based and correlates to turns, so we keep
+		// only those where checkpoint_number <= forkTurnIndex + 1.
 		await dbRun(db,
 			`INSERT INTO checkpoints (session_id, checkpoint_number, title, overview, history, work_done, technical_details, important_files, next_steps, created_at)
 			SELECT ?, checkpoint_number, title, overview, history, work_done, technical_details, important_files, next_steps, created_at
 			FROM checkpoints
-			WHERE session_id = ?`,
-			[newSessionId, sourceSessionId],
+			WHERE session_id = ? AND checkpoint_number <= ?`,
+			[newSessionId, sourceSessionId, forkTurnIndex + 1],
 		);
 
 		await dbExec(db, 'COMMIT');
diff --git a/src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts b/src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts
index 9106ded0cf8..893e6e90f3f 100644
--- a/src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts
+++ b/src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts
@@ -1270,15 +1270,22 @@ export class AgentHostSessionHandler extends Disposable implements IChatSessionC
 		}
 
 		// Determine the turn index to fork at. If a specific request is
-		// provided, find its position in the protocol state's turn list.
-		// Otherwise fork the entire session.
+		// provided, fork BEFORE it (keeping turns up to the previous one).
+		// This matches the non-contributed path in ForkConversationAction
+		// which uses `requestIndex - 1`. If no request is provided, fork
+		// the entire session.
 		const protocolState = this._clientState.getSessionState(backendSession.toString());
 		let turnIndex: number | undefined;
 		if (request) {
-			turnIndex = protocolState?.turns.findIndex(t => t.id === request.id);
-			if (turnIndex === undefined || turnIndex < 0) {
+			const requestIdx = protocolState?.turns.findIndex(t => t.id === request.id);
+			if (requestIdx === undefined || requestIdx < 0) {
 				throw new Error(`Cannot fork: turn for request ${request.id} not found in protocol state`);
 			}
+			// Fork before this request — keep turns [0..requestIdx-1]
+			turnIndex = requestIdx - 1;
+			if (turnIndex < 0) {
+				throw new Error('Cannot fork: cannot fork before the first request');
+			}
 		} else if (protocolState && protocolState.turns.length > 0) {
 			turnIndex = protocolState.turns.length - 1;
 		}