--- a/dsh-host-apiproxy/lib/index.js
+++ b/dsh-host-apiproxy/lib/index.js
@@ -0,0 +0,0 @@
 });
 /** session.fork response value (the child session id). */
 const sessionForkValueSchema = z$1.object({ sessionId: sessionIdSchema });
+/** session.truncate request payload (atSeq discards that event and everything after from this session log). */
+const sessionTruncateRequestSchema = z$1.object({
+	sessionId: sessionIdSchema,
+	atSeq: z$1.number().int().nonnegative()
+});
+/** session.truncate response value (the new last seq after the cut). */
+const sessionTruncateValueSchema = z$1.object({ lastSeq: z$1.number().int().nonnegative() });
 /** session.history request payload (beforeSeq/maxMessages page backwards from the window tail). */
 const sessionHistoryRequestSchema = z$1.object({
 	sessionId: sessionIdSchema,
@@ -0,0 +0,0 @@
 				}
 				return ok(request, { sessionId: childId });
 			},
+			async truncate(request) {
+				const { sessionId, atSeq } = request.payload || {};
+				if (typeof atSeq !== "number" || !Number.isSafeInteger(atSeq) || atSeq < 0) return err(request, { code: "invalid-arg", message: "atSeq must be a non-negative safe integer", details: {} });
+				// truncate mutates the live session log; require the attached Session instance
+				// (readSessionState returns an immutable {id,header,events} snapshot with no truncate).
+				const session = ctx.sessions.get(sessionId);
+				if (session === void 0) return err(request, { code: "session-not-found", message: `session "${sessionId}" not found`, details: { sessionId } });
+				try {
+					const newSeq = session.truncate(atSeq);
+					// truncate leaves the token-meter's per-session incremental fold cursor
+					// stale (cursor > log length), so the next measure/compaction folds an
+					// assistant/message whose step/start was already cut and throws
+					// "assistant/message at seq ... has no matching step/start event".
+					// Drop the cached fold so measure re-folds the shortened log from scratch.
+					const meter = ctx.get && ctx.get("tokenMeter");
+					if (meter && typeof meter.states === "object" && typeof meter.states.delete === "function") { try { meter.states.delete(session); } catch (me) {} }
+					const persist = ctx.get && ctx.get("sessionPersistence");
+					// truncate may roll the cut back to a clean step boundary (newSeq < atSeq);
+					// persist the SAME cut so the durable artifact matches the live log.
+					if (persist && typeof persist.truncate === "function") { try { await persist.truncate(sessionId, newSeq); } catch (pe) {} }
+					if (ctx.sessions && typeof ctx.sessions.flush === "function") { try { ctx.sessions.flush(session); } catch (fe) {} }
+					broadcast({ type: "session/subscribed", sessionId, lastSeq: newSeq - 1 });
+					broadcast({ type: "session/reload", sessionId, lastSeq: newSeq - 1 });
+					return ok(request, { lastSeq: newSeq - 1 });
+				} catch (error) {
+					return err(request, { code: "internal", message: String(error), details: {} });
+				}
+			},
 			async prompt(request) {
 				const { sessionId, mode, content, clientTimeZone } = request.payload;
 				const canonicalTimeZone = clientTimeZone === void 0 ? void 0 : canonicalClientTimeZone(clientTimeZone);
@@ -0,0 +0,0 @@
 		schema: sessionForkRequestSchema,
 		invoke: (api, r) => api.sessions.fork(r)
 	},
+	"session.truncate": {
+		schema: sessionTruncateRequestSchema,
+		invoke: (api, r) => api.sessions.truncate(r)
+	},
 	"session.prompt": {
 		schema: sessionPromptRequestSchema,
 		invoke: (api, r) => api.sessions.prompt(r)
@@ -0,0 +0,0 @@
 	"session.selectModel": sessionSelectModelValueSchema,
 	"session.rename": sessionRenameValueSchema,
 	"session.fork": sessionForkValueSchema,
+	"session.truncate": sessionTruncateValueSchema,
 	"session.prompt": sessionPromptValueSchema,
 	"session.attachment": sessionAttachmentValueSchema,
 	"session.updateQueue": sessionUpdateQueueValueSchema,
@@ -0,0 +0,0 @@
 		selectModel: (payload, signal) => this.callUnary("session.selectModel", payload, signal),
 		rename: (payload, signal) => this.callUnary("session.rename", payload, signal),
 		fork: (payload, signal) => this.callUnary("session.fork", payload, signal),
+		truncate: (payload, signal) => this.callUnary("session.truncate", payload, signal),
 		prompt: (payload, signal) => this.callUnary("session.prompt", payload, signal),
 		attachment: (payload, signal) => this.callUnary("session.attachment", payload, signal),
 		updateQueue: (payload, signal) => this.callUnary("session.updateQueue", payload, signal),