diff --git a/src/storage/mysql.ts b/src/storage/mysql.ts
index b3005bb5f4..948cf6df75 100644
--- a/src/storage/mysql.ts
+++ b/src/storage/mysql.ts
@@ -1794,7 +1794,10 @@ export class MysqlBackend implements StorageBackend {
             if (c !== null) content[hash] = c;
         }
         mkdirSync(opts.archiveDir, { recursive: true });
-        appendFileSync(exportPath, JSON.stringify({ version: 1, sessionId, archivedAt: Date.now(), session: row, content, messages, mediaMaps, blocks, refs: refsRows, deadRefs, ccrEntries: ccr, sessionExt: ext, checkpoints }) + "\n");
+        // sqlite twin: appended as the last step inside the sweep
+        // transaction, so a failed sweep leaves no line for a session that
+        // is still active.
+        const archiveLine = JSON.stringify({ version: 1, sessionId, archivedAt: Date.now(), session: row, content, messages, mediaMaps, blocks, refs: refsRows, deadRefs, ccrEntries: ccr, sessionExt: ext, checkpoints }) + "\n";
         let contentRowsDeleted = 0;
         await this.tx(async (c) => {
             const rel: string[] = [];
@@ -1823,6 +1826,7 @@ export class MysqlBackend implements StorageBackend {
             // #4796 follow-up: scoped collection (see collectZeroRefIn) —
             // the reported count is what THIS archive collected.
             contentRowsDeleted = await this.collectZeroRefIn(c, rel);
+            appendFileSync(exportPath, archiveLine);
         });
         loggerLog("info", `[#3931] archived ${sessionId} → ${exportPath} (${contentRowsDeleted} content rows collected)`);
         return { ok: true, exportPath, contentRowsDeleted };
@@ -1842,11 +1846,12 @@ export class MysqlBackend implements StorageBackend {
             sessionExt: Record<string, unknown>[];
             checkpoints: Record<string, unknown>[];
         };
+        // sqlite twin: newest matching line wins.
         let lineNo = -1;
         let rawLines: string[] = [];
         try {
             rawLines = readFileSync(exportPath, "utf8").split("\n");
-            for (let i = 0; i < rawLines.length; i++) {
+            for (let i = rawLines.length - 1; i >= 0; i--) {
                 if (!rawLines[i].trim()) continue;
                 let candidate: { sessionId?: string };
                 try {
@@ -1929,11 +1934,15 @@ export class MysqlBackend implements StorageBackend {
             );
         });
         const kept: string[] = [];
+        let ownLines = 0;
         for (const l of rawLines) {
             if (!l.trim()) continue;
             try {
                 const head = JSON.parse(l) as { sessionId?: string };
-                if (head.sessionId === sessionId) continue;
+                if (head.sessionId === sessionId) {
+                    if (++ownLines === 2) loggerLog("warn", `[storage] archive log holds more than one line for session ${sessionId}: the newest was restored, older ones are dropped`);
+                    continue;
+                }
             } catch {
                 continue;
             }
diff --git a/src/storage/store.ts b/src/storage/store.ts
index 97c674f438..be95a5a3b9 100644
--- a/src/storage/store.ts
+++ b/src/storage/store.ts
@@ -2079,7 +2079,15 @@ export class UnifiedStore {
         // One JSONL line per archived session: append-only (cheap, crash =
         // at worst one truncated line which the reader skips), restore
         // rewrites the file without the restored session's lines.
-        appendFileSync(exportPath, JSON.stringify({ version: 1, sessionId, archivedAt: Date.now(), session: row, content, messages, mediaMaps, blocks, refs: refsRows, deadRefs, ccrEntries: ccr, sessionExt: ext, checkpoints }) + "\n");
+        // Serialized here, appended as the LAST step inside the sweep
+        // transaction below: a sweep that cannot start or fails midway
+        // (write lock held by another process, constraint error) must not
+        // leave a line behind for a session that is still active — that
+        // stale line would shadow the next successful archive on restore,
+        // and every failed attempt would add another full copy. The line
+        // still reaches the log before COMMIT, so a committed sweep always
+        // has its export.
+        const archiveLine = JSON.stringify({ version: 1, sessionId, archivedAt: Date.now(), session: row, content, messages, mediaMaps, blocks, refs: refsRows, deadRefs, ccrEntries: ccr, sessionExt: ext, checkpoints }) + "\n";
         // --- sweep the row bodies (refcount discipline; pinned checkpoints survive) ---
         let contentRowsDeleted = 0;
         this.db.transaction(() => {
@@ -2101,6 +2109,7 @@ export class UnifiedStore {
             // #4796 follow-up: scoped collection (see collectZeroRefIn) —
             // the reported count is what THIS archive collected.
             contentRowsDeleted = this.collectZeroRefIn(rel);
+            appendFileSync(exportPath, archiveLine);
         });
         loggerLog("info", `[#3901] archived ${sessionId} → ${exportPath} (${contentRowsDeleted} content rows collected)`);
         return { ok: true, exportPath, contentRowsDeleted };
@@ -2127,12 +2136,15 @@ export class UnifiedStore {
         };
         // Scan the JSONL for the session's line (skip malformed/truncated
         // trailing lines — an archive append interrupted by a crash must not
-        // block restoring OTHER sessions).
+        // block restoring OTHER sessions). Newest first: when the log holds
+        // more than one line for this session (an archive whose sweep never
+        // committed, then a later successful one), the last line appended is
+        // the state the session was actually archived in.
         let lineNo = -1;
         let rawLines: string[] = [];
         try {
             rawLines = readFileSync(exportPath, "utf8").split("\n");
-            for (let i = 0; i < rawLines.length; i++) {
+            for (let i = rawLines.length - 1; i >= 0; i--) {
                 if (!rawLines[i].trim()) continue;
                 let candidate: { sessionId?: string };
                 try {
@@ -2195,11 +2207,15 @@ export class UnifiedStore {
         // archive can theoretically appear more than once across reinstalls;
         // rewrite removes them all). Other sessions' lines are untouched.
         const kept: string[] = [];
+        let ownLines = 0;
         for (const l of rawLines) {
             if (!l.trim()) continue;
             try {
                 const head = JSON.parse(l) as { sessionId?: string };
-                if (head.sessionId === sessionId) continue;
+                if (head.sessionId === sessionId) {
+                    if (++ownLines === 2) loggerLog("warn", `[storage] archive log holds more than one line for session ${sessionId}: the newest was restored, older ones are dropped`);
+                    continue;
+                }
             } catch { continue; }
             kept.push(l);
         }
diff --git a/tests/storage-archive-failed-sweep.test.ts b/tests/storage-archive-failed-sweep.test.ts
new file mode 100644
index 0000000000..04a603f352
--- /dev/null
+++ b/tests/storage-archive-failed-sweep.test.ts
@@ -0,0 +1,215 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { planIngest, type LegacySessionLike } from "../src/storage/ingest.ts";
+import { UnifiedStore } from "../src/storage/store.ts";
+import { openSqliteDatabase } from "../src/storage/driver.ts";
+import { MysqlBackend, type Mysql2Module } from "../src/storage/mysql.ts";
+
+/**
+ * archiveSession used to append the session's export line to archive.jsonl
+ * BEFORE the sweep transaction ran. A sweep that then failed (write lock held
+ * by another process, constraint error) left the line behind while the
+ * session stayed active and kept growing. The next successful archive
+ * appended a second line; restoreArchived took the FIRST matching line (the
+ * stale one), then dropped every line of that session — the newer export,
+ * with everything said since the failed attempt, was gone.
+ *
+ * Pinned here, for both backends:
+ *  - a failed sweep leaves no line in the archive log;
+ *  - when the log does hold several lines for one session (logs written by
+ *    earlier versions), restore takes the newest.
+ */
+
+process.env.BILI_PERSIST_ZSTD = "0";
+process.env.BILI_STORAGE_UNIFIED = "full";
+process.env.BILI_CONFIG_FILE = path.join(
+    fs.mkdtempSync(path.join(os.tmpdir(), "bili-archive-failed-sweep-")),
+    "no-config.json",
+);
+
+interface CoreMsg { id: string; role: string; contentType: string; text: string }
+const mkMsg = (id: string, text: string): CoreMsg => ({ id, role: "user", contentType: "text", text });
+
+const mkSession = (id: string, msgs: CoreMsg[], lastSeen = 2_000): LegacySessionLike => ({
+    id,
+    createdAt: 1_000,
+    lastSeen,
+    meta: { protocol: "openai" },
+    stats: {},
+    metadata: {},
+    state: { blocks: [], nextBlockId: 1, nextRunId: 1, messageRefs: { byRaw: {}, byRef: {} } },
+    blockContents: new Map(),
+    lastMessages: msgs as unknown as LegacySessionLike["lastMessages"],
+    lastMessagesFolded: false,
+    pluginSnapshot: undefined,
+    contentStore: undefined,
+} as unknown as LegacySessionLike);
+
+const two = [mkMsg("m1", "one"), mkMsg("m2", "two")];
+const five = [...two, mkMsg("m3", "three"), mkMsg("m4", "four"), mkMsg("m5", "five")];
+
+const logLines = (file: string): string[] => (fs.existsSync(file) ? fs.readFileSync(file, "utf8").split("\n").filter((l) => l.trim() !== "") : []);
+const linesOf = (file: string, sessionId: string): number => logLines(file).filter((l) => (JSON.parse(l) as { sessionId?: string }).sessionId === sessionId).length;
+
+test("sqlite: a failed sweep leaves no archive line; the later archive restores in full", () => {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "archive-failed-sweep-"));
+    const dbPath = path.join(dir, "index.db");
+    const archiveDir = path.join(dir, "archive");
+    const file = path.join(archiveDir, UnifiedStore.ARCHIVE_LOG_NAME);
+    const store = UnifiedStore.open(dbPath);
+    try {
+        store.ingestSession(planIngest(mkSession("sess-A", two)));
+        store.ingestSession(planIngest(mkSession("sess-B", [mkMsg("b1", "other session")])));
+        assert.ok(store.archiveSession("sess-B", { archiveDir }).ok);
+        const otherLine = logLines(file)[0];
+
+        // Make the sweep fail at its last statement, after the row deletes.
+        const side = openSqliteDatabase(dbPath).db;
+        side.exec("CREATE TRIGGER fail_archive BEFORE UPDATE OF status ON sessions WHEN NEW.status = 'archived' BEGIN SELECT RAISE(ABORT, 'injected sweep failure'); END");
+        assert.throws(() => store.archiveSession("sess-A", { archiveDir }), /injected sweep failure/);
+        side.exec("DROP TRIGGER fail_archive");
+        side.close();
+
+        assert.equal((store.getSessionRow("sess-A") as unknown as { status?: string }).status, "active", "the session is still active");
+        assert.equal(store.getMessages("sess-A").length, 2, "the sweep rolled back");
+        assert.equal(linesOf(file, "sess-A"), 0, "a failed sweep leaves no line behind");
+        assert.deepEqual(logLines(file), [otherLine], "the other session's line is untouched");
+
+        // The session lives on, then archives for real.
+        store.ingestSession(planIngest(mkSession("sess-A", five, 3_000)));
+        assert.equal(store.getMessages("sess-A").length, 5);
+        assert.ok(store.archiveSession("sess-A", { archiveDir }).ok);
+        assert.equal(linesOf(file, "sess-A"), 1, "exactly one line for the one committed archive");
+        assert.equal(store.getMessages("sess-A").length, 0);
+
+        assert.deepEqual(store.restoreArchived("sess-A", { archiveDir }), { ok: true });
+        assert.deepEqual(store.getMessages("sess-A").map((m) => m.msgId), ["m1", "m2", "m3", "m4", "m5"], "the messages added after the failed attempt are restored");
+        assert.deepEqual(logLines(file), [otherLine], "only the restored session's line is removed");
+    } finally {
+        store.close();
+        fs.rmSync(dir, { recursive: true, force: true });
+    }
+});
+
+test("sqlite: with several lines for one session in the log, restore takes the newest", () => {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "archive-newest-line-"));
+    const archiveDir = path.join(dir, "archive");
+    const file = path.join(archiveDir, UnifiedStore.ARCHIVE_LOG_NAME);
+    const store = UnifiedStore.open(path.join(dir, "index.db"));
+    try {
+        // A log as earlier versions could leave it: the stale line of an
+        // archive whose sweep never committed, then the real one.
+        store.ingestSession(planIngest(mkSession("sess-A", two)));
+        assert.ok(store.archiveSession("sess-A", { archiveDir }).ok);
+        const staleLine = logLines(file)[0];
+        assert.deepEqual(store.restoreArchived("sess-A", { archiveDir }), { ok: true });
+        store.ingestSession(planIngest(mkSession("sess-A", five, 3_000)));
+        store.ingestSession(planIngest(mkSession("sess-B", [mkMsg("b1", "other session")])));
+        assert.ok(store.archiveSession("sess-B", { archiveDir }).ok);
+        assert.ok(store.archiveSession("sess-A", { archiveDir }).ok);
+        const [otherLine, newestLine] = logLines(file);
+        fs.writeFileSync(file, [staleLine, otherLine, newestLine].join("\n") + "\n");
+
+        assert.deepEqual(store.restoreArchived("sess-A", { archiveDir }), { ok: true });
+        assert.deepEqual(store.getMessages("sess-A").map((m) => m.msgId), ["m1", "m2", "m3", "m4", "m5"], "restored from the newest line, not the first");
+        assert.deepEqual(logLines(file), [otherLine], "every line of the restored session is removed, nothing else");
+    } finally {
+        store.close();
+        fs.rmSync(dir, { recursive: true, force: true });
+    }
+});
+
+/** A mysql2 stand-in: the pool answers the archive reads from fixed rows,
+ *  the transaction connection records what it is asked and can be told to
+ *  fail one statement. */
+function fakeMysql(opts: { messages: Record<string, unknown>[]; failOn?: () => RegExp | null }) {
+    const calls: Array<{ sql: string; params: unknown[] }> = [];
+    const sessionRow = {
+        session_id: "sess-A", status: "active", namespace: "proxy", created_at: 1_000, updated_at: 2_000, next_block_id: 1, next_run_id: 1,
+        meta_json: "{}", stats_json: "{}", metadata_json: "{}", kstate_json: "{}", tail_json: "{}",
+    };
+    const pool = {
+        async query(sql: string, params: unknown[] = []) {
+            if (/FROM sessions WHERE session_id = \?/.test(sql)) return [[sessionRow]];
+            if (/FROM messages WHERE session_id = \?/.test(sql)) return [opts.messages];
+            if (/SELECT bytes FROM content WHERE hash = \?/.test(sql)) return [[{ bytes: `body of ${String(params[0])}` }]];
+            return [[]];
+        },
+        async getConnection() {
+            return {
+                async beginTransaction() { calls.push({ sql: "begin", params: [] }); },
+                async commit() { calls.push({ sql: "commit", params: [] }); },
+                async rollback() { calls.push({ sql: "rollback", params: [] }); },
+                release() { /* nothing to free */ },
+                async query(sql: string, params: unknown[] = []) {
+                    calls.push({ sql, params });
+                    const fail = opts.failOn?.() ?? null;
+                    if (fail !== null && fail.test(sql)) throw new Error("injected sweep failure");
+                    return [{ affectedRows: 1 }];
+                },
+            };
+        },
+    };
+    const driver = { createPool: () => pool } as unknown as Mysql2Module;
+    return { backend: new MysqlBackend("unused", driver), calls };
+}
+
+test("mysql: a failed sweep leaves no archive line; a committed one writes it before COMMIT", async () => {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "archive-failed-sweep-mysql-"));
+    const archiveDir = path.join(dir, "archive");
+    const file = path.join(archiveDir, MysqlBackend.ARCHIVE_LOG_NAME);
+    try {
+        let fail: RegExp | null = /^UPDATE sessions SET status = 'archived'/;
+        const { backend, calls } = fakeMysql({
+            messages: [{ seq: 1, msg_id: "m1", ref: null, ref_num: null, role: "user", kind: "message", content_hash: "h1", identity: "i1", retired_at: null, ts: null }],
+            failOn: () => fail,
+        });
+        await assert.rejects(backend.archiveSession("sess-A", { archiveDir }), /injected sweep failure/);
+        assert.equal(calls[calls.length - 1].sql, "rollback");
+        assert.equal(linesOf(file, "sess-A"), 0, "a failed sweep leaves no line behind");
+
+        fail = null;
+        calls.length = 0;
+        let linesAtCommit = -1;
+        const realPush = calls.push.bind(calls);
+        calls.push = (...items) => {
+            for (const it of items) if (it.sql === "commit") linesAtCommit = linesOf(file, "sess-A");
+            return realPush(...items);
+        };
+        assert.equal((await backend.archiveSession("sess-A", { archiveDir })).ok, true);
+        assert.equal(linesAtCommit, 1, "the export line is on disk before the sweep commits");
+        assert.equal(linesOf(file, "sess-A"), 1);
+    } finally {
+        fs.rmSync(dir, { recursive: true, force: true });
+    }
+});
+
+test("mysql: with several lines for one session in the log, restore takes the newest", async () => {
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), "archive-newest-line-mysql-"));
+    const archiveDir = path.join(dir, "archive");
+    const file = path.join(archiveDir, MysqlBackend.ARCHIVE_LOG_NAME);
+    const source = UnifiedStore.open(path.join(dir, "source.db"));
+    try {
+        // Build the two exports with the sqlite twin (same line format).
+        source.ingestSession(planIngest(mkSession("sess-A", two)));
+        assert.ok(source.archiveSession("sess-A", { archiveDir }).ok);
+        const staleLine = logLines(file)[0];
+        assert.deepEqual(source.restoreArchived("sess-A", { archiveDir }), { ok: true });
+        source.ingestSession(planIngest(mkSession("sess-A", five, 3_000)));
+        assert.ok(source.archiveSession("sess-A", { archiveDir }).ok);
+        const newestLine = logLines(file)[0];
+        fs.writeFileSync(file, [staleLine, newestLine].join("\n") + "\n");
+
+        const { backend, calls } = fakeMysql({ messages: [] });
+        assert.deepEqual(await backend.restoreArchived("sess-A", { archiveDir }), { ok: true });
+        const restored = calls.filter((c) => c.sql.startsWith("INSERT INTO messages")).map((c) => c.params[2]);
+        assert.deepEqual(restored, ["m1", "m2", "m3", "m4", "m5"], "restored from the newest line, not the first");
+        assert.deepEqual(logLines(file), [], "every line of the restored session is removed");
+    } finally {
+        source.close();
+        fs.rmSync(dir, { recursive: true, force: true });
+    }
+});