diff --git a/Sources/PokeTokenBar/Core/LocalUsageCache.swift b/Sources/PokeTokenBar/Core/LocalUsageCache.swift
index ee6da8fe..5e30f8cf 100644
--- a/Sources/PokeTokenBar/Core/LocalUsageCache.swift
+++ b/Sources/PokeTokenBar/Core/LocalUsageCache.swift
@@ -8,7 +8,25 @@ import Foundation
 actor LocalUsageCache {
     static let shared = LocalUsageCache()
 
-    private struct Blob: Codable { let mtime: Date; let size: Int; let entries: [LocalUsageReader.Entry] }
+    private struct Blob: Codable {
+        let mtime: Date
+        let size: Int
+        let entries: [LocalUsageReader.Entry]
+        /// Bytes already parsed, for append-only logs. Nil on blobs from older cache files.
+        let offset: Int?
+
+        init(mtime: Date, size: Int, entries: [LocalUsageReader.Entry], offset: Int? = nil) {
+            self.mtime = mtime; self.size = size; self.entries = entries; self.offset = offset
+        }
+
+        init(from decoder: Decoder) throws {
+            let c = try decoder.container(keyedBy: CodingKeys.self)
+            mtime = try c.decode(Date.self, forKey: .mtime)
+            size = try c.decode(Int.self, forKey: .size)
+            entries = try c.decode([LocalUsageReader.Entry].self, forKey: .entries)
+            offset = try c.decodeIfPresent(Int.self, forKey: .offset)
+        }
+    }
     private struct CodexBlob: Codable {
         let mtime: Date
         let size: Int
@@ -165,7 +183,10 @@ actor LocalUsageCache {
         let roots = claudeRoots ?? claudeRoot.map { [$0] } ?? LocalUsageReader.claudeProjectRoots
         var all: [LocalUsageReader.Entry] = []
         for root in roots {
-            all += collect(root: root, since: modifiedSince, cache: &claudeCache) {
+            all += collect(root: root, since: modifiedSince, cache: &claudeCache,
+                           incremental: { url, offset in
+                LocalUsageReader.parseClaudeFile(url, fromOffset: offset, fmt: fmt)
+            }) {
                 LocalUsageReader.parseClaudeFile($0, fmt: fmt)
             }
         }
@@ -266,8 +287,12 @@ actor LocalUsageCache {
 
     /// `include` 는 blob 캐시 조회 **전에** 평가된다 — 파일 밖 상태(옆 파일 등)에 의존하는 판정을
     /// 캐시에 굳히지 않기 위해서다.
+    /// `incremental` parses from a byte offset for append-only logs. When the cached blob has an
+    /// offset and the file grew, only the new tail is read and merged with the cached entries;
+    /// otherwise the file is parsed from the start and the resulting offset recorded.
     private func collect(root: URL, since: Date, cache: inout [String: Blob],
                          allowJSON: Bool = false, include: ((URL) -> Bool)? = nil,
+                         incremental: ((URL, Int) -> (entries: [LocalUsageReader.Entry], newOffset: Int)?)? = nil,
                          parse: (URL) -> [LocalUsageReader.Entry]?) -> [LocalUsageReader.Entry] {
         let fm = FileManager.default
         guard let en = fm.enumerator(
@@ -286,6 +311,21 @@ actor LocalUsageCache {
             let key = url.path
             if let blob = cache[key], blob.mtime == mtime, blob.size == size {
                 result.append(contentsOf: blob.entries)            // 변경 없음 → 재파싱 안 함
+            } else if let incremental {
+                let prior = cache[key]
+                // Tail-parse only when the file strictly grew; a same-size rewrite or a
+                // truncation is re-read from the start.
+                let from = prior.flatMap { p in size > p.size ? p.offset : nil } ?? 0
+                if let tail = incremental(url, from) {
+                    let entries = from > 0
+                        ? LocalUsageReader.dedupKeepMax((prior?.entries ?? []) + tail.entries)
+                        : tail.entries
+                    cache[key] = Blob(mtime: mtime, size: size, entries: entries, offset: tail.newOffset)
+                    dirty = true
+                    result.append(contentsOf: entries)
+                } else if let prior {
+                    result.append(contentsOf: prior.entries)
+                }
             } else if let entries = parse(url) {
                 cache[key] = Blob(mtime: mtime, size: size, entries: entries)
                 dirty = true
diff --git a/Sources/PokeTokenBar/Core/LocalUsageReader.swift b/Sources/PokeTokenBar/Core/LocalUsageReader.swift
index 51011ce0..598d7e43 100644
--- a/Sources/PokeTokenBar/Core/LocalUsageReader.swift
+++ b/Sources/PokeTokenBar/Core/LocalUsageReader.swift
@@ -368,17 +368,55 @@ enum LocalUsageReader {
 
     /// Claude 파일 하나를 파싱(파일 내 dedup). 캐시가 파일 단위로 호출.
     static func parseClaudeFile(_ url: URL, fmt: DateFormatter) -> [Entry] {
-        guard let text = try? String(contentsOf: url, encoding: .utf8) else { return [] }
+        parseClaudeFile(url, fromOffset: 0, fmt: fmt)?.entries ?? []
+    }
+
+    /// Parses only the bytes after `fromOffset`. Session logs are append-only and the active
+    /// one can be hundreds of MB, so the cache re-reads just the new tail each refresh instead
+    /// of the whole file. `newOffset` stops at the last complete line so a half-written record
+    /// is retried next time. Nil means the file could not be read.
+    static func parseClaudeFile(_ url: URL, fromOffset: Int, fmt: DateFormatter)
+        -> (entries: [Entry], newOffset: Int)?
+    {
+        guard let handle = try? FileHandle(forReadingFrom: url) else { return nil }
+        defer { try? handle.close() }
+        guard (try? handle.seek(toOffset: UInt64(fromOffset))) != nil,
+              let data = try? handle.readToEnd() else { return nil }
+        let parsed = parseClaudeData(data, fmt: fmt)
+        return (parsed.entries, fromOffset + parsed.consumed)
+    }
+
+    private static let usageNeedle = Data("\"usage\"".utf8)
+    private static let assistantNeedle = Data("\"assistant\"".utf8)
+
+    /// Byte-level line scan: no whole-file String materialization or per-line Substring
+    /// allocation. `consumed` is the byte count up to and including the last newline.
+    static func parseClaudeData(_ data: Data, fmt: DateFormatter) -> (entries: [Entry], consumed: Int) {
         var out: [Entry] = []
-        for line in text.split(separator: "\n", omittingEmptySubsequences: true) {
-            guard line.contains("\"usage\""), line.contains("\"assistant\"") else { continue }
+        var consumed = 0
+        var lineStart = data.startIndex
+        func parse(_ line: Data.SubSequence) {
+            guard !line.isEmpty,
+                  line.range(of: usageNeedle) != nil,
+                  line.range(of: assistantNeedle) != nil,
+                  let text = String(data: line, encoding: .utf8) else { return }
             // 라인마다 autoreleasepool — JSONSerialization 이 만드는 autoreleased NSDictionary/NSString 가
             // 수천 파일·수만 라인에 걸쳐 배출 없이 누적돼 콜드 파싱 피크를 키우던 것을 즉시 배출.
             autoreleasepool {
-                if let e = parseClaudeLine(String(line), fmt: fmt) { out.append(e) }
+                if let e = parseClaudeLine(text, fmt: fmt) { out.append(e) }
             }
         }
-        return dedupKeepMax(out)
+        while lineStart < data.endIndex {
+            guard let nl = data[lineStart...].firstIndex(of: 0x0A) else { break }
+            parse(data[lineStart..<nl])
+            consumed = nl - data.startIndex + 1
+            lineStart = nl + 1
+        }
+        // A trailing line with no newline is either a finished file's last record or a record
+        // still being written. Parse it now but leave it out of `consumed` so an incremental
+        // re-read picks it up again; dedup by id makes the repeat harmless.
+        if lineStart < data.endIndex { parse(data[lineStart...]) }
+        return (dedupKeepMax(out), consumed)
     }
 
     /// `modifiedSince` 이후 파일에서 Claude 사용 엔트리(전역 dedup) — 테스트/캐시 미사용 경로.
diff --git a/Sources/PokeTokenBar/Core/Models.swift b/Sources/PokeTokenBar/Core/Models.swift
index 5c445f00..e15ca119 100644
--- a/Sources/PokeTokenBar/Core/Models.swift
+++ b/Sources/PokeTokenBar/Core/Models.swift
@@ -566,23 +566,43 @@ struct ProviderSnapshot: Sendable, Identifiable {
 
 enum ISO8601Parser {
     /// resets_at 은 마이크로초("...034464+00:00") 또는 밀리초("....303Z") 형태 — 둘 다 처리.
-    /// ISO8601DateFormatter 는 non-Sendable 이라 호출마다 생성 (파싱 빈도 낮음).
     static func date(from string: String) -> Date? {
-        let fractional = ISO8601DateFormatter()
-        fractional.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
-        if let d = fractional.date(from: string) { return d }
-        // 소수점 자릿수가 3자리가 아니면 3자리로 절단 후 재시도
-        if let dotIndex = string.firstIndex(of: ".") {
-            let afterDot = string.index(after: dotIndex)
-            if let tzIndex = string[afterDot...].firstIndex(where: { $0 == "+" || $0 == "-" || $0 == "Z" }) {
-                let frac = String(string[afterDot..<tzIndex]).prefix(3)
-                let padded = String(frac).padding(toLength: 3, withPad: "0", startingAt: 0)
-                let rebuilt = String(string[..<dotIndex]) + "." + padded + String(string[tzIndex...])
-                if let d = fractional.date(from: rebuilt) { return d }
+        shared.date(from: string)
+    }
+
+    private static let shared = LockedParser()
+
+    /// This is also the per-line timestamp parser for every local session log, so it runs
+    /// millions of times on a large history. Allocating an `ISO8601DateFormatter` per call was
+    /// the dominant cost of a cold scan. The formatters are non-Sendable and callers (the
+    /// provider caches) run concurrently, so one shared pair is kept behind a lock.
+    private final class LockedParser: @unchecked Sendable {
+        private let lock = NSLock()
+        private let fractional: ISO8601DateFormatter
+        private let plain: ISO8601DateFormatter
+
+        init() {
+            fractional = ISO8601DateFormatter()
+            fractional.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+            plain = ISO8601DateFormatter()
+            plain.formatOptions = [.withInternetDateTime]
+        }
+
+        func date(from string: String) -> Date? {
+            lock.lock()
+            defer { lock.unlock() }
+            if let d = fractional.date(from: string) { return d }
+            // 소수점 자릿수가 3자리가 아니면 3자리로 절단 후 재시도
+            if let dotIndex = string.firstIndex(of: ".") {
+                let afterDot = string.index(after: dotIndex)
+                if let tzIndex = string[afterDot...].firstIndex(where: { $0 == "+" || $0 == "-" || $0 == "Z" }) {
+                    let frac = String(string[afterDot..<tzIndex]).prefix(3)
+                    let padded = String(frac).padding(toLength: 3, withPad: "0", startingAt: 0)
+                    let rebuilt = String(string[..<dotIndex]) + "." + padded + String(string[tzIndex...])
+                    if let d = fractional.date(from: rebuilt) { return d }
+                }
             }
+            return plain.date(from: string)
         }
-        let plain = ISO8601DateFormatter()
-        plain.formatOptions = [.withInternetDateTime]
-        return plain.date(from: string)
     }
 }
diff --git a/Tests/PokeTokenBarTests/LocalUsageCacheTests.swift b/Tests/PokeTokenBarTests/LocalUsageCacheTests.swift
index 4352eba4..c0251851 100644
--- a/Tests/PokeTokenBarTests/LocalUsageCacheTests.swift
+++ b/Tests/PokeTokenBarTests/LocalUsageCacheTests.swift
@@ -240,6 +240,86 @@ final class LocalUsageCacheTests: XCTestCase {
         XCTAssertEqual(second.map(\.output), [999])
     }
 
+    // MARK: Incremental (append-only) Claude parsing
+
+    private func writeRaw(_ name: String, _ text: String, mtime: Date) throws {
+        let url = root.appendingPathComponent(name)
+        try text.write(to: url, atomically: true, encoding: .utf8)
+        try FileManager.default.setAttributes([.modificationDate: mtime], ofItemAtPath: url.path)
+    }
+
+    /// The active session log is appended to constantly and can be hundreds of MB. When a
+    /// cached file has only grown, just the new tail is parsed. Proven the same way as the
+    /// unchanged-file test: the already-parsed prefix is rewritten in place at the same length
+    /// and must NOT be picked up, while the appended line must be.
+    func testAppendedFileParsesOnlyTheNewTail() async throws {
+        let t0 = Date(timeIntervalSince1970: floor(Date().timeIntervalSince1970) - 3600)
+        try writeRaw("a.jsonl", claudeLine(id: "1", output: 111) + "\n", mtime: t0)
+        let cache = makeCache()
+        let first = await cache.claudeEntries(modifiedSince: since)
+        XCTAssertEqual(first.map(\.output), [111])
+
+        try writeRaw("a.jsonl",
+                     claudeLine(id: "1", output: 222) + "\n" + claudeLine(id: "2", output: 333) + "\n",
+                     mtime: t0.addingTimeInterval(10))
+        let second = await cache.claudeEntries(modifiedSince: since)
+        XCTAssertEqual(Set(second.map(\.output)), [111, 333],
+                       "prefix must come from the cache (111, not 222); only the tail is parsed")
+    }
+
+    /// A shrunk file is not append-only growth: it is re-read from the start.
+    func testTruncatedFileIsReparsedFromStart() async throws {
+        let t0 = Date(timeIntervalSince1970: floor(Date().timeIntervalSince1970) - 3600)
+        try writeRaw("a.jsonl",
+                     claudeLine(id: "1", output: 111) + "\n" + claudeLine(id: "2", output: 222) + "\n",
+                     mtime: t0)
+        let cache = makeCache()
+        _ = await cache.claudeEntries(modifiedSince: since)
+
+        try writeRaw("a.jsonl", claudeLine(id: "3", output: 333) + "\n", mtime: t0.addingTimeInterval(10))
+        let second = await cache.claudeEntries(modifiedSince: since)
+        XCTAssertEqual(second.map(\.output), [333])
+    }
+
+    /// A record still being written (no trailing newline) is parsed if complete, but the offset
+    /// stops before it so the next refresh re-reads it once it is finished.
+    func testHalfWrittenTrailingLineIsRetriedOnNextRefresh() async throws {
+        let t0 = Date(timeIntervalSince1970: floor(Date().timeIntervalSince1970) - 3600)
+        let full = claudeLine(id: "2", output: 222)
+        let partial = String(full.prefix(full.count / 2))
+        try writeRaw("a.jsonl", claudeLine(id: "1", output: 111) + "\n" + partial, mtime: t0)
+        let cache = makeCache()
+        let first = await cache.claudeEntries(modifiedSince: since)
+        XCTAssertEqual(first.map(\.output), [111], "a truncated JSON line is not an entry")
+
+        try writeRaw("a.jsonl", claudeLine(id: "1", output: 111) + "\n" + full + "\n",
+                     mtime: t0.addingTimeInterval(10))
+        let second = await cache.claudeEntries(modifiedSince: since)
+        XCTAssertEqual(Set(second.map(\.output)), [111, 222])
+    }
+
+    /// Cache files written before `offset` existed still load; those blobs fall back to a full
+    /// parse the first time their file changes.
+    func testBlobWithoutOffsetStillDecodes() async throws {
+        let t0 = Date(timeIntervalSince1970: floor(Date().timeIntervalSince1970) - 3600)
+        try writeRaw("a.jsonl", claudeLine(id: "1", output: 111) + "\n", mtime: t0)
+        let cache = makeCache()
+        _ = await cache.claudeEntries(modifiedSince: since)   // first save is never debounced
+
+        // Strip the offset from every Claude blob on disk, as an older release would have written.
+        let raw = try Data(contentsOf: cacheFile)
+        let decompressed = (try? (raw as NSData).decompressed(using: .zlib) as Data) ?? raw
+        var json = try XCTUnwrap(JSONSerialization.jsonObject(with: decompressed) as? [String: Any])
+        var claude = try XCTUnwrap(json["claude"] as? [String: [String: Any]])
+        for (k, var blob) in claude { blob.removeValue(forKey: "offset"); claude[k] = blob }
+        json["claude"] = claude
+        try JSONSerialization.data(withJSONObject: json).write(to: cacheFile, options: .atomic)
+
+        let reloaded = makeCache()
+        let entries = await reloaded.claudeEntries(modifiedSince: since)
+        XCTAssertEqual(entries.map(\.output), [111])
+    }
+
     func testCodexCacheDropsForkedReplayBurst() async throws {
         try writeFile("rollout-child.jsonl", lines: forkedCodexLines())