diff --git a/Sources/CodexBarCore/Vendored/CostUsage/CostUsageJsonl.swift b/Sources/CodexBarCore/Vendored/CostUsage/CostUsageJsonl.swift
index c5132737a..5105f9759 100644
--- a/Sources/CodexBarCore/Vendored/CostUsage/CostUsageJsonl.swift
+++ b/Sources/CodexBarCore/Vendored/CostUsage/CostUsageJsonl.swift
@@ -23,15 +23,24 @@ enum CostUsageJsonl {
             try handle.seek(toOffset: UInt64(startOffset))
         }
 
-        var buffer = Data()
-        buffer.reserveCapacity(64 * 1024)
-
         var current = Data()
         current.reserveCapacity(4 * 1024)
         var lineBytes = 0
         var truncated = false
         var bytesRead: Int64 = 0
 
+        func appendSegment(_ segment: Data.SubSequence) {
+            guard !segment.isEmpty else { return }
+            lineBytes += segment.count
+            guard !truncated else { return }
+            if lineBytes > maxLineBytes || lineBytes > prefixBytes {
+                truncated = true
+                current.removeAll(keepingCapacity: true)
+                return
+            }
+            current.append(contentsOf: segment)
+        }
+
         func flushLine() {
             guard lineBytes > 0 else { return }
             let line = Line(bytes: current, wasTruncated: truncated)
@@ -49,23 +58,14 @@ enum CostUsageJsonl {
             }
 
             bytesRead += Int64(chunk.count)
-            buffer.append(chunk)
-
-            while true {
-                guard let nl = buffer.firstIndex(of: 0x0A) else { break }
-                let linePart = buffer[..<nl]
-                buffer.removeSubrange(...nl)
-
-                lineBytes += linePart.count
-                if !truncated {
-                    if lineBytes > maxLineBytes || lineBytes > prefixBytes {
-                        truncated = true
-                        current.removeAll(keepingCapacity: true)
-                    } else {
-                        current.append(contentsOf: linePart)
-                    }
-                }
+            var segmentStart = chunk.startIndex
+            while let nl = chunk[segmentStart...].firstIndex(of: 0x0A) {
+                appendSegment(chunk[segmentStart..<nl])
                 flushLine()
+                segmentStart = chunk.index(after: nl)
+            }
+            if segmentStart < chunk.endIndex {
+                appendSegment(chunk[segmentStart..<chunk.endIndex])
             }
         }
 
diff --git a/Tests/CodexBarTests/CostUsageJsonlPerformanceTests.swift b/Tests/CodexBarTests/CostUsageJsonlPerformanceTests.swift
new file mode 100644
index 000000000..7e08ae0fe
--- /dev/null
+++ b/Tests/CodexBarTests/CostUsageJsonlPerformanceTests.swift
@@ -0,0 +1,204 @@
+import Foundation
+import Testing
+@testable import CodexBarCore
+
+@Suite(.serialized)
+struct CostUsageJsonlPerformanceTests {
+    @Test
+    func scannerBenchmarkBeatsFrontBufferBaseline() throws {
+        let root = FileManager.default.temporaryDirectory.appendingPathComponent(
+            "codexbar-cost-usage-bench-\(UUID().uuidString)",
+            isDirectory: true)
+        try FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: root) }
+
+        let fileURL = root.appendingPathComponent("scanner-benchmark.jsonl", isDirectory: false)
+        let lineCount = 20000
+        let line = #"{"type":"assistant","message":{"usage":{"input_tokens":1,"output_tokens":2}}}"#
+        let fixture = makeBenchmarkFixture(line: line, lineCount: lineCount)
+        try fixture.write(to: fileURL)
+
+        let maxLineBytes = 8192
+        let prefixBytes = 8192
+
+        let currentSummary = try summarizeScan(
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: CostUsageJsonl.scan)
+        let baselineSummary = try summarizeScan(
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: scanWithFrontBufferBaseline)
+
+        #expect(currentSummary == baselineSummary)
+        #expect(currentSummary.lineCount == lineCount)
+        #expect(currentSummary.truncatedCount == 0)
+
+        // Warm up both code paths before timing.
+        _ = try summarizeScan(
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: CostUsageJsonl.scan)
+        _ = try summarizeScan(
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: scanWithFrontBufferBaseline)
+
+        let currentFastest = try fastestScanDurationNanoseconds(
+            runs: 3,
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: CostUsageJsonl.scan)
+        let baselineFastest = try fastestScanDurationNanoseconds(
+            runs: 3,
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: scanWithFrontBufferBaseline)
+
+        let speedup = Double(baselineFastest) / Double(currentFastest)
+        #expect(speedup >= 5.0)
+    }
+}
+
+private struct JsonlScanSummary: Equatable {
+    let lineCount: Int
+    let truncatedCount: Int
+    let payloadByteCount: Int
+    let endOffset: Int64
+}
+
+private typealias JsonlScanner = (
+    _ fileURL: URL,
+    _ offset: Int64,
+    _ maxLineBytes: Int,
+    _ prefixBytes: Int,
+    _ onLine: (CostUsageJsonl.Line) -> Void) throws -> Int64
+
+private func makeBenchmarkFixture(line: String, lineCount: Int) -> Data {
+    let lineBytes = Data(line.utf8)
+    var data = Data()
+    data.reserveCapacity((lineBytes.count + 1) * lineCount)
+    for _ in 0..<lineCount {
+        data.append(lineBytes)
+        data.append(0x0A)
+    }
+    return data
+}
+
+private func summarizeScan(
+    fileURL: URL,
+    maxLineBytes: Int,
+    prefixBytes: Int,
+    scanner: JsonlScanner) throws -> JsonlScanSummary
+{
+    var lineCount = 0
+    var truncatedCount = 0
+    var payloadByteCount = 0
+
+    let endOffset = try scanner(fileURL, 0, maxLineBytes, prefixBytes) { line in
+        lineCount += 1
+        payloadByteCount += line.bytes.count
+        if line.wasTruncated {
+            truncatedCount += 1
+        }
+    }
+
+    return JsonlScanSummary(
+        lineCount: lineCount,
+        truncatedCount: truncatedCount,
+        payloadByteCount: payloadByteCount,
+        endOffset: endOffset)
+}
+
+private func fastestScanDurationNanoseconds(
+    runs: Int,
+    fileURL: URL,
+    maxLineBytes: Int,
+    prefixBytes: Int,
+    scanner: JsonlScanner) throws -> UInt64
+{
+    var fastest = UInt64.max
+    for _ in 0..<runs {
+        let startedAt = DispatchTime.now().uptimeNanoseconds
+        _ = try summarizeScan(
+            fileURL: fileURL,
+            maxLineBytes: maxLineBytes,
+            prefixBytes: prefixBytes,
+            scanner: scanner)
+        let elapsed = DispatchTime.now().uptimeNanoseconds - startedAt
+        fastest = min(fastest, elapsed)
+    }
+    return fastest
+}
+
+@discardableResult
+private func scanWithFrontBufferBaseline(
+    fileURL: URL,
+    offset: Int64 = 0,
+    maxLineBytes: Int,
+    prefixBytes: Int,
+    onLine: (CostUsageJsonl.Line) -> Void) throws
+    -> Int64
+{
+    let handle = try FileHandle(forReadingFrom: fileURL)
+    defer { try? handle.close() }
+
+    let startOffset = max(0, offset)
+    if startOffset > 0 {
+        try handle.seek(toOffset: UInt64(startOffset))
+    }
+
+    var buffer = Data()
+    buffer.reserveCapacity(64 * 1024)
+
+    var current = Data()
+    current.reserveCapacity(4 * 1024)
+    var lineBytes = 0
+    var truncated = false
+    var bytesRead: Int64 = 0
+
+    func flushLine() {
+        guard lineBytes > 0 else { return }
+        onLine(.init(bytes: current, wasTruncated: truncated))
+        current.removeAll(keepingCapacity: true)
+        lineBytes = 0
+        truncated = false
+    }
+
+    while true {
+        let chunk = try handle.read(upToCount: 256 * 1024) ?? Data()
+        if chunk.isEmpty {
+            flushLine()
+            break
+        }
+
+        bytesRead += Int64(chunk.count)
+        buffer.append(chunk)
+
+        while true {
+            guard let nl = buffer.firstIndex(of: 0x0A) else { break }
+            let linePart = buffer[..<nl]
+            buffer.removeSubrange(...nl)
+
+            lineBytes += linePart.count
+            if !truncated {
+                if lineBytes > maxLineBytes || lineBytes > prefixBytes {
+                    truncated = true
+                    current.removeAll(keepingCapacity: true)
+                } else {
+                    current.append(contentsOf: linePart)
+                }
+            }
+
+            flushLine()
+        }
+    }
+
+    return startOffset + bytesRead
+}
diff --git a/Tests/CodexBarTests/CostUsageScannerTests.swift b/Tests/CodexBarTests/CostUsageScannerTests.swift
index 98985bd6a..ecad8208d 100644
--- a/Tests/CodexBarTests/CostUsageScannerTests.swift
+++ b/Tests/CodexBarTests/CostUsageScannerTests.swift
@@ -838,6 +838,60 @@ struct CostUsageScannerTests {
         #expect(report.data[0].outputTokens == 15)
         #expect(report.data[0].totalTokens == 45)
     }
+
+    @Test
+    func jsonlScannerHandlesLinesAcrossReadChunks() throws {
+        let env = try CostUsageTestEnvironment()
+        defer { env.cleanup() }
+
+        let fileURL = env.root.appendingPathComponent("large-lines.jsonl", isDirectory: false)
+        let largeLine = String(repeating: "x", count: 300_000)
+        let contents = "\(largeLine)\nsmall\n"
+        try contents.write(to: fileURL, atomically: true, encoding: .utf8)
+
+        var scanned: [(count: Int, truncated: Bool)] = []
+        let endOffset = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            maxLineBytes: 400_000,
+            prefixBytes: 400_000)
+        { line in
+            scanned.append((line.bytes.count, line.wasTruncated))
+        }
+
+        #expect(endOffset == Int64(Data(contents.utf8).count))
+        #expect(scanned.count == 2)
+        #expect(scanned[0].count == 300_000)
+        #expect(scanned[0].truncated == false)
+        #expect(scanned[1].count == 5)
+        #expect(scanned[1].truncated == false)
+    }
+
+    @Test
+    func jsonlScannerMarksPrefixLimitedLinesAsTruncated() throws {
+        let env = try CostUsageTestEnvironment()
+        defer { env.cleanup() }
+
+        let fileURL = env.root.appendingPathComponent("truncated-lines.jsonl", isDirectory: false)
+        let shortLine = "ok"
+        let longLine = String(repeating: "a", count: 2000)
+        let contents = "\(shortLine)\n\(longLine)\n"
+        try contents.write(to: fileURL, atomically: true, encoding: .utf8)
+
+        var scanned: [CostUsageJsonl.Line] = []
+        _ = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            maxLineBytes: 10000,
+            prefixBytes: 64)
+        { line in
+            scanned.append(line)
+        }
+
+        #expect(scanned.count == 2)
+        #expect(String(data: scanned[0].bytes, encoding: .utf8) == "ok")
+        #expect(scanned[0].wasTruncated == false)
+        #expect(scanned[1].bytes.isEmpty)
+        #expect(scanned[1].wasTruncated == true)
+    }
 }
 
 private struct CostUsageTestEnvironment {