fix: retain incomplete JSONL tails

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## 0.43.1 — Unreleased
 
+### Fixed
+- Cost usage: retain an incomplete trailing JSONL record for the next scan so active Codex, Claude, and Pi sessions do not lose newly appended token usage.
+
 ## 0.43.0 — 2026-07-14
 
 ### Added

Sources/CodexBarCore/Vendored/CostUsage/CostUsageJsonl.swift

@@ -47,6 +47,7 @@ enum CostUsageJsonl {
         var lineBytes = 0
         var truncated = false
         var bytesRead: Int64 = 0
+        var committedOffset = startOffset
 
         func appendSegment(_ bytes: UnsafePointer<UInt8>, count: Int) {
             guard count > 0 else { return }
@@ -71,17 +72,26 @@ enum CostUsageJsonl {
             truncated = false
         }
 
+        func hasCompleteJSONTail() -> Bool {
+            guard !truncated, lineBytes == current.count else { return false }
+            return (try? JSONSerialization.jsonObject(with: current, options: [.fragmentsAllowed])) != nil
+        }
+
         while true {
             try checkCancellation?()
             let reachedEOF = try autoreleasepool {
                 let chunk = try handle.read(upToCount: 256 * 1024) ?? Data()
                 if chunk.isEmpty {
-                    flushLine()
+                    if hasCompleteJSONTail() {
+                        flushLine()
+                        committedOffset = startOffset + bytesRead
+                    }
                     return true
                 }
 
                 try checkCancellation?()
                 bytesRead += Int64(chunk.count)
+                let chunkStartOffset = startOffset + bytesRead - Int64(chunk.count)
                 chunk.withUnsafeBytes { rawBuffer in
                     guard let base = rawBuffer.bindMemory(to: UInt8.self).baseAddress else { return }
                     var segmentStart = 0
@@ -90,6 +100,7 @@ enum CostUsageJsonl {
                         if base[index] == 0x0A {
                             appendSegment(base.advanced(by: segmentStart), count: index - segmentStart)
                             flushLine()
+                            committedOffset = chunkStartOffset + Int64(index + 1)
                             segmentStart = index + 1
                         }
                         index += 1
@@ -104,6 +115,6 @@ enum CostUsageJsonl {
             try checkCancellation?()
         }
 
-        return startOffset + bytesRead
+        return committedOffset
     }
 }

Tests/CodexBarTests/CostUsageJsonlScannerTests.swift

@@ -58,6 +58,69 @@ struct CostUsageJsonlScannerTests {
         #expect(scanned[1].wasTruncated == true)
     }
 
+    @Test
+    func `jsonl scanner retries an incomplete final record after append`() throws {
+        let root = try self.makeTemporaryRoot()
+        defer { try? FileManager.default.removeItem(at: root) }
+
+        let fileURL = root.appendingPathComponent("appending.jsonl", isDirectory: false)
+        let initial = #"{"type":"message","id":"partial"#
+        try initial.write(to: fileURL, atomically: true, encoding: .utf8)
+
+        var firstPass: [String] = []
+        let resumeOffset = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            maxLineBytes: 1024,
+            prefixBytes: 1024)
+        { line in
+            firstPass.append(String(decoding: line.bytes, as: UTF8.self))
+        }
+
+        #expect(firstPass.isEmpty)
+        #expect(resumeOffset == 0)
+
+        let completion = #""}"# + "\n"
+        let handle = try FileHandle(forWritingTo: fileURL)
+        defer { try? handle.close() }
+        try handle.seekToEnd()
+        try handle.write(contentsOf: Data(completion.utf8))
+
+        var secondPass: [String] = []
+        let endOffset = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            offset: resumeOffset,
+            maxLineBytes: 1024,
+            prefixBytes: 1024)
+        { line in
+            secondPass.append(String(decoding: line.bytes, as: UTF8.self))
+        }
+
+        #expect(secondPass == [initial + String(completion.dropLast())])
+        #expect(endOffset == Int64(Data((initial + completion).utf8).count))
+    }
+
+    @Test
+    func `jsonl scanner accepts a complete final record without newline`() throws {
+        let root = try self.makeTemporaryRoot()
+        defer { try? FileManager.default.removeItem(at: root) }
+
+        let fileURL = root.appendingPathComponent("final-record.jsonl", isDirectory: false)
+        let record = #"{"type":"message","id":"complete"}"#
+        try record.write(to: fileURL, atomically: true, encoding: .utf8)
+
+        var scanned: [String] = []
+        let endOffset = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            maxLineBytes: 1024,
+            prefixBytes: 1024)
+        { line in
+            scanned.append(String(decoding: line.bytes, as: UTF8.self))
+        }
+
+        #expect(scanned == [record])
+        #expect(endOffset == Int64(Data(record.utf8).count))
+    }
+
     private func makeTemporaryRoot() throws -> URL {
         let root = FileManager.default.temporaryDirectory.appendingPathComponent(
             "codexbar-cost-usage-jsonl-\(UUID().uuidString)",