diff --git a/Sources/Writing/NextSentence/NextSentenceClaude.swift b/Sources/Writing/NextSentence/NextSentenceClaude.swift
index 791531db0..fff4e12a6 100644
--- a/Sources/Writing/NextSentence/NextSentenceClaude.swift
+++ b/Sources/Writing/NextSentence/NextSentenceClaude.swift
@@ -42,8 +42,15 @@ final class NextSentenceClaude: @unchecked Sendable {
     }
 
     private let pool: NextSentenceWarmPool<Kind, NextSentenceLineProcess>
+    private let scheduleTimeout: (TimeInterval, NextSentenceCallbackBox) -> Void
 
-    init() {
+    init(
+        spawn: ((Kind, String?) -> NextSentenceLineProcess?)? = nil,
+        background: @escaping (@escaping @Sendable () -> Void) -> Void = { DispatchQueue.global(qos: .utility).async(execute: $0) },
+        scheduleTimeout: @escaping (TimeInterval, NextSentenceCallbackBox) -> Void = { delay, callback in
+            DispatchQueue.global().asyncAfter(deadline: .now() + delay) { callback.call() }
+        }
+    ) {
         var json: [Kind: String] = [:]
         for kind in Kind.allCases {
             guard let schema = kind.schema,
@@ -51,7 +58,11 @@ final class NextSentenceClaude: @unchecked Sendable {
             json[kind] = String(decoding: data, as: UTF8.self)
         }
         let schemas = json
-        pool = NextSentenceWarmPool(spawn: { Self.spawn($0, schema: schemas[$0]) })
+        self.scheduleTimeout = scheduleTimeout
+        pool = NextSentenceWarmPool(spawn: { kind in
+            if let spawn { return spawn(kind, schemas[kind]) }
+            return Self.spawn(kind, schema: schemas[kind])
+        }, background: background)
     }
 
     static func binary(userDefaults: UserDefaults = .standard) -> URL? {
@@ -79,9 +90,11 @@ final class NextSentenceClaude: @unchecked Sendable {
         let finish: (Result<String, Error>) -> Void = { result in
             guard once.claim() else { return }
             process.onLine(nil)
+            process.onOutputEnd(nil)
             self.pool.finished(process)
             completion(result)
         }
+        let ended = { finish(.failure(Failure.failed("Claude stopped before returning an answer."))) }
         process.onLine { line in
             switch line["type"] as? String {
             case "stream_event":
@@ -99,7 +112,9 @@ final class NextSentenceClaude: @unchecked Sendable {
             default: break
             }
         }
-        send(prompt: prompt, image: image, to: process, timeout: 60, onTimeout: { finish(.failure(Failure.failed("Claude took too long."))) })
+        process.onOutputEnd(ended)
+        send(prompt: prompt, image: image, to: process, timeout: 60, onSendFailure: ended,
+             onTimeout: { finish(.failure(Failure.failed("Claude took too long."))) })
     }
 
     /// A JSON answer matching `kind`'s schema. `completion` runs on a background queue.
@@ -110,9 +125,11 @@ final class NextSentenceClaude: @unchecked Sendable {
         let finish: (Result<[String: Any], Error>) -> Void = { result in
             guard once.claim() else { return }
             process.onLine(nil)
+            process.onOutputEnd(nil)
             self.pool.finished(process)
             completion(result)
         }
+        let ended = { finish(.failure(Failure.failed("Claude stopped before returning an answer."))) }
         process.onLine { line in
             // Structured output arrives as a StructuredOutput tool call, then a result line.
             if line["type"] as? String == "assistant",
@@ -127,7 +144,9 @@ final class NextSentenceClaude: @unchecked Sendable {
                 finish(.failure(Failure.failed("Claude didn't return an answer.")))
             }
         }
-        send(prompt: prompt, image: image, to: process, timeout: timeout, onTimeout: { finish(.failure(Failure.failed("Claude took too long."))) })
+        process.onOutputEnd(ended)
+        send(prompt: prompt, image: image, to: process, timeout: timeout, onSendFailure: ended,
+             onTimeout: { finish(.failure(Failure.failed("Claude took too long."))) })
     }
 
     // MARK: Processes
@@ -150,7 +169,8 @@ final class NextSentenceClaude: @unchecked Sendable {
         return process
     }
 
-    private func send(prompt: String, image: Data?, to process: NextSentenceLineProcess, timeout: TimeInterval, onTimeout: @escaping () -> Void) {
+    private func send(prompt: String, image: Data?, to process: NextSentenceLineProcess, timeout: TimeInterval,
+                      onSendFailure: () -> Void, onTimeout: @escaping () -> Void) {
         var content: [[String: Any]] = []
         if let image {
             content.append(["type": "image", "source": ["type": "base64", "media_type": "image/jpeg", "data": image.base64EncodedString()]])
@@ -159,10 +179,10 @@ final class NextSentenceClaude: @unchecked Sendable {
         do {
             try process.send(["type": "user", "message": ["role": "user", "content": content]])
         } catch {
-            return onTimeout()
+            return onSendFailure()
         }
         let timeoutBox = NextSentenceCallbackBox(onTimeout)
-        DispatchQueue.global().asyncAfter(deadline: .now() + timeout) { timeoutBox.call() }
+        scheduleTimeout(timeout, timeoutBox)
     }
 
     private static func failure(_ line: [String: Any], _ process: NextSentenceLineProcess) -> Failure {
diff --git a/Sources/Writing/NextSentence/NextSentenceLineProcess.swift b/Sources/Writing/NextSentence/NextSentenceLineProcess.swift
index 8ee9af798..e9ac1311d 100644
--- a/Sources/Writing/NextSentence/NextSentenceLineProcess.swift
+++ b/Sources/Writing/NextSentence/NextSentenceLineProcess.swift
@@ -13,6 +13,12 @@ final class NextSentenceLineProcess: @unchecked Sendable {
     private var buffer = Data()
     private var errorTail = Data()
     private var lineHandler: (([String: Any]) -> Void)?
+    // The following state belongs to queue, alongside the stdout buffer.
+    private var outputEnded = false
+    private var endHandler: EndCallback?
+    private var endWaiters: [CheckedContinuation<Void, Never>] = []
+    private var exited = false
+    private var exitWaiters: [CheckedContinuation<Void, Never>] = []
 
     init(executable: URL, arguments: [String], environment: [String: String] = [:], workingDirectory: URL? = nil, label: String) {
         queue = DispatchQueue(label: "transcripted.next-sentence.\(label)")
@@ -28,8 +34,18 @@ final class NextSentenceLineProcess: @unchecked Sendable {
     var isRunning: Bool { process.isRunning }
 
     func start() throws {
+        process.terminationHandler = { [weak self] _ in
+            guard let self else { return }
+            self.queue.async {
+                self.exited = true
+                let waiters = self.exitWaiters
+                self.exitWaiters.removeAll()
+                for waiter in waiters { waiter.resume() }
+            }
+        }
         output.fileHandleForReading.readabilityHandler = { [weak self] handle in
             let data = handle.availableData
+            if data.isEmpty { handle.readabilityHandler = nil }
             guard let self else { return }
             self.queue.async { self.receive(data) }
         }
@@ -49,6 +65,37 @@ final class NextSentenceLineProcess: @unchecked Sendable {
         lock.lock(); lineHandler = handler; lock.unlock()
     }
 
+    /// Runs after stdout's final JSON frame, including when the child ended
+    /// before this handler was installed. No child output is included.
+    func onOutputEnd(_ handler: (() -> Void)?) {
+        let callback = handler.map(EndCallback.init)
+        queue.async {
+            self.endHandler = callback
+            if self.outputEnded { callback?.call() }
+        }
+    }
+
+    /// Waits until buffered output and its end callback have been delivered.
+    func waitForOutputEnd() async {
+        await withCheckedContinuation { continuation in
+            queue.async {
+                if self.outputEnded { continuation.resume() }
+                else { self.endWaiters.append(continuation) }
+            }
+        }
+    }
+
+    /// Waits for a started child to exit, then drains its output callbacks.
+    func waitForExit() async {
+        await withCheckedContinuation { continuation in
+            queue.async {
+                if self.exited { continuation.resume() }
+                else { self.exitWaiters.append(continuation) }
+            }
+        }
+        await waitForOutputEnd()
+    }
+
     func send(_ object: [String: Any]) throws {
         let data = try JSONSerialization.data(withJSONObject: object)
         try input.fileHandleForWriting.write(contentsOf: data + Data([0x0A]))
@@ -63,17 +110,44 @@ final class NextSentenceLineProcess: @unchecked Sendable {
         output.fileHandleForReading.readabilityHandler = nil
         errors.fileHandleForReading.readabilityHandler = nil
         if process.isRunning { process.terminate() }
+        queue.async { self.finishOutput() }
     }
 
     private func receive(_ data: Data) {
-        guard !data.isEmpty else { return }
+        guard !outputEnded else { return }
+        if data.isEmpty {
+            // A final complete JSON object need not have a trailing newline.
+            if !buffer.isEmpty { emit(buffer); buffer.removeAll() }
+            finishOutput()
+            return
+        }
         buffer.append(data)
         while let newline = buffer.firstIndex(of: 0x0A) {
             let line = buffer[buffer.startIndex..<newline]
             buffer.removeSubrange(buffer.startIndex...newline)
-            guard let object = try? JSONSerialization.jsonObject(with: line) as? [String: Any] else { continue }
-            lock.lock(); let handler = lineHandler; lock.unlock()
-            handler?(object)
+            emit(line)
         }
     }
+
+    private func emit(_ line: Data) {
+        guard let object = try? JSONSerialization.jsonObject(with: line) as? [String: Any] else { return }
+        lock.lock(); let handler = lineHandler; lock.unlock()
+        handler?(object)
+    }
+
+    private func finishOutput() {
+        guard !outputEnded else { return }
+        outputEnded = true
+        endHandler?.call()
+        let waiters = endWaiters
+        endWaiters.removeAll()
+        for waiter in waiters { waiter.resume() }
+    }
+
+    /// Carries registration across queues; callbacks are invoked only on queue.
+    private final class EndCallback: @unchecked Sendable {
+        private let handler: () -> Void
+        init(_ handler: @escaping () -> Void) { self.handler = handler }
+        func call() { handler() }
+    }
 }
diff --git a/Tests/NextSentenceExitTests.swift b/Tests/NextSentenceExitTests.swift
new file mode 100644
index 000000000..1b0ecc346
--- /dev/null
+++ b/Tests/NextSentenceExitTests.swift
@@ -0,0 +1,161 @@
+import Foundation
+
+func testNextSentenceExit() async {
+    for structured in [false, true] {
+        await runSuite(structured ? "Structured Claude child exit completes before timeout" : "Streaming Claude child exit completes before timeout") {
+            let fixture = NextSentenceExitFixture(script: "IFS= read -r request\nexit 7\n")
+            let result = NextSentenceExitResult()
+            assertEqual(fixture.started, false, "constructing the client must not start a child")
+            if structured {
+                fixture.client.structured(.alternatives, prompt: "synthetic request", image: nil) {
+                    result.record($0.map { _ in true })
+                }
+            } else {
+                fixture.client.stream(prompt: "synthetic request", image: nil, onText: { _ in }) {
+                    result.record($0.map { _ in true })
+                }
+            }
+            await fixture.waitForExit()
+            assertEqual(result.outcomes, ["ended"], "exit must finish the request before the timeout callback")
+            fixture.timeouts.fire()
+            assertEqual(result.outcomes, ["ended"], "the later timeout must not complete the request again")
+            await fixture.cleanup()
+        }
+    }
+
+    for structured in [false, true] {
+        await runSuite(structured ? "Structured final JSON wins over EOF without a trailing newline" : "Streaming final result wins over EOF") {
+            let payload = structured
+                ? #"{"type":"result","is_error":false,"structured_output":{"answer":"synthetic"}}"#
+                : #"{"type":"stream_event","event":{"type":"content_block_delta","delta":{"type":"text_delta","text":"synthetic"}}}"#
+                    + "\n" + #"{"type":"result","is_error":false}"# + "\n"
+            let fixture = NextSentenceExitFixture(script: "IFS= read -r request\nprintf '%s' '\(payload)'\n")
+            let result = NextSentenceExitResult()
+            if structured {
+                fixture.client.structured(.alternatives, prompt: "synthetic request", image: nil) {
+                    result.record($0.map { $0["answer"] as? String == "synthetic" })
+                }
+            } else {
+                fixture.client.stream(prompt: "synthetic request", image: nil, onText: { _ in }) {
+                    result.record($0.map { $0 == "synthetic" })
+                }
+            }
+            await fixture.waitForExit()
+            assertEqual(result.outcomes, ["success"], "the final parsed answer must win before output ends")
+            fixture.timeouts.fire()
+            assertEqual(result.outcomes, ["success"], "EOF and timeout must not replace a successful answer")
+            await fixture.cleanup()
+        }
+    }
+
+    await runSuite("A late output-end observer still sees the exited child") {
+        let fixture = NextSentenceExitFixture(script: "exit 0\n")
+        let process = fixture.start()
+        await fixture.waitForExit()
+        let seen = NextSentenceExitResult()
+        process?.onOutputEnd { seen.markEnd() }
+        await process?.waitForOutputEnd()
+        assertEqual(seen.outcomes, ["ended"], "output closure must be replayed after handler registration")
+        await fixture.cleanup()
+    }
+}
+
+private final class NextSentenceExitResult: @unchecked Sendable {
+    private let lock = NSLock()
+    private var values: [String] = []
+    var outcomes: [String] { lock.lock(); defer { lock.unlock() }; return values }
+    func markEnd() { lock.lock(); values.append("ended"); lock.unlock() }
+    func record(_ result: Result<Bool, Error>) {
+        let outcome: String
+        switch result {
+        case .success(true): outcome = "success"
+        case .success(false): outcome = "unexpected_success"
+        case .failure(let error):
+            if case NextSentenceClaude.Failure.failed(let message) = error,
+               message == "Claude stopped before returning an answer." { outcome = "ended" }
+            else { outcome = "other_failure" }
+        }
+        lock.lock(); values.append(outcome); lock.unlock()
+    }
+}
+
+private final class NextSentenceExitTimeouts: @unchecked Sendable {
+    private let lock = NSLock()
+    private var callbacks: [NextSentenceCallbackBox] = []
+    func store(_ callback: NextSentenceCallbackBox) { lock.lock(); callbacks.append(callback); lock.unlock() }
+    func fire() {
+        lock.lock(); let pending = callbacks; callbacks.removeAll(); lock.unlock()
+        for callback in pending { callback.call() }
+    }
+}
+
+private final class NextSentenceExitFixture {
+    let root = FileManager.default.temporaryDirectory
+        .appendingPathComponent("NextSentenceExit-\(UUID().uuidString)", isDirectory: true)
+    let timeouts = NextSentenceExitTimeouts()
+    private let process: NextSentenceLineProcess
+    private let startState = NextSentenceExitStartState()
+    private let prepared: Bool
+    let client: NextSentenceClaude
+    var started: Bool { startState.started }
+
+    init(script: String) {
+        let child = root.appendingPathComponent("child.sh")
+        var ready = false
+        do {
+            try FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+            try script.write(to: child, atomically: true, encoding: .utf8)
+            ready = true
+        } catch { recordFailure("could not prepare the owned synthetic child") }
+        prepared = ready
+        let isPrepared = ready
+        let process = NextSentenceLineProcess(
+            executable: URL(fileURLWithPath: "/bin/sh"), arguments: [child.path],
+            environment: ["HOME": root.path, "PATH": "/usr/bin:/bin", "ENV": "/dev/null", "BASH_ENV": "/dev/null"],
+            workingDirectory: root, label: "synthetic-exit"
+        )
+        self.process = process
+        let state = startState
+        let timeouts = timeouts
+        client = NextSentenceClaude(spawn: { _, _ in
+            guard isPrepared else { return nil }
+            do { try process.start(); state.didStart(); return process }
+            catch { return nil }
+        }, background: { _ in }, scheduleTimeout: { _, callback in timeouts.store(callback) })
+    }
+
+    func start() -> NextSentenceLineProcess? {
+        guard prepared else { return nil }
+        do { try process.start(); startState.didStart(); return process }
+        catch { recordFailure("could not start the owned synthetic child"); return nil }
+    }
+
+    func waitForExit() async {
+        guard started else { recordFailure("the synthetic child was not started"); return }
+        await process.waitForExit()
+    }
+
+    func cleanup() async {
+        client.stop()
+        if started { process.terminate(); await process.waitForExit() }
+        let resolved = root.resolvingSymlinksInPath()
+        let parent = FileManager.default.temporaryDirectory.resolvingSymlinksInPath()
+        let prefix = "NextSentenceExit-"
+        guard root.lastPathComponent.hasPrefix(prefix),
+              UUID(uuidString: String(root.lastPathComponent.dropFirst(prefix.count))) != nil,
+              resolved.deletingLastPathComponent() == parent,
+              resolved.lastPathComponent == root.lastPathComponent else {
+            return recordFailure("refusing to remove a synthetic child outside its owned root")
+        }
+        guard FileManager.default.fileExists(atPath: root.path) else { return }
+        do { try FileManager.default.removeItem(at: root) }
+        catch { recordFailure("synthetic child fixture cleanup failed") }
+    }
+}
+
+private final class NextSentenceExitStartState: @unchecked Sendable {
+    private let lock = NSLock()
+    private var value = false
+    var started: Bool { lock.lock(); defer { lock.unlock() }; return value }
+    func didStart() { lock.lock(); value = true; lock.unlock() }
+}
diff --git a/scripts/entrypoints/run-tests.sh b/scripts/entrypoints/run-tests.sh
index e4c366a4e..7fa74c949 100755
--- a/scripts/entrypoints/run-tests.sh
+++ b/scripts/entrypoints/run-tests.sh
@@ -485,6 +485,8 @@ APP_SOURCES=(
     "Sources/Writing/NextSentence/NextSentencePrivacy.swift"
     "Sources/Writing/NextSentence/NextSentenceWarmPool.swift"
     "Sources/Writing/NextSentence/NextSentenceTetheredProcess.swift"
+    "Sources/Writing/NextSentence/NextSentenceLineProcess.swift"
+    "Sources/Writing/NextSentence/NextSentenceClaude.swift"
     "Sources/Writing/NextSentence/NextSentenceSemanticIndex.swift"
     "Sources/TranscriptedWriting/Core/ScreenMemory/DefaultExcludedApps.swift"
     "Sources/TranscriptedWriting/Core/Text/SecretRules.swift"