diff --git a/src/emitter.rs b/src/emitter.rs
new file mode 100644
index 0000000..f5d67ed
--- /dev/null
+++ b/src/emitter.rs
@@ -0,0 +1,184 @@
+use crate::cli::OutputFormat;
+use crate::error::ClaudePrintError;
+use crate::transcript::TranscriptResult;
+use std::io::Write;
+use std::path::PathBuf;
+use std::sync::mpsc;
+use std::thread;
+use std::time::Duration;
+
+/// Emit a successful response.
+///
+/// `text`: writes `{response_text}\n` to stdout.
+/// `json`: writes a single-line JSON result object.
+/// `stream-json`: no-op — the reader thread handles all output.
+pub fn emit_success(
+    writer: &mut dyn Write,
+    result: &TranscriptResult,
+    format: &OutputFormat,
+    claude_version: &str,
+    duration_ms: u64,
+) -> std::io::Result<()> {
+    match format {
+        OutputFormat::Text => {
+            writeln!(writer, "{}", result.text)?;
+        }
+        OutputFormat::Json => {
+            let obj = serde_json::json!({
+                "type": "result",
+                "subtype": "success",
+                "is_error": false,
+                "result": result.text,
+                "session_id": result.session_id,
+                "num_turns": result.num_turns as u64,
+                "duration_ms": duration_ms,
+                "cost_usd": 0,
+                "claude_version": claude_version,
+                "usage": {
+                    "input_tokens": result.usage.input_tokens,
+                    "output_tokens": result.usage.output_tokens,
+                    "cache_creation_input_tokens": result.usage.cache_creation_input_tokens,
+                    "cache_read_input_tokens": result.usage.cache_read_input_tokens,
+                }
+            });
+            writeln!(writer, "{}", serde_json::to_string(&obj).unwrap())?;
+        }
+        OutputFormat::StreamJson => {
+            // Reader thread handles all output; nothing to emit here on success.
+        }
+    }
+    Ok(())
+}
+
+/// Emit an error result.
+///
+/// `text`: message to stderr only.
+/// `json`: JSON error object to stdout.
+/// `stream-json` after inject: JSON error object to stdout.
+/// `stream-json` before inject: message to stderr only (same as text).
+pub fn emit_error(
+    stdout: &mut dyn Write,
+    stderr: &mut dyn Write,
+    error: &ClaudePrintError,
+    format: &OutputFormat,
+    claude_version: &str,
+    stream_json_after_inject: bool,
+) -> std::io::Result<()> {
+    let write_json = match format {
+        OutputFormat::Json => true,
+        OutputFormat::StreamJson => stream_json_after_inject,
+        OutputFormat::Text => false,
+    };
+
+    if write_json {
+        let obj = serde_json::json!({
+            "type": "result",
+            "subtype": error.subtype(),
+            "is_error": true,
+            "error_message": error.message(),
+            "claude_version": claude_version,
+        });
+        writeln!(stdout, "{}", serde_json::to_string(&obj).unwrap())?;
+    } else {
+        writeln!(stderr, "error: {}", error.message())?;
+    }
+    Ok(())
+}
+
+/// Handle for the stream-json reader thread.
+pub struct StreamJsonHandle {
+    /// Send `()` to signal "drain remaining lines then exit".
+    /// Drop without sending to signal "exit immediately".
+    pub drain_tx: mpsc::SyncSender<()>,
+    pub join_handle: thread::JoinHandle<()>,
+}
+
+/// Spawn a stream-json reader thread writing to stdout.
+pub fn spawn_stream_json_reader(transcript_path: PathBuf, start_offset: u64) -> StreamJsonHandle {
+    spawn_stream_json_reader_to(transcript_path, start_offset, Box::new(std::io::stdout()))
+}
+
+/// Spawn a stream-json reader thread writing to the given writer (testable).
+pub fn spawn_stream_json_reader_to(
+    transcript_path: PathBuf,
+    start_offset: u64,
+    writer: Box<dyn Write + Send + 'static>,
+) -> StreamJsonHandle {
+    let (drain_tx, drain_rx) = mpsc::sync_channel(1);
+    let join_handle = thread::spawn(move || {
+        stream_json_reader_loop(transcript_path, start_offset, writer, drain_rx);
+    });
+    StreamJsonHandle {
+        drain_tx,
+        join_handle,
+    }
+}
+
+fn stream_json_reader_loop(
+    transcript_path: PathBuf,
+    start_offset: u64,
+    mut writer: Box<dyn Write + Send + 'static>,
+    drain_rx: mpsc::Receiver<()>,
+) {
+    use std::fs::File;
+    use std::io::{BufRead, BufReader, Seek, SeekFrom};
+
+    // Open the file, waiting if it doesn't exist yet.
+    let file = loop {
+        match File::open(&transcript_path) {
+            Ok(f) => break f,
+            Err(_) => match drain_rx.try_recv() {
+                Ok(()) => return,
+                Err(mpsc::TryRecvError::Disconnected) => return,
+                Err(mpsc::TryRecvError::Empty) => {
+                    thread::sleep(Duration::from_millis(5));
+                }
+            },
+        }
+    };
+
+    let mut reader = BufReader::new(file);
+    if reader.seek(SeekFrom::Start(start_offset)).is_err() {
+        let _ = drain_rx.recv();
+        return;
+    }
+
+    let mut draining = false;
+    let mut line = String::new();
+
+    loop {
+        line.clear();
+        match reader.read_line(&mut line) {
+            Ok(0) => {
+                if draining {
+                    break;
+                }
+                match drain_rx.try_recv() {
+                    Ok(()) => {
+                        draining = true;
+                    }
+                    Err(mpsc::TryRecvError::Disconnected) => return,
+                    Err(mpsc::TryRecvError::Empty) => {
+                        thread::sleep(Duration::from_millis(5));
+                    }
+                }
+            }
+            Ok(_) => {
+                let trimmed = line.trim_end_matches('\n').trim_end_matches('\r');
+                if !trimmed.is_empty() {
+                    let _ = writeln!(writer, "{}", trimmed);
+                }
+            }
+            Err(_) => {
+                if draining {
+                    break;
+                }
+                match drain_rx.try_recv() {
+                    Ok(()) => draining = true,
+                    Err(mpsc::TryRecvError::Disconnected) => return,
+                    Err(mpsc::TryRecvError::Empty) => {}
+                }
+            }
+        }
+    }
+}
diff --git a/src/error.rs b/src/error.rs
index 697165b..d86038b 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -13,3 +13,41 @@ pub enum Error {
 }
 
 pub type Result<T> = std::result::Result<T, Error>;
+
+/// User-facing error type with exit code and JSON subtype mapping.
+#[derive(Debug)]
+pub enum ClaudePrintError {
+    Setup(String),        // exit 2
+    Timeout,              // exit 124
+    Interrupted,          // exit 130
+    AssistantError(String), // exit 1
+}
+
+impl ClaudePrintError {
+    pub fn exit_code(&self) -> i32 {
+        match self {
+            ClaudePrintError::Setup(_) => 2,
+            ClaudePrintError::Timeout => 124,
+            ClaudePrintError::Interrupted => 130,
+            ClaudePrintError::AssistantError(_) => 1,
+        }
+    }
+
+    pub fn subtype(&self) -> &'static str {
+        match self {
+            ClaudePrintError::Setup(_) => "internal_error",
+            ClaudePrintError::Timeout => "timeout",
+            ClaudePrintError::Interrupted => "interrupted",
+            ClaudePrintError::AssistantError(_) => "assistant_error",
+        }
+    }
+
+    pub fn message(&self) -> &str {
+        match self {
+            ClaudePrintError::Setup(m) => m,
+            ClaudePrintError::Timeout => "operation timed out",
+            ClaudePrintError::Interrupted => "interrupted by signal",
+            ClaudePrintError::AssistantError(m) => m,
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index cd2e658..056d6a4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,6 @@
 pub mod cli;
 pub mod config;
+pub mod emitter;
 pub mod error;
 pub mod event_loop;
 pub mod hook;
diff --git a/tests/emitter.rs b/tests/emitter.rs
new file mode 100644
index 0000000..25563c4
--- /dev/null
+++ b/tests/emitter.rs
@@ -0,0 +1,233 @@
+use claude_print::cli::OutputFormat;
+use claude_print::emitter::{emit_error, emit_success, spawn_stream_json_reader_to};
+use claude_print::error::ClaudePrintError;
+use claude_print::transcript::{AggregatedUsage, TranscriptResult};
+use std::io::Write;
+use std::sync::{Arc, Mutex};
+use tempfile::TempDir;
+
+fn make_result(text: &str) -> TranscriptResult {
+    TranscriptResult {
+        text: text.to_string(),
+        num_turns: 2,
+        usage: AggregatedUsage {
+            input_tokens: 100,
+            output_tokens: 50,
+            cache_creation_input_tokens: 10,
+            cache_read_input_tokens: 5,
+        },
+        session_id: Some("test-session-id".to_string()),
+        is_error: false,
+        used_fallback: false,
+    }
+}
+
+struct CaptureWriter(Arc<Mutex<Vec<u8>>>);
+
+impl Write for CaptureWriter {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.0.lock().unwrap().extend_from_slice(buf);
+        Ok(buf.len())
+    }
+    fn flush(&mut self) -> std::io::Result<()> {
+        Ok(())
+    }
+}
+
+fn capture() -> (Arc<Mutex<Vec<u8>>>, CaptureWriter) {
+    let buf = Arc::new(Mutex::new(Vec::new()));
+    let writer = CaptureWriter(Arc::clone(&buf));
+    (buf, writer)
+}
+
+// ── text format ──────────────────────────────────────────────────────────────
+
+#[test]
+fn test_text_correct_string_trailing_newline() {
+    let result = make_result("hello world");
+    let (buf, mut writer) = capture();
+    emit_success(&mut writer, &result, &OutputFormat::Text, "2.1.168", 0).unwrap();
+    let output = buf.lock().unwrap().clone();
+    assert_eq!(output, b"hello world\n");
+}
+
+#[test]
+fn test_text_no_extra_whitespace() {
+    let result = make_result("response");
+    let (buf, mut writer) = capture();
+    emit_success(&mut writer, &result, &OutputFormat::Text, "1.0", 0).unwrap();
+    let output = buf.lock().unwrap();
+    let s = std::str::from_utf8(&output).unwrap();
+    assert_eq!(s.trim_end_matches('\n'), "response");
+    assert!(s.ends_with('\n'));
+    assert!(!s.starts_with(' '));
+}
+
+// ── json format ──────────────────────────────────────────────────────────────
+
+#[test]
+fn test_json_valid_with_required_fields() {
+    let result = make_result("the answer");
+    let (buf, mut writer) = capture();
+    emit_success(&mut writer, &result, &OutputFormat::Json, "2.1.168", 4200).unwrap();
+    let output = buf.lock().unwrap().clone();
+    let v: serde_json::Value = serde_json::from_slice(&output).unwrap();
+
+    assert_eq!(v["type"], "result");
+    assert_eq!(v["subtype"], "success");
+    assert_eq!(v["is_error"], false);
+    assert_eq!(v["result"], "the answer");
+    assert!(v.get("session_id").is_some());
+    assert!(v.get("num_turns").is_some());
+    assert!(v.get("duration_ms").is_some());
+    assert!(v.get("cost_usd").is_some());
+    assert!(v.get("usage").is_some());
+    assert!(v.get("claude_version").is_some());
+}
+
+#[test]
+fn test_json_claude_version_included() {
+    let result = make_result("text");
+    let (buf, mut writer) = capture();
+    emit_success(&mut writer, &result, &OutputFormat::Json, "2.1.168", 0).unwrap();
+    let output = buf.lock().unwrap().clone();
+    let v: serde_json::Value = serde_json::from_slice(&output).unwrap();
+    assert_eq!(v["claude_version"], "2.1.168");
+}
+
+#[test]
+fn test_json_usage_fields_are_integers() {
+    let result = make_result("text");
+    let (buf, mut writer) = capture();
+    emit_success(&mut writer, &result, &OutputFormat::Json, "1.0", 0).unwrap();
+    let output = buf.lock().unwrap().clone();
+    let v: serde_json::Value = serde_json::from_slice(&output).unwrap();
+    let usage = &v["usage"];
+    assert!(usage["input_tokens"].is_u64(), "input_tokens must be integer");
+    assert!(usage["output_tokens"].is_u64(), "output_tokens must be integer");
+    assert!(usage["cache_creation_input_tokens"].is_u64());
+    assert!(usage["cache_read_input_tokens"].is_u64());
+}
+
+// ── error result ─────────────────────────────────────────────────────────────
+
+#[test]
+fn test_error_result_is_error_true_and_subtype() {
+    let err = ClaudePrintError::Timeout;
+    let (out_buf, mut stdout) = capture();
+    let (_, mut stderr) = capture();
+    emit_error(&mut stdout, &mut stderr, &err, &OutputFormat::Json, "1.0", false).unwrap();
+    let output = out_buf.lock().unwrap().clone();
+    let v: serde_json::Value = serde_json::from_slice(&output).unwrap();
+    assert_eq!(v["is_error"], true);
+    assert_eq!(v["subtype"], "timeout");
+}
+
+#[test]
+fn test_error_exit_code_nonzero() {
+    assert_ne!(ClaudePrintError::Setup("x".to_string()).exit_code(), 0);
+    assert_ne!(ClaudePrintError::Timeout.exit_code(), 0);
+    assert_ne!(ClaudePrintError::Interrupted.exit_code(), 0);
+    assert_ne!(ClaudePrintError::AssistantError("x".to_string()).exit_code(), 0);
+}
+
+#[test]
+fn test_error_subtypes() {
+    assert_eq!(ClaudePrintError::Setup("x".to_string()).subtype(), "internal_error");
+    assert_eq!(ClaudePrintError::Timeout.subtype(), "timeout");
+    assert_eq!(ClaudePrintError::Interrupted.subtype(), "interrupted");
+    assert_eq!(ClaudePrintError::AssistantError("x".to_string()).subtype(), "assistant_error");
+}
+
+#[test]
+fn test_error_exit_codes() {
+    assert_eq!(ClaudePrintError::Setup("x".to_string()).exit_code(), 2);
+    assert_eq!(ClaudePrintError::Timeout.exit_code(), 124);
+    assert_eq!(ClaudePrintError::Interrupted.exit_code(), 130);
+    assert_eq!(ClaudePrintError::AssistantError("x".to_string()).exit_code(), 1);
+}
+
+#[test]
+fn test_text_error_goes_to_stderr_not_stdout() {
+    let err = ClaudePrintError::Setup("missing binary".to_string());
+    let (out_buf, mut stdout) = capture();
+    let (err_buf, mut stderr) = capture();
+    emit_error(&mut stdout, &mut stderr, &err, &OutputFormat::Text, "1.0", false).unwrap();
+    assert!(out_buf.lock().unwrap().is_empty(), "text error must not write to stdout");
+    assert!(!err_buf.lock().unwrap().is_empty(), "text error must write to stderr");
+}
+
+// ── zero token counts ─────────────────────────────────────────────────────────
+
+#[test]
+fn test_zero_token_counts_when_fallback() {
+    let result = TranscriptResult {
+        text: "fallback text".to_string(),
+        num_turns: 0,
+        usage: AggregatedUsage::default(),
+        session_id: None,
+        is_error: false,
+        used_fallback: true,
+    };
+    let (buf, mut writer) = capture();
+    emit_success(&mut writer, &result, &OutputFormat::Json, "1.0", 0).unwrap();
+    let output = buf.lock().unwrap().clone();
+    let v: serde_json::Value = serde_json::from_slice(&output).unwrap();
+    let usage = &v["usage"];
+    assert!(usage.get("input_tokens").is_some(), "usage must be present");
+    assert_eq!(usage["input_tokens"], 0);
+    assert_eq!(usage["output_tokens"], 0);
+    assert_eq!(usage["cache_creation_input_tokens"], 0);
+    assert_eq!(usage["cache_read_input_tokens"], 0);
+}
+
+// ── stream-json ───────────────────────────────────────────────────────────────
+
+#[test]
+fn test_stream_json_each_line_parses_as_json() {
+    let dir = TempDir::new().unwrap();
+    let path = dir.path().join("transcript.jsonl");
+
+    let lines = vec![
+        r#"{"type":"assistant","message":{"id":"msg-1","content":[{"type":"text","text":"hi"}],"usage":{"input_tokens":10,"output_tokens":5}}}"#,
+        r#"{"type":"result","is_error":false,"session_id":"abc123"}"#,
+    ];
+    {
+        let mut f = std::fs::File::create(&path).unwrap();
+        for line in &lines {
+            writeln!(f, "{}", line).unwrap();
+        }
+    }
+
+    let output_buf = Arc::new(Mutex::new(Vec::new()));
+    let writer = Box::new(CaptureWriter(Arc::clone(&output_buf)));
+
+    let handle = spawn_stream_json_reader_to(path, 0, writer);
+    handle.drain_tx.send(()).unwrap();
+    handle.join_handle.join().unwrap();
+
+    let output = output_buf.lock().unwrap().clone();
+    let text = std::str::from_utf8(&output).unwrap();
+    let output_lines: Vec<&str> = text.lines().filter(|l| !l.is_empty()).collect();
+
+    assert_eq!(output_lines.len(), lines.len(), "should forward all lines");
+    for line in &output_lines {
+        let _: serde_json::Value = serde_json::from_str(line)
+            .unwrap_or_else(|_| panic!("line is not valid JSON: {line}"));
+    }
+}
+
+#[test]
+fn test_stream_json_disconnect_exits_immediately() {
+    let dir = TempDir::new().unwrap();
+    let path = dir.path().join("transcript.jsonl");
+    std::fs::write(&path, b"").unwrap();
+
+    let output_buf = Arc::new(Mutex::new(Vec::new()));
+    let writer = Box::new(CaptureWriter(Arc::clone(&output_buf)));
+
+    let handle = spawn_stream_json_reader_to(path, 0, writer);
+    // Drop drain_tx without sending — thread should exit immediately
+    drop(handle.drain_tx);
+    handle.join_handle.join().unwrap(); // must not hang
+}