diff --git a/crates/beat-core/src/lib.rs b/crates/beat-core/src/lib.rs
index 7f1968c..8c02a11 100644
--- a/crates/beat-core/src/lib.rs
+++ b/crates/beat-core/src/lib.rs
@@ -7,6 +7,7 @@
 //! `beat-data`, behind the trait ports defined in [`ports`] and [`discovery`].
 
 pub mod discovery;
+pub mod output;
 pub mod pipeline;
 pub mod ports;
 pub mod segment;
@@ -14,6 +15,7 @@ pub mod segment;
 pub use discovery::{
     FsEvent, ProcessedLedger, Stability, StabilityPolicy, StabilityTracker, is_supported_video,
 };
+pub use output::{OutputWriter, format_timestamp, render_json, render_markdown};
 pub use pipeline::process_decoded;
 pub use ports::{AudioSource, AudioWindow, DecodedFrame, FrameSource, LlmClient};
 pub use segment::{AudioChunk, FrameSample, Signals, segment, select_representatives};
diff --git a/crates/beat-core/src/output.rs b/crates/beat-core/src/output.rs
new file mode 100644
index 0000000..b274144
--- /dev/null
+++ b/crates/beat-core/src/output.rs
@@ -0,0 +1,77 @@
+//! Output rendering and the writer port.
+//!
+//! Rendering is pure (produces strings matching the readme contract) and lives
+//! here; the actual atomic filesystem write lives in `beat-data`'s
+//! `FsOutputWriter`, behind the [`OutputWriter`] port.
+
+use beat_entities::{Error, OutputFormat, Result, VideoResult};
+use std::fmt::Write;
+
+/// Persist a result in the requested formats to its sidecar destination(s).
+pub trait OutputWriter {
+    /// Write `result` in every requested format. `source_path` is the original
+    /// video path, used to derive the sidecar location and stem.
+    fn write(
+        &self,
+        result: &VideoResult,
+        source_path: &std::path::Path,
+        formats: &[OutputFormat],
+    ) -> Result<()>;
+}
+
+/// Render the canonical JSON sidecar body (pretty, with a trailing newline).
+pub fn render_json(result: &VideoResult) -> Result<String> {
+    let mut s = serde_json::to_string_pretty(result)
+        .map_err(|e| Error::Parse(format!("json render: {e}")))?;
+    s.push('\n');
+    Ok(s)
+}
+
+/// Render the canonical Markdown sidecar body.
+pub fn render_markdown(result: &VideoResult) -> String {
+    let mut o = String::new();
+    let _ = writeln!(o, "# {}\n", result.source);
+    let _ = writeln!(o, "{}\n", result.summary);
+    let _ = writeln!(o, "## Beats\n");
+    for b in &result.beats {
+        let _ = writeln!(
+            o,
+            "### {} — {} → {}",
+            b.index,
+            format_timestamp(b.start_seconds),
+            format_timestamp(b.end_seconds),
+        );
+        let _ = writeln!(o, "**Characters:** {}", b.characters.join(", "));
+        let _ = writeln!(o, "**Activity:** {}\n", b.activity);
+        let _ = writeln!(o, "{}\n", b.description);
+    }
+    // Collapse the trailing blank line to a single terminating newline.
+    o.truncate(o.trim_end().len());
+    o.push('\n');
+    o
+}
+
+/// Format whole seconds as `HH:MM:SS`, truncating fractional seconds to match
+/// the readme (`47.5 → 00:00:47`).
+pub fn format_timestamp(seconds: f64) -> String {
+    let total = seconds.max(0.0) as u64;
+    format!(
+        "{:02}:{:02}:{:02}",
+        total / 3600,
+        (total % 3600) / 60,
+        total % 60
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn timestamps_truncate() {
+        assert_eq!(format_timestamp(0.0), "00:00:00");
+        assert_eq!(format_timestamp(47.5), "00:00:47");
+        assert_eq!(format_timestamp(3661.0), "01:01:01");
+        assert_eq!(format_timestamp(5421.0), "01:30:21");
+    }
+}
diff --git a/crates/beat-core/tests/fixtures/holiday.beat.json b/crates/beat-core/tests/fixtures/holiday.beat.json
new file mode 100644
index 0000000..8ada981
--- /dev/null
+++ b/crates/beat-core/tests/fixtures/holiday.beat.json
@@ -0,0 +1,18 @@
+{
+  "source": "holiday.mp4",
+  "duration_seconds": 5421.0,
+  "summary": "A family road trip along the coast, ending at a beach campsite at dusk.",
+  "beats": [
+    {
+      "index": 0,
+      "start_seconds": 0.0,
+      "end_seconds": 47.5,
+      "characters": [
+        "man in red jacket",
+        "two children"
+      ],
+      "activity": "Loading luggage into a car in a driveway.",
+      "description": "A man and two children pack suitcases into the boot of a parked car on a sunny morning."
+    }
+  ]
+}
diff --git a/crates/beat-core/tests/fixtures/holiday.beat.md b/crates/beat-core/tests/fixtures/holiday.beat.md
new file mode 100644
index 0000000..b46f4e8
--- /dev/null
+++ b/crates/beat-core/tests/fixtures/holiday.beat.md
@@ -0,0 +1,11 @@
+# holiday.mp4
+
+A family road trip along the coast, ending at a beach campsite at dusk.
+
+## Beats
+
+### 0 — 00:00:00 → 00:00:47
+**Characters:** man in red jacket, two children
+**Activity:** Loading luggage into a car in a driveway.
+
+A man and two children pack suitcases into the boot of a parked car on a sunny morning.
diff --git a/crates/beat-core/tests/golden.rs b/crates/beat-core/tests/golden.rs
new file mode 100644
index 0000000..eb7f89b
--- /dev/null
+++ b/crates/beat-core/tests/golden.rs
@@ -0,0 +1,38 @@
+//! Golden-file tests pinning the JSON/Markdown output to the readme contract.
+
+use beat_core::{render_json, render_markdown};
+use beat_entities::{Beat, VideoResult};
+
+fn holiday() -> VideoResult {
+    VideoResult {
+        source: "holiday.mp4".into(),
+        duration_seconds: 5421.0,
+        summary: "A family road trip along the coast, ending at a beach campsite at dusk.".into(),
+        beats: vec![Beat {
+            index: 0,
+            start_seconds: 0.0,
+            end_seconds: 47.5,
+            characters: vec!["man in red jacket".into(), "two children".into()],
+            activity: "Loading luggage into a car in a driveway.".into(),
+            description:
+                "A man and two children pack suitcases into the boot of a parked car on a sunny morning."
+                    .into(),
+        }],
+    }
+}
+
+#[test]
+fn json_matches_readme_golden() {
+    assert_eq!(
+        render_json(&holiday()).unwrap(),
+        include_str!("fixtures/holiday.beat.json")
+    );
+}
+
+#[test]
+fn markdown_matches_readme_golden() {
+    assert_eq!(
+        render_markdown(&holiday()),
+        include_str!("fixtures/holiday.beat.md")
+    );
+}
diff --git a/crates/beat-data/src/lib.rs b/crates/beat-data/src/lib.rs
index e0c4752..5b7c52e 100644
--- a/crates/beat-data/src/lib.rs
+++ b/crates/beat-data/src/lib.rs
@@ -5,10 +5,12 @@
 pub mod decode;
 pub mod ledger;
 pub mod llm;
+pub mod output;
 pub mod watcher;
 
 #[cfg(feature = "ffmpeg")]
 pub use decode::{FfmpegAudioSource, FfmpegFrameSource};
 pub use ledger::RedbLedger;
 pub use llm::HttpLlmClient;
+pub use output::FsOutputWriter;
 pub use watcher::FsWatcher;
diff --git a/crates/beat-data/src/output.rs b/crates/beat-data/src/output.rs
new file mode 100644
index 0000000..4bc1416
--- /dev/null
+++ b/crates/beat-data/src/output.rs
@@ -0,0 +1,162 @@
+//! Filesystem sidecar writer: renders via `beat-core` and writes atomically.
+
+use beat_core::{OutputWriter, render_json, render_markdown};
+use beat_entities::{Config, Error, OutputFormat, Result, VideoResult};
+use std::fs;
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+/// Writes `<stem>.beat.json` / `<stem>.beat.md` sidecars.
+pub struct FsOutputWriter {
+    /// `None` means "alongside the source"; `Some(dir)` is a flat output dir.
+    output_dir: Option<PathBuf>,
+}
+
+impl FsOutputWriter {
+    /// Build from configuration. `output_dir = "alongside"` writes next to the
+    /// source; any other value is treated as an absolute output directory.
+    pub fn new(config: &Config) -> Self {
+        let output_dir = match config.output_dir.as_str() {
+            "alongside" => None,
+            other => Some(PathBuf::from(other)),
+        };
+        Self { output_dir }
+    }
+
+    /// Destination path for a source + extension, e.g. `<stem>.beat.json`.
+    fn sidecar_path(&self, source_path: &Path, ext: &str) -> Result<PathBuf> {
+        let stem = source_path
+            .file_stem()
+            .ok_or_else(|| Error::Config(format!("no file stem: {}", source_path.display())))?;
+        let mut name = stem.to_os_string();
+        name.push(format!(".beat.{ext}"));
+        let dir = match &self.output_dir {
+            None => source_path
+                .parent()
+                .unwrap_or_else(|| Path::new("."))
+                .to_path_buf(),
+            Some(d) => d.clone(),
+        };
+        Ok(dir.join(name))
+    }
+}
+
+impl OutputWriter for FsOutputWriter {
+    fn write(
+        &self,
+        result: &VideoResult,
+        source_path: &Path,
+        formats: &[OutputFormat],
+    ) -> Result<()> {
+        for fmt in formats {
+            let (ext, body) = match fmt {
+                OutputFormat::Json => ("json", render_json(result)?),
+                OutputFormat::Markdown => ("md", render_markdown(result)),
+            };
+            let dest = self.sidecar_path(source_path, ext)?;
+            if let Some(parent) = dest.parent() {
+                fs::create_dir_all(parent).map_err(|e| Error::io(parent, e))?;
+            }
+            atomic_write(&dest, body.as_bytes())?;
+            tracing::info!(dest = %dest.display(), ?fmt, "wrote sidecar");
+        }
+        Ok(())
+    }
+}
+
+/// Write to a uniquely-named temp file in the destination directory, fsync,
+/// then rename over the destination — atomic on a single filesystem, so a
+/// crash never leaves a half-written sidecar.
+fn atomic_write(dest: &Path, bytes: &[u8]) -> Result<()> {
+    let dir = dest.parent().unwrap_or_else(|| Path::new("."));
+    let file_name = dest
+        .file_name()
+        .and_then(|s| s.to_str())
+        .unwrap_or("sidecar");
+    let tmp = dir.join(format!(".{file_name}.tmp.{}", std::process::id()));
+
+    let mut f = fs::File::create(&tmp).map_err(|e| Error::io(&tmp, e))?;
+    f.write_all(bytes).map_err(|e| Error::io(&tmp, e))?;
+    f.sync_all().map_err(|e| Error::io(&tmp, e))?;
+    fs::rename(&tmp, dest).map_err(|e| Error::io(dest, e))?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use beat_entities::{Beat, LlmConfig, SegmentationConfig};
+
+    fn result() -> VideoResult {
+        VideoResult {
+            source: "clip.mp4".into(),
+            duration_seconds: 12.0,
+            summary: "A short clip.".into(),
+            beats: vec![Beat {
+                index: 0,
+                start_seconds: 0.0,
+                end_seconds: 12.0,
+                characters: vec!["cat".into()],
+                activity: "sleeping".into(),
+                description: "A cat sleeps.".into(),
+            }],
+        }
+    }
+
+    fn config(output_dir: &str) -> Config {
+        Config {
+            watch: vec![],
+            formats: vec![OutputFormat::Json, OutputFormat::Markdown],
+            output_dir: output_dir.into(),
+            llm: LlmConfig {
+                endpoint: "http://x/v1".into(),
+                model: "m".into(),
+                max_concurrent_requests: 1,
+            },
+            segmentation: SegmentationConfig::default(),
+            discovery: Default::default(),
+        }
+    }
+
+    #[test]
+    fn writes_alongside_with_no_temp_leftover() {
+        let dir = tempfile::tempdir().unwrap();
+        let source = dir.path().join("clip.mp4");
+        fs::write(&source, b"fake").unwrap();
+
+        let writer = FsOutputWriter::new(&config("alongside"));
+        writer
+            .write(&result(), &source, &[OutputFormat::Json, OutputFormat::Markdown])
+            .unwrap();
+
+        let json = dir.path().join("clip.beat.json");
+        let md = dir.path().join("clip.beat.md");
+        assert!(json.exists());
+        assert!(md.exists());
+
+        // JSON round-trips back to an equal VideoResult.
+        let read: VideoResult = serde_json::from_str(&fs::read_to_string(&json).unwrap()).unwrap();
+        assert_eq!(read, result());
+
+        // No leftover temp files.
+        let leftovers: Vec<_> = fs::read_dir(dir.path())
+            .unwrap()
+            .filter_map(|e| e.ok())
+            .filter(|e| e.file_name().to_string_lossy().contains(".tmp."))
+            .collect();
+        assert!(leftovers.is_empty(), "temp files left behind");
+    }
+
+    #[test]
+    fn writes_into_absolute_output_dir() {
+        let src_dir = tempfile::tempdir().unwrap();
+        let out_dir = tempfile::tempdir().unwrap();
+        let source = src_dir.path().join("movie.mkv");
+
+        let writer = FsOutputWriter::new(&config(out_dir.path().to_str().unwrap()));
+        writer.write(&result(), &source, &[OutputFormat::Json]).unwrap();
+
+        assert!(out_dir.path().join("movie.beat.json").exists());
+        assert!(!src_dir.path().join("movie.beat.json").exists());
+    }
+}
diff --git a/doc/plan/05-output/01-output.md b/doc/plan/05-output/01-output.md
new file mode 100644
index 0000000..60ad83e
--- /dev/null
+++ b/doc/plan/05-output/01-output.md
@@ -0,0 +1,35 @@
+# 05 — Output
+
+## Purpose
+Persist each video's `VideoResult` as a JSON and/or Markdown sidecar, one per source video.
+
+## Entity
+`VideoResult { source, duration_seconds, summary, beats }` (beat-entities) serialises to the exact
+readme JSON shape. `source` is the file *name*; `duration_seconds` is `f64` (so `5421` renders as
+`5421.0` — a documented, intentional deviation from the readme's integer).
+
+## Rendering (beat-core — pure)
+- `render_json`: `serde_json::to_string_pretty` + trailing newline.
+- `render_markdown`: matches the readme — `# {source}`, summary, `## Beats`, then per beat
+  `### {index} — HH:MM:SS → HH:MM:SS`, `**Characters:** a, b`, `**Activity:** …`, blank line,
+  description. The trailing blank line is collapsed to a single terminating newline.
+- `format_timestamp`: whole-second `HH:MM:SS`, truncating fractions (`47.5 → 00:00:47`).
+
+These are pure and locked by golden-file tests (`crates/beat-core/tests/fixtures/holiday.beat.{json,md}`),
+so any whitespace drift fails CI.
+
+## Writing (beat-data `FsOutputWriter`, implements `OutputWriter`)
+- `output_dir = "alongside"` → sidecar in the source's parent dir; any other value → a flat output
+  directory (`create_dir_all` on first run). (`mirror` mode keyed off watch roots is future work.)
+- Naming: `<stem>.beat.json` / `<stem>.beat.md`.
+- **Atomic write**: render to a uniquely-named temp file in the destination directory, `sync_all`,
+  then `rename` over the target. A kill mid-write leaves either the old sidecar or the new one — never
+  a half-written file. A complete sidecar is also the discovery fast-skip signal.
+
+## CLI
+`--format` overrides config formats via `OutputFormat::from_str` (`json`, `markdown`, `md`). Wiring
+lands with the daemon/one-shot integration (stage 05 lifecycle).
+
+## Tests
+Golden JSON/MD byte-equality; `format_timestamp` cases; `tempfile`-based atomic-write tests
+(alongside + absolute dir, round-trip, no leftover temp files).