diff --git a/tines-core/src/frag.rs b/tines-core/src/frag.rs
new file mode 100644
index 0000000..b6af1ba
--- /dev/null
+++ b/tines-core/src/frag.rs
@@ -0,0 +1,397 @@
+//! Fragmentation and reassembly of `Observation`s over a byte-oriented link.
+
+use crate::consts;
+use crate::{Frame, Observation, NodeId, ObservationId, Tick, WireError};
+use heapless::Vec;
+
+/// Header allowance per fragment (bytes), pinned.
+pub const FRAG_OVERHEAD: usize = 64;
+
+/// Encodes `obs` as one Obs frame if it fits in `mtu`, else as N Frag frames.
+/// Returns the number of frames; writes each via `emit(frame_bytes)`.
+pub fn fragment(
+    from: NodeId,
+    obs: &Observation,
+    mtu: usize,
+    emit: &mut dyn FnMut(&[u8]),
+) -> Result<usize, WireError> {
+    // 1. Try to send the whole Observation as a single Obs frame.
+    let mut buf = [0u8; consts::MAX_FRAME];
+    let frame = Frame::Obs {
+        from,
+        obs: obs.clone(),
+    };
+    if let Ok(n) = frame.encode(&mut buf) {
+        if n <= mtu {
+            emit(&buf[..n]);
+            return Ok(1);
+        }
+    }
+
+    // 2. Encode the Observation alone and split it into Frag frames.
+    let mut obs_buf = [0u8; consts::MAX_FRAME * 2];
+    let obs_bytes = postcard::to_slice(obs, &mut obs_buf).map_err(|_| WireError::TooLarge)?;
+
+    let chunk = mtu.saturating_sub(FRAG_OVERHEAD);
+    if chunk == 0 {
+        return Err(WireError::TooLarge);
+    }
+    #[allow(clippy::manual_div_ceil)]
+    let count = (obs_bytes.len() + chunk - 1) / chunk;
+    if count > 64 {
+        return Err(WireError::TooLarge);
+    }
+
+    for i in 0..count {
+        let start = i * chunk;
+        let end = core::cmp::min(start + chunk, obs_bytes.len());
+        let data = &obs_bytes[start..end];
+        let frag_data = heapless::Vec::from_slice(data).map_err(|_| WireError::TooLarge)?;
+        let frame = Frame::Frag {
+            from,
+            id: obs.id(),
+            index: i as u8,
+            count: count as u8,
+            data: frag_data,
+        };
+        let mut fb = [0u8; consts::MAX_FRAME];
+        let n = frame.encode(&mut fb).map_err(|_| WireError::TooLarge)?;
+        emit(&fb[..n]);
+    }
+    Ok(count)
+}
+
+/// A single-slot reassembler for inbound Frag streams.
+pub struct Reassembler {
+    id: Option<ObservationId>,
+    count: u8,
+    received: u64,
+    started_at: Tick,
+    buf: Vec<u8, { consts::MAX_FRAME * 2 }>,
+    offs: [u16; 64],
+    lens: [u16; 64],
+}
+
+impl Reassembler {
+    pub fn new() -> Self {
+        Reassembler {
+            id: None,
+            count: 0,
+            received: 0,
+            started_at: 0,
+            buf: heapless::Vec::new(),
+            offs: [u16::MAX; 64],
+            lens: [0u16; 64],
+        }
+    }
+
+    pub fn feed(&mut self, id: ObservationId, index: u8, count: u8, data: &[u8], now: Tick) -> Option<Observation> {
+        // 1. Drop malformed indices: count must be in 1..=64 and index < count.
+        //    `index >= 64` is guarded too so the `1 << index` bitmap never
+        //    shifts out of bounds on adversarial wire input.
+        if count == 0 || count > 64 || (index as usize) >= 64 || index >= count {
+            return None;
+        }
+
+        // 2. Reset the slot if it is empty, belongs to a different id/count, or
+        //    has exceeded the reassembly timeout.
+        let need_reset = self.id.is_none()
+            || self.id != Some(id)
+            || self.count != count
+            || now.saturating_sub(self.started_at) > consts::REASSEMBLY_TIMEOUT_MS;
+        if need_reset {
+            self.id = Some(id);
+            self.count = count;
+            self.received = 0;
+            self.started_at = now;
+            self.buf.clear();
+            self.offs = [u16::MAX; 64];
+            self.lens = [0u16; 64];
+        }
+
+        // 3. Append the fragment data to the flat buffer in arrival order and
+        //    record where this index's bytes live.
+        let start = self.buf.len();
+        if self.buf.len().saturating_add(data.len()) > self.buf.capacity() {
+            return None;
+        }
+        self.buf.extend_from_slice(data).expect("buf capacity check above");
+        self.offs[index as usize] = start as u16;
+        self.lens[index as usize] = data.len() as u16;
+        self.received |= 1u64 << index;
+
+        // 4. Not all fragments have arrived yet.
+        let complete: u64 = if count >= 64 { u64::MAX } else { (1u64 << count) - 1 };
+        if self.received != complete {
+            return None;
+        }
+
+        // 5. Reassemble in index order and decode.
+        let mut recon = [0u8; consts::MAX_FRAME * 2];
+        let mut total = 0usize;
+        for i in 0..count {
+            let o = self.offs[i as usize] as usize;
+            let l = self.lens[i as usize] as usize;
+            let frag = &self.buf[o..o + l];
+            recon[total..total + l].copy_from_slice(frag);
+            total += l;
+        }
+
+        match postcard::from_bytes::<Observation>(&recon[..total]) {
+            Ok(obs) if obs.id() == id => {
+                self.reset_slot();
+                Some(obs)
+            }
+            _ => {
+                self.reset_slot();
+                None
+            }
+        }
+    }
+}
+
+impl Default for Reassembler {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Reassembler {
+    /// Clears a completed/aborted slot so it is ready for a fresh stream.
+    fn reset_slot(&mut self) {
+        self.id = None;
+        self.count = 0;
+        self.received = 0;
+        self.started_at = 0;
+        self.buf.clear();
+        self.offs = [u16::MAX; 64];
+        self.lens = [0u16; 64];
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::wire::{Frame, WireError};
+    use crate::{Payload, Topic};
+    use std::vec::Vec;
+
+    fn obs_with_payload(observer: [u8; 16], payload_len: usize) -> Observation {
+        Observation {
+            observer,
+            seq: 1,
+            age_ms: 0,
+            observed_at: None,
+            topic: Topic::from_slice(b"door/3").unwrap(),
+            payload: Payload::from_slice(&std::vec![0u8; payload_len]).unwrap(),
+        }
+    }
+
+    #[test]
+    fn small_obs_is_one_obs_frame() {
+        let obs = obs_with_payload([1; 16], 10);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let n = fragment([1; 16], &obs, 1500, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment small obs");
+        assert_eq!(n, 1);
+        assert_eq!(frames.len(), 1);
+        match Frame::decode(&frames[0]).unwrap() {
+            Frame::Obs { .. } => {}
+            _ => panic!("expected a single Obs frame"),
+        }
+    }
+
+    #[test]
+    fn large_obs_is_fragmented_and_reassembles() {
+        let obs = obs_with_payload([2; 16], 1024);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let n = fragment([2; 16], &obs, 200, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment large obs");
+        assert!(n >= 6, "expected >= 6 frames, got {n}");
+        for f in &frames {
+            match Frame::decode(f).unwrap() {
+                Frame::Frag { count, .. } => {
+                    assert_eq!(count, n as u8, "all frags must share the same count");
+                }
+                other => panic!("expected only Frag frames, got {other:?}"),
+            }
+        }
+        let mut r = Reassembler::new();
+        let mut last: Option<Observation> = None;
+        for f in frames.iter() {
+            if let Frame::Frag { id, index, count, data, .. } = Frame::decode(f).unwrap() {
+                last = r.feed(id, index, count, &data, 0);
+            }
+        }
+        let reassembled = last.expect("reassembled obs");
+        assert_eq!(reassembled, obs, "reassembled observation must equal original");
+    }
+
+    #[test]
+    fn out_of_order_reassembles() {
+        let obs = obs_with_payload([3; 16], 1024);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let _n = fragment([3; 16], &obs, 200, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment large obs");
+        let mut r = Reassembler::new();
+        let mut last: Option<Observation> = None;
+        for f in frames.iter().rev() {
+            if let Frame::Frag { id, index, count, data, .. } = Frame::decode(f).unwrap() {
+                last = r.feed(id, index, count, &data, 0);
+            }
+        }
+        let reassembled = last.expect("reassembled obs");
+        assert_eq!(reassembled, obs);
+    }
+
+    #[test]
+    fn duplicate_fragment_is_harmless() {
+        let obs = obs_with_payload([4; 16], 1024);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let n = fragment([4; 16], &obs, 200, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment large obs");
+        let frags: Vec<(ObservationId, u8, u8, Vec<u8>)> = frames
+            .iter()
+            .map(|f| {
+                if let Frame::Frag { id, index, count, data, .. } = Frame::decode(f).unwrap() {
+                    (id, index, count, data.to_vec())
+                } else {
+                    panic!("expected Frag frames");
+                }
+            })
+            .collect();
+        let mut r = Reassembler::new();
+        let mut last: Option<Observation> = None;
+        let (id0, idx0, cnt0, data0) = &frags[0];
+        // feed index 0 twice
+        r.feed(*id0, *idx0, *cnt0, data0, 0);
+        r.feed(*id0, *idx0, *cnt0, data0, 0);
+        for i in 1..n as usize {
+            let (id, index, count, data) = &frags[i];
+            last = r.feed(*id, *index, *count, data, 0);
+        }
+        let reassembled = last.expect("reassembled obs");
+        assert_eq!(reassembled, obs);
+    }
+
+    #[test]
+    fn wrong_count_is_dropped() {
+        let obs = obs_with_payload([5; 16], 1024);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let n = fragment([5; 16], &obs, 200, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment large obs");
+        let frags: Vec<(ObservationId, u8, u8, Vec<u8>)> = frames
+            .iter()
+            .map(|f| {
+                if let Frame::Frag { id, index, count, data, .. } = Frame::decode(f).unwrap() {
+                    (id, index, count, data.to_vec())
+                } else {
+                    panic!("expected Frag frames");
+                }
+            })
+            .collect();
+        let mut r = Reassembler::new();
+        let mut got_some = false;
+        for i in 0..n as usize {
+            let (id, index, _count, data) = &frags[i];
+            // The changing count keeps resetting the slot, so fragments never
+            // accumulate and the stream can never complete (no panic).
+            let a = r.feed(*id, *index, 2, data, 0);
+            let b = r.feed(*id, *index, 3, data, 0);
+            if a.is_some() || b.is_some() {
+                got_some = true;
+            }
+        }
+        assert!(!got_some, "a wrong-count stream must never complete");
+    }
+
+    #[test]
+    fn index_ge_count_is_ignored() {
+        let mut r = Reassembler::new();
+        let id = [6; 16];
+        // index 5 with count 3
+        assert!(r.feed(id, 5, 3, &[1, 2, 3], 0).is_none());
+        // index == count is also invalid
+        assert!(r.feed(id, 3, 3, &[1, 2, 3], 0).is_none());
+    }
+
+    #[test]
+    fn timeout_discards_partial() {
+        let obs = obs_with_payload([7; 16], 200);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let n = fragment([7; 16], &obs, 200, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment large obs");
+        let frags: Vec<(ObservationId, u8, u8, Vec<u8>)> = frames
+            .iter()
+            .map(|f| {
+                if let Frame::Frag { id, index, count, data, .. } = Frame::decode(f).unwrap() {
+                    (id, index, count, data.to_vec())
+                } else {
+                    panic!("expected Frag frames");
+                }
+            })
+            .collect();
+        assert_eq!(frags.len(), n as usize);
+        let mut r = Reassembler::new();
+        let (id0, idx0, cnt0, data0) = &frags[0];
+        // feed index 0 at now=0 (incomplete)
+        r.feed(*id0, *idx0, *cnt0, data0, 0);
+        // feed index 1 at now=timeout+1 -> slot restarts with only index 1, returns None
+        let (id1, idx1, cnt1, data1) = &frags[1];
+        assert!(r.feed(*id1, *idx1, *cnt1, data1, consts::REASSEMBLY_TIMEOUT_MS + 1).is_none());
+        // re-feed index 0 late: proves index 0 from now=0 was discarded, but the
+        // restarted slot now holds index 1, so the stream completes to the original
+        let (idc, idxc, cntc, datac) = &frags[0];
+        let late = r.feed(*idc, *idxc, *cntc, datac, consts::REASSEMBLY_TIMEOUT_MS + 2);
+        assert_eq!(late, Some(obs.clone()), "late re-feed must complete the restarted stream");
+        // sanity: a fresh feed of the whole set from now=0 also completes
+        let mut r2 = Reassembler::new();
+        let mut complete: Option<Observation> = None;
+        for (id, index, count, data) in &frags {
+            complete = r2.feed(*id, *index, *count, data, 0);
+        }
+        assert_eq!(complete, Some(obs));
+    }
+
+    #[test]
+    fn reassembled_bytes_must_match_id() {
+        let obs = obs_with_payload([8; 16], 1024);
+        let mut frames: Vec<Vec<u8>> = Vec::new();
+        let n = fragment([8; 16], &obs, 200, &mut |b| frames.push(b.to_vec()))
+            .expect("fragment large obs");
+        let mut frags: Vec<(ObservationId, u8, u8, Vec<u8>)> = frames
+            .iter()
+            .map(|f| {
+                if let Frame::Frag { id, index, count, data, .. } = Frame::decode(f).unwrap() {
+                    (id, index, count, data.to_vec())
+                } else {
+                    panic!("expected Frag frames");
+                }
+            })
+            .collect();
+        // corrupt one byte in the payload region of fragment 0
+        frags[0].3[0] ^= 0xFF;
+        let mut r = Reassembler::new();
+        let mut last: Option<Observation> = None;
+        for i in 0..n as usize {
+            let (id, index, count, data) = &frags[i];
+            last = r.feed(*id, *index, *count, data, 0);
+        }
+        assert!(last.is_none(), "corrupted reassembly must not yield an obs");
+    }
+
+    #[test]
+    fn fragment_too_large_returns_error() {
+        let obs = obs_with_payload([9; 16], consts::MAX_PAYLOAD);
+        // mtu smaller than header -> chunk == 0 -> TooLarge
+        let got = fragment([9; 16], &obs, FRAG_OVERHEAD, &mut |_| {}).is_err();
+        assert!(got, "chunk == 0 must return an error");
+    }
+
+    #[test]
+    fn decode_garbage_is_not_a_frame() {
+        // smoke check that wire decode of the module's dependency behaves
+        assert!(Frame::decode(&[0xFFu8, 0xFF, 0xFF]).is_err());
+        assert!(WireError::Malformed == WireError::Malformed);
+    }
+}
diff --git a/tines-core/src/lib.rs b/tines-core/src/lib.rs
index 455cf3a..8257be3 100644
--- a/tines-core/src/lib.rs
+++ b/tines-core/src/lib.rs
@@ -6,6 +6,7 @@ extern crate alloc;
 
 pub mod bloom;
 pub mod consts;
+pub mod frag;
 pub mod observation;
 pub mod store;
 #[cfg(feature = "alloc")]
@@ -14,6 +15,7 @@ pub mod transport;
 pub mod wire;
 
 pub use bloom::Bloom;
+pub use frag::{fragment, Reassembler};
 pub use observation::{Observation, NodeId, ObservationId, Payload, Tick, Topic};
 pub use store::{Store, TrimPolicy, StoreError};
 pub use transport::{Event, FrameBuf, SendError, Transport};