diff --git a/Cargo.lock b/Cargo.lock
index 821bd7d..f2cc3ef 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1799,6 +1799,7 @@ dependencies = [
  "anyhow",
  "bytes",
  "crc32fast",
+ "md-5 0.11.0",
  "parking_lot",
  "proptest",
  "tempfile",
diff --git a/crates/nzb-decode/Cargo.toml b/crates/nzb-decode/Cargo.toml
index 08b740c..ab7451c 100644
--- a/crates/nzb-decode/Cargo.toml
+++ b/crates/nzb-decode/Cargo.toml
@@ -10,6 +10,7 @@ readme = "README.md"
 [dependencies]
 bytes = "1"
 crc32fast = "1"
+md-5 = "0.11"
 yenc-simd = "0.1"
 tracing = "0.1"
 anyhow = "1"
diff --git a/crates/nzb-decode/src/lib.rs b/crates/nzb-decode/src/lib.rs
index 23649a4..029fa23 100644
--- a/crates/nzb-decode/src/lib.rs
+++ b/crates/nzb-decode/src/lib.rs
@@ -7,8 +7,10 @@
 
 pub mod assembler;
 pub mod cache;
+pub mod slice_map;
 pub mod yenc;
 
 pub use assembler::FileAssembler;
 pub use cache::ArticleCache;
+pub use slice_map::{FileSlices, SliceDigest, SliceLayout, SliceState, SliceVerifier};
 pub use yenc::{YencDecodeResult, decode_yenc};
diff --git a/crates/nzb-decode/src/slice_map.rs b/crates/nzb-decode/src/slice_map.rs
new file mode 100644
index 0000000..0572331
--- /dev/null
+++ b/crates/nzb-decode/src/slice_map.rs
@@ -0,0 +1,315 @@
+//! In-flight PAR2 slice verification (WI-144 core).
+//!
+//! As decoded articles land in the assembler, the bytes that make up each PAR2
+//! recovery slice arrive too — out of order, at arbitrary offsets, from many
+//! workers. This module turns those writes into a per-slice verdict
+//! (`Verified` / `Damaged` / `Unknown`) so the post-download PAR2 verify pass
+//! can be skipped for a clean job (WI-147) and damage can be compared to
+//! recovery blocks during the download (WI-145).
+//!
+//! MD5 is not associative, so a slice cannot be hashed incrementally from
+//! out-of-order fragments. Instead each slice tracks how many of its bytes
+//! have been written; once a slice's whole byte range is filled the caller
+//! reads it back (from the assembler's file, page-cache-hot) and calls
+//! [`SliceVerifier::verify`], which pads the final partial slice with zeros —
+//! matching how PAR2 IFSC checksums are computed — and compares MD5 and CRC32.
+//!
+//! The verifier holds only PAR2 geometry as plain data ([`SliceLayout`]), so
+//! this crate needs no PAR2 parser: the download engine builds the layout from
+//! a parsed `rust-par2` file set and the NZB-file-to-PAR2-file mapping.
+
+use crc32fast::Hasher as Crc32;
+use md5::{Digest, Md5};
+
+/// A 16-byte MD5 digest.
+pub type Md5Hash = [u8; 16];
+
+/// The verdict for one slice.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SliceState {
+    /// Not yet fully written, or written but not yet checked.
+    Unknown,
+    /// Fully written and its MD5 and CRC32 match the PAR2 checksums.
+    Verified,
+    /// Fully written but a checksum mismatched, or a byte is known missing.
+    Damaged,
+}
+
+/// The expected checksums for one slice, from a PAR2 IFSC packet.
+#[derive(Debug, Clone, Copy)]
+pub struct SliceDigest {
+    pub md5: Md5Hash,
+    pub crc32: u32,
+}
+
+/// One file's slice checksums, in slice order, with the file's total size.
+#[derive(Debug, Clone)]
+pub struct FileSlices {
+    pub size: u64,
+    pub slices: Vec<SliceDigest>,
+}
+
+/// PAR2 geometry the verifier needs, in the recovery set's file order. Built by
+/// the caller from a parsed `rust-par2` `Par2FileSet` (its `slice_size`,
+/// `file_order`, and each file's IFSC `slices`).
+#[derive(Debug, Clone)]
+pub struct SliceLayout {
+    pub slice_size: u64,
+    pub files: Vec<FileSlices>,
+}
+
+#[derive(Debug)]
+struct SliceCell {
+    file_index: usize,
+    /// Byte offset of this slice within its file.
+    start: u64,
+    /// Actual byte length (the last slice of a file may be shorter).
+    len: u64,
+    /// Bytes written into this slice's range so far.
+    filled: u64,
+    expected: SliceDigest,
+    state: SliceState,
+}
+
+/// Tracks slice fill as writes arrive and verifies each slice once complete.
+#[derive(Debug)]
+pub struct SliceVerifier {
+    slice_size: u64,
+    cells: Vec<SliceCell>,
+    /// Global index of the first slice of each file, plus a trailing sentinel
+    /// equal to `cells.len()` so `[file_index]..[file_index + 1]` is the file's
+    /// slice range for any valid `file_index`.
+    file_bounds: Vec<usize>,
+}
+
+fn overlap(a0: u64, a1: u64, b0: u64, b1: u64) -> u64 {
+    a1.min(b1).saturating_sub(a0.max(b0))
+}
+
+impl SliceVerifier {
+    /// Build a verifier for a PAR2 set. Every slice starts `Unknown`.
+    pub fn new(layout: &SliceLayout) -> Self {
+        let mut cells = Vec::new();
+        let mut file_bounds = Vec::with_capacity(layout.files.len() + 1);
+        for (file_index, file) in layout.files.iter().enumerate() {
+            file_bounds.push(cells.len());
+            for (i, digest) in file.slices.iter().enumerate() {
+                let start = i as u64 * layout.slice_size;
+                let len = file.size.saturating_sub(start).min(layout.slice_size);
+                cells.push(SliceCell {
+                    file_index,
+                    start,
+                    len,
+                    filled: 0,
+                    expected: *digest,
+                    state: SliceState::Unknown,
+                });
+            }
+        }
+        file_bounds.push(cells.len());
+        Self {
+            slice_size: layout.slice_size,
+            cells,
+            file_bounds,
+        }
+    }
+
+    /// Record that `len` bytes were written to `file_index` at `offset`.
+    /// Returns the global indices of any slices that became fully filled with
+    /// this write and are ready to [`verify`](Self::verify).
+    pub fn mark_written(&mut self, file_index: usize, offset: u64, len: u64) -> Vec<usize> {
+        let mut completed = Vec::new();
+        let Some(&start_g) = self.file_bounds.get(file_index) else {
+            return completed;
+        };
+        let end_g = self.file_bounds[file_index + 1];
+        let (w0, w1) = (offset, offset + len);
+        for gi in start_g..end_g {
+            let cell = &mut self.cells[gi];
+            if cell.state != SliceState::Unknown {
+                continue;
+            }
+            let ov = overlap(cell.start, cell.start + cell.len, w0, w1);
+            if ov == 0 {
+                continue;
+            }
+            cell.filled = (cell.filled + ov).min(cell.len);
+            if cell.filled == cell.len && cell.len > 0 {
+                completed.push(gi);
+            }
+        }
+        completed
+    }
+
+    /// Which file and byte range a global slice covers, so the caller can read
+    /// the slice's bytes back to verify it. Returns `(file_index, offset, len)`.
+    pub fn slice_range(&self, global_index: usize) -> Option<(usize, u64, u64)> {
+        self.cells
+            .get(global_index)
+            .map(|c| (c.file_index, c.start, c.len))
+    }
+
+    /// Verify a completed slice against its PAR2 checksums. `bytes` is the
+    /// slice's actual bytes (`len` from [`slice_range`](Self::slice_range));
+    /// they are zero-padded to the slice size before hashing, as PAR2 does.
+    pub fn verify(&mut self, global_index: usize, bytes: &[u8]) -> SliceState {
+        let slice_size = self.slice_size as usize;
+        let Some(cell) = self.cells.get_mut(global_index) else {
+            return SliceState::Unknown;
+        };
+        let mut padded = bytes.to_vec();
+        padded.resize(slice_size, 0);
+
+        let md5: Md5Hash = Md5::digest(&padded).into();
+        let mut crc = Crc32::new();
+        crc.update(&padded);
+        let crc = crc.finalize();
+
+        cell.state = if md5 == cell.expected.md5 && crc == cell.expected.crc32 {
+            SliceState::Verified
+        } else {
+            SliceState::Damaged
+        };
+        cell.state
+    }
+
+    /// Mark a slice damaged without hashing — used when the ledger confirms an
+    /// article overlapping the slice is missing.
+    pub fn mark_damaged(&mut self, global_index: usize) {
+        if let Some(cell) = self.cells.get_mut(global_index) {
+            cell.state = SliceState::Damaged;
+        }
+    }
+
+    /// The verdict of every slice, in global index order.
+    pub fn states(&self) -> Vec<SliceState> {
+        self.cells.iter().map(|c| c.state).collect()
+    }
+
+    /// Total number of slices in the set.
+    pub fn total_slices(&self) -> usize {
+        self.cells.len()
+    }
+
+    /// Number of slices currently marked `Damaged`.
+    pub fn damaged_count(&self) -> usize {
+        self.cells
+            .iter()
+            .filter(|c| c.state == SliceState::Damaged)
+            .count()
+    }
+
+    /// True once no slice is still `Unknown` — the map is decided.
+    pub fn is_complete(&self) -> bool {
+        self.cells.iter().all(|c| c.state != SliceState::Unknown)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn digest_of(slice: &[u8], slice_size: usize) -> SliceDigest {
+        let mut padded = slice.to_vec();
+        padded.resize(slice_size, 0);
+        let md5: Md5Hash = Md5::digest(&padded).into();
+        let mut crc = Crc32::new();
+        crc.update(&padded);
+        SliceDigest {
+            md5,
+            crc32: crc.finalize(),
+        }
+    }
+
+    /// Build a one-file layout plus the payload for `size` bytes at
+    /// `slice_size`, with correct per-slice checksums.
+    fn one_file(size: usize, slice_size: usize) -> (SliceLayout, Vec<u8>) {
+        let payload: Vec<u8> = (0..size).map(|i| (i % 251) as u8).collect();
+        let slices = payload
+            .chunks(slice_size)
+            .map(|c| digest_of(c, slice_size))
+            .collect();
+        let layout = SliceLayout {
+            slice_size: slice_size as u64,
+            files: vec![FileSlices {
+                size: size as u64,
+                slices,
+            }],
+        };
+        (layout, payload)
+    }
+
+    #[test]
+    fn in_order_writes_verify_all_slices() {
+        let (layout, payload) = one_file(250, 100); // slices of 100, 100, 50
+        let mut v = SliceVerifier::new(&layout);
+        assert_eq!(v.total_slices(), 3);
+
+        // Three articles, one per slice, in order.
+        for (i, chunk) in payload.chunks(100).enumerate() {
+            let completed = v.mark_written(0, (i * 100) as u64, chunk.len() as u64);
+            assert_eq!(completed, vec![i]);
+            let (_f, off, len) = v.slice_range(i).unwrap();
+            assert_eq!(
+                v.verify(i, &payload[off as usize..(off + len) as usize]),
+                SliceState::Verified
+            );
+        }
+        assert!(v.is_complete());
+        assert_eq!(v.damaged_count(), 0);
+        assert_eq!(v.states(), vec![SliceState::Verified; 3]);
+    }
+
+    #[test]
+    fn article_spanning_two_slices_and_out_of_order() {
+        let (layout, payload) = one_file(300, 100);
+        let mut v = SliceVerifier::new(&layout);
+
+        // A write covering [50, 250) straddles slices 0, 1 and 2 but completes
+        // only the fully covered middle slice 1.
+        assert_eq!(v.mark_written(0, 50, 200), vec![1]);
+        // Fill the head of slice 0 and the tail of slice 2, out of order.
+        assert_eq!(v.mark_written(0, 250, 50), vec![2]);
+        assert_eq!(v.mark_written(0, 0, 50), vec![0]);
+
+        for gi in 0..3 {
+            let (_f, off, len) = v.slice_range(gi).unwrap();
+            assert_eq!(
+                v.verify(gi, &payload[off as usize..(off + len) as usize]),
+                SliceState::Verified
+            );
+        }
+        assert!(v.is_complete());
+    }
+
+    #[test]
+    fn corrupt_bytes_mark_slice_damaged() {
+        let (layout, payload) = one_file(200, 100);
+        let mut v = SliceVerifier::new(&layout);
+
+        v.mark_written(0, 0, 100);
+        let mut corrupt = payload[0..100].to_vec();
+        corrupt[0] ^= 0xff;
+        assert_eq!(v.verify(0, &corrupt), SliceState::Damaged);
+        assert_eq!(v.damaged_count(), 1);
+        assert!(!v.is_complete()); // slice 1 still Unknown
+    }
+
+    #[test]
+    fn partial_fill_does_not_complete() {
+        let (layout, _payload) = one_file(200, 100);
+        let mut v = SliceVerifier::new(&layout);
+        // Only half of slice 0.
+        assert!(v.mark_written(0, 0, 50).is_empty());
+        assert_eq!(v.states()[0], SliceState::Unknown);
+    }
+
+    #[test]
+    fn mark_damaged_sets_state_directly() {
+        let (layout, _payload) = one_file(150, 100);
+        let mut v = SliceVerifier::new(&layout);
+        v.mark_damaged(1);
+        assert_eq!(v.states()[1], SliceState::Damaged);
+        assert_eq!(v.damaged_count(), 1);
+    }
+}