diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index dca3504..7b0b5c3 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -28,8 +28,9 @@ jobs:
         # dependency a failure (CLAWHDF5_REQUIRE_INTEROP below).
         run: |
           apt-get update
-          # cmake builds libz-ng-sys (clawhdf5-format's default `fast-deflate`);
-          # rust:latest does not ship it.
+          # cmake builds libz-ng-sys for the opt-in `fast-deflate` (zlib-ng)
+          # steps in ci-test.sh; rust:latest does not ship it. The default
+          # build (pure-Rust zlib-rs) does not need it.
           apt-get install -y --no-install-recommends python3 python3-venv cmake
           python3 -m venv /opt/interop
           /opt/interop/bin/pip install --no-cache-dir h5py numpy netCDF4 xarray
@@ -75,19 +76,6 @@ jobs:
           command -v rustup >/dev/null || curl -sSf --retry 5 https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain none
           rustup toolchain install stable --profile minimal --component clippy
           echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
-      - name: Build dependencies
-        # libz-ng-sys (clawhdf5-format's default `fast-deflate`) needs cmake.
-        # In the Docker runner the job is root and can install it; a host
-        # runner cannot, so say what is missing rather than fail inside a
-        # build script.
-        run: |
-          if command -v cmake >/dev/null; then cmake --version | head -1; exit 0; fi
-          if [ "$(id -u)" = 0 ]; then
-            apt-get update -qq && apt-get install -y -qq cmake
-          else
-            echo "::error::cmake is not installed on this runner host (needed by libz-ng-sys)"
-            exit 1
-          fi
       - name: Confirm aarch64
         run: |
           test "$(uname -m)" = aarch64
diff --git a/BENCHMARKS.md b/BENCHMARKS.md
index 1d782e4..969d1ed 100644
--- a/BENCHMARKS.md
+++ b/BENCHMARKS.md
@@ -1185,6 +1185,69 @@ cargo run --release --bin ephemeral_perf
 
 ---
 
+## Deflate backend: zlib-rs vs zlib-ng
+
+Measured 2026-09-23 on tank (AMD Ryzen 7 7800X3D, 8C/16T). The default
+deflate backend is now **zlib-rs**, a pure-Rust port of zlib-ng; zlib-ng (C,
+built with cmake) was the default before and is still available as
+`fast-deflate`. Both builds were compiled once into separate target
+directories and run **alternately, three rounds each**; figures are medians.
+
+```bash
+# zlib-rs (default)
+cargo bench -p clawhdf5-filters --bench deflate_bench
+cargo bench -p clawhdf5-bench --bench h5bench_write --features libhdf5-compare -- '^write_2d_chunked/'
+cargo run --release -p clawhdf5-bench --bin read_harness
+# zlib-ng: add --features fast-deflate (filters) or clawhdf5-format/fast-deflate (bench)
+```
+
+| Workload | zlib-rs | zlib-ng | rs / ng |
+|---|---:|---:|---:|
+| HDF5 chunked write, deflate-6, 512×512 f32 | 1.458 ms | 1.484 ms | 0.98 |
+| HDF5 chunked write, deflate-6, 128×128 f32 | 157.6 µs | 152.8 µs | 1.03 |
+| HDF5 chunked write, deflate-6, 32×32 f32 | 62.9 µs | 60.2 µs | 1.05 |
+| HDF5 read, 64 MB chunked + deflate, full | 64.4 ms | 65.2 ms | 0.99 |
+| HDF5 read, 64×64 window (1 chunk) | 0.18 ms | 0.17 ms | 1.06 |
+| HDF5 read, 512×512 window (4–9 chunks) | 4.10 ms | 4.10 ms | 1.00 |
+| HDF5 read, one row / one column | 1.00 / 2.01 ms | 0.95 / 1.95 ms | 1.05 / 1.03 |
+| Raw inflate, 8 MB f64 | 5.92 ms | 6.06 ms | 0.98 |
+| Raw inflate, 1 MB sine | 82.8 µs | 68.6 µs | 1.21 |
+| Raw deflate-6, 8 MB f64 / 1 MB sine | 92.2 / 2.01 ms | 83.1 / 1.84 ms | 1.11 / 1.09 |
+
+Compressed output is **byte-identical** between the two at levels 1, 6 and 9
+on all three inputs, so files do not change size. libhdf5 1.14.6 took 51.4 ms
+for the 512×512 write in the same session (35× the zlib-rs figure).
+
+On the HDF5 paths zlib-rs is within 6% of zlib-ng everywhere, and ahead on the
+largest write. The raw codec loops show zlib-ng still slightly faster at
+compression (~10%), which chunked writes do not expose because encoding runs
+in parallel across chunks.
+
+**Two findings along the way.** The first measurement had zlib-rs 1.2–1.9×
+slower on single-chunk reads and 3.7× slower on a 1 MB inflate — slower even
+than miniz_oxide. Neither was zlib-rs's fault:
+
+1. **Runtime CPU detection was off.** zlib-rs needs its `std` feature to
+   detect and use SIMD at runtime; flate2 turns it on through its default
+   `runtime_detection` feature, which our `default-features = false` flate2
+   dependency was disabling. With it, a 1 MB inflate goes 282 → 83 µs.
+   `clawhdf5-format/zlib-rs` and `clawhdf5-filters/zlib-rs` now enable it.
+2. **The codec was fed through a 32 KiB buffer.** Both deflate paths used
+   flate2's streaming `read::ZlibDecoder` / `write::ZlibEncoder`. A chunk's
+   decompressed size is known, so they now hand the codec the whole input in
+   one call, into an output buffer sized up front. Worth ~5% on chunked
+   writes and ~10% on zlib-ng's 1 MB inflate. It also closed a hole: the
+   streaming reader returned a truncated stream's bytes without an error, so
+   a truncated chunk read back short; it is now an error.
+
+| 1 MB inflate, same build otherwise | zlib-rs | zlib-ng |
+|---|---:|---:|
+| streaming reader, no runtime detection | 284.1 µs | 76.7 µs |
+| one-shot, no runtime detection | 282.3 µs | 68.5 µs |
+| one-shot + runtime detection (shipped) | **82.8 µs** | **68.6 µs** |
+
+---
+
 ## h5bench-Equivalent I/O Benchmarks
 
 Criterion harness mirroring h5bench serial workloads. clawhdf5 benchmarks dated 2026-07-01;
diff --git a/CHANGELOG.md b/CHANGELOG.md
index cdccb42..e437f17 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,14 @@
 ## Unreleased
 
 ### Upgrade Notes
+- **The default build no longer compiles any C.** Deflate now defaults to the
+  pure-Rust zlib-rs instead of zlib-ng, so building the core crates needs
+  neither cmake nor a C compiler. Speed on HDF5 reads and writes is within 6%
+  of zlib-ng, and compressed output is byte-identical. To keep zlib-ng, enable
+  `fast-deflate` (on `clawhdf5`, `clawhdf5-format` or `clawhdf5-filters`); it
+  overrides zlib-rs wherever it is on.
+- **A truncated deflate chunk is now an error.** It used to read back short,
+  with no error.
 - **New stores use the int8 vector index by default.**
   `MemoryConfig::quantized_index` now defaults to `true`: a quarter of the
   index memory, builds 1.8x (x86-64) and 2.3x (Raspberry Pi 5) faster, and
@@ -13,6 +21,28 @@
   `quantized_index = false`, or pass `create --f32-index` to the CLI, to opt
   out. The CLI's `--quantized-index` is still accepted but is now a no-op.
 
+### Build
+- **Pure-Rust default.** `clawhdf5-format`, `clawhdf5-filters` and the
+  `clawhdf5` facade default to the `zlib-rs` deflate backend; `fast-deflate`
+  (zlib-ng) is opt-in. No crate in the default dependency tree of the core
+  crates compiles C, and `ci-test.sh` now fails if one appears. The facade's
+  `fast-deflate` was on by default and is now off. See `BENCHMARKS.md`,
+  "Deflate backend".
+- `zlib-rs` also enables flate2's `runtime_detection`. Without it zlib-rs has
+  no `std`, cannot detect SIMD at runtime, and inflates 3.5x slower; the
+  workspace builds flate2 with `default-features = false`, which had been
+  switching it off.
+- CI keeps zlib-ng building and tested; the arm64 job no longer needs cmake.
+
+### Correctness
+- `clawhdf5-format`: **a truncated deflate chunk read back short, with no
+  error.** The deflate filter used flate2's streaming reader, which returns the
+  bytes it has when the input runs out before the end-of-stream marker. It now
+  decodes in one pass into a buffer sized to the chunk and reports a
+  truncated stream as `DecompressionError`. Same fix in `clawhdf5-filters`,
+  where output longer than the stated size was also silently cut off; it is
+  now an error.
+
 ### Defaults
 - `clawhdf5-agent`: `MemoryConfig::quantized_index` defaults to `true` for new
   stores. The reason it had been off — that int8 search was slower on ARM —
@@ -26,6 +56,10 @@
   knew to ask; it now only ever switches the default off.
 
 ### Performance
+- `clawhdf5-format`, `clawhdf5-filters`: both deflate paths hand the codec the
+  whole chunk in one call, into a buffer allocated once, instead of streaming
+  it through a 32 KiB buffer: about 5% on chunked writes and 10% on zlib-ng's
+  1 MB inflate.
 - `clawhdf5-accel`: **`dot_i8` has aarch64 kernels** — `SDOT` for CPUs with
   the ARMv8.2 dot-product extension (Cortex-A76 and later, Neoverse-N1, every
   Apple Silicon generation) and plain NEON (`vmull_s8` + `vpadalq_s16`) for
diff --git a/CLAUDE.md b/CLAUDE.md
index 6165035..6b9fa24 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -27,7 +27,11 @@ Cargo workspace with 16 crates under `crates/` (plus `libaec-sys`, an internal F
 | `clawhdf5-bench` | Benchmark suite |
 
 ## Key Features
-- Zero-dependency HDF5 read/write (no libhdf5 C library required)
+- Zero-C-dependency HDF5 read/write: no libhdf5, and deflate defaults to
+  pure-Rust zlib-rs (`fast-deflate` opts into zlib-ng, which needs cmake).
+  `ci-test.sh` fails if a C-building crate enters the core crates' default
+  tree. flate2 must keep `runtime_detection` with zlib-rs — without it zlib-rs
+  loses SIMD and inflates 3.5x slower.
 - HNSW vector index for semantic similarity search over agent memories — the
   `clawhdf5-agent` `hnsw` feature is **on by default**, so `hybrid_search` uses
   the approximate `clawhdf5-ann` index for the vector stage (the index mirrors
@@ -128,8 +132,9 @@ cargo test --workspace
 
 Keep workflows free of JavaScript actions (`actions/checkout`, `actions/cache`,
 …): `rust:latest` has no `node`, and not every runner reaches GitHub, where
-they are fetched from. Check out with plain `git` instead. Both jobs need
-`cmake` for `libz-ng-sys` (from `clawhdf5-format`'s default `fast-deflate`).
+they are fetched from. Check out with plain `git` instead. The `test` job
+installs `cmake` for the opt-in `fast-deflate` (zlib-ng) steps; the default
+build needs no C toolchain, so `test-arm64` does not.
 All runners are on `gitea-runner` 3.5.0, from `docker.gitea.com/act_runner`
 — `gitea/act_runner:latest` on Docker Hub is frozen at 0.6.1.
 
diff --git a/README.md b/README.md
index 990b1c3..cf93565 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # ClawhDF5
 
-**The memory layer AI agents deserve. One file. Pure Rust. No libhdf5.**
+**The memory layer AI agents deserve. One file. Pure Rust. Zero C dependencies.**
 
 [![License: MIT](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
 [![Rust](https://img.shields.io/badge/rust-1.94%2B-orange.svg)](https://www.rust-lang.org)
@@ -11,7 +11,7 @@
 ClawHDF5 is a pure-Rust HDF5 implementation combined with a research-grade agent memory engine. It gives AI agents persistent, searchable, integrity-checked memory — all stored in a single portable file.
 
 > **Two things live here:**
-> - **A general-purpose, pure-Rust HDF5 library** — no libhdf5, NetCDF-4 support, SIMD/GPU acceleration. See the **[Crate Map](#crate-map)** and **[BENCHMARKS.md](BENCHMARKS.md)** for the libhdf5 head-to-head numbers.
+> - **A general-purpose, pure-Rust HDF5 library** — zero C dependencies, NetCDF-4 support, SIMD/GPU acceleration. See the **[Crate Map](#crate-map)** and **[BENCHMARKS.md](BENCHMARKS.md)** for the libhdf5 head-to-head numbers.
 > - **An agent memory layer built on top of it** — vector search, knowledge graph, hippocampal-style consolidation, in `clawhdf5-agent`.
 
 The crates are not on crates.io yet, so depend on them from git:
@@ -22,10 +22,16 @@ clawhdf5       = { git = "https://git.redclaw.dev/quantumclaw/clawhdf5" }  # cor
 clawhdf5-agent = { git = "https://git.redclaw.dev/quantumclaw/clawhdf5" }  # + agent memory layer
 ```
 
-> **C dependencies, precisely:** the HDF5 format code is pure Rust and never
-> links libhdf5. The default deflate backend is zlib-ng (`fast-deflate`), a C
-> library built from source, so a default build needs `cmake` and a C
-> compiler. Opt-in codecs (`zstd`, `szip`) and BLAS backends link C too.
+> **C dependencies, precisely:** the core crates (`clawhdf5`, `clawhdf5-agent`,
+> `-format`, `-io`, `-filters`, `-ann`, `-accel`, `-netcdf4`, `-cli`) build no C
+> code by default — no libhdf5, and deflate is the pure-Rust
+> [zlib-rs](https://github.com/trifectatechfoundation/zlib-rs), which matches
+> zlib-ng on HDF5 reads and writes and produces byte-identical output
+> ([BENCHMARKS.md § Deflate backend](BENCHMARKS.md#deflate-backend-zlib-rs-vs-zlib-ng)).
+> CI fails if a C-building crate enters their default dependency tree. C comes
+> in only when you ask for it: `fast-deflate` (zlib-ng, needs cmake), `zstd`,
+> `szip`, the BLAS backends, `clawhdf5-migrate` (bundled SQLite) and the
+> Node.js bindings.
 
 > **New here?** Start with the **[Quickstart Guide](docs/QUICKSTART.md)** · See **[Use Cases](docs/USE_CASES.md)** · Read **[Benchmarks](BENCHMARKS.md)**
 
@@ -117,6 +123,12 @@ Figures below are from an independent reproduction run on a second machine (AMD
 | Sequential read (100K f32) | 23.3 µs | 63.6 µs | **2.7×** |
 | Sequential write (100K f32) | 210 µs | 189 µs | **≈ tie** |
 
+The chunked-write row was re-measured on the same machine on 2026-09-23, after
+the default deflate backend became pure-Rust zlib-rs: 1.46 ms against
+libhdf5's 51.4 ms (**35×**), and 1.48 ms with zlib-ng. libhdf5's own time on
+that machine moved from 65.0 to 51.4 ms between the two dates, which is most
+of the difference from 45×; compare same-day numbers only.
+
 ### Vector Search
 
 **HNSW (the default backend for `hybrid_search`)** — `search_harness`, clustered
@@ -564,14 +576,15 @@ setting existed keep their `f32` index; opt out for new stores with
 | `deflate` | yes | Deflate compression |
 | `checksum` | yes | Jenkins lookup3 verification |
 | `provenance` | yes | SHA-256 provenance attributes |
-| `fast-deflate` | **yes** | zlib-ng backend for faster deflate (C; needs `cmake`) |
+| `zlib-rs` | **yes** | Pure-Rust deflate backend ([zlib-rs](https://github.com/trifectatechfoundation/zlib-rs)) |
+| `fast-deflate` | no | zlib-ng deflate backend instead (C; needs `cmake`). Overrides `zlib-rs` when both are on |
 | `system-zlib-decompress` | **yes** | Use Apple's system libz for decompression (macOS only; no effect elsewhere) |
 | `parallel` | no | Parallel chunk encoding + compression (rayon) |
 | `fast-checksum` | no | crc32fast-accelerated checksums |
 | `lz4` | no | LZ4 block compression filter (id 32004) |
 | `zstd` | no | Zstandard compression filter (id 32015) |
 | `pcodec` | no | Pcodec lossless numerical codec (id 32023, via `pco` crate) |
-| `system-zlib` / `zlib-rs` | no | Alternative zlib backends for deflate |
+| `system-zlib` | no | System zlib backend for deflate (C) |
 | `blake3_hash` | no | BLAKE3 content hashing for provenance |
 | `szip` | no | SZIP filter (id 4) via libaec (C, through the internal `libaec-sys` crate) |
 
@@ -601,7 +614,7 @@ setting existed keep their `f32` index; opt out for new stores with
 ## Building
 
 ```bash
-# Default (needs cmake + a C compiler for zlib-ng)
+# Default (pure Rust: no cmake or C compiler needed)
 cargo build --workspace
 
 # Agent memory with all accelerations (Linux)
@@ -723,5 +736,5 @@ MIT
 
 <p align="center">
   <em>Built by <a href="https://git.redclaw.dev/quantumclaw">RedClaw Systems</a></em><br>
-  <em>~86,000 lines of Rust. No libhdf5. One file to remember everything.</em>
+  <em>~86,000 lines of Rust. Zero C dependencies. One file to remember everything.</em>
 </p>
diff --git a/crates/clawhdf5-filters/Cargo.toml b/crates/clawhdf5-filters/Cargo.toml
index a2202b8..07fdeff 100644
--- a/crates/clawhdf5-filters/Cargo.toml
+++ b/crates/clawhdf5-filters/Cargo.toml
@@ -25,8 +25,12 @@ name = "compression_bench"
 harness = false
 
 [features]
-default = ["fast-deflate"]
+# Pure-Rust zlib-rs by default; `fast-deflate` (zlib-ng, C) overrides it.
+default = ["zlib-rs"]
 fast-deflate = ["flate2/zlib-ng"]
 system-zlib = ["flate2/zlib-default"]
-zlib-rs = ["flate2/zlib-rs"]
+# `runtime_detection` gives zlib-rs `std`, which it needs to detect and use
+# SIMD at runtime. flate2 enables it by default, but we build flate2 with
+# default-features = false, and without it zlib-rs inflates 3.5x slower.
+zlib-rs = ["flate2/zlib-rs", "flate2/runtime_detection"]
 apple-compression = []
diff --git a/crates/clawhdf5-filters/README.md b/crates/clawhdf5-filters/README.md
index 4ccd3fd..45fcf30 100644
--- a/crates/clawhdf5-filters/README.md
+++ b/crates/clawhdf5-filters/README.md
@@ -8,16 +8,18 @@ Filter and compression pipeline for clawhdf5.
 ## Features
 
 - DEFLATE compression/decompression
-- Fast deflate via zlib-ng (`fast-deflate` feature)
+- Pure-Rust deflate via zlib-rs (default, `zlib-rs` feature)
+- zlib-ng instead, if you want it (`fast-deflate` feature; C, needs cmake)
 - Apple Compression framework support (`apple-compression` feature)
 
 ## Usage
 
 ```rust
-use clawhdf5_filters::{deflate_decode, deflate_encode};
+use clawhdf5_filters::{deflate_compress, deflate_decompress};
 
-let compressed = deflate_encode(&data, 6).unwrap();
-let decompressed = deflate_decode(&compressed).unwrap();
+let compressed = deflate_compress(&data, 6).unwrap();
+// The second argument bounds the output: the expected decompressed size.
+let decompressed = deflate_decompress(&compressed, data.len()).unwrap();
 ```
 
 ## License
diff --git a/crates/clawhdf5-filters/src/fast_deflate.rs b/crates/clawhdf5-filters/src/fast_deflate.rs
index 6b4c65a..6ce268f 100644
--- a/crates/clawhdf5-filters/src/fast_deflate.rs
+++ b/crates/clawhdf5-filters/src/fast_deflate.rs
@@ -1,12 +1,13 @@
-//! Fast deflate backends: Apple Compression Framework and zlib-ng.
+//! Deflate backends: Apple Compression Framework, zlib-ng and zlib-rs.
 //!
 //! Backend selection priority (decompression & compression):
 //! 1. Apple Compression Framework (macOS only, `apple-compression` feature)
-//! 2. flate2 with zlib-ng backend (`fast-deflate` feature) or miniz_oxide (default)
+//! 2. flate2 with zlib-ng (`fast-deflate`), else zlib-rs (`zlib-rs`, the
+//!    default), else miniz_oxide
 //!
 //! The Apple Compression Framework uses hardware-accelerated zlib on Apple Silicon
-//! and is typically the fastest option on macOS. zlib-ng is the fastest portable
-//! option and what C HDF5 uses internally.
+//! and is typically the fastest option on macOS. zlib-rs is a pure-Rust port of
+//! zlib-ng; see `BENCHMARKS.md` for how the two compare.
 
 // ---------------------------------------------------------------------------
 // Apple Compression Framework FFI (macOS only)
@@ -243,65 +244,117 @@ mod apple {
 }
 
 // ---------------------------------------------------------------------------
-// Streaming decompression via flate2 (uses zlib-ng when fast-deflate enabled)
+// One-shot (de)compression via flate2 (whichever backend flate2 was built with)
+//
+// The whole input goes to the codec in one call, into an output buffer sized
+// up front. `flate2::read::ZlibDecoder` / `write::ZlibEncoder` stream through a
+// 32 KiB buffer instead, which cost zlib-rs up to 3.7x against zlib-ng on a
+// 1 MB chunk. clawhdf5-format's deflate filter does the same; see
+// `BENCHMARKS.md`, "Deflate backend".
 // ---------------------------------------------------------------------------
 
-/// Streaming decompress with pre-allocated output buffer.
-///
-/// When the output size is known (typical for HDF5 chunks), this avoids
-/// dynamic reallocation by writing directly into a pre-sized buffer.
+/// Decompress into a buffer pre-sized to `output_size`, the expected
+/// decompressed length (known for HDF5 chunks). Output longer than that is an
+/// error, as is a stream that ends early.
 pub(crate) fn flate2_decompress_preallocated(
     data: &[u8],
     output_size: usize,
 ) -> Result<Vec<u8>, String> {
-    use std::io::Read;
-    let mut decoder = flate2::read::ZlibDecoder::new(data);
-    let mut output = vec![0u8; output_size];
-    let mut total_read = 0;
-
-    loop {
-        match decoder.read(&mut output[total_read..]) {
-            Ok(0) => break,
-            Ok(n) => total_read += n,
-            Err(e) => return Err(e.to_string()),
-        }
-    }
-    output.truncate(total_read);
-    Ok(output)
+    inflate_bounded(data, output_size, output_size)
 }
 
 /// Absolute ceiling on decompressed output when the caller has no size hint,
 /// preventing unbounded allocation from a hostile/corrupted zlib stream.
 const MAX_DECOMPRESS_SIZE: usize = 256 * 1024 * 1024;
 
-/// Streaming decompress with dynamic sizing (when output size is unknown).
-///
-/// Bounded by [`MAX_DECOMPRESS_SIZE`] since there is no chunk-size hint to
-/// validate against here — an unbounded `read_to_end` would let a hostile
-/// zlib stream force arbitrarily large allocation (a "zlib bomb").
+/// Decompress with no size hint, bounded by [`MAX_DECOMPRESS_SIZE`] so a
+/// hostile zlib stream cannot force arbitrarily large allocation (a "zlib
+/// bomb").
 pub(crate) fn flate2_decompress_streaming(data: &[u8]) -> Result<Vec<u8>, String> {
-    use std::io::Read;
-    let decoder = flate2::read::ZlibDecoder::new(data);
-    let mut result = Vec::new();
-    decoder
-        .take(MAX_DECOMPRESS_SIZE as u64 + 1)
-        .read_to_end(&mut result)
-        .map_err(|e| e.to_string())?;
-    if result.len() > MAX_DECOMPRESS_SIZE {
-        return Err(format!(
-            "decompressed output exceeds {} MiB limit",
-            MAX_DECOMPRESS_SIZE / 1024 / 1024
-        ));
-    }
-    Ok(result)
+    let hint = data.len().saturating_mul(4).min(1 << 20);
+    inflate_bounded(data, hint, MAX_DECOMPRESS_SIZE).map_err(|e| {
+        if e.ends_with("exceeds size limit") {
+            format!(
+                "decompressed output exceeds {} MiB limit",
+                MAX_DECOMPRESS_SIZE / 1024 / 1024
+            )
+        } else {
+            e
+        }
+    })
 }
 
-/// Compress data using flate2 (zlib-ng when fast-deflate enabled, else miniz_oxide).
+/// Inflate a zlib stream, starting from `size_hint` bytes of output and
+/// failing past `limit`.
+fn inflate_bounded(data: &[u8], size_hint: usize, limit: usize) -> Result<Vec<u8>, String> {
+    use flate2::{Decompress, FlushDecompress, Status};
+
+    // One byte of headroom past the limit distinguishes an over-size stream
+    // from one that legitimately ends exactly at the limit.
+    let max_capacity = limit.saturating_add(1);
+    let mut out = Vec::new();
+    out.try_reserve_exact(size_hint.clamp(1, max_capacity))
+        .map_err(|e| format!("deflate: cannot allocate output: {e}"))?;
+
+    let mut inflater = Decompress::new(true);
+    loop {
+        let (in_before, out_before) = (inflater.total_in(), inflater.total_out());
+        let status = inflater
+            .decompress_vec(
+                &data[in_before as usize..],
+                &mut out,
+                FlushDecompress::Finish,
+            )
+            .map_err(|e| format!("deflate: {e}"))?;
+        if out.len() > limit {
+            return Err("deflate: output exceeds size limit".into());
+        }
+        match status {
+            Status::StreamEnd => return Ok(out),
+            Status::Ok | Status::BufError if out.len() == out.capacity() => {
+                let grow = out.capacity().min(max_capacity - out.capacity()).max(1);
+                out.try_reserve_exact(grow)
+                    .map_err(|e| format!("deflate: cannot allocate output: {e}"))?;
+            }
+            Status::Ok | Status::BufError => {
+                if inflater.total_in() as usize >= data.len()
+                    || (inflater.total_in(), inflater.total_out()) == (in_before, out_before)
+                {
+                    return Err("deflate: truncated stream".into());
+                }
+            }
+        }
+    }
+}
+
+/// Compress data using flate2 (zlib-ng, zlib-rs or miniz_oxide; see module docs).
 pub(crate) fn flate2_compress(data: &[u8], level: u32) -> Result<Vec<u8>, String> {
-    use std::io::Write;
-    let mut encoder = flate2::write::ZlibEncoder::new(Vec::new(), flate2::Compression::new(level));
-    encoder.write_all(data).map_err(|e| e.to_string())?;
-    encoder.finish().map_err(|e| e.to_string())
+    use flate2::{Compress, Compression, FlushCompress, Status};
+
+    // zlib's compressBound, plus the zlib header and trailer.
+    let bound = data.len() + (data.len() >> 12) + (data.len() >> 14) + (data.len() >> 25) + 13 + 6;
+    let mut out = Vec::new();
+    out.try_reserve_exact(bound)
+        .map_err(|e| format!("deflate: cannot allocate output: {e}"))?;
+
+    let mut deflater = Compress::new(Compression::new(level), true);
+    loop {
+        let (in_before, out_before) = (deflater.total_in(), deflater.total_out());
+        let status = deflater
+            .compress_vec(&data[in_before as usize..], &mut out, FlushCompress::Finish)
+            .map_err(|e| format!("deflate: {e}"))?;
+        match status {
+            Status::StreamEnd => return Ok(out),
+            Status::Ok | Status::BufError if out.len() == out.capacity() => out
+                .try_reserve(out.capacity().max(4096))
+                .map_err(|e| format!("deflate: cannot allocate output: {e}"))?,
+            Status::Ok | Status::BufError => {
+                if (deflater.total_in(), deflater.total_out()) == (in_before, out_before) {
+                    return Err("deflate: encoder made no progress".into());
+                }
+            }
+        }
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -312,7 +365,7 @@ pub(crate) fn flate2_compress(data: &[u8], level: u32) -> Result<Vec<u8>, String
 ///
 /// Selection order:
 /// 1. Apple Compression Framework (macOS + `apple-compression` feature)
-/// 2. flate2 (zlib-ng with `fast-deflate`, otherwise miniz_oxide)
+/// 2. flate2 (zlib-ng with `fast-deflate`, else zlib-rs, else miniz_oxide)
 ///
 /// When `output_hint` > 0, pre-allocates the output buffer for zero-copy
 /// decompression (avoids reallocation).
@@ -344,7 +397,7 @@ pub fn decompress(data: &[u8], output_hint: usize) -> Result<Vec<u8>, String> {
 ///
 /// Selection order:
 /// 1. Apple Compression Framework (macOS + `apple-compression` feature)
-/// 2. flate2 (zlib-ng with `fast-deflate`, otherwise miniz_oxide)
+/// 2. flate2 (zlib-ng with `fast-deflate`, else zlib-rs, else miniz_oxide)
 pub fn compress(data: &[u8], level: u32) -> Result<Vec<u8>, String> {
     #[cfg(all(target_os = "macos", feature = "apple-compression"))]
     {
@@ -377,9 +430,19 @@ pub fn active_backend() -> &'static str {
     {
         "zlib-ng"
     }
+    // flate2 prefers a C zlib over zlib-rs when both are enabled.
+    #[cfg(all(
+        not(all(target_os = "macos", feature = "apple-compression")),
+        not(feature = "fast-deflate"),
+        feature = "zlib-rs"
+    ))]
+    {
+        "zlib-rs"
+    }
     #[cfg(not(any(
         all(target_os = "macos", feature = "apple-compression"),
-        feature = "fast-deflate"
+        feature = "fast-deflate",
+        feature = "zlib-rs"
     )))]
     {
         "miniz_oxide"
@@ -436,7 +499,7 @@ mod tests {
     fn backend_name_is_set() {
         let name = active_backend();
         assert!(
-            ["miniz_oxide", "zlib-ng", "apple-compression"].contains(&name),
+            ["miniz_oxide", "zlib-rs", "zlib-ng", "apple-compression"].contains(&name),
             "unexpected backend: {name}"
         );
     }
diff --git a/crates/clawhdf5-filters/src/lib.rs b/crates/clawhdf5-filters/src/lib.rs
index ae67589..d9d18eb 100644
--- a/crates/clawhdf5-filters/src/lib.rs
+++ b/crates/clawhdf5-filters/src/lib.rs
@@ -2,12 +2,14 @@
 //!
 //! Provides deflate (zlib) decompression/compression with multiple backend options:
 //!
-//! - **Default**: `miniz_oxide` (pure Rust, no C dependencies)
-//! - **`fast-deflate` feature**: `zlib-ng` via flate2 (~2-3x faster, matches C HDF5)
+//! - **Default (`zlib-rs` feature)**: `zlib-rs` via flate2 (pure Rust, no C
+//!   dependencies)
+//! - **`fast-deflate` feature**: `zlib-ng` via flate2 (C, built with cmake)
 //! - **`apple-compression` feature**: Apple Compression Framework on macOS
 //!   (hardware-accelerated on Apple Silicon)
+//! - With none of the above: `miniz_oxide` (pure Rust, slower)
 //!
-//! Backend priority: apple-compression > zlib-ng > miniz_oxide.
+//! Backend priority: apple-compression > zlib-ng > zlib-rs > miniz_oxide.
 
 pub mod fast_deflate;
 
@@ -115,7 +117,7 @@ mod tests {
     fn backend_reports_name() {
         let name = deflate_backend();
         assert!(
-            ["miniz_oxide", "zlib-ng", "apple-compression"].contains(&name),
+            ["miniz_oxide", "zlib-rs", "zlib-ng", "apple-compression"].contains(&name),
             "unexpected backend: {name}"
         );
     }
diff --git a/crates/clawhdf5-format/Cargo.toml b/crates/clawhdf5-format/Cargo.toml
index 7263497..fe38392 100644
--- a/crates/clawhdf5-format/Cargo.toml
+++ b/crates/clawhdf5-format/Cargo.toml
@@ -32,7 +32,10 @@ name = "bench"
 harness = false
 
 [features]
-default = ["std", "checksum", "deflate", "provenance", "fast-deflate", "system-zlib-decompress"]
+# Deflate backend: `zlib-rs` (pure Rust) by default. `fast-deflate` selects
+# zlib-ng instead (C, built with cmake); flate2 prefers a C zlib whenever one
+# is enabled, so turning it on anywhere in the build overrides the default.
+default = ["std", "checksum", "deflate", "provenance", "zlib-rs", "system-zlib-decompress"]
 std = []
 checksum = []
 deflate = ["flate2"]
@@ -42,7 +45,10 @@ fast-checksum = ["crc32fast"]
 fast-deflate = ["flate2/zlib-ng"]
 system-zlib = ["flate2/zlib-default"]
 system-zlib-decompress = []
-zlib-rs = ["flate2/zlib-rs"]
+# `runtime_detection` gives zlib-rs `std`, which it needs to detect and use
+# SIMD at runtime. flate2 enables it by default, but we build flate2 with
+# default-features = false, and without it zlib-rs inflates 3.5x slower.
+zlib-rs = ["flate2/zlib-rs", "flate2/runtime_detection"]
 lz4 = ["lz4_flex"]
 zstd = ["dep:zstd"]
 blake3_hash = ["blake3"]
diff --git a/crates/clawhdf5-format/src/filters.rs b/crates/clawhdf5-format/src/filters.rs
index 9979d3f..c10cea9 100644
--- a/crates/clawhdf5-format/src/filters.rs
+++ b/crates/clawhdf5-format/src/filters.rs
@@ -629,21 +629,70 @@ fn deflate_decompress(data: &[u8], expected_bytes: usize) -> Result<Vec<u8>, For
         // Fall through to flate2 on error
     }
 
-    use std::io::Read;
-    let decoder = flate2::read::ZlibDecoder::new(data);
-    let mut result = Vec::with_capacity(limit.min(1 << 20));
-    // Read one byte past the limit so an over-size stream is distinguishable
+    // A chunk's decompressed size is known, so allocate it once; without one,
+    // start from a multiple of the input and grow.
+    let size_hint = if expected_bytes != 0 {
+        expected_bytes
+    } else {
+        data.len().saturating_mul(4).min(1 << 20)
+    };
+    inflate_bounded(data, size_hint, limit).map_err(FormatError::DecompressionError)
+}
+
+/// Inflate a zlib stream into a buffer sized up front, handing the decoder the
+/// whole input at once.
+///
+/// `flate2::read::ZlibDecoder` feeds its input through a 32 KiB buffer and
+/// grows the output as it goes; on single chunks that cost zlib-rs up to 3.7x
+/// against zlib-ng (`BENCHMARKS.md`, "Deflate backend"). Output beyond `limit`
+/// is an error, as is a stream that ends before its end-of-stream marker (the
+/// streaming reader returned the bytes it had and no error).
+#[cfg(feature = "deflate")]
+pub(crate) fn inflate_bounded(
+    data: &[u8],
+    size_hint: usize,
+    limit: usize,
+) -> Result<Vec<u8>, String> {
+    use flate2::{Decompress, FlushDecompress, Status};
+
+    // One byte of headroom past the limit distinguishes an over-size stream
     // from one that legitimately ends exactly at the limit.
-    decoder
-        .take(limit as u64 + 1)
-        .read_to_end(&mut result)
-        .map_err(|e| FormatError::DecompressionError(e.to_string()))?;
-    if result.len() > limit {
-        return Err(FormatError::DecompressionError(
-            "deflate: output exceeds size limit".into(),
-        ));
+    let max_capacity = limit.saturating_add(1);
+    let mut out = Vec::new();
+    out.try_reserve_exact(size_hint.clamp(1, max_capacity))
+        .map_err(|e| format!("deflate: cannot allocate output: {e}"))?;
+
+    let mut inflater = Decompress::new(true);
+    loop {
+        let (in_before, out_before) = (inflater.total_in(), inflater.total_out());
+        let status = inflater
+            .decompress_vec(
+                &data[in_before as usize..],
+                &mut out,
+                FlushDecompress::Finish,
+            )
+            .map_err(|e| format!("deflate: {e}"))?;
+        if out.len() > limit {
+            return Err("deflate: output exceeds size limit".into());
+        }
+        match status {
+            Status::StreamEnd => return Ok(out),
+            Status::Ok | Status::BufError if out.len() == out.capacity() => {
+                // Out of room: double, up to the limit.
+                let grow = out.capacity().min(max_capacity - out.capacity()).max(1);
+                out.try_reserve_exact(grow)
+                    .map_err(|e| format!("deflate: cannot allocate output: {e}"))?;
+            }
+            Status::Ok | Status::BufError => {
+                // Room left, so the decoder stopped for want of input.
+                if inflater.total_in() as usize >= data.len()
+                    || (inflater.total_in(), inflater.total_out()) == (in_before, out_before)
+                {
+                    return Err("deflate: truncated stream".into());
+                }
+            }
+        }
     }
-    Ok(result)
 }
 
 /// Direct FFI to Apple's system libz for fast decompression.
@@ -722,14 +771,41 @@ fn deflate_decompress(_data: &[u8], _expected_bytes: usize) -> Result<Vec<u8>, F
 /// Compress data with zlib.
 #[cfg(feature = "deflate")]
 fn deflate_compress(data: &[u8], level: u32) -> Result<Vec<u8>, FormatError> {
-    use std::io::Write;
-    let mut encoder = flate2::write::ZlibEncoder::new(Vec::new(), flate2::Compression::new(level));
-    encoder
-        .write_all(data)
-        .map_err(|e| FormatError::CompressionError(e.to_string()))?;
-    encoder
-        .finish()
-        .map_err(|e| FormatError::CompressionError(e.to_string()))
+    deflate_bounded(data, level).map_err(FormatError::CompressionError)
+}
+
+/// Deflate `data` into a zlib stream in one pass, into a buffer sized for the
+/// worst case up front (the same reasoning as [`inflate_bounded`]).
+#[cfg(feature = "deflate")]
+pub(crate) fn deflate_bounded(data: &[u8], level: u32) -> Result<Vec<u8>, String> {
+    use flate2::{Compress, Compression, FlushCompress, Status};
+
+    // zlib's compressBound, plus the zlib header and trailer.
+    let bound = data.len() + (data.len() >> 12) + (data.len() >> 14) + (data.len() >> 25) + 13 + 6;
+    let mut out = Vec::new();
+    out.try_reserve_exact(bound)
+        .map_err(|e| format!("deflate: cannot allocate output: {e}"))?;
+
+    let mut deflater = Compress::new(Compression::new(level), true);
+    loop {
+        let (in_before, out_before) = (deflater.total_in(), deflater.total_out());
+        let status = deflater
+            .compress_vec(&data[in_before as usize..], &mut out, FlushCompress::Finish)
+            .map_err(|e| format!("deflate: {e}"))?;
+        match status {
+            Status::StreamEnd => return Ok(out),
+            // The bound should make running out of room unreachable; grow
+            // rather than fail if it happens.
+            Status::Ok | Status::BufError if out.len() == out.capacity() => out
+                .try_reserve(out.capacity().max(4096))
+                .map_err(|e| format!("deflate: cannot allocate output: {e}"))?,
+            Status::Ok | Status::BufError => {
+                if (deflater.total_in(), deflater.total_out()) == (in_before, out_before) {
+                    return Err("deflate: encoder made no progress".into());
+                }
+            }
+        }
+    }
 }
 
 #[cfg(not(feature = "deflate"))]
@@ -1833,6 +1909,74 @@ mod tests {
         assert!(deflate_decompress(&compressed, 64).is_err());
     }
 
+    #[cfg(feature = "deflate")]
+    fn noisy_bytes(n: usize) -> Vec<u8> {
+        // Compressible but not trivially so.
+        (0..n)
+            .map(|i| ((i as f64 * 0.01).sin() * 127.0 + 128.0) as u8 ^ (i as u8 & 3))
+            .collect()
+    }
+
+    #[test]
+    #[cfg(feature = "deflate")]
+    fn deflate_decompress_accepts_output_exactly_at_chunk_size() {
+        let data = noisy_bytes(100_000);
+        let compressed = deflate_compress(&data, 6).unwrap();
+        assert_eq!(deflate_decompress(&compressed, data.len()).unwrap(), data);
+        // One byte short of the real size is over the limit.
+        assert!(deflate_decompress(&compressed, data.len() - 1).is_err());
+    }
+
+    #[test]
+    #[cfg(feature = "deflate")]
+    fn deflate_decompress_without_size_grows_the_buffer() {
+        // No chunk size: the output starts at 4x the input and has to grow.
+        let data = vec![7u8; 3 * 1024 * 1024];
+        let compressed = deflate_compress(&data, 6).unwrap();
+        assert!(compressed.len() * 4 < data.len());
+        assert_eq!(deflate_decompress(&compressed, 0).unwrap(), data);
+    }
+
+    #[test]
+    #[cfg(feature = "deflate")]
+    fn deflate_decompress_rejects_truncated_stream() {
+        // The streaming reader this replaced returned the bytes it had and no
+        // error, so a truncated chunk read back short.
+        let data = noisy_bytes(100_000);
+        let compressed = deflate_compress(&data, 6).unwrap();
+        for cut in [compressed.len() - 1, compressed.len() / 2, 3] {
+            assert!(
+                deflate_decompress(&compressed[..cut], data.len()).is_err(),
+                "truncated to {cut} of {} bytes",
+                compressed.len()
+            );
+        }
+    }
+
+    #[test]
+    #[cfg(feature = "deflate")]
+    fn deflate_compress_roundtrips_incompressible_data() {
+        // Random-looking input compresses to slightly more than it started
+        // as; the output must still fit the pre-sized buffer (or grow).
+        let mut x = 0x9E37_79B9_7F4A_7C15u64;
+        let data: Vec<u8> = (0..200_000)
+            .map(|_| {
+                x ^= x << 13;
+                x ^= x >> 7;
+                x ^= x << 17;
+                x as u8
+            })
+            .collect();
+        for level in [0, 1, 6, 9] {
+            let compressed = deflate_compress(&data, level).unwrap();
+            assert_eq!(deflate_decompress(&compressed, data.len()).unwrap(), data);
+        }
+        assert_eq!(
+            deflate_decompress(&deflate_compress(&[], 6).unwrap(), 0).unwrap(),
+            Vec::<u8>::new()
+        );
+    }
+
     #[test]
     #[cfg(feature = "zstd")]
     fn zstd_decompress_rejects_output_exceeding_chunk_size() {
diff --git a/crates/clawhdf5/Cargo.toml b/crates/clawhdf5/Cargo.toml
index ee5326f..40dbec2 100644
--- a/crates/clawhdf5/Cargo.toml
+++ b/crates/clawhdf5/Cargo.toml
@@ -30,9 +30,10 @@ name = "parallel_bench"
 harness = false
 
 [features]
-default = ["mmap", "fast-deflate", "provenance"]
+default = ["mmap", "provenance"]
 mmap = ["clawhdf5-io/mmap"]
 parallel = ["clawhdf5-format/parallel", "rayon"]
+# zlib-ng (C, needs cmake) instead of the default pure-Rust zlib-rs.
 fast-deflate = ["clawhdf5-format/fast-deflate"]
 apple-compression = []
 zstd = ["clawhdf5-format/zstd"]
diff --git a/scripts/ci-test.sh b/scripts/ci-test.sh
index f04dd09..4df9e33 100755
--- a/scripts/ci-test.sh
+++ b/scripts/ci-test.sh
@@ -77,6 +77,35 @@ run_step "cargo clippy (ann parallel)" cargo clippy \
     --features parallel \
     -- -D warnings
 
+# zlib-ng is opt-in (`fast-deflate`; the default is pure-Rust zlib-rs), so
+# nothing above builds it. Keep it compiling and passing.
+run_step "cargo clippy (fast-deflate / zlib-ng)" cargo clippy \
+    -p clawhdf5-format -p clawhdf5-filters -p clawhdf5 \
+    --all-targets \
+    --features clawhdf5-format/fast-deflate,clawhdf5-filters/fast-deflate \
+    -- -D warnings
+
+# The README promises that the core crates build no C by default. Hold it to
+# that: fail if a crate that compiles C (a *-sys crate, cc or cmake) enters the
+# default dependency tree of any of them. clawhdf5-migrate (bundled SQLite),
+# clawhdf5-napi (Node) and clawhdf5-gpu (graphics drivers) are exempt.
+no_c_in_default_build() {
+    local crate found=0
+    for crate in clawhdf5-format clawhdf5-io clawhdf5-filters clawhdf5 \
+        clawhdf5-agent clawhdf5-ann clawhdf5-accel clawhdf5-netcdf4 clawhdf5-cli; do
+        local c_deps
+        c_deps=$(cargo tree -q -p "$crate" -e normal,build --prefix none \
+            | grep -E '^([a-z0-9_-]+-sys|cc|cmake) v' | sort -u)
+        if [ -n "$c_deps" ]; then
+            echo "$crate pulls in C by default:"
+            echo "$c_deps" | sed 's/^/    /'
+            found=1
+        fi
+    done
+    return $found
+}
+run_step "no C in the default build (core crates)" no_c_in_default_build
+
 # 4. Tests (exclude clawhdf5-py)
 run_step "cargo test" cargo test \
     --workspace \
@@ -90,6 +119,10 @@ run_step "cargo test (ann parallel)" cargo test \
     -p clawhdf5-ann \
     --features parallel
 
+run_step "cargo test (fast-deflate / zlib-ng)" cargo test \
+    -p clawhdf5-format -p clawhdf5-filters -p clawhdf5 \
+    --features clawhdf5-format/fast-deflate,clawhdf5-filters/fast-deflate
+
 # 5. Python interop suites. The h5py writer tests are #[ignore]d so a plain
 #    `cargo test` stays hermetic; run them explicitly here.
 #    On a PEP 668 "externally managed" system h5py can only live in a