diff --git a/b3sum/src/main.rs b/b3sum/src/main.rs
index 3810bfe..2a9b737 100644
--- a/b3sum/src/main.rs
+++ b/b3sum/src/main.rs
@@ -308,7 +308,7 @@ fn maybe_memmap_file(file: &File) -> Result<Option<memmap::Mmap>> {
 fn write_hex_output(mut output: blake3::OutputReader, args: &Args) -> Result<()> {
     // Encoding multiples of the block size is most efficient.
     let mut len = args.len()?;
-    let mut block = [0; blake3::BLOCK_LEN];
+    let mut block = [0; blake3::guts::BLOCK_LEN];
     while len > 0 {
         output.fill(&mut block);
         let hex_str = hex::encode(&block[..]);
diff --git a/benches/bench.rs b/benches/bench.rs
index 90dbc42..9038a94 100644
--- a/benches/bench.rs
+++ b/benches/bench.rs
@@ -4,8 +4,9 @@ extern crate test;
 
 use arrayref::array_ref;
 use arrayvec::ArrayVec;
+use blake3::guts::{BLOCK_LEN, CHUNK_LEN};
 use blake3::platform::{Platform, MAX_SIMD_DEGREE};
-use blake3::{BLOCK_LEN, CHUNK_LEN, OUT_LEN};
+use blake3::OUT_LEN;
 use rand::prelude::*;
 use test::Bencher;
 
diff --git a/src/guts.rs b/src/guts.rs
index e06721c..ecde326 100644
--- a/src/guts.rs
+++ b/src/guts.rs
@@ -6,6 +6,9 @@
 //! We could stabilize something like this module in the future. If you have a
 //! use case for it, please let us know by filing a GitHub issue.
 
+pub const BLOCK_LEN: usize = 64;
+pub const CHUNK_LEN: usize = 1024;
+
 #[derive(Clone, Debug)]
 pub struct ChunkState(crate::ChunkState);
 
diff --git a/src/lib.rs b/src/lib.rs
index 6ac32a1..cff55d2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -79,7 +79,7 @@ mod test;
 #[doc(hidden)]
 pub mod guts;
 
-// The platform module is pub for benchmarks only. It is not stable.
+/// Undocumented and unstable, for benchmarks only.
 #[doc(hidden)]
 pub mod platform;
 
@@ -128,14 +128,8 @@ pub const OUT_LEN: usize = 32;
 /// The number of bytes in a key, 32.
 pub const KEY_LEN: usize = 32;
 
-// These constants are pub for incremental use cases like `bao`, as well as
-// tests and benchmarks. Most callers should not need them.
-#[doc(hidden)]
-pub const BLOCK_LEN: usize = 64;
-#[doc(hidden)]
-pub const CHUNK_LEN: usize = 1024;
-#[doc(hidden)]
-pub const MAX_DEPTH: usize = 54; // 2^54 * CHUNK_LEN = 2^64
+const MAX_DEPTH: usize = 54; // 2^54 * CHUNK_LEN = 2^64
+use guts::{BLOCK_LEN, CHUNK_LEN};
 
 // While iterating the compression function within a chunk, the CV is
 // represented as words, to avoid doing two extra endianness conversions for
@@ -540,7 +534,7 @@ impl fmt::Debug for ChunkState {
 //   use full-width SIMD vectors for parent hashing. Without parallel parent
 //   hashing, we lose about 10% of overall throughput on AVX2 and AVX-512.
 
-// pub for benchmarks
+/// Undocumented and unstable, for benchmarks only.
 #[doc(hidden)]
 #[derive(Clone, Copy)]
 pub enum IncrementCounter {
diff --git a/test_vectors/src/lib.rs b/test_vectors/src/lib.rs
index 129bd16..ec769d1 100644
--- a/test_vectors/src/lib.rs
+++ b/test_vectors/src/lib.rs
@@ -1,9 +1,9 @@
-use blake3::{BLOCK_LEN, CHUNK_LEN};
+use blake3::guts::{BLOCK_LEN, CHUNK_LEN};
 use serde::{Deserialize, Serialize};
 
 // A non-multiple of 4 is important, since one possible bug is to fail to emit
 // partial words.
-pub const OUTPUT_LEN: usize = 2 * blake3::BLOCK_LEN + 3;
+pub const OUTPUT_LEN: usize = 2 * BLOCK_LEN + 3;
 
 pub const TEST_CASES: &[usize] = &[
     0,