diff --git a/nodedb-wal/Cargo.toml b/nodedb-wal/Cargo.toml
index c6e084769..50358353e 100644
--- a/nodedb-wal/Cargo.toml
+++ b/nodedb-wal/Cargo.toml
@@ -63,8 +63,6 @@ getrandom_03 = { workspace = true }
 getrandom_02 = { workspace = true }
 
 [dev-dependencies]
-tokio = { workspace = true }
-fluxbench = { workspace = true }
 tracing-subscriber = { workspace = true }
 # Named again here so the diagnostics integration tests can act as the host —
 # call `init` and read reports back. Same published version as the optional
@@ -73,6 +71,18 @@ faultbox = { workspace = true }
 tempfile = { workspace = true }
 rand = { workspace = true }
 
+# Native-only dev-dependencies. `tokio = { workspace = true }` inherits
+# `features = ["full"]` and `fluxbench` declares its own normal
+# `tokio = { features = ["full"] }`; tokio rejects fs/io-std/net/process/
+# rt-multi-thread/signal on wasm, and Cargo unifies features across the whole
+# `cargo test` invocation, so either entry breaks every wasm32 test target in
+# the build. Neither is reachable from a wasm test: this crate has no tokio call
+# sites, and `fluxbench` is used only by benches/wal_throughput.rs, which
+# `cargo test` does not build. Native test and bench builds keep both.
+[target.'cfg(not(target_arch = "wasm32"))'.dev-dependencies]
+tokio = { workspace = true }
+fluxbench = { workspace = true }
+
 [target.'cfg(target_os = "linux")'.dev-dependencies]
 io-uring = { workspace = true }
 
diff --git a/nodedb-wal/src/double_write/raw_io.rs b/nodedb-wal/src/double_write/raw_io.rs
index 11a3f570f..7833ece91 100644
--- a/nodedb-wal/src/double_write/raw_io.rs
+++ b/nodedb-wal/src/double_write/raw_io.rs
@@ -30,6 +30,16 @@ pub(crate) fn full_capacity_slice(buf: &AlignedBuf) -> &[u8] {
 }
 
 /// `pwrite`-retry helper that handles short writes.
+///
+/// Reports [`WalError::Unsupported`] on wasm32 rather than falling back to a
+/// seek-and-write. A fallback would mirror the slot successfully and the writer
+/// would then report `DwbProtection::Active`, but `recover_record` is a stub on
+/// that target (slot recovery needs `pread`), so nothing could ever read the
+/// slot back. The protection would be claimed and unusable. Failing here keeps
+/// the writer's own degradation signal honest: the append still succeeds
+/// (`DoubleWriteBuffer` failures are deliberately not fatal to the WAL), while
+/// `dwb_protection()` reports `Degraded(WriteFailed)` and every record is
+/// counted as unprotected.
 pub(crate) fn pwrite_all(file: &File, data: &[u8], offset: u64) -> Result<()> {
     #[cfg(not(target_arch = "wasm32"))]
     {
diff --git a/nodedb-wal/src/segment/atomic_io.rs b/nodedb-wal/src/segment/atomic_io.rs
index 5162579c6..e6341ecd6 100644
--- a/nodedb-wal/src/segment/atomic_io.rs
+++ b/nodedb-wal/src/segment/atomic_io.rs
@@ -48,6 +48,12 @@ use crate::error::{Result, WalError};
 /// before the directory entry is persisted causes the file to "disappear"
 /// on reboot. Calling fsync on the directory fd ensures the metadata
 /// (filename, inode pointer) is on stable storage.
+///
+/// The exception is `wasm32-wasip1`: wasi preview1 has no directory fsync, so on
+/// that target this is a documented no-op and the durability guarantee above
+/// does not hold there. The alternative was failing every caller that renames
+/// and fsyncs on that target, which turns a weaker guarantee into an unusable
+/// one.
 pub fn fsync_directory(dir: &Path) -> Result<()> {
     // Crash injection: the directory entry never reaches stable storage.
     // Every caller must treat this as a durability failure, not a warning.
@@ -55,9 +61,23 @@ pub fn fsync_directory(dir: &Path) -> Result<()> {
         std::io::Error::other(format!("failpoint wal::fsync_directory: {detail}"))
     ));
 
-    let dir_file = fs::File::open(dir).map_err(WalError::Io)?;
-    dir_file.sync_all().map_err(WalError::Io)?;
-    Ok(())
+    #[cfg(target_arch = "wasm32")]
+    {
+        // `File::sync_all` on a directory is not implemented by wasi preview1
+        // and there is no weaker syscall with the same guarantee. The rename
+        // itself still succeeds; what is lost on this target is the assurance
+        // that the rename survives a host crash. The failpoint above still
+        // fires here, so crash injection keeps working.
+        let _ = dir;
+        Ok(())
+    }
+
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        let dir_file = fs::File::open(dir).map_err(WalError::Io)?;
+        dir_file.sync_all().map_err(WalError::Io)?;
+        Ok(())
+    }
 }
 
 fn invalid_input(detail: String) -> WalError {
@@ -103,6 +123,10 @@ fn tmp_name(name: &str) -> String {
 /// 4. `fsync_directory(dir)` — forces the directory entry durable so the new
 ///    name survives power loss.
 ///
+/// Step 4 is a no-op on `wasm32-wasip1`, which has no directory fsync: there the
+/// rename is atomic with respect to the process but its durability across a host
+/// crash is not assured. Steps 1-3 are unchanged on every target.
+///
 /// `name` must be one plain path component; anything else returns
 /// `InvalidInput` before a byte is written. Both paths are built here from
 /// `dir`, so the rename stays inside one filesystem directory and the single
diff --git a/nodedb-wal/src/writer/flush.rs b/nodedb-wal/src/writer/flush.rs
index aac2c1cdb..6bf52683c 100644
--- a/nodedb-wal/src/writer/flush.rs
+++ b/nodedb-wal/src/writer/flush.rs
@@ -1,13 +1,20 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use crate::error::Result;
-// Only the unix `pwrite` path constructs an error value directly; elsewhere
-// failures propagate as `Result` from calls that build their own.
-#[cfg(unix)]
+// The unix `pwrite` arm and the wasm seek+write arm both construct an error
+// value directly; elsewhere failures propagate as `Result` from calls that
+// build their own.
+#[cfg(any(unix, target_arch = "wasm32"))]
 use crate::error::WalError;
 
 use super::core::WalWriter;
 
+// The write arms below cover unix and wasm32. A target in neither family would
+// compile no arm at all and reinstate the silent success this file's wasi arm
+// exists to prevent, so fail the build instead of writing nothing.
+#[cfg(not(any(unix, target_arch = "wasm32")))]
+compile_error!("nodedb-wal has no WAL write arm for this target family");
+
 impl WalWriter {
     /// Flush the aligned buffer to the file.
     ///
@@ -42,8 +49,12 @@ impl WalWriter {
             self.buffer.as_slice()
         };
 
-        // Use pwrite to write at the exact offset, retrying on short writes.
-        #[cfg(unix)]
+        // Write at the exact offset. The unix path uses `pwrite` and retries
+        // short writes. `cfg(unix)` is false on `wasm32-wasip1` — its
+        // `target_family` is `wasm`, not `unix` — so wasi needs its own arm:
+        // without one this function advanced `file_offset`, cleared the buffer
+        // and reported the flush with nothing written at all.
+        #[cfg(all(unix, not(target_arch = "wasm32")))]
         {
             use std::os::unix::io::AsRawFd;
             let fd = self.file.as_raw_fd();
@@ -59,7 +70,8 @@ impl WalWriter {
                     )
                 };
                 if written < 0 {
-                    return Err(write_error(
+                    return Err(classify_write_error(
+                        std::io::Error::last_os_error(),
                         "WAL segment append",
                         write_offset,
                         remaining.len() as u64,
@@ -79,6 +91,43 @@ impl WalWriter {
             }
         }
 
+        // This arm covers every wasm32 target, including
+        // `wasm32-unknown-unknown`, where libc defines no `pwrite` at all, and
+        // std's wasi positional API (`std::os::wasi::fs`) is still unstable.
+        // The segment is only ever appended to, so seeking to `file_offset` and
+        // writing is equivalent to a positional write. An error returns before
+        // the shared bookkeeping below, so a failed flush leaves the buffer and
+        // `file_offset` untouched for a retry — the same property the `pwrite`
+        // arm has.
+        #[cfg(target_arch = "wasm32")]
+        {
+            use std::io::{Seek as _, SeekFrom, Write as _};
+            self.file
+                .seek(SeekFrom::Start(self.file_offset))
+                .map_err(WalError::Io)?;
+            // Crash injection: this arm's write fails with a full device. It
+            // sits inside the arm, so a test that arms it proves the arm is what
+            // ran — and it is classified exactly as the real failure below is,
+            // by the same call, which is the only way to reach that classifier
+            // on a runtime whose writes cannot be made to fail on demand.
+            nodedb_types::fail_point_err!("wal::wasm_flush_write", |_detail: String| {
+                classify_write_error(
+                    std::io::Error::new(std::io::ErrorKind::StorageFull, "device full"),
+                    "WAL segment append",
+                    self.file_offset,
+                    data.len() as u64,
+                )
+            });
+            self.file.write_all(data).map_err(|err| {
+                classify_write_error(
+                    err,
+                    "WAL segment append",
+                    self.file_offset,
+                    data.len() as u64,
+                )
+            })?;
+        }
+
         self.file_offset += data.len() as u64;
         self.buffer.clear();
 
@@ -90,7 +139,7 @@ impl WalWriter {
     }
 }
 
-/// Classify the current `errno` from a failed WAL write.
+/// Classify a failed WAL write.
 ///
 /// A full device is called out separately from generic I/O failure: it is not
 /// transient, retrying cannot succeed, and the caller must stop acknowledging
@@ -98,14 +147,22 @@ impl WalWriter {
 /// where the batch stalled and how much of it never reached the file, which is
 /// what a report needs to describe the write that could not complete.
 ///
-/// Gated to match its only call site: the `pwrite` loop is unix-only, so on
-/// other targets (wasm32) this would be dead code and a `-D warnings` build
-/// would reject it.
-#[cfg(unix)]
-fn write_error(context: &'static str, offset: u64, pending: u64) -> WalError {
-    let err = std::io::Error::last_os_error();
-    #[cfg(unix)]
-    if err.raw_os_error() == Some(libc::ENOSPC) {
+/// Takes the error rather than reading `errno` again: the wasm arm is handed a
+/// real `io::Error` by `write_all`, and re-deriving it from the thread's errno
+/// would classify whatever happened to be there last.
+///
+/// Keyed on `ErrorKind::StorageFull` rather than on `libc::ENOSPC`: std maps the
+/// full-device errno to that kind on every target that has a filesystem, and
+/// `libc` defines no constants at all for `wasm32-unknown-unknown`, which this
+/// crate is also compiled for.
+#[cfg(any(unix, target_arch = "wasm32"))]
+fn classify_write_error(
+    err: std::io::Error,
+    context: &'static str,
+    offset: u64,
+    pending: u64,
+) -> WalError {
+    if err.kind() == std::io::ErrorKind::StorageFull {
         let out_of_space = WalError::OutOfSpace { context };
         crate::diag::out_of_space(&out_of_space, context, offset, pending);
         return out_of_space;
diff --git a/nodedb-wal/tests/wasi_append.rs b/nodedb-wal/tests/wasi_append.rs
new file mode 100644
index 000000000..c353823e5
--- /dev/null
+++ b/nodedb-wal/tests/wasi_append.rs
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: Apache-2.0
+
+//! The WAL's write path on `wasm32-wasip1`, the target the wasm test job runs.
+//!
+//! `WalWriter::flush_buffer` wrote the batch with a `#[cfg(unix)]` arm and
+//! nothing else. `cfg(unix)` is false on this target — `target_family` is `wasm`
+//! — so the write was not compiled at all, while the statements after it still
+//! ran: the offset advanced, the buffer was cleared and the flush reported
+//! success. The test below is the difference between "the append landed" and
+//! "the writer said it landed", and it is decided by the file, not by the
+//! return value.
+//!
+//! `fsync_directory` had the same shape of problem: it opened the directory and
+//! called `sync_all`, which wasi preview1 cannot do, so every caller that
+//! fsyncs a directory after a rename failed on this target.
+//!
+//! Notes for whoever runs this:
+//!
+//! - Run it with `--nocapture`. A panic aborts the process on this target, so
+//!   without it a failed assertion arrives as a wasm trap and the message that
+//!   says which byte count disagreed is lost.
+//! - `wasmtime --dir=.` preopens the working directory (Cargo runs the test
+//!   binary from the package root), and `tempfile`'s default base is
+//!   `std::env::temp_dir()`, which is `unimplemented!()` in std for wasi. A
+//!   directory under the preopen is the only place a wasm test can write.
+//! - A panicking test skips its own cleanup, so an aborted run leaves the
+//!   tempdir behind next to the package.
+//! - The failure-path test needs the crate's own failpoints, so it runs with
+//!   `--features failpoints`. The other two run either way.
+//!
+//! wasm-only on purpose: the native append path is covered by `tests/wal_suite`.
+
+#![cfg(target_arch = "wasm32")]
+
+use nodedb_wal::WalWriter;
+use nodedb_wal::segment::atomic_io::{atomic_write_fsync, fsync_directory};
+use std::fs;
+
+fn tempdir() -> tempfile::TempDir {
+    tempfile::Builder::new()
+        .tempdir_in(".")
+        .expect("the wasm test runner preopens the working directory")
+}
+
+const PAYLOAD: &[u8] = b"wasip1 append probe";
+
+#[test]
+fn an_appended_record_reaches_the_file() {
+    let dir = tempdir();
+    let path = dir.path().join("append.wal");
+
+    let mut writer = WalWriter::open_without_direct_io(&path).expect("open the segment");
+    writer.append(1, 0, 0, 0, PAYLOAD).expect("append");
+    writer.sync().expect("sync");
+
+    let counted = writer.file_offset();
+    let written = fs::read(&path).expect("read the segment back");
+
+    assert_eq!(
+        written.len() as u64,
+        counted,
+        "sync() reported success with {counted} bytes written, but the segment holds {} bytes",
+        written.len()
+    );
+    assert!(
+        written.windows(PAYLOAD.len()).any(|w| w == PAYLOAD),
+        "the segment does not contain the appended payload"
+    );
+}
+
+#[test]
+fn a_checkpoint_can_fsync_its_directory() {
+    let dir = tempdir();
+
+    fsync_directory(dir.path()).expect("wasi preview1 has no directory fsync, not a failure");
+    atomic_write_fsync(dir.path(), "payload.ckpt", b"hello wasi")
+        .expect("atomic_write_fsync renames then fsyncs the directory");
+    assert_eq!(
+        fs::read(dir.path().join("payload.ckpt")).expect("read back"),
+        b"hello wasi",
+        "the checkpoint bytes did not survive the round trip"
+    );
+}
+
+/// A failed write is reported as a failure and advances nothing, so the batch is
+/// retried byte-for-byte at the same offset.
+///
+/// The injection is `wal::wasm_flush_write`, which sits *inside* the wasi write
+/// arm: with the arm removed the injection cannot fire and this test fails, so
+/// it is a chokepoint for the arm rather than for the wrapper above it. The
+/// error it produces goes through the same `classify_write_error` call the real
+/// `write_all` failure does, which is the only way to reach that classifier on a
+/// runtime whose writes cannot be made to fail on demand. It carries the error
+/// kind a full device produces, so what the assertion pins is the classifier's
+/// rule — not the runtime's errno translation, which std owns and which a real
+/// full device would exercise.
+///
+/// Needs the feature, and is compiled out without it rather than weakened:
+/// without `failpoints` the injection expands to nothing and these assertions
+/// would describe a successful flush.
+///
+/// ```text
+/// CARGO_TARGET_WASM32_WASIP1_RUNNER="wasmtime --dir=." \
+///   cargo test -p nodedb-wal --target wasm32-wasip1 --features failpoints \
+///   --test wasi_append -- --nocapture
+/// ```
+#[cfg(feature = "failpoints")]
+#[test]
+fn a_failed_flush_reports_failure_and_advances_nothing() {
+    use nodedb_wal::error::WalError;
+
+    let dir = tempdir();
+    let path = dir.path().join("failed.wal");
+
+    let mut writer = WalWriter::open_without_direct_io(&path).expect("open the segment");
+    writer.append(1, 0, 0, 0, PAYLOAD).expect("append");
+    let before = writer.file_offset();
+
+    let guard = nodedb_types::fail_point::FailGuard::fail("wal::wasm_flush_write", "device full");
+    let result = writer.sync();
+    drop(guard);
+
+    let err = result.expect_err("an armed write failpoint must not be reported as a flush");
+    assert!(
+        matches!(err, WalError::OutOfSpace { .. }),
+        "a full device must be classified as OutOfSpace on this target too, got {err:?}"
+    );
+    assert_eq!(
+        writer.file_offset(),
+        before,
+        "a failed flush advanced the offset, so a retry would skip the batch"
+    );
+    assert_eq!(
+        fs::metadata(&path).map(|m| m.len()).unwrap_or(0),
+        0,
+        "a failed flush left bytes in the segment"
+    );
+}