diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 1362609..9681c60 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -54,6 +54,9 @@ jobs:
     - name: wstd tests
       run: cargo test -p wstd -p wstd-axum --target wasm32-wasip2 -- --nocapture
 
+    - name: p3 wstd tests
+      run: cargo test -p wstd -p wstd-axum --target wasm32-wasip2 --no-default-features --features p3 -- --nocapture
+
     - name: test-programs tests
       run: cargo test -p test-programs -- --nocapture
       if: steps.creds.outcome == 'success'
diff --git a/Cargo.toml b/Cargo.toml
index 386afa8..90a90cd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,9 +13,16 @@ repository.workspace = true
 rust-version.workspace = true
 
 [features]
-default = ["json"]
+default = ["json", "p2"]
 json = ["dep:serde", "dep:serde_json"]
 
+# WASI backend selection. Exactly one of these must be enabled.
+p2 = ["dep:wasip2"]
+p3 = ["dep:wasip3"]
+
+[build-dependencies]
+cfg_aliases.workspace = true
+
 [dependencies]
 anyhow.workspace = true
 async-task.workspace = true
@@ -27,9 +34,11 @@ http.workspace = true
 itoa.workspace = true
 pin-project-lite.workspace = true
 slab.workspace = true
-wasip2.workspace = true
 wstd-macro.workspace = true
 
+wasip2 = { workspace = true, optional = true }
+wasip3 = { workspace = true, optional = true }
+
 # optional
 serde = { workspace = true, optional = true }
 serde_json = { workspace = true, optional = true }
@@ -43,6 +52,62 @@ humantime.workspace = true
 serde = { workspace = true, features = ["derive"] }
 serde_json.workspace = true
 
+[[test]]
+name = "http_first_byte_timeout"
+required-features = ["p2"]
+
+[[test]]
+name = "http_get"
+required-features = ["p2"]
+
+[[test]]
+name = "http_get_json"
+required-features = ["p2"]
+
+[[test]]
+name = "http_handle_error_code"
+required-features = ["p2"]
+
+[[test]]
+name = "http_post"
+required-features = ["p2"]
+
+[[test]]
+name = "http_post_json"
+required-features = ["p2"]
+
+[[test]]
+name = "http_timeout"
+required-features = ["p2"]
+
+[[test]]
+name = "sleep"
+required-features = ["p2"]
+
+[[example]]
+name = "complex_http_client"
+required-features = ["p2"]
+
+[[example]]
+name = "http_client"
+required-features = ["p2"]
+
+[[example]]
+name = "http_server"
+required-features = ["p2"]
+
+[[example]]
+name = "http_server_proxy"
+required-features = ["p2"]
+
+[[example]]
+name = "tcp_echo_server"
+required-features = ["p2"]
+
+[[example]]
+name = "tcp_stream_client"
+required-features = ["p2"]
+
 [workspace]
 members = [
     "axum",
@@ -72,6 +137,7 @@ async-task = "4.7"
 axum = { version = "0.8.6", default-features = false }
 bytes = "1.10.1"
 cargo_metadata = "0.22"
+cfg_aliases = "0.2"
 clap = { version = "4.5.26", features = ["derive"] }
 futures-core = "0.3.19"
 futures-lite = "1.12.0"
@@ -95,13 +161,13 @@ test-programs = { path = "test-programs" }
 tower-service = "0.3.3"
 ureq = { version = "3.1", default-features = false, features = ["json"] }
 wasip2 = "1.0"
-wstd = { path = ".", version = "=0.6.8" }
+wasip3 = "0.8"
+wstd = { path = ".", version = "=0.6.8", default-features = false }
 wstd-axum = { path = "./axum", version = "=0.6.8" }
 wstd-axum-macro = { path = "./axum/macro", version = "=0.6.8" }
 wstd-macro = { path = "./macro", version = "=0.6.8" }
 
 [package.metadata.docs.rs]
-all-features = true
 targets = [
     "wasm32-wasip2"
 ]
diff --git a/axum/Cargo.toml b/axum/Cargo.toml
index 154a021..a27ce78 100644
--- a/axum/Cargo.toml
+++ b/axum/Cargo.toml
@@ -13,12 +13,32 @@ rust-version.workspace = true
 [dependencies]
 axum.workspace = true
 tower-service.workspace = true
-wstd.workspace = true
+wstd = { workspace = true, features = ["json"] }
 wstd-axum-macro.workspace = true
 
+[features]
+default = ["p2"]
+p2 = ["wstd/p2"]
+p3 = ["wstd/p3"]
+
+[build-dependencies]
+cfg_aliases.workspace = true
+
 [dev-dependencies]
 anyhow.workspace = true
 futures-concurrency.workspace = true
 serde = { workspace = true, features = ["derive"] }
 serde_qs.workspace = true
 axum = { workspace = true, features = ["query", "json", "macros"] }
+
+[[example]]
+name = "hello_world"
+required-features = ["p2"]
+
+[[example]]
+name = "hello_world_nomacro"
+required-features = ["p2"]
+
+[[example]]
+name = "weather"
+required-features = ["p2"]
diff --git a/axum/build.rs b/axum/build.rs
new file mode 100644
index 0000000..5a5f457
--- /dev/null
+++ b/axum/build.rs
@@ -0,0 +1,10 @@
+use cfg_aliases::cfg_aliases;
+
+fn main() {
+    cfg_aliases! {
+        // TODO https://github.com/bytecodealliance/wstd/issues/147: Swap these
+        // to use `target_env` instead.
+        p2: { feature = "p2" },
+        p3: { feature = "p3" },
+    }
+}
diff --git a/axum/src/lib.rs b/axum/src/lib.rs
index 3272b91..c730fd2 100644
--- a/axum/src/lib.rs
+++ b/axum/src/lib.rs
@@ -1,3 +1,4 @@
+#![cfg(p2)]
 //! Support for the [`axum`] web server framework in wasi-http components, via
 //! [`wstd`].
 //!
diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..5a5f457
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,10 @@
+use cfg_aliases::cfg_aliases;
+
+fn main() {
+    cfg_aliases! {
+        // TODO https://github.com/bytecodealliance/wstd/issues/147: Swap these
+        // to use `target_env` instead.
+        p2: { feature = "p2" },
+        p3: { feature = "p3" },
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index ebc673d..64ac672 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -18,7 +18,7 @@
 //! **TCP echo server**
 //!
 //! ```rust,no_run
-#![doc = include_str!("../examples/tcp_echo_server.rs")]
+#![cfg_attr(p2, doc = include_str!("../examples/tcp_echo_server.rs"))]
 //! ```
 //!
 //! **HTTP Client**
@@ -30,7 +30,7 @@
 //! **HTTP Server**
 //!
 //! ```rust,no_run
-#![doc = include_str!("../examples/http_server.rs")]
+#![cfg_attr(p2, doc = include_str!("../examples/http_server.rs"))]
 //! ```
 //!
 //! # Design Decisions
@@ -55,30 +55,52 @@
 //! These are unique capabilities provided by WASI 0.2, and because this library
 //! is specific to that are exposed from here.
 
+// Exactly one WASI backend must be selected. See the `p2`/`p3` features.
+#[cfg(all(p2, p3))]
+compile_error!(
+    "the `p2` and `p3` features are mutually exclusive — enable exactly one WASI backend"
+);
+#[cfg(not(any(p2, p3)))]
+compile_error!("exactly one of the `p2` or `p3` features must be enabled");
+
+#[cfg(p2)]
 pub mod future;
+#[cfg(p2)]
 #[macro_use]
 pub mod http;
+#[cfg(p2)]
 pub mod io;
 pub mod iter;
+#[cfg(p2)]
 pub mod net;
+#[cfg(p2)]
 pub mod rand;
+#[cfg(p2)]
 pub mod runtime;
+#[cfg(p2)]
 pub mod task;
+#[cfg(p2)]
 pub mod time;
 
-pub use wstd_macro::attr_macro_http_server as http_server;
-pub use wstd_macro::attr_macro_main as main;
-pub use wstd_macro::attr_macro_test as test;
+#[cfg(p2)]
+pub use wstd_macro::{
+    attr_macro_http_server as http_server, attr_macro_main as main, attr_macro_test as test,
+};
 
-// Re-export the wasip2 crate for use only by `wstd-macro` macros. The proc
-// macros need to generate code that uses these definitions, but we don't want
-// to treat it as part of our public API with regards to semver, so we keep it
-// under `__internal` as well as doc(hidden) to indicate it is private.
+// Re-export the active WASI backend crate for use only by `wstd-macro` macros.
+// The proc macros need to generate code that uses these definitions, but we
+// don't want to treat it as part of our public API with regards to semver, so
+// we keep it under `__internal` as well as doc(hidden) to indicate it is
+// private.
 #[doc(hidden)]
 pub mod __internal {
+    #[cfg(p2)]
     pub use wasip2;
+    #[cfg(p3)]
+    pub use wasip3;
 }
 
+#[cfg(p2)]
 pub mod prelude {
     pub use crate::future::FutureExt as _;
     pub use crate::io::AsyncRead as _;