diff --git a/PROTOCOL.md b/PROTOCOL.md
index ac47812..a0f1154 100644
--- a/PROTOCOL.md
+++ b/PROTOCOL.md
@@ -516,8 +516,13 @@ builds `/plugins/notes/source/`:
   its caller ends (under the normal process stopping deadline).
 - A call to `cargo.build` gets a process ID, a folder in `/proc`, and journal
   entries, like other calls.
-- `/plugins/cargo/sdk/` shows the source of the SDK. The dependency
-  `sausage-plugin = "0.1"` points to the same files in the host's cache.
+- `/plugins/cargo/sdk/` is the SDK used by builds. The dependency
+  `sausage-plugin = "0.1"` points directly to this folder, including its macros
+  and interface crate. Edits apply to the next build without a host restart.
+- The host seeds a missing SDK folder from its embedded copy at startup or
+  before a build. An existing folder is never overwritten or repaired, even
+  after a host upgrade. To reset to the host's bundled SDK, remove the whole
+  folder; the next startup or build re-seeds it. Back up local edits first.
 - Compiled crates stay in the host's cache, outside the world, and all worlds
   share them. So a second build compiles only what changed.
 - Build scripts can compile C: `CC_wasm32_wasip2` and `AR_wasm32_wasip2`
diff --git a/README.md b/README.md
index 6a672f5..f623eaa 100644
--- a/README.md
+++ b/README.md
@@ -97,6 +97,12 @@ config/boot.yaml             the services that start with the world
 config/agent.yaml            the configuration of the agent
 ```
 
+Rust builds use the SDK in `plugins/cargo/sdk/` directly. The host seeds this
+folder only when it is missing; restarts and host upgrades preserve existing
+files. SDK edits take effect on the next `cargo.build`, without a restart.
+To reset to the host's bundled SDK, back up any edits and remove the whole
+folder; the next startup or build re-seeds it.
+
 Each process also sees `/proc` (the processes and the state of the core),
 `/self` (its own folder in `/proc`), and `/run` (shared files in memory).
 
diff --git a/cli/src/cargo.rs b/cli/src/cargo.rs
index 717f6c6..fe935d8 100644
--- a/cli/src/cargo.rs
+++ b/cli/src/cargo.rs
@@ -3,7 +3,6 @@ use crate::{Host, world::World};
 use anyhow::{Context, Result, bail};
 use include_dir::{Dir, include_dir};
 use serde_json::{Value, json};
-use sha2::{Digest, Sha256};
 use std::time::Duration;
 use std::{
     path::{Path, PathBuf},
@@ -169,10 +168,9 @@ pub struct Cargo {
     runner: PathBuf,
     target: PathBuf,
     sdk: PathBuf,
-    sdk_files: Arc<std::collections::BTreeMap<PathBuf, Vec<u8>>>,
 }
 impl Cargo {
-    /// Installs changed SDK files and selects the current sausage executable as runner.
+    /// Seeds a missing world SDK and selects the current sausage executable as runner.
     pub fn new(world: World, cache: &Path) -> Result<Self> {
         let mut runner = std::env::current_exe()?;
         if runner
@@ -183,9 +181,23 @@ impl Cargo {
         }
         Self::with_runner(world, runner, cache)
     }
-    /// Installs the SDK and accepts an explicit sausage binary, including for init integrations.
+    /// Seeds a missing world SDK and accepts an explicit sausage binary, including for init integrations.
     pub fn with_runner(world: World, runner: PathBuf, cache: &Path) -> Result<Self> {
         let sdk = world.plugin_dir(ID).join("sdk");
+        Self::seed_sdk(&sdk)?;
+        Ok(Self {
+            sdk,
+            target: std::path::absolute(cache)?.join("cargo-target"),
+            world,
+            runner,
+            builds: Arc::new(tokio::sync::Mutex::new(())),
+        })
+    }
+    /// Seeds only an absent SDK; an existing world owns all of its SDK files.
+    fn seed_sdk(sdk: &Path) -> Result<()> {
+        if sdk.try_exists()? {
+            return Ok(());
+        }
         let mut files = std::collections::BTreeMap::new();
         collect_files(&SDK, Path::new("src"), &mut files);
         collect_files(&MACROS, Path::new("macros/src"), &mut files);
@@ -223,26 +235,23 @@ impl Cargo {
             }
             files.insert(destination, document.to_string().into_bytes());
         }
-        let mut hash = Sha256::new();
+        let parent = sdk.parent().context("SDK has no parent")?;
+        std::fs::create_dir_all(parent).with_context(|| format!("create {}", parent.display()))?;
+        let staging = tempfile::Builder::new()
+            .prefix("seed-sdk-")
+            .tempdir_in(parent)
+            .with_context(|| format!("stage SDK in {}", parent.display()))?;
         for (path, bytes) in &files {
-            let name = path.to_str().unwrap().as_bytes();
-            hash.update((name.len() as u64).to_le_bytes());
-            hash.update(name);
-            hash.update((bytes.len() as u64).to_le_bytes());
-            hash.update(bytes);
-            write_changed(&sdk.join(path), bytes)?;
+            write_changed(&staging.path().join(path), bytes)?;
         }
-        let cached = std::path::absolute(cache)?
-            .join("sdk")
-            .join(format!("{:x}", hash.finalize()));
-        Ok(Self {
-            sdk: cached,
-            sdk_files: Arc::new(files),
-            target: std::path::absolute(cache)?.join("cargo-target"),
-            world,
-            runner,
-            builds: Arc::new(tokio::sync::Mutex::new(())),
-        })
+        // Another builder may have seeded the world while we prepared the files.
+        if !sdk.try_exists()?
+            && let Err(error) = std::fs::rename(staging.path(), sdk)
+            && !sdk.try_exists()?
+        {
+            return Err(error).with_context(|| format!("seed {}", sdk.display()));
+        }
+        Ok(())
     }
     /// Registers cargo.build as a cancellable native process.
     pub fn register(self, host: &Host) {
@@ -310,27 +319,8 @@ impl Cargo {
         let source = source.to_path_buf();
         let copy = work.clone();
         let sdk = self.sdk.clone();
-        let sdk_files = self.sdk_files.clone();
         let build_guard = tokio::task::spawn_blocking(move || -> Result<_> {
-            // Host startup installs only the readable world copy. Builds populate the cache.
-            if !sdk.is_dir() {
-                let parent = sdk.parent().unwrap();
-                std::fs::create_dir_all(parent)
-                    .with_context(|| format!("create {}", parent.display()))?;
-                let staging = tempfile::Builder::new()
-                    .prefix("install-")
-                    .tempdir_in(parent)
-                    .with_context(|| format!("stage SDK in {}", parent.display()))?;
-                for (path, bytes) in sdk_files.iter() {
-                    write_changed(&staging.path().join(path), bytes)?;
-                }
-                // Another builder can publish the same complete SDK first.
-                if let Err(error) = std::fs::rename(staging.path(), &sdk)
-                    && !sdk.is_dir()
-                {
-                    return Err(error).with_context(|| format!("install {}", sdk.display()));
-                }
-            }
+            Self::seed_sdk(&sdk)?;
             if cleanup.exists() {
                 std::fs::remove_dir_all(&cleanup)
                     .with_context(|| format!("remove {}", cleanup.display()))?;
@@ -659,7 +649,7 @@ fn write_changed(path: &Path, bytes: &[u8]) -> Result<()> {
     }
     Ok(())
 }
-/// Collects embedded source files for deterministic hashing and installation.
+/// Collects embedded source files for seeding a missing world SDK.
 fn collect_files(
     directory: &Dir<'_>,
     destination: &Path,
diff --git a/cli/tests/host/cargo.rs b/cli/tests/host/cargo.rs
index e860322..120b262 100644
--- a/cli/tests/host/cargo.rs
+++ b/cli/tests/host/cargo.rs
@@ -278,14 +278,7 @@ async fn manifest_and_nested_sources() {
     assert!(manifest.contains("optional = true"));
     let document: toml_edit::DocumentMut = manifest.parse().unwrap();
     let sdk = Path::new(document["target"]["cfg(target_arch = \"wasm32\")"]["dependencies"]["sausage_plugin"]["path"].as_str().unwrap());
-    assert_eq!(sdk.parent().unwrap(), Path::new(CACHE).join("sdk"));
-    let hash = sdk.file_name().unwrap().to_str().unwrap();
-    assert_eq!(hash.len(), 64);
-    assert!(hash.bytes().all(|byte| byte.is_ascii_hexdigit()));
-    assert_eq!(
-        std::fs::read(sdk.join("src/lib.rs")).unwrap(),
-        std::fs::read(valid.world.0.join("plugins/cargo/sdk/src/lib.rs")).unwrap()
-    );
+    assert_eq!(sdk, valid.world.0.join("plugins/cargo/sdk"));
     assert_eq!(
         std::fs::read_to_string(root.join("src/target/data")).unwrap(),
         "nested target"
@@ -483,7 +476,7 @@ async fn source_copy() {
     assert!(
         std::fs::read_to_string(valid.world.0.join("state/cargo/work/Cargo.toml"))
             .unwrap()
-            .contains(Path::new(CACHE).join("sdk").to_str().unwrap())
+            .contains(valid.world.0.join("plugins/cargo/sdk").to_str().unwrap())
     );
 }
 /// PROTOCOL 11: compiler failures return compiler diagnostics.
@@ -637,6 +630,96 @@ async fn unchanged_sdk() {
         modified
     );
 }
+/// PROTOCOL 11: an SDK edit is used by the next build on the same running host.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn world_sdk_edits_apply_without_restart() {
+    tokio::time::timeout(std::time::Duration::from_secs(180), async {
+        let world = TestWorld::new();
+        world.simple("sdk-probe", "fn main() {}\n#[test]\nfn world_sdk_probe() { assert_eq!(sausage_plugin::world_sdk_probe(), 1); }\n");
+        let manifest = world.0.join("plugins/sdk-probe/source/Cargo.toml");
+        let mut text = std::fs::read_to_string(&manifest).unwrap();
+        text.push_str("\n[dependencies]\nsausage-plugin = '0.1'\n");
+        std::fs::write(manifest, text).unwrap();
+        let host = Host::start_with_cache(&world.0, Path::new(CACHE)).await.unwrap();
+        let library = world.0.join("plugins/cargo/sdk/src/lib.rs");
+        let original = std::fs::read_to_string(&library).unwrap();
+        std::fs::write(&library, format!("{original}\npub fn world_sdk_probe() -> u8 {{ 1 }}\n")).unwrap();
+        let params = json!({"plugin":"sdk-probe", "timeout_secs":120, "test_timeout_secs":10});
+        let first = host.call("cargo.build", params.clone()).await.unwrap();
+        assert_eq!(first["tests"]["passed"], 1);
+        // Only the SDK changes: Cargo must not reuse its old compiled version.
+        std::fs::write(&library, format!("{original}\npub fn world_sdk_probe() -> u8 {{ 2 }}\n")).unwrap();
+        let second = host.call("cargo.build", params).await.unwrap_err();
+        assert_eq!(second["code"], -32004);
+        assert_eq!(second["data"]["tests"]["failed"], 1);
+        assert!(second["data"]["tests"]["output"].as_str().unwrap().contains("world_sdk_probe"));
+        host.stop().await;
+    }).await.expect("SDK edit regression timed out");
+}
+/// PROTOCOL 11: startup seeds a missing SDK, but never repairs or overwrites an existing one.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn world_sdk_survives_restart_and_missing_sdk_is_reseeded() {
+    tokio::time::timeout(std::time::Duration::from_secs(30), async {
+        let world = TestWorld::new();
+        let sdk = world.0.join("plugins/cargo/sdk");
+        let host = Host::start_with_cache(&world.0, Path::new(CACHE))
+            .await
+            .unwrap();
+        let library = sdk.join("src/lib.rs");
+        let original = std::fs::read(&library).unwrap();
+        let manifest = sdk.join("Cargo.toml");
+        let edited = b"// world SDK edit must survive restart\n";
+        std::fs::write(&library, edited).unwrap();
+        std::fs::write(&manifest, "# edited manifest\n").unwrap();
+        let removed = sdk.join("macros/src/lib.rs");
+        std::fs::remove_file(&removed).unwrap();
+        host.stop().await;
+        let host = Host::start_with_cache(&world.0, Path::new(CACHE))
+            .await
+            .unwrap();
+        assert_eq!(std::fs::read(&library).unwrap(), edited);
+        assert_eq!(
+            std::fs::read_to_string(&manifest).unwrap(),
+            "# edited manifest\n"
+        );
+        assert!(!removed.exists(), "existing SDK must not be repaired");
+        host.stop().await;
+        std::fs::remove_dir_all(&sdk).unwrap();
+        let host = Host::start_with_cache(&world.0, Path::new(CACHE))
+            .await
+            .unwrap();
+        assert_eq!(std::fs::read(&library).unwrap(), original);
+        assert!(removed.is_file());
+        assert!(sdk.join("interface/src/lib.rs").is_file());
+        host.stop().await;
+    })
+    .await
+    .expect("SDK restart regression timed out");
+}
+/// PROTOCOL 11: removing the SDK while the host is running re-seeds it on the next build.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn world_sdk_missing_at_build_is_reseeded() {
+    tokio::time::timeout(std::time::Duration::from_secs(180), async {
+        let world = TestWorld::new();
+        world.simple("reseed-probe", "fn main() {}\n");
+        let host = Host::start_with_cache(&world.0, Path::new(CACHE))
+            .await
+            .unwrap();
+        let sdk = world.0.join("plugins/cargo/sdk");
+        let original = std::fs::read(sdk.join("src/lib.rs")).unwrap();
+        std::fs::remove_dir_all(&sdk).unwrap();
+        host.call(
+            "cargo.build",
+            json!({"plugin":"reseed-probe", "timeout_secs":120, "test_timeout_secs":10}),
+        )
+        .await
+        .unwrap();
+        assert_eq!(std::fs::read(sdk.join("src/lib.rs")).unwrap(), original);
+        host.stop().await;
+    })
+    .await
+    .expect("SDK re-seed build regression timed out");
+}
 /// PROTOCOL 11: init can build a plugin without a running host.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn standalone_builder() {