rivet-dev · NathanFlurry · Jul 8, 2026
diff --git a/crates/execution/assets/runners/wasi-module.js b/crates/execution/assets/runners/wasi-module.js
@@ -54,6 +54,7 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
   const __agentOSWasiOpenDirectory = 2;
   const __agentOSWasiOpenExclusive = 4;
   const __agentOSWasiOpenTruncate = 8;
+  const __agentOSWasiFdflagsAppend = 1;
   const __agentOSWasiRightFdRead = 1n << 1n;
   const __agentOSWasiRightFdWrite = 1n << 6n;
   const __agentOSWasiDefaultRightsBase = 0xffffffffffffffffn;
@@ -396,6 +397,14 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
       }
     }
 
+    _hasReadRights(rights) {
+      try {
+        return (BigInt(rights) & __agentOSWasiRightFdRead) !== 0n;
+      } catch {
+        return true;
+      }
+    }
+
     _writeUint32(ptr, value) {
       try {
         this._memoryView().setUint32(Number(ptr) >>> 0, Number(value) >>> 0, true);
@@ -779,6 +788,9 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
     }
 
     _mapFsError(error) {
+      __agentOSWasiDebug(
+        `fs error code=${String(error?.code ?? "")} message=${String(error?.message ?? error)}`,
+      );
       switch (error?.code) {
         case "EACCES":
         case "EPERM":
@@ -827,6 +839,7 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
         refCount: 1,
         open: true,
         readOnly: entry.readOnly === true,
+        append: entry.append === true,
       };
     }
 
@@ -1380,19 +1393,28 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
               return this._measureWasiPhase("writeResultPtr", () => this._writeUint32(nwrittenPtr, written));
             }
           }
+          const position = handle.append
+            ? this._measureWasiPhase("appendFstat", () =>
+                Number(__agentOSFs().fstatSync(handle.targetFd).size ?? 0)
+              )
+            : null;
           const written = this._measureWasiPhase("writeSync", () =>
             __agentOSFs().writeSync(
               handle.targetFd,
               bytes,
               0,
               bytes.length,
-              null,
+              position,
             )
           );
           return this._measureWasiPhase("writeResultPtr", () => this._writeUint32(nwrittenPtr, written));
         }
         if (handle?.kind === "guest-file" && typeof handle.targetFd === "number") {
-          const position = handle.append ? null : (handle.position ?? 0);
+          const position = handle.append
+            ? this._measureWasiPhase("appendFstat", () =>
+                Number(__agentOSFs().fstatSync(handle.targetFd).size ?? 0)
+              )
+            : (handle.position ?? 0);
           const written = this._measureWasiPhase("writeSync", () =>
             __agentOSFs().writeSync(
               handle.targetFd,
@@ -1469,7 +1491,11 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
         }
         if (entry.kind === "file") {
           this._clearStatCache();
-          const position = typeof entry.offset === "number" ? entry.offset : null;
+          const position = entry.append
+            ? this._measureWasiPhase("appendFstat", () =>
+                Number(__agentOSFs().fstatSync(entry.realFd).size ?? 0)
+              )
+            : (typeof entry.offset === "number" ? entry.offset : null);
           const written = this._measureWasiPhase("writeSync", () =>
             __agentOSFs().writeSync(
               entry.realFd,
@@ -1479,7 +1505,11 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
               position,
             )
           );
-          if (typeof entry.offset === "number") {
+          if (entry.append) {
+            entry.offset = this._measureWasiPhase("appendFstat", () =>
+              Number(__agentOSFs().fstatSync(entry.realFd).size ?? 0)
+            );
+          } else if (typeof entry.offset === "number") {
             entry.offset += written;
           }
           return this._measureWasiPhase("writeResultPtr", () => this._writeUint32(nwrittenPtr, written));
@@ -2206,8 +2236,12 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
           this._clearStatCache();
         }
         const fsConstants = __agentOSFs().constants ?? {};
+        const requestedFdFlags = Number(_fdflags) >>> 0;
+        const append = (requestedFdFlags & __agentOSWasiFdflagsAppend) !== 0;
         let openFlags = requestedWriteAccess
-          ? fsConstants.O_RDWR ?? 2
+          ? (this._hasReadRights(requestedRightsBase)
+              ? fsConstants.O_RDWR ?? 2
+              : fsConstants.O_WRONLY ?? 1)
           : fsConstants.O_RDONLY ?? 0;
         if ((requestedFlags & __agentOSWasiOpenCreate) !== 0) {
           openFlags |= fsConstants.O_CREAT ?? 64;
@@ -2218,6 +2252,9 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
         if ((requestedFlags & __agentOSWasiOpenTruncate) !== 0) {
           openFlags |= fsConstants.O_TRUNC ?? 512;
         }
+        if (append) {
+          openFlags |= fsConstants.O_APPEND ?? 1024;
+        }
         if (openDirectory) {
           openFlags |= fsConstants.O_DIRECTORY ?? 0;
         }
@@ -2235,10 +2272,13 @@ if (typeof globalThis !== "undefined" && typeof globalThis.__agentOSWasiModule =
             hostPath: fsPath,
             readOnly: resolved.readOnly === true,
             realFd,
-            offset: 0,
+            offset: append
+              ? Number(__agentOSFs().fstatSync(realFd).size ?? 0)
+              : 0,
+            append,
             rightsBase: requestedRightsBase & allowedRightsInheriting,
             rightsInheriting: requestedRightsInheriting & allowedRightsInheriting,
-            fdFlags: (Number(_fdflags) >>> 0) & 0xffff,
+            fdFlags: requestedFdFlags & 0xffff,
           });
         });
         return this._measureWasiPhase("writeOpenedFd", () => this._writeUint32(openedFdPtr, openedFd));

diff --git a/crates/native-sidecar/src/execution.rs b/crates/native-sidecar/src/execution.rs
@@ -6608,6 +6608,23 @@ where
         }
         let resolved = resolved;
         record_execute_phase("child_process_resolve_execution", phase_start.elapsed());
+        let parent_sync_roots = {
+            let vm = self.vms.get(vm_id).ok_or_else(|| missing_vm_error(vm_id))?;
+            let parent = vm
+                .active_processes
+                .get(process_id)
+                .ok_or_else(|| missing_process_error(vm_id, process_id))?;
+            (parent.host_write_dirty_recursive()
+                || !parent.clean_host_writes_are_observable_recursive())
+            .then(|| (parent.host_cwd.clone(), parent.guest_cwd.clone()))
+        };
+        if let Some((host_cwd, guest_cwd)) = parent_sync_roots {
+            let vm = self
+                .vms
+                .get_mut(vm_id)
+                .ok_or_else(|| missing_vm_error(vm_id))?;
+            sync_process_host_roots_to_kernel(vm, &host_cwd, &guest_cwd)?;
+        }
         let (parent_kernel_pid, child_process_id) = {
             let vm = self
                 .vms
@@ -7111,6 +7128,29 @@ where
         }
         let resolved = resolved;
         record_execute_phase("child_process_resolve_execution", phase_start.elapsed());
+        let parent_sync_roots = {
+            let vm = self.vms.get(vm_id).ok_or_else(|| missing_vm_error(vm_id))?;
+            let root = vm
+                .active_processes
+                .get(process_id)
+                .ok_or_else(|| missing_process_error(vm_id, process_id))?;
+            let parent =
+                Self::active_process_by_path(root, current_process_path).ok_or_else(|| {
+                    SidecarError::InvalidState(format!(
+                        "unknown child process path {current_process_label} during nested spawn"
+                    ))
+                })?;
+            (parent.host_write_dirty_recursive()
+                || !parent.clean_host_writes_are_observable_recursive())
+            .then(|| (parent.host_cwd.clone(), parent.guest_cwd.clone()))
+        };
+        if let Some((host_cwd, guest_cwd)) = parent_sync_roots {
+            let vm = self
+                .vms
+                .get_mut(vm_id)
+                .ok_or_else(|| missing_vm_error(vm_id))?;
+            sync_process_host_roots_to_kernel(vm, &host_cwd, &guest_cwd)?;
+        }
 
         let sidecar_requests = self.sidecar_requests.clone();
         let vm = self
@@ -9837,17 +9877,25 @@ fn collect_process_host_sync_roots(
 fn sync_process_host_writes_to_kernel(
     vm: &mut VmState,
     process: &ActiveProcess,
+) -> Result<(), SidecarError> {
+    sync_process_host_roots_to_kernel(vm, &process.host_cwd, &process.guest_cwd)
+}
+
+fn sync_process_host_roots_to_kernel(
+    vm: &mut VmState,
+    process_host_cwd: &Path,
+    process_guest_cwd: &str,
 ) -> Result<(), SidecarError> {
     if vm.root_filesystem_mode != RootFilesystemMode::ReadOnly {
         let shadow_root = vm.cwd.clone();
         sync_host_directory_tree_to_kernel(vm, &shadow_root, "/")?;
     }
 
     if !path_is_within_root(
-        &normalize_host_path(&process.host_cwd),
+        &normalize_host_path(process_host_cwd),
         &normalize_host_path(&vm.cwd),
     ) {
-        sync_host_directory_tree_to_kernel(vm, &process.host_cwd, &process.guest_cwd)?;
+        sync_host_directory_tree_to_kernel(vm, process_host_cwd, process_guest_cwd)?;
     }
 
     Ok(())

diff --git a/crates/native-sidecar/tests/service.rs b/crates/native-sidecar/tests/service.rs
@@ -1349,7 +1349,7 @@ ykAheWCsAteSEWVc0w==\n\
                 return fallback;
             }
 
-            let vendored = repo_root.join("packages/core/commands");
+            let vendored = repo_root.join("packages/runtime-core/commands");
             if vendored.exists() {
                 let staged = temp_dir("agentos-native-sidecar-vendored-commands");
                 for command in ["bash", "cat", "mkdir", "printf", "sh"] {
@@ -1372,6 +1372,29 @@ ykAheWCsAteSEWVc0w==\n\
                 return staged;
             }
 
+            let legacy_vendored = repo_root.join("packages/core/commands");
+            if legacy_vendored.exists() {
+                let staged = temp_dir("agentos-native-sidecar-vendored-commands");
+                for command in ["bash", "cat", "mkdir", "printf", "sh"] {
+                    let source = legacy_vendored.join(command);
+                    let target = staged.join(command);
+                    fs::copy(&source, &target).unwrap_or_else(|error| {
+                        panic!(
+                            "copy vendored command {} -> {}: {error}",
+                            source.display(),
+                            target.display()
+                        )
+                    });
+                    let mut permissions = fs::metadata(&target)
+                        .expect("stat staged vendored command")
+                        .permissions();
+                    permissions.set_mode(0o755);
+                    fs::set_permissions(&target, permissions)
+                        .expect("chmod staged vendored command");
+                }
+                return staged;
+            }
+
             panic!(
                 "registry WASM commands are required for service fs regression tests: expected {}, {}, or {}",
                 copied.display(),

diff --git a/docs-internal/registry-parity-worklist.md b/docs-internal/registry-parity-worklist.md
@@ -138,14 +138,14 @@ so a reader sees the whole board at a glance.
 
 ## P0 — Runtime / VM correctness
 
-### 1. brush-shell `>>` append truncates instead of appending
+### 1. brush-shell `>>` append truncates instead of appending — DONE
 - **Broken:** `execSync` with `>>` onto a write-only file overwrites instead of
   appends. `expected 'changed' to be 'originalchanged'`. (issue: rivet-dev/agentos#1657)
 - **Objective:** `>>` opens `O_WRONLY|O_APPEND` against the kernel VFS and appends,
   identical to bash on Linux.
-- **Proof:** `bridge-child-process.test.ts` "append redirection … succeeds like
-  Linux" passes un-skipped, plus a direct VFS append test.
-- **rev:** `fix(runtime): honor >> append mode in guest shell VFS redirection`
+- **Proof:** `bridge-child-process.test.ts` append redirection tests pass
+  un-skipped; direct kernel append and native sidecar append regressions pass.
+- **rev:** `ouxrzutq` — `fix(runtime): honor >> append mode in guest shell VFS redirection`
 
 ### 2. brush-shell `cat < file` stdin redirection fails (exit 1)
 - **Broken:** `cat < stdin-input.txt` exits 1 — input redirection from a VFS path