diff --git a/internal/sandbox/fsbridge.go b/internal/sandbox/fsbridge.go
index 951f1378..6188f78b 100644
--- a/internal/sandbox/fsbridge.go
+++ b/internal/sandbox/fsbridge.go
@@ -50,8 +50,9 @@ func (b *FsBridge) ReadFile(ctx context.Context, path string) (string, error) {
 }
 
 // WriteFile writes content to a file inside the container, creating directories as needed.
+// When append is true, content is appended (shell >>); otherwise the file is overwritten (shell >).
 // Matching TS FsBridge.writeFile().
-func (b *FsBridge) WriteFile(ctx context.Context, path, content string) error {
+func (b *FsBridge) WriteFile(ctx context.Context, path, content string, appendMode bool) error {
 	resolved := b.resolvePath(path)
 
 	// Create parent directory
@@ -60,8 +61,12 @@ func (b *FsBridge) WriteFile(ctx context.Context, path, content string) error {
 		_, _, _, _ = b.dockerExec(ctx, nil, "mkdir", "-p", dir)
 	}
 
+	redir := ">"
+	if appendMode {
+		redir = ">>"
+	}
 	// Write content via stdin pipe
-	_, stderr, exitCode, err := b.dockerExec(ctx, []byte(content), "sh", "-c", fmt.Sprintf("cat > %q", resolved))
+	_, stderr, exitCode, err := b.dockerExec(ctx, []byte(content), "sh", "-c", fmt.Sprintf("cat %s %q", redir, resolved))
 	if err != nil {
 		return fmt.Errorf("fsbridge write: %w", err)
 	}
diff --git a/internal/tools/edit.go b/internal/tools/edit.go
index f7afeb26..7ad59266 100644
--- a/internal/tools/edit.go
+++ b/internal/tools/edit.go
@@ -14,13 +14,13 @@ import (
 // EditTool performs search-and-replace edits on files.
 // Supports context file interceptor and sandbox routing.
 type EditTool struct {
-	workspace        string
-	restrict         bool
-	deniedPrefixes   []string // path prefixes to deny access to (e.g. .goclaw)
-	sandboxMgr       sandbox.Manager
-	contextFileIntc  *ContextFileInterceptor
-	memIntc          *MemoryInterceptor
-	permStore store.ConfigPermissionStore // nil = no group write restriction
+	workspace       string
+	restrict        bool
+	deniedPrefixes  []string // path prefixes to deny access to (e.g. .goclaw)
+	sandboxMgr      sandbox.Manager
+	contextFileIntc *ContextFileInterceptor
+	memIntc         *MemoryInterceptor
+	permStore       store.ConfigPermissionStore // nil = no group write restriction
 }
 
 // DenyPaths adds path prefixes that edit must reject.
@@ -215,7 +215,7 @@ func (t *EditTool) executeInSandbox(ctx context.Context, path, oldStr, newStr st
 		return result
 	}
 
-	if err := bridge.WriteFile(ctx, containerPath, newContent); err != nil {
+	if err := bridge.WriteFile(ctx, containerPath, newContent, false); err != nil {
 		return ErrorResult(fmt.Sprintf("failed to write file: %v", err) + MaybeFsBridgeHint(err))
 	}
 
diff --git a/internal/tools/filesystem_write.go b/internal/tools/filesystem_write.go
index 15becbcb..c9efba34 100644
--- a/internal/tools/filesystem_write.go
+++ b/internal/tools/filesystem_write.go
@@ -13,14 +13,14 @@ import (
 
 // WriteFileTool writes content to a file, optionally through a sandbox container.
 type WriteFileTool struct {
-	workspace        string
-	restrict         bool
-	deniedPrefixes   []string // path prefixes to deny access to (e.g. .goclaw)
-	sandboxMgr       sandbox.Manager
-	contextFileIntc  *ContextFileInterceptor // nil = no virtual FS routing
-	memIntc          *MemoryInterceptor      // nil = no memory routing
-	permStore     store.ConfigPermissionStore // nil = no group write restriction
-	workspaceIntc *WorkspaceInterceptor      // nil = no team workspace validation
+	workspace       string
+	restrict        bool
+	deniedPrefixes  []string // path prefixes to deny access to (e.g. .goclaw)
+	sandboxMgr      sandbox.Manager
+	contextFileIntc *ContextFileInterceptor     // nil = no virtual FS routing
+	memIntc         *MemoryInterceptor          // nil = no memory routing
+	permStore       store.ConfigPermissionStore // nil = no group write restriction
+	workspaceIntc   *WorkspaceInterceptor       // nil = no team workspace validation
 }
 
 // DenyPaths adds path prefixes that write_file must reject.
@@ -148,7 +148,7 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Resul
 	// Sandbox routing (sandboxKey from ctx — thread-safe)
 	sandboxKey := ToolSandboxKeyFromCtx(ctx)
 	if t.sandboxMgr != nil && sandboxKey != "" {
-		return t.executeInSandbox(ctx, path, content, sandboxKey, deliver)
+		return t.executeInSandbox(ctx, path, content, sandboxKey, deliver, appendMode)
 	}
 
 	// Host execution — use per-user workspace from context if available
@@ -222,7 +222,7 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Resul
 	return result
 }
 
-func (t *WriteFileTool) executeInSandbox(ctx context.Context, path, content, sandboxKey string, deliver bool) *Result {
+func (t *WriteFileTool) executeInSandbox(ctx context.Context, path, content, sandboxKey string, deliver, appendMode bool) *Result {
 	bridge, err := t.getFsBridge(ctx, sandboxKey)
 	if err != nil {
 		return ErrorResult(fmt.Sprintf("sandbox error: %v", err))
@@ -234,11 +234,19 @@ func (t *WriteFileTool) executeInSandbox(ctx context.Context, path, content, san
 	}
 	containerPath := ResolveSandboxPath(path, containerCwd)
 
-	if err := bridge.WriteFile(ctx, containerPath, content); err != nil {
-		return ErrorResult(fmt.Sprintf("failed to write file: %v", err) + MaybeFsBridgeHint(err))
+	if err := bridge.WriteFile(ctx, containerPath, content, appendMode); err != nil {
+		verb := "write"
+		if appendMode {
+			verb = "append to"
+		}
+		return ErrorResult(fmt.Sprintf("failed to %s file: %v", verb, err) + MaybeFsBridgeHint(err))
 	}
 
-	msg := fmt.Sprintf("File written: %s (%d bytes)", path, len(content))
+	verb := "written"
+	if appendMode {
+		verb = "appended"
+	}
+	msg := fmt.Sprintf("File %s: %s (%d bytes)", verb, path, len(content))
 	if deliver {
 		msg += ". File will be automatically delivered to the user — do NOT send it again via message tool."
 	}