🥇 export from upstream (f22ca70)

compose/docker-compose.yml

@@ -6,6 +6,7 @@ services:
     volumes:
       - ${SANDCAGE_WORKSPACE}:${SANDCAGE_CONTAINER_DIR}
       - ${SANDCAGE_HOME}/.claude:/home/agent/.claude
+      - ${SANDCAGE_HOME}/.claude.json:/home/agent/.claude.json
       - ${SANDCAGE_GLOBAL_JUSTFILE}:/home/agent/.justfile:ro
     environment:
       - HOME=/home/agent
@@ -32,6 +33,7 @@ services:
     volumes:
       - ${SANDCAGE_WORKSPACE}:${SANDCAGE_CONTAINER_DIR}
       - ${SANDCAGE_HOME}/.claude:/home/agent/.claude
+      - ${SANDCAGE_HOME}/.claude.json:/home/agent/.claude.json
       - ${SANDCAGE_HOME}/.codex:/home/agent/.codex
       - ${SANDCAGE_GLOBAL_JUSTFILE}:/home/agent/.justfile:ro
     environment:

crates/sandcage/src/init.rs

@@ -51,7 +51,14 @@ pub fn preseed_with_home(home: &Path) -> Result<()> {
         println!("sandcage: seeded {} from template", settings_dest.display());
     }
 
-    // 4. Create an empty Justfile if absent
+    // 4. Seed .claude.json so the bind mount targets a file, not a directory
+    let claude_json = sandcage_home.join(".claude.json");
+    if !claude_json.exists() {
+        std::fs::write(&claude_json, "{}\n")
+            .map_err(|e| InitError::WriteFileFailed(claude_json.clone(), e))?;
+    }
+
+    // 5. Create an empty Justfile if absent
     let justfile = sandcage_home.join("Justfile");
     if !justfile.exists() {
         std::fs::write(&justfile, "")