diff --git a/tools/aconfig/protos/aconfig.proto b/tools/aconfig/protos/aconfig.proto
index 9e193ec..1a80b04 100644
--- a/tools/aconfig/protos/aconfig.proto
+++ b/tools/aconfig/protos/aconfig.proto
@@ -46,6 +46,7 @@
 message flag_declarations {
   optional string package = 1;
   repeated flag_declaration flag = 2;
+  optional string container = 3;
 };
 
 message flag_value {
@@ -79,7 +80,7 @@
   repeated tracepoint trace = 8;
   optional bool is_fixed_read_only = 9;
   optional bool is_exported = 10;
-
+  optional string container = 11;
 }
 
 message parsed_flags {
diff --git a/tools/aconfig/src/codegen.rs b/tools/aconfig/src/codegen.rs
index b7fb08f..fef7a3f 100644
--- a/tools/aconfig/src/codegen.rs
+++ b/tools/aconfig/src/codegen.rs
@@ -38,6 +38,10 @@
     s.split('.').all(is_valid_name_ident)
 }
 
+pub fn is_valid_container_ident(s: &str) -> bool {
+    is_valid_name_ident(s) || s.split('.').all(is_valid_name_ident)
+}
+
 pub fn create_device_config_ident(package: &str, flag_name: &str) -> Result<String> {
     ensure!(is_valid_package_ident(package), "bad package");
     ensure!(is_valid_name_ident(flag_name), "bad flag name");
@@ -85,6 +89,29 @@
     }
 
     #[test]
+    fn test_is_valid_container_ident() {
+        assert!(is_valid_container_ident("foo.bar"));
+        assert!(is_valid_container_ident("foo.bar_baz"));
+        assert!(is_valid_container_ident("foo.bar.a123"));
+        assert!(is_valid_container_ident("foo"));
+        assert!(is_valid_container_ident("foo_bar_123"));
+
+        assert!(!is_valid_container_ident(""));
+        assert!(!is_valid_container_ident("foo._bar"));
+        assert!(!is_valid_container_ident("_foo"));
+        assert!(!is_valid_container_ident("123_foo"));
+        assert!(!is_valid_container_ident("foo-bar"));
+        assert!(!is_valid_container_ident("foo-b\u{00e5}r"));
+        assert!(!is_valid_container_ident("foo.bar.123"));
+        assert!(!is_valid_container_ident(".foo.bar"));
+        assert!(!is_valid_container_ident("foo.bar."));
+        assert!(!is_valid_container_ident("."));
+        assert!(!is_valid_container_ident(".."));
+        assert!(!is_valid_container_ident("foo..bar"));
+        assert!(!is_valid_container_ident("foo.__bar"));
+    }
+
+    #[test]
     fn test_create_device_config_ident() {
         assert_eq!(
             "com.foo.bar.some_flag",
diff --git a/tools/aconfig/src/commands.rs b/tools/aconfig/src/commands.rs
index 47e90ac..d04c0cf 100644
--- a/tools/aconfig/src/commands.rs
+++ b/tools/aconfig/src/commands.rs
@@ -57,6 +57,7 @@
 
 pub fn parse_flags(
     package: &str,
+    container: Option<&str>,
     declarations: Vec<Input>,
     values: Vec<Input>,
     default_permission: ProtoFlagPermission,
@@ -79,12 +80,24 @@
             package,
             flag_declarations.package()
         );
+        if let Some(c) = container {
+            ensure!(
+                c == flag_declarations.container(),
+                "failed to parse {}: expected container {}, got {}",
+                input.source,
+                c,
+                flag_declarations.container()
+            );
+        }
         for mut flag_declaration in flag_declarations.flag.into_iter() {
             crate::protos::flag_declaration::verify_fields(&flag_declaration)
                 .with_context(|| input.error_context())?;
 
             // create ParsedFlag using FlagDeclaration and default values
             let mut parsed_flag = ProtoParsedFlag::new();
+            if let Some(c) = container {
+                parsed_flag.set_container(c.to_string());
+            }
             parsed_flag.set_package(package.to_string());
             parsed_flag.set_name(flag_declaration.take_name());
             parsed_flag.set_namespace(flag_declaration.take_namespace());
@@ -262,9 +275,10 @@
         DumpFormat::Text => {
             for parsed_flag in parsed_flags.parsed_flag.into_iter() {
                 let line = format!(
-                    "{}.{}: {:?} + {:?}\n",
+                    "{}.{} [{}]: {:?} + {:?}\n",
                     parsed_flag.package(),
                     parsed_flag.name(),
+                    parsed_flag.container(),
                     parsed_flag.permission(),
                     parsed_flag.state()
                 );
@@ -276,9 +290,10 @@
                 let sources: Vec<_> =
                     parsed_flag.trace.iter().map(|tracepoint| tracepoint.source()).collect();
                 let line = format!(
-                    "{}.{}: {:?} + {:?} ({})\n",
+                    "{}.{} [{}]: {:?} + {:?} ({})\n",
                     parsed_flag.package(),
                     parsed_flag.name(),
+                    parsed_flag.container(),
                     parsed_flag.permission(),
                     parsed_flag.state(),
                     sources.join(", ")
@@ -377,6 +392,7 @@
 
         let flags_bytes = crate::commands::parse_flags(
             "com.first",
+            None,
             declaration,
             value,
             ProtoFlagPermission::READ_ONLY,
@@ -391,9 +407,72 @@
     }
 
     #[test]
+    fn test_parse_flags_package_mismatch_between_declaration_and_command_line() {
+        let first_flag = r#"
+        package: "com.declaration.package"
+        container: "first.container"
+        flag {
+            name: "first"
+            namespace: "first_ns"
+            description: "This is the description of the first flag."
+            bug: "123"
+        }
+        "#;
+        let declaration =
+            vec![Input { source: "memory".to_string(), reader: Box::new(first_flag.as_bytes()) }];
+
+        let value: Vec<Input> = vec![];
+
+        let error = crate::commands::parse_flags(
+            "com.argument.package",
+            Some("first.container"),
+            declaration,
+            value,
+            ProtoFlagPermission::READ_WRITE,
+        )
+        .unwrap_err();
+        assert_eq!(
+            format!("{:?}", error),
+            "failed to parse memory: expected package com.argument.package, got com.declaration.package"
+        );
+    }
+
+    #[test]
+    fn test_parse_flags_container_mismatch_between_declaration_and_command_line() {
+        let first_flag = r#"
+        package: "com.first"
+        container: "declaration.container"
+        flag {
+            name: "first"
+            namespace: "first_ns"
+            description: "This is the description of the first flag."
+            bug: "123"
+        }
+        "#;
+        let declaration =
+            vec![Input { source: "memory".to_string(), reader: Box::new(first_flag.as_bytes()) }];
+
+        let value: Vec<Input> = vec![];
+
+        let error = crate::commands::parse_flags(
+            "com.first",
+            Some("argument.container"),
+            declaration,
+            value,
+            ProtoFlagPermission::READ_WRITE,
+        )
+        .unwrap_err();
+        assert_eq!(
+            format!("{:?}", error),
+            "failed to parse memory: expected container argument.container, got declaration.container"
+        );
+    }
+
+    #[test]
     fn test_parse_flags_override_fixed_read_only() {
         let first_flag = r#"
         package: "com.first"
+        container: "com.first.container"
         flag {
             name: "first"
             namespace: "first_ns"
@@ -419,6 +498,7 @@
         }];
         let error = crate::commands::parse_flags(
             "com.first",
+            Some("com.first.container"),
             declaration,
             value,
             ProtoFlagPermission::READ_WRITE,
@@ -451,7 +531,9 @@
         let input = parse_test_flags_as_input();
         let bytes = dump_parsed_flags(vec![input], DumpFormat::Text).unwrap();
         let text = std::str::from_utf8(&bytes).unwrap();
-        assert!(text.contains("com.android.aconfig.test.disabled_ro: READ_ONLY + DISABLED"));
+        assert!(
+            text.contains("com.android.aconfig.test.disabled_ro [system]: READ_ONLY + DISABLED")
+        );
     }
 
     #[test]
diff --git a/tools/aconfig/src/main.rs b/tools/aconfig/src/main.rs
index 7e44baf..af0368a 100644
--- a/tools/aconfig/src/main.rs
+++ b/tools/aconfig/src/main.rs
@@ -42,6 +42,8 @@
         .subcommand(
             Command::new("create-cache")
                 .arg(Arg::new("package").long("package").required(true))
+                // TODO(b/312769710): Make this argument required.
+                .arg(Arg::new("container").long("container"))
                 .arg(Arg::new("declarations").long("declarations").action(ArgAction::Append))
                 .arg(Arg::new("values").long("values").action(ArgAction::Append))
                 .arg(
@@ -119,6 +121,13 @@
         .ok_or(anyhow!("internal error: required argument '{}' not found", arg_name))
 }
 
+fn get_optional_arg<'a, T>(matches: &'a ArgMatches, arg_name: &str) -> Option<&'a T>
+where
+    T: Any + Clone + Send + Sync + 'static,
+{
+    matches.get_one::<T>(arg_name)
+}
+
 fn open_zero_or_more_files(matches: &ArgMatches, arg_name: &str) -> Result<Vec<Input>> {
     let mut opened_files = vec![];
     for path in matches.get_many::<String>(arg_name).unwrap_or_default() {
@@ -167,12 +176,20 @@
     match matches.subcommand() {
         Some(("create-cache", sub_matches)) => {
             let package = get_required_arg::<String>(sub_matches, "package")?;
+            let container =
+                get_optional_arg::<String>(sub_matches, "container").map(|c| c.as_str());
             let declarations = open_zero_or_more_files(sub_matches, "declarations")?;
             let values = open_zero_or_more_files(sub_matches, "values")?;
             let default_permission =
                 get_required_arg::<protos::ProtoFlagPermission>(sub_matches, "default-permission")?;
-            let output = commands::parse_flags(package, declarations, values, *default_permission)
-                .context("failed to create cache")?;
+            let output = commands::parse_flags(
+                package,
+                container,
+                declarations,
+                values,
+                *default_permission,
+            )
+            .context("failed to create cache")?;
             let path = get_required_arg::<String>(sub_matches, "cache")?;
             write_output_to_file_or_stdout(path, &output)?;
         }
diff --git a/tools/aconfig/src/protos.rs b/tools/aconfig/src/protos.rs
index a5a5342..c11596b 100644
--- a/tools/aconfig/src/protos.rs
+++ b/tools/aconfig/src/protos.rs
@@ -111,11 +111,16 @@
 
     pub fn verify_fields(pdf: &ProtoFlagDeclarations) -> Result<()> {
         ensure_required_fields!("flag declarations", pdf, "package");
+        // TODO(b/312769710): Make the container field required.
 
         ensure!(
             codegen::is_valid_package_ident(pdf.package()),
             "bad flag declarations: bad package"
         );
+        ensure!(
+            !pdf.has_container() || codegen::is_valid_container_ident(pdf.container()),
+            "bad flag declarations: bad container"
+        );
         for flag_declaration in pdf.flag.iter() {
             super::flag_declaration::verify_fields(flag_declaration)?;
         }
@@ -207,6 +212,10 @@
         );
 
         ensure!(codegen::is_valid_package_ident(pf.package()), "bad parsed flag: bad package");
+        ensure!(
+            !pf.has_container() || codegen::is_valid_container_ident(pf.container()),
+            "bad parsed flag: bad container"
+        );
         ensure!(codegen::is_valid_name_ident(pf.name()), "bad parsed flag: bad name");
         ensure!(codegen::is_valid_name_ident(pf.namespace()), "bad parsed flag: bad namespace");
         ensure!(!pf.description().is_empty(), "bad parsed flag: empty description");
@@ -303,6 +312,7 @@
         let flag_declarations = flag_declarations::try_from_text_proto(
             r#"
 package: "com.foo.bar"
+container: "system"
 flag {
     name: "first"
     namespace: "first_ns"
@@ -321,6 +331,7 @@
         )
         .unwrap();
         assert_eq!(flag_declarations.package(), "com.foo.bar");
+        assert_eq!(flag_declarations.container(), "system");
         let first = flag_declarations.flag.iter().find(|pf| pf.name() == "first").unwrap();
         assert_eq!(first.name(), "first");
         assert_eq!(first.namespace(), "first_ns");
@@ -336,9 +347,26 @@
         assert!(second.is_fixed_read_only());
         assert!(!second.is_exported());
 
+        // valid input: missing container in flag declarations is supported
+        let flag_declarations = flag_declarations::try_from_text_proto(
+            r#"
+package: "com.foo.bar"
+flag {
+    name: "first"
+    namespace: "first_ns"
+    description: "This is the description of the first flag."
+    bug: "123"
+}
+"#,
+        )
+        .unwrap();
+        assert_eq!(flag_declarations.container(), "");
+        assert!(!flag_declarations.has_container());
+
         // bad input: missing package in flag declarations
         let error = flag_declarations::try_from_text_proto(
             r#"
+container: "system"
 flag {
     name: "first"
     namespace: "first_ns"
@@ -358,6 +386,7 @@
         let error = flag_declarations::try_from_text_proto(
             r#"
 package: "com.foo.bar"
+container: "system"
 flag {
     name: "first"
     description: "This is the description of the first flag."
@@ -376,6 +405,7 @@
         let error = flag_declarations::try_from_text_proto(
             r#"
 package: "_com.FOO__BAR"
+container: "system"
 flag {
     name: "first"
     namespace: "first_ns"
@@ -395,6 +425,7 @@
         let error = flag_declarations::try_from_text_proto(
             r#"
 package: "com.foo.bar"
+container: "system"
 flag {
     name: "FIRST"
     namespace: "first_ns"
@@ -414,6 +445,7 @@
         let error = flag_declarations::try_from_text_proto(
             r#"
 package: "com.foo.bar"
+container: "system"
 flag {
     name: "first"
     namespace: "first_ns"
@@ -428,6 +460,7 @@
         let error = flag_declarations::try_from_text_proto(
             r#"
 package: "com.foo.bar"
+container: "system"
 flag {
     name: "first"
     namespace: "first_ns"
@@ -439,6 +472,25 @@
         )
         .unwrap_err();
         assert!(format!("{:?}", error).contains("bad flag declaration: exactly one bug required"));
+
+        // bad input: invalid container name in flag declaration
+        let error = flag_declarations::try_from_text_proto(
+            r#"
+package: "com.foo.bar"
+container: "__bad_bad_container.com"
+flag {
+    name: "first"
+    namespace: "first_ns"
+    description: "This is the description of the first flag."
+    bug: "123"
+    bug: "abc"
+}
+"#,
+        )
+        .unwrap_err();
+        assert!(format!("{:?}", error).contains("bad flag declarations: bad container"));
+
+        // TODO(b/312769710): Verify error when container is missing.
     }
 
     #[test]
@@ -553,6 +605,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 parsed_flag {
     package: "com.second"
@@ -573,6 +626,7 @@
         permission: READ_ONLY
     }
     is_fixed_read_only: true
+    container: "system"
 }
 "#;
         let parsed_flags = try_from_binary_proto_from_text_proto(text_proto).unwrap();
@@ -607,6 +661,7 @@
     description: "This is the description of the first flag."
     state: DISABLED
     permission: READ_ONLY
+    container: "system"
 }
 "#;
         let error = try_from_binary_proto_from_text_proto(text_proto).unwrap_err();
@@ -625,6 +680,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let error = try_from_binary_proto_from_text_proto(text_proto).unwrap_err();
@@ -645,6 +701,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 parsed_flag {
     package: "aaa.aaa"
@@ -659,6 +716,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let error = try_from_binary_proto_from_text_proto(text_proto).unwrap_err();
@@ -682,6 +740,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 parsed_flag {
     package: "com.foo"
@@ -696,6 +755,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let error = try_from_binary_proto_from_text_proto(text_proto).unwrap_err();
@@ -719,6 +779,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 parsed_flag {
     package: "com.foo"
@@ -733,6 +794,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let error = try_from_binary_proto_from_text_proto(text_proto).unwrap_err();
@@ -760,6 +822,7 @@
         state: ENABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let parsed_flags = try_from_binary_proto_from_text_proto(text_proto).unwrap();
@@ -786,6 +849,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 parsed_flag {
     package: "com.second"
@@ -800,6 +864,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let expected = try_from_binary_proto_from_text_proto(text_proto).unwrap();
@@ -818,6 +883,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let first = try_from_binary_proto_from_text_proto(text_proto).unwrap();
@@ -836,6 +902,7 @@
         state: DISABLED
         permission: READ_ONLY
     }
+    container: "system"
 }
 "#;
         let second = try_from_binary_proto_from_text_proto(text_proto).unwrap();
diff --git a/tools/aconfig/src/test.rs b/tools/aconfig/src/test.rs
index 9f598d0..8df4493 100644
--- a/tools/aconfig/src/test.rs
+++ b/tools/aconfig/src/test.rs
@@ -43,6 +43,7 @@
   }
   is_fixed_read_only: false
   is_exported: false
+  container: "system"
 }
 parsed_flag {
   package: "com.android.aconfig.test"
@@ -59,6 +60,7 @@
   }
   is_fixed_read_only: false
   is_exported: true
+  container: "system"
 }
 parsed_flag {
   package: "com.android.aconfig.test"
@@ -80,6 +82,7 @@
   }
   is_fixed_read_only: false
   is_exported: true
+  container: "system"
 }
 parsed_flag {
   package: "com.android.aconfig.test"
@@ -101,6 +104,7 @@
   }
   is_fixed_read_only: false
   is_exported: false
+  container: "system"
 }
 parsed_flag {
   package: "com.android.aconfig.test"
@@ -122,6 +126,7 @@
   }
   is_fixed_read_only: true
   is_exported: false
+  container: "system"
 }
 parsed_flag {
   package: "com.android.aconfig.test"
@@ -148,6 +153,7 @@
   }
   is_fixed_read_only: false
   is_exported: false
+  container: "system"
 }
 parsed_flag {
   package: "com.android.aconfig.test"
@@ -169,12 +175,14 @@
   }
   is_fixed_read_only: false
   is_exported: false
+  container: "system"
 }
 "#;
 
     pub fn parse_test_flags() -> ProtoParsedFlags {
         let bytes = crate::commands::parse_flags(
             "com.android.aconfig.test",
+            Some("system"),
             vec![Input {
                 source: "tests/test.aconfig".to_string(),
                 reader: Box::new(include_bytes!("../tests/test.aconfig").as_slice()),
diff --git a/tools/aconfig/tests/test.aconfig b/tools/aconfig/tests/test.aconfig
index 8a1a913..e44d154 100644
--- a/tools/aconfig/tests/test.aconfig
+++ b/tools/aconfig/tests/test.aconfig
@@ -1,4 +1,5 @@
 package: "com.android.aconfig.test"
+container: "system"
 
 # This flag's final value is calculated from:
 # - test.aconfig: DISABLED + READ_WRITE (default)