diff --git a/codex-rs/context-fragments/src/lib.rs b/codex-rs/context-fragments/src/lib.rs
index f141275e00..e8e63c46b7 100644
--- a/codex-rs/context-fragments/src/lib.rs
+++ b/codex-rs/context-fragments/src/lib.rs
@@ -17,3 +17,6 @@ pub use fragment::ContextualUserFragment;
 pub use fragment::RenderedFragment;
 
 pub use recap_prompt::RecapPrompt;
+
+/// Shared classification for compaction summaries produced by core and filtered by Guardian.
+pub const COMPACTION_SUMMARY_KIND: &str = "compaction.summary";
diff --git a/codex-rs/core/src/agent/control/spawn.rs b/codex-rs/core/src/agent/control/spawn.rs
index cc68bfdd3b..21f6963664 100644
--- a/codex-rs/core/src/agent/control/spawn.rs
+++ b/codex-rs/core/src/agent/control/spawn.rs
@@ -17,6 +17,7 @@ use crate::context::ContextualUserFragment;
 use crate::context::CurrentTimeReminder;
 use crate::context::CurrentTimeUnavailable;
 use crate::context::DeveloperInstructions;
+use crate::context::GuardianApprovedAction;
 use crate::context::ManagedDeveloperInstructions;
 use crate::context::ModelInstructions;
 use crate::context::MultiAgentModeInstructions;
@@ -141,7 +142,9 @@ fn retain_forked_developer_message(item: &mut ResponseItem, usage_hint_texts: &[
         // Persisted role hints can predate the current bundled wording and lack markers.
         if matches!(
             content_item.kind().as_str(),
-            "guardian.approved_action" | "multi_agent.role_instructions" | "multi_agent.usage_hint"
+            GuardianApprovedAction::KIND
+                | MultiAgentRoleInstructions::KIND
+                | MultiAgentUsageHint::KIND
         ) {
             return false;
         }
diff --git a/codex-rs/core/src/context/compaction_summary.rs b/codex-rs/core/src/context/compaction_summary.rs
index 5c576275f9..dc2c768ea1 100644
--- a/codex-rs/core/src/context/compaction_summary.rs
+++ b/codex-rs/core/src/context/compaction_summary.rs
@@ -7,6 +7,8 @@ pub(crate) struct CompactionSummary {
 }
 
 impl CompactionSummary {
+    pub(crate) const KIND: &str = codex_context_fragments::COMPACTION_SUMMARY_KIND;
+
     pub(crate) fn new(summary: impl Into<String>) -> Self {
         Self {
             summary: summary.into(),
@@ -16,7 +18,7 @@ impl CompactionSummary {
 
 impl ContextualUserFragment for CompactionSummary {
     fn content_kind(&self) -> ContentItemKind {
-        ContentItemKind("compaction.summary".to_string())
+        ContentItemKind(Self::KIND.to_string())
     }
 
     fn role(&self) -> &'static str {
diff --git a/codex-rs/core/src/context/contextual_user_message.rs b/codex-rs/core/src/context/contextual_user_message.rs
index a74f10a616..130fe03c56 100644
--- a/codex-rs/core/src/context/contextual_user_message.rs
+++ b/codex-rs/core/src/context/contextual_user_message.rs
@@ -14,10 +14,12 @@ use super::LegacyUnifiedExecProcessLimitWarning;
 use super::RecommendedPluginsInstructions;
 use super::SubagentNotification;
 use super::TurnAborted;
+use super::UnsupportedMedia;
 use super::UserGoalUpdate;
 use super::UserInstructions;
 use super::UserShellCommand;
 use super::world_state::EnvironmentsState;
+use crate::image_preparation::ImagePreparationError;
 
 const CONTEXTUAL_USER_FRAGMENT_MATCHERS: &[fn(&str) -> bool] = &[
     UserInstructions::matches_text,
@@ -73,9 +75,9 @@ pub(crate) fn is_user_authorization_message(item: &ResponseItem) -> bool {
                                 kind.0.as_str(),
                                 "" | "unknown"
                                     // Media preparation can replace real user input.
-                                    | "images.preparation_error"
-                                    | "images.unsupported"
-                                    | "audio.unsupported"
+                                    | ImagePreparationError::KIND
+                                    | UnsupportedMedia::IMAGE_KIND
+                                    | UnsupportedMedia::AUDIO_KIND
                             )
                     })
             })
diff --git a/codex-rs/core/src/context/guardian_approved_action.rs b/codex-rs/core/src/context/guardian_approved_action.rs
index b206b058b3..df7f754c08 100644
--- a/codex-rs/core/src/context/guardian_approved_action.rs
+++ b/codex-rs/core/src/context/guardian_approved_action.rs
@@ -8,6 +8,8 @@ pub(crate) struct GuardianApprovedAction {
 }
 
 impl GuardianApprovedAction {
+    pub(crate) const KIND: &str = "guardian.approved_action";
+
     pub(crate) fn new(approved_action_json: String) -> Self {
         Self {
             approved_action_json,
@@ -17,7 +19,7 @@ impl GuardianApprovedAction {
 
 impl ContextualUserFragment for GuardianApprovedAction {
     fn content_kind(&self) -> ContentItemKind {
-        ContentItemKind("guardian.approved_action".to_string())
+        ContentItemKind(Self::KIND.to_string())
     }
 
     fn role(&self) -> &'static str {
diff --git a/codex-rs/core/src/context/memory.rs b/codex-rs/core/src/context/memory.rs
index be4e0f63d5..9e0c9d1ecf 100644
--- a/codex-rs/core/src/context/memory.rs
+++ b/codex-rs/core/src/context/memory.rs
@@ -11,6 +11,11 @@ pub enum MemoryContextFragment {
     ExtractionEvidence(String),
 }
 
+impl MemoryContextFragment {
+    /// Content classification for memory read-path instructions.
+    pub const READ_INSTRUCTIONS_KIND: &str = "memories.instructions";
+}
+
 impl ContextualUserFragment for MemoryContextFragment {
     fn role(&self) -> &'static str {
         match self {
@@ -21,7 +26,7 @@ impl ContextualUserFragment for MemoryContextFragment {
     fn content_kind(&self) -> ContentItemKind {
         ContentItemKind(
             match self {
-                Self::ReadInstructions(_) => "memories.instructions",
+                Self::ReadInstructions(_) => Self::READ_INSTRUCTIONS_KIND,
                 Self::ExtractionEvidence(_) => "memories.extraction_evidence",
             }
             .to_string(),
diff --git a/codex-rs/core/src/context/multi_agent_usage_hint.rs b/codex-rs/core/src/context/multi_agent_usage_hint.rs
index f08c3fa657..665b221acf 100644
--- a/codex-rs/core/src/context/multi_agent_usage_hint.rs
+++ b/codex-rs/core/src/context/multi_agent_usage_hint.rs
@@ -8,6 +8,8 @@ pub(crate) struct MultiAgentUsageHint {
 }
 
 impl MultiAgentUsageHint {
+    pub(crate) const KIND: &str = "multi_agent.usage_hint";
+
     pub(crate) fn new(text: &str) -> Self {
         Self {
             text: text.to_string(),
@@ -17,7 +19,7 @@ impl MultiAgentUsageHint {
 
 impl ContextualUserFragment for MultiAgentUsageHint {
     fn content_kind(&self) -> ContentItemKind {
-        ContentItemKind("multi_agent.usage_hint".to_string())
+        ContentItemKind(Self::KIND.to_string())
     }
 
     fn role(&self) -> &'static str {
diff --git a/codex-rs/core/src/context/unsupported_media.rs b/codex-rs/core/src/context/unsupported_media.rs
index 8b225649c0..2b30865b8b 100644
--- a/codex-rs/core/src/context/unsupported_media.rs
+++ b/codex-rs/core/src/context/unsupported_media.rs
@@ -8,14 +8,17 @@ pub(crate) struct UnsupportedMedia {
 }
 
 impl UnsupportedMedia {
+    pub(crate) const IMAGE_KIND: &str = "images.unsupported";
+    pub(crate) const AUDIO_KIND: &str = "audio.unsupported";
+
     pub(crate) const IMAGE: Self = Self {
         text: "image content omitted because you do not support image input",
-        content_kind: "images.unsupported",
+        content_kind: Self::IMAGE_KIND,
     };
 
     pub(crate) const AUDIO: Self = Self {
         text: "audio content omitted because you do not support audio input",
-        content_kind: "audio.unsupported",
+        content_kind: Self::AUDIO_KIND,
     };
 }
 
diff --git a/codex-rs/core/src/context/user_goal.rs b/codex-rs/core/src/context/user_goal.rs
index 76d7c1c532..5fbf484663 100644
--- a/codex-rs/core/src/context/user_goal.rs
+++ b/codex-rs/core/src/context/user_goal.rs
@@ -23,6 +23,7 @@ pub enum UserGoalUpdate {
 }
 
 impl UserGoalUpdate {
+    pub(crate) const KIND: &str = "user.goal";
     pub(crate) const OMITTED_OBJECTIVE_KIND: &str = "user.goal.omitted";
 
     /// Captures accepted goal instructions at a compaction input boundary.
@@ -53,7 +54,7 @@ impl UserGoalUpdate {
         let [ContentItem::InputText { text }] = content.as_slice() else {
             return None;
         };
-        (role == "user" && matches!(kind.0.as_str(), "user.goal" | Self::OMITTED_OBJECTIVE_KIND))
+        (role == "user" && matches!(kind.0.as_str(), Self::KIND | Self::OMITTED_OBJECTIVE_KIND))
             .then_some(text)
     }
 }
@@ -71,7 +72,7 @@ impl ContextualUserFragment for UserGoalUpdate {
             } if serde_json::json!(objective).to_string().len() > MAX_OBJECTIVE_BYTES => {
                 Self::OMITTED_OBJECTIVE_KIND
             }
-            Self::Set { .. } | Self::Clear => "user.goal",
+            Self::Set { .. } | Self::Clear => Self::KIND,
         };
         ContentItemKind(kind.to_owned())
     }
diff --git a/codex-rs/core/src/image_preparation.rs b/codex-rs/core/src/image_preparation.rs
index ae7e97f9db..3142613b04 100644
--- a/codex-rs/core/src/image_preparation.rs
+++ b/codex-rs/core/src/image_preparation.rs
@@ -107,6 +107,8 @@ pub(crate) enum ImagePreparationError {
 }
 
 impl ImagePreparationError {
+    pub(crate) const KIND: &str = "images.preparation_error";
+
     fn placeholder(&self) -> &'static str {
         match self {
             ImagePreparationError::RemoteUrlUnsupported => REMOTE_IMAGE_URL_PLACEHOLDER,
@@ -285,7 +287,7 @@ async fn prepare_message_content(
                     warn!(%error, "failed to prepare message image");
                     *item = AnnotatedContent::text(
                         error.placeholder(),
-                        ContentItemKind("images.preparation_error".to_string()),
+                        ContentItemKind(ImagePreparationError::KIND.to_string()),
                         codex_protocol::models::ContentItemMetadata::harness(),
                     );
                 }
diff --git a/codex-rs/ext/memories/src/extension.rs b/codex-rs/ext/memories/src/extension.rs
index 3db27e4984..337e63e11d 100644
--- a/codex-rs/ext/memories/src/extension.rs
+++ b/codex-rs/ext/memories/src/extension.rs
@@ -94,7 +94,7 @@ impl ContextContributor for MemoriesExtension {
                 .map(|instructions| {
                     PromptFragment::developer_policy(
                         instructions,
-                        ContentItemKind("memories.instructions".to_string()),
+                        ContentItemKind(MemoryContextFragment::READ_INSTRUCTIONS_KIND.to_string()),
                     )
                 })
                 .collect()
diff --git a/codex-rs/guardian-context/src/history.rs b/codex-rs/guardian-context/src/history.rs
index d9aad1bec2..1c0c56146b 100644
--- a/codex-rs/guardian-context/src/history.rs
+++ b/codex-rs/guardian-context/src/history.rs
@@ -13,7 +13,9 @@ use std::io;
 use std::io::Write;
 use std::sync::Arc;
 
+use codex_context_fragments::COMPACTION_SUMMARY_KIND;
 use codex_protocol::models::ContentItem;
+use codex_protocol::models::InternalChatMessageMetadataPassthrough;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::models::executed_tool_call_metadata_bytes;
 
@@ -68,9 +70,18 @@ impl TranscriptHistory {
                     | ResponseItem::ContextCompaction { .. }
                     | ResponseItem::CompactionTrigger { .. }
             )
-            || matches!(item, ResponseItem::Message { internal_chat_message_metadata_passthrough: Some(metadata), .. }
-                if metadata.content_item_kinds.as_ref().is_some_and(|kinds|
-                    kinds.iter().any(|kind| kind.0 == "compaction.summary")))
+            || matches!(
+                item,
+                ResponseItem::Message {
+                    internal_chat_message_metadata_passthrough: Some(
+                        InternalChatMessageMetadataPassthrough {
+                            content_item_kinds: Some(kinds),
+                            ..
+                        }
+                    ),
+                    ..
+                } if kinds.iter().any(|kind| kind.as_str() == COMPACTION_SUMMARY_KIND)
+            )
         {
             return;
         }
diff --git a/codex-rs/guardian-context/src/history_tests.rs b/codex-rs/guardian-context/src/history_tests.rs
index 4748ad192e..21fec31483 100644
--- a/codex-rs/guardian-context/src/history_tests.rs
+++ b/codex-rs/guardian-context/src/history_tests.rs
@@ -20,6 +20,23 @@ fn tool(text: &str) -> ResponseItem {
     .unwrap()
 }
 
+#[test]
+fn compaction_summary_is_excluded_without_dropping_user_messages() {
+    let before = message("Earlier user message");
+    let summary: ResponseItem = serde_json::from_value(json!({
+        "type": "message", "role": "user",
+        "content": [{"type": "input_text", "text": "Summary text"}],
+        "internal_chat_message_metadata_passthrough": {
+            "content_item_kinds": [COMPACTION_SUMMARY_KIND]
+        }
+    }))
+    .unwrap();
+    let after = message("Summary text");
+    let mut history = TranscriptHistory::default();
+    history.reset([before.clone().into(), summary.into(), after.clone().into()].iter());
+    assert_eq!(history.items().collect::<Vec<_>>(), vec![&before, &after]);
+}
+
 #[test]
 fn rollback_keeps_the_earlier_prefix_or_clears_an_evicted_boundary() {
     let items = [message("keep"), message("roll back"), tool("later")];
diff --git a/codex-rs/prompts/src/multi_agent_instructions.rs b/codex-rs/prompts/src/multi_agent_instructions.rs
index 415f52dd2a..d75b261e0a 100644
--- a/codex-rs/prompts/src/multi_agent_instructions.rs
+++ b/codex-rs/prompts/src/multi_agent_instructions.rs
@@ -32,9 +32,14 @@ pub enum MultiAgentRoleInstructions {
     },
 }
 
+impl MultiAgentRoleInstructions {
+    /// Content classification shared by role producers and inherited-history filtering.
+    pub const KIND: &str = "multi_agent.role_instructions";
+}
+
 impl ContextualUserFragment for MultiAgentRoleInstructions {
     fn content_kind(&self) -> ContentItemKind {
-        ContentItemKind("multi_agent.role_instructions".to_string())
+        ContentItemKind(Self::KIND.to_string())
     }
 
     fn role(&self) -> &'static str {