diff --git a/AGENTS.md b/AGENTS.md
index 3a287a5991..3ee7390387 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -41,6 +41,18 @@ In the codex-rs folder where the rust code lives:
   - When extracting code from a large module, move the related tests and module/type docs toward
     the new implementation so the invariants stay close to the code that owns them.
 
+### Model-visible context fragments
+
+- Model-visible prompt context should go through the shared fragment abstractions described in `docs/model-visible-context.md`.
+- Every named fragment type should implement `ModelVisibleContextFragment` and set `type Role`.
+- If a fragment represents durable turn/session state that should be rebuilt correctly across resume/fork/compaction/backtracking, implement `ModelVisibleContextFragment::build(...)`.
+- If a fragment is contextual-user, it must provide stable detection: prefer `contextual_user_markers()` when fixed markers are sufficient, and override `matches_contextual_user_text()` only for genuinely custom matching (for example AGENTS.md).
+- Choose the role intentionally: developer guidance belongs in `DeveloperContextRole`; contextual user-role state belongs in `ContextualUserContextRole`.
+- Use contextual-user fragments for contextual user-role state that must be parsed as context rather than literal user intent.
+- Runtime/session-prefix fragments that are not turn-state diffs should usually leave `ModelVisibleContextFragment::build(...)` as `None`.
+- Prefer dedicated typed fragments over plain strings. Developer-only one-off text is acceptable only when it is truly isolated, does not need contextual-user detection, and does not participate in turn-state diff reconstruction.
+- Do not hand-construct model-visible `ResponseItem::Message` payloads in new turn-state code; use fragment conversion helpers from `codex-rs/core/src/model_visible_context.rs`.
+
 Run `just fmt` (in `codex-rs` directory) automatically after you have finished making Rust code changes; do not ask for approval to run it. Additionally, run the tests:
 
 1. Run the test for the specific project that was changed. For example, if changes were made in `codex-rs/tui`, run `cargo test -p codex-tui`.
diff --git a/codex-rs/core/src/lib.rs b/codex-rs/core/src/lib.rs
index 29436a0d7f..d72b207d8b 100644
--- a/codex-rs/core/src/lib.rs
+++ b/codex-rs/core/src/lib.rs
@@ -64,6 +64,7 @@ pub mod mention_syntax;
 mod mentions;
 pub mod message_history;
 mod model_provider_info;
+mod model_visible_context;
 pub mod path_utils;
 pub mod personality_migration;
 pub mod plugins;
diff --git a/codex-rs/core/src/model_visible_context.rs b/codex-rs/core/src/model_visible_context.rs
new file mode 100644
index 0000000000..e913ebe06b
--- /dev/null
+++ b/codex-rs/core/src/model_visible_context.rs
@@ -0,0 +1,290 @@
+//! Shared model-visible context abstractions.
+//!
+//! Use this path for any injected prompt context, whether it renders in the
+//! developer envelope or the contextual-user envelope.
+//!
+//! This module keeps only the shared rendering, role, marker, and turn-context
+//! parameter helpers that fragment implementations can reuse.
+//!
+//! Contributor guide:
+//!
+//! - If the model should not see the data, do not add a fragment.
+//! - If it should, prefer a typed fragment that implements
+//!   `ModelVisibleContextFragment`.
+//! - Choose the role intentionally:
+//!   - `DeveloperContextRole` for developer guidance/policy
+//!   - `ContextualUserContextRole` for contextual user-role state that must be
+//!     parsed as context rather than literal user intent
+//! - If the fragment is durable turn/session state that should rebuild across
+//!   resume, compaction, backtracking, or fork, implement `build(...)`.
+//!   `reference_context_item` is the baseline already represented in
+//!   model-visible history; compare against it to avoid duplicates, and use
+//!   `TurnContextDiffParams` for other runtime/session inputs such as
+//!   `previous_turn_settings`.
+//! - If the fragment is a runtime/session-prefix marker rather than turn-state
+//!   context, leave `build(...)` as `None`.
+//! - Contextual-user fragments must have stable detection. Prefer
+//!   `contextual_user_markers()`; override `matches_contextual_user_text()`
+//!   only when matching is genuinely custom.
+//! - Keep logic fragment-local. The fragment type should own rendering,
+//!   state/diff inspection, and contextual-user detection when applicable.
+
+#![allow(dead_code)]
+
+use crate::codex::PreviousTurnSettings;
+use crate::codex::TurnContext;
+use crate::plugins::PluginCapabilitySummary;
+use crate::shell::Shell;
+use codex_execpolicy::Policy;
+use codex_protocol::models::ContentItem;
+use codex_protocol::models::ResponseInputItem;
+use codex_protocol::models::ResponseItem;
+use codex_protocol::protocol::TurnContextItem;
+
+pub(crate) const SKILL_OPEN_TAG: &str = "<skill>";
+pub(crate) const SKILL_CLOSE_TAG: &str = "</skill>";
+pub(crate) const JS_REPL_INSTRUCTIONS_OPEN_TAG: &str = "<js_repl_instructions>";
+pub(crate) const JS_REPL_INSTRUCTIONS_CLOSE_TAG: &str = "</js_repl_instructions>";
+pub(crate) const CHILD_AGENTS_INSTRUCTIONS_OPEN_TAG: &str = "<child_agents_instructions>";
+pub(crate) const CHILD_AGENTS_INSTRUCTIONS_CLOSE_TAG: &str = "</child_agents_instructions>";
+pub(crate) const USER_SHELL_COMMAND_OPEN_TAG: &str = "<user_shell_command>";
+pub(crate) const USER_SHELL_COMMAND_CLOSE_TAG: &str = "</user_shell_command>";
+pub(crate) const TURN_ABORTED_OPEN_TAG: &str = "<turn_aborted>";
+pub(crate) const TURN_ABORTED_CLOSE_TAG: &str = "</turn_aborted>";
+pub(crate) const SUBAGENTS_OPEN_TAG: &str = "<subagents>";
+pub(crate) const SUBAGENTS_CLOSE_TAG: &str = "</subagents>";
+pub(crate) const SUBAGENT_NOTIFICATION_OPEN_TAG: &str = "<subagent_notification>";
+pub(crate) const SUBAGENT_NOTIFICATION_CLOSE_TAG: &str = "</subagent_notification>";
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) enum ModelVisibleMessageRole {
+    Developer,
+    User,
+}
+
+impl ModelVisibleMessageRole {
+    const fn as_str(self) -> &'static str {
+        match self {
+            Self::Developer => "developer",
+            Self::User => "user",
+        }
+    }
+}
+
+pub(crate) trait ModelVisibleContextRole {
+    const MESSAGE_ROLE: ModelVisibleMessageRole;
+}
+
+pub(crate) struct DeveloperContextRole;
+
+impl ModelVisibleContextRole for DeveloperContextRole {
+    const MESSAGE_ROLE: ModelVisibleMessageRole = ModelVisibleMessageRole::Developer;
+}
+
+pub(crate) struct ContextualUserContextRole;
+
+impl ModelVisibleContextRole for ContextualUserContextRole {
+    const MESSAGE_ROLE: ModelVisibleMessageRole = ModelVisibleMessageRole::User;
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) struct ContextualUserFragmentMarkers {
+    start_marker: &'static str,
+    end_marker: &'static str,
+}
+
+impl ContextualUserFragmentMarkers {
+    pub(crate) const fn new(start_marker: &'static str, end_marker: &'static str) -> Self {
+        Self {
+            start_marker,
+            end_marker,
+        }
+    }
+
+    pub(crate) fn matches_text(self, text: &str) -> bool {
+        let trimmed = text.trim_start();
+        let starts_with_marker = trimmed
+            .get(..self.start_marker.len())
+            .is_some_and(|candidate| candidate.eq_ignore_ascii_case(self.start_marker));
+        let trimmed = trimmed.trim_end();
+        let ends_with_marker = trimmed
+            .get(trimmed.len().saturating_sub(self.end_marker.len())..)
+            .is_some_and(|candidate| candidate.eq_ignore_ascii_case(self.end_marker));
+        starts_with_marker && ends_with_marker
+    }
+
+    pub(crate) fn wrap_body(self, body: String) -> String {
+        format!("{}\n{}\n{}", self.start_marker, body, self.end_marker)
+    }
+}
+
+pub(crate) fn model_visible_content_item(text: String) -> ContentItem {
+    ContentItem::InputText { text }
+}
+
+pub(crate) fn model_visible_message<R: ModelVisibleContextRole>(text: String) -> ResponseItem {
+    ResponseItem::Message {
+        id: None,
+        role: R::MESSAGE_ROLE.as_str().to_owned(),
+        content: vec![model_visible_content_item(text)],
+        end_turn: None,
+        phase: None,
+    }
+}
+
+pub(crate) fn model_visible_response_input_item<R: ModelVisibleContextRole>(
+    text: String,
+) -> ResponseInputItem {
+    ResponseInputItem::Message {
+        role: R::MESSAGE_ROLE.as_str().to_owned(),
+        content: vec![model_visible_content_item(text)],
+    }
+}
+
+pub(crate) struct TurnContextDiffParams<'a> {
+    pub(crate) shell: &'a Shell,
+    pub(crate) previous_turn_settings: Option<&'a PreviousTurnSettings>,
+    pub(crate) exec_policy: &'a Policy,
+    pub(crate) personality_feature_enabled: bool,
+    pub(crate) base_instructions: Option<&'a str>,
+    pub(crate) plugin_capability_summaries: Option<&'a [PluginCapabilitySummary]>,
+}
+
+impl<'a> TurnContextDiffParams<'a> {
+    pub(crate) fn new(
+        shell: &'a Shell,
+        previous_turn_settings: Option<&'a PreviousTurnSettings>,
+        exec_policy: &'a Policy,
+        personality_feature_enabled: bool,
+        base_instructions: Option<&'a str>,
+        plugin_capability_summaries: Option<&'a [PluginCapabilitySummary]>,
+    ) -> Self {
+        Self {
+            shell,
+            previous_turn_settings,
+            exec_policy,
+            personality_feature_enabled,
+            base_instructions,
+            plugin_capability_summaries,
+        }
+    }
+}
+
+/// Implement this for any model-visible prompt fragment, regardless of which
+/// envelope it renders into.
+pub(crate) trait ModelVisibleContextFragment: Sized {
+    type Role: ModelVisibleContextRole;
+
+    fn render_text(&self) -> String;
+
+    /// Build the fragment from the current turn state and an optional baseline
+    /// context item that represents the turn state already reflected in
+    /// model-visible history.
+    ///
+    /// Implementations that are not turn-state fragments should leave the
+    /// default `None`.
+    fn build(
+        _turn_context: &TurnContext,
+        _reference_context_item: Option<&TurnContextItem>,
+        _params: &TurnContextDiffParams<'_>,
+    ) -> Option<Self> {
+        None
+    }
+
+    /// Stable markers used to recognize contextual-user fragments in persisted
+    /// history. Developer fragments should keep the default `None`.
+    fn contextual_user_markers() -> Option<ContextualUserFragmentMarkers> {
+        None
+    }
+
+    fn matches_contextual_user_text(text: &str) -> bool {
+        Self::contextual_user_markers().is_some_and(|markers| markers.matches_text(text))
+    }
+
+    fn wrap_contextual_user_body(body: String) -> String {
+        let Some(markers) = Self::contextual_user_markers() else {
+            panic!("contextual-user fragments using wrap_contextual_user_body must define markers");
+        };
+        markers.wrap_body(body)
+    }
+
+    fn into_content_item(self) -> ContentItem {
+        model_visible_content_item(self.render_text())
+    }
+
+    fn into_message(self) -> ResponseItem {
+        model_visible_message::<Self::Role>(self.render_text())
+    }
+
+    fn into_response_input_item(self) -> ResponseInputItem {
+        model_visible_response_input_item::<Self::Role>(self.render_text())
+    }
+}
+
+pub(crate) struct DeveloperTextFragment {
+    text: String,
+}
+
+impl DeveloperTextFragment {
+    pub(crate) fn new(text: impl Into<String>) -> Self {
+        Self { text: text.into() }
+    }
+}
+
+pub(crate) struct ContextualUserTextFragment {
+    text: String,
+}
+
+impl ContextualUserTextFragment {
+    pub(crate) fn new(text: impl Into<String>) -> Self {
+        Self { text: text.into() }
+    }
+}
+
+impl ModelVisibleContextFragment for DeveloperTextFragment {
+    type Role = DeveloperContextRole;
+
+    fn render_text(&self) -> String {
+        self.text.clone()
+    }
+}
+
+impl ModelVisibleContextFragment for ContextualUserTextFragment {
+    type Role = ContextualUserContextRole;
+
+    fn render_text(&self) -> String {
+        self.text.clone()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn contextual_user_markers_match_case_insensitive_wrapped_text() {
+        let markers = ContextualUserFragmentMarkers::new("<example>", "</example>");
+        let text = "  <EXAMPLE>\nbody\n</EXAMPLE>  ";
+
+        assert_eq!(markers.matches_text(text), true);
+    }
+
+    #[test]
+    fn developer_role_message_uses_developer_wire_role() {
+        let message = model_visible_message::<DeveloperContextRole>("hi".to_owned());
+
+        assert_eq!(
+            message,
+            ResponseItem::Message {
+                id: None,
+                role: "developer".to_owned(),
+                content: vec![ContentItem::InputText {
+                    text: "hi".to_owned()
+                }],
+                end_turn: None,
+                phase: None,
+            }
+        );
+    }
+}
diff --git a/docs/model-visible-context.md b/docs/model-visible-context.md
new file mode 100644
index 0000000000..07be2f14ea
--- /dev/null
+++ b/docs/model-visible-context.md
@@ -0,0 +1,115 @@
+# Model-visible context
+
+This document defines the shared foundation for model-visible prompt context in
+`codex-rs`.
+
+The goal of this layer is simple: if the model should see some structured
+context, represent it as a typed fragment instead of hand-assembling ad hoc
+message payloads. That keeps rendering, role choice, and contextual-user
+detection consistent before higher-level assembly code starts composing larger
+prompt envelopes.
+
+The shared foundation lives in
+[`codex-rs/core/src/model_visible_context.rs`](../codex-rs/core/src/model_visible_context.rs).
+
+Follow-up work will build on this foundation by introducing concrete fragment
+types and wiring them into turn-state assembly, history parsing, and prompt
+diffing. Those integrations are intentionally out of scope for this document.
+
+## Core concepts
+
+### Fragment role
+
+Every named fragment type implements `ModelVisibleContextFragment` and declares
+its role with `type Role`.
+
+Use:
+
+- `DeveloperContextRole` for developer guidance and policy text
+- `ContextualUserContextRole` for contextual user-role state that should be
+  parsed as context rather than literal user intent
+
+Choosing the right role is part of the fragment contract, not a later callsite
+decision.
+
+### Fragment rendering
+
+`ModelVisibleContextFragment` owns the text rendering for the fragment via
+`render_text()`. Shared helpers convert that rendered text into the standard
+model-visible payload shapes:
+
+- `into_content_item()`
+- `into_message()`
+- `into_response_input_item()`
+
+New model-visible context should use these conversions instead of
+hand-constructing `ResponseItem::Message` payloads.
+
+### Turn-state rebuild hook
+
+Fragments that represent durable turn or session state can implement:
+
+`ModelVisibleContextFragment::build(...)`
+
+That hook receives:
+
+- the current `TurnContext`
+- an optional `reference_context_item`
+- `TurnContextDiffParams`
+
+The intended use is:
+
+- compare current state against the persisted baseline in
+  `reference_context_item`
+- emit `Some(fragment)` only when the current prompt state needs to be injected
+- leave the default `None` for fragments that are not rebuilt from turn state
+
+This hook exists so later integrations can rebuild context correctly across
+resume, compaction, backtracking, and fork.
+
+### Contextual-user detection
+
+Contextual-user fragments share the `user` role with real user messages, so
+they need stable detection.
+
+Preferred path:
+
+- implement `contextual_user_markers()` when the fragment has fixed wrappers
+
+Fallback path:
+
+- override `matches_contextual_user_text()` when matching is genuinely custom
+
+The foundation intentionally keeps that detection API close to fragment
+definitions so later history-parsing code can rely on it consistently.
+
+## Included shared helpers
+
+The foundation module currently provides:
+
+- `ModelVisibleContextRole`
+- `DeveloperContextRole`
+- `ContextualUserContextRole`
+- `ContextualUserFragmentMarkers`
+- `TurnContextDiffParams`
+- `DeveloperTextFragment`
+- `ContextualUserTextFragment`
+- shared wrapper/tag constants for current contextual marker shapes
+
+These helpers are intentionally generic. They should stay reusable across
+future fragment definitions rather than reflecting any one prompt assembly path.
+
+## Contributor guidance
+
+When adding model-visible context:
+
+1. Decide whether the model should see it at all.
+2. If it should, prefer a typed fragment over a plain string.
+3. Choose the role intentionally.
+4. If the fragment is durable turn/session state, implement `build(...)`.
+5. If the fragment is contextual-user, provide stable detection.
+6. Use the shared conversion helpers instead of custom message assembly.
+
+Developer-only one-off text can still be acceptable when it is truly isolated
+and does not need contextual-user detection or turn-state reconstruction, but
+that should be the exception rather than the default.