Add model-visible context layout snapshot tests (#12073)

## Summary
- add a dedicated `core/tests/suite/model_visible_layout.rs` snapshot
suite to materialize model-visible request layout in high-value
scenarios
- add three reviewer-focused snapshot scenarios:
  - turn-level context updates (cwd / permissions / personality)
  - first post-resume turn with model hydration + personality change
- first post-resume turn where pre-turn model override matches rollout
model
- wire the new suite into `core/tests/suite/mod.rs`
- commit generated `insta` snapshots under `core/tests/suite/snapshots/`

## Why
This creates a stable, reviewable baseline of model-visible context
layout against `main` before follow-on context-management refactors. It
lets subsequent PRs show focused snapshot diffs for behavior changes
instead of introducing the test surface and behavior changes at once.

## Testing
- `just fmt`
- `INSTA_UPDATE=always cargo test -p codex-core model_visible_layout`

codex-rs/core/tests/suite/mod.rs

@@ -85,6 +85,7 @@ mod model_info_overrides;
 mod model_overrides;
 mod model_switching;
 mod model_tools;
+mod model_visible_layout;
 mod models_cache_ttl;
 mod models_etag_responses;
 mod otel;

codex-rs/core/tests/suite/model_visible_layout.rs

@@ -0,0 +1,443 @@
+#![allow(clippy::expect_used)]
+
+use std::fs;
+use std::sync::Arc;
+
+use anyhow::Result;
+use codex_core::config::types::Personality;
+use codex_core::features::Feature;
+use codex_core::protocol::AskForApproval;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::Op;
+use codex_core::protocol::SandboxPolicy;
+use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::user_input::UserInput;
+use core_test_support::context_snapshot;
+use core_test_support::context_snapshot::ContextSnapshotOptions;
+use core_test_support::context_snapshot::ContextSnapshotRenderMode;
+use core_test_support::responses::ResponsesRequest;
+use core_test_support::responses::ev_assistant_message;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_response_created;
+use core_test_support::responses::mount_sse_once;
+use core_test_support::responses::mount_sse_sequence;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::skip_if_no_network;
+use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
+
+const PRETURN_CONTEXT_DIFF_CWD: &str = "PRETURN_CONTEXT_DIFF_CWD";
+
+fn context_snapshot_options() -> ContextSnapshotOptions {
+    ContextSnapshotOptions::default()
+        .render_mode(ContextSnapshotRenderMode::KindWithTextPrefix { max_chars: 96 })
+}
+
+fn format_labeled_requests_snapshot(
+    scenario: &str,
+    sections: &[(&str, &ResponsesRequest)],
+) -> String {
+    context_snapshot::format_labeled_requests_snapshot(
+        scenario,
+        sections,
+        &context_snapshot_options(),
+    )
+}
+
+fn agents_message_count(request: &ResponsesRequest) -> usize {
+    request
+        .message_input_texts("user")
+        .iter()
+        .filter(|text| text.starts_with("# AGENTS.md instructions for "))
+        .count()
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn snapshot_model_visible_layout_turn_overrides() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let responses = mount_sse_sequence(
+        &server,
+        vec![
+            sse(vec![
+                ev_response_created("resp-1"),
+                ev_assistant_message("msg-1", "turn one complete"),
+                ev_completed("resp-1"),
+            ]),
+            sse(vec![
+                ev_response_created("resp-2"),
+                ev_assistant_message("msg-2", "turn two complete"),
+                ev_completed("resp-2"),
+            ]),
+        ],
+    )
+    .await;
+
+    let mut builder = test_codex()
+        .with_model("gpt-5.2-codex")
+        .with_config(|config| {
+            config.features.enable(Feature::Personality);
+            config.personality = Some(Personality::Pragmatic);
+        });
+    let test = builder.build(&server).await?;
+    let preturn_context_diff_cwd = test.cwd_path().join(PRETURN_CONTEXT_DIFF_CWD);
+    fs::create_dir_all(&preturn_context_diff_cwd)?;
+
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "first turn".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: test.cwd_path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::new_read_only_policy(),
+            model: test.session_configured.model.clone(),
+            effort: test.config.model_reasoning_effort,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "second turn with context updates".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: preturn_context_diff_cwd,
+            approval_policy: AskForApproval::OnRequest,
+            sandbox_policy: SandboxPolicy::new_read_only_policy(),
+            model: test.session_configured.model.clone(),
+            effort: test.config.model_reasoning_effort,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: Some(Personality::Friendly),
+        })
+        .await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let requests = responses.requests();
+    assert_eq!(requests.len(), 2, "expected two requests");
+    insta::assert_snapshot!(
+        "model_visible_layout_turn_overrides",
+        format_labeled_requests_snapshot(
+            "Second turn changes cwd, approval policy, and personality while keeping model constant.",
+            &[
+                ("First Request (Baseline)", &requests[0]),
+                ("Second Request (Turn Overrides)", &requests[1]),
+            ]
+        )
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+// TODO(ccunningham): Diff `user_instructions` and emit updates when AGENTS.md content changes
+// (for example after cwd changes), then update this test to assert refreshed AGENTS content.
+async fn snapshot_model_visible_layout_cwd_change_does_not_refresh_agents() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let responses = mount_sse_sequence(
+        &server,
+        vec![
+            sse(vec![
+                ev_response_created("resp-1"),
+                ev_assistant_message("msg-1", "turn one complete"),
+                ev_completed("resp-1"),
+            ]),
+            sse(vec![
+                ev_response_created("resp-2"),
+                ev_assistant_message("msg-2", "turn two complete"),
+                ev_completed("resp-2"),
+            ]),
+        ],
+    )
+    .await;
+
+    let mut builder = test_codex().with_model("gpt-5.2-codex");
+    let test = builder.build(&server).await?;
+    let cwd_one = test.cwd_path().join("agents_one");
+    let cwd_two = test.cwd_path().join("agents_two");
+    fs::create_dir_all(&cwd_one)?;
+    fs::create_dir_all(&cwd_two)?;
+    fs::write(
+        cwd_one.join("AGENTS.md"),
+        "# AGENTS one\n\n<INSTRUCTIONS>\nTurn one agents instructions.\n</INSTRUCTIONS>\n",
+    )?;
+    fs::write(
+        cwd_two.join("AGENTS.md"),
+        "# AGENTS two\n\n<INSTRUCTIONS>\nTurn two agents instructions.\n</INSTRUCTIONS>\n",
+    )?;
+
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "first turn in agents_one".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: cwd_one.clone(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::new_read_only_policy(),
+            model: test.session_configured.model.clone(),
+            effort: test.config.model_reasoning_effort,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "second turn in agents_two".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: cwd_two,
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::new_read_only_policy(),
+            model: test.session_configured.model.clone(),
+            effort: test.config.model_reasoning_effort,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let requests = responses.requests();
+    assert_eq!(requests.len(), 2, "expected two requests");
+    assert_eq!(
+        agents_message_count(&requests[0]),
+        1,
+        "expected exactly one AGENTS message in first request"
+    );
+    assert_eq!(
+        agents_message_count(&requests[1]),
+        1,
+        "expected AGENTS to refresh after cwd change, but current behavior only keeps history AGENTS"
+    );
+    insta::assert_snapshot!(
+        "model_visible_layout_cwd_change_does_not_refresh_agents",
+        format_labeled_requests_snapshot(
+            "Second turn changes cwd to a directory with different AGENTS.md; current behavior does not emit refreshed AGENTS instructions.",
+            &[
+                ("First Request (agents_one)", &requests[0]),
+                ("Second Request (agents_two cwd)", &requests[1]),
+            ]
+        )
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn snapshot_model_visible_layout_resume_with_personality_change() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let mut initial_builder = test_codex().with_config(|config| {
+        config.model = Some("gpt-5.2".to_string());
+    });
+    let initial = initial_builder.build(&server).await?;
+    let codex = Arc::clone(&initial.codex);
+    let home = initial.home.clone();
+    let rollout_path = initial
+        .session_configured
+        .rollout_path
+        .clone()
+        .expect("rollout path");
+
+    let initial_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-initial"),
+            ev_assistant_message("msg-1", "recorded before resume"),
+            ev_completed("resp-initial"),
+        ]),
+    )
+    .await;
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "seed resume history".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&codex, |event| matches!(event, EventMsg::TurnComplete(_))).await;
+    let initial_request = initial_mock.single_request();
+
+    let resumed_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-resume"),
+            ev_assistant_message("msg-2", "first resumed turn"),
+            ev_completed("resp-resume"),
+        ]),
+    )
+    .await;
+
+    let mut resume_builder = test_codex().with_config(|config| {
+        config.model = Some("gpt-5.2-codex".to_string());
+        config.features.enable(Feature::Personality);
+        config.personality = Some(Personality::Pragmatic);
+    });
+    let resumed = resume_builder.resume(&server, home, rollout_path).await?;
+    resumed
+        .codex
+        .submit(Op::UserTurn {
+            items: vec![UserInput::Text {
+                text: "resume and change personality".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            cwd: resumed.cwd_path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::new_read_only_policy(),
+            model: resumed.session_configured.model.clone(),
+            effort: resumed.config.model_reasoning_effort,
+            summary: ReasoningSummary::Auto,
+            collaboration_mode: None,
+            personality: Some(Personality::Friendly),
+        })
+        .await?;
+    wait_for_event(&resumed.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let resumed_request = resumed_mock.single_request();
+    insta::assert_snapshot!(
+        "model_visible_layout_resume_with_personality_change",
+        format_labeled_requests_snapshot(
+            "First post-resume turn where resumed config model differs from rollout and personality changes.",
+            &[
+                ("Last Request Before Resume", &initial_request),
+                ("First Request After Resume", &resumed_request),
+            ]
+        )
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn snapshot_model_visible_layout_resume_override_matches_rollout_model() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let mut initial_builder = test_codex().with_config(|config| {
+        config.model = Some("gpt-5.2".to_string());
+    });
+    let initial = initial_builder.build(&server).await?;
+    let codex = Arc::clone(&initial.codex);
+    let home = initial.home.clone();
+    let rollout_path = initial
+        .session_configured
+        .rollout_path
+        .clone()
+        .expect("rollout path");
+
+    let initial_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-initial"),
+            ev_assistant_message("msg-1", "recorded before resume"),
+            ev_completed("resp-initial"),
+        ]),
+    )
+    .await;
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "seed resume history".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&codex, |event| matches!(event, EventMsg::TurnComplete(_))).await;
+    let initial_request = initial_mock.single_request();
+
+    let resumed_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-resume"),
+            ev_assistant_message("msg-2", "first resumed turn"),
+            ev_completed("resp-resume"),
+        ]),
+    )
+    .await;
+
+    let mut resume_builder = test_codex().with_config(|config| {
+        config.model = Some("gpt-5.2-codex".to_string());
+    });
+    let resumed = resume_builder.resume(&server, home, rollout_path).await?;
+    resumed
+        .codex
+        .submit(Op::OverrideTurnContext {
+            cwd: None,
+            approval_policy: None,
+            sandbox_policy: None,
+            windows_sandbox_level: None,
+            model: Some("gpt-5.2".to_string()),
+            effort: None,
+            summary: None,
+            collaboration_mode: None,
+            personality: None,
+        })
+        .await?;
+    resumed
+        .codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "first resumed turn after model override".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&resumed.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let resumed_request = resumed_mock.single_request();
+    insta::assert_snapshot!(
+        "model_visible_layout_resume_override_matches_rollout_model",
+        format_labeled_requests_snapshot(
+            "First post-resume turn where pre-turn override sets model to rollout model; no model-switch update should appear.",
+            &[
+                ("Last Request Before Resume", &initial_request),
+                ("First Request After Resume + Override", &resumed_request),
+            ]
+        )
+    );
+
+    Ok(())
+}

codex-rs/core/tests/suite/snapshots/all__suite__model_visible_layout__model_visible_layout_cwd_change_does_not_refresh_agents.snap

@@ -0,0 +1,24 @@
+---
+source: core/tests/suite/model_visible_layout.rs
+expression: "format_labeled_requests_snapshot(\"Second turn changes cwd to a directory with different AGENTS.md; current behavior does not emit refreshed AGENTS instructions.\",\n&[(\"First Request (agents_one)\", &requests[0]),\n(\"Second Request (agents_two cwd)\", &requests[1]),])"
+---
+Scenario: Second turn changes cwd to a directory with different AGENTS.md; current behavior does not emit refreshed AGENTS instructions.
+
+## First Request (agents_one)
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+04:message/developer:<PERMISSIONS_INSTRUCTIONS>
+05:message/user:first turn in agents_one
+
+## Second Request (agents_two cwd)
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+04:message/developer:<PERMISSIONS_INSTRUCTIONS>
+05:message/user:first turn in agents_one
+06:message/assistant:turn one complete
+07:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+08:message/user:second turn in agents_two

codex-rs/core/tests/suite/snapshots/all__suite__model_visible_layout__model_visible_layout_resume_override_matches_rollout_model.snap

@@ -0,0 +1,22 @@
+---
+source: core/tests/suite/model_visible_layout.rs
+expression: "format_labeled_requests_snapshot(\"First post-resume turn where pre-turn override sets model to rollout model; no model-switch update should appear.\",\n&[(\"Last Request Before Resume\", &initial_request),\n(\"First Request After Resume + Override\", &resumed_request),])"
+---
+Scenario: First post-resume turn where pre-turn override sets model to rollout model; no model-switch update should appear.
+
+## Last Request Before Resume
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:seed resume history
+
+## First Request After Resume + Override
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:seed resume history
+04:message/assistant:recorded before resume
+05:message/developer:<PERMISSIONS_INSTRUCTIONS>
+06:message/user:<AGENTS_MD>
+07:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+08:message/user:first resumed turn after model override

codex-rs/core/tests/suite/snapshots/all__suite__model_visible_layout__model_visible_layout_resume_with_personality_change.snap

@@ -0,0 +1,26 @@
+---
+source: core/tests/suite/model_visible_layout.rs
+expression: "format_labeled_requests_snapshot(\"First post-resume turn where resumed config model differs from rollout and personality changes.\",\n&[(\"Last Request Before Resume\", &initial_request),\n(\"First Request After Resume\", &resumed_request),])"
+---
+Scenario: First post-resume turn where resumed config model differs from rollout and personality changes.
+
+## Last Request Before Resume
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:seed resume history
+
+## First Request After Resume
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:seed resume history
+04:message/assistant:recorded before resume
+05:message/developer:<PERMISSIONS_INSTRUCTIONS>
+06:message/developer:<personality_spec> The user has requested a new communication style. Future messages should adhe...
+07:message/user:<AGENTS_MD>
+08:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+09:message/developer:<PERMISSIONS_INSTRUCTIONS>
+10:message/developer:<model_switch>\nThe user was previously using a different model. Please continue the conversatio...
+11:message/developer:<personality_spec> The user has requested a new communication style. Future messages should adhe...
+12:message/user:resume and change personality

codex-rs/core/tests/suite/snapshots/all__suite__model_visible_layout__model_visible_layout_turn_overrides.snap

@@ -0,0 +1,24 @@
+---
+source: core/tests/suite/model_visible_layout.rs
+expression: "format_labeled_requests_snapshot(\"Second turn changes cwd, approval policy, and personality while keeping model constant.\",\n&[(\"First Request (Baseline)\", &requests[0]),\n(\"Second Request (Turn Overrides)\", &requests[1]),])"
+---
+Scenario: Second turn changes cwd, approval policy, and personality while keeping model constant.
+
+## First Request (Baseline)
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/developer:<PERMISSIONS_INSTRUCTIONS>
+04:message/user:first turn
+
+## Second Request (Turn Overrides)
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/developer:<PERMISSIONS_INSTRUCTIONS>
+04:message/user:first turn
+05:message/assistant:turn one complete
+06:message/user:<ENVIRONMENT_CONTEXT:cwd=PRETURN_CONTEXT_DIFF_CWD>
+07:message/developer:<PERMISSIONS_INSTRUCTIONS>
+08:message/developer:<personality_spec> The user has requested a new communication style. Future messages should adhe...
+09:message/user:second turn with context updates