core: snapshot fork startup context injection (#15443)

## Summary
- add a snapshot-style core test for fork startup context injection
followed by first-turn diff injection
- capture the current duplicated startup-plus-turn context behavior
without changing runtime logic

## Testing
- not run locally; relying on CI
- just fmt

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/codex_tests.rs

@@ -64,17 +64,31 @@ use codex_execpolicy::NetworkRuleProtocol;
 use codex_execpolicy::Policy;
 use codex_network_proxy::NetworkProxyConfig;
 use codex_otel::TelemetryAuthMode;
+use codex_protocol::config_types::CollaborationMode;
+use codex_protocol::config_types::ModeKind;
+use codex_protocol::config_types::Settings;
 use codex_protocol::models::BaseInstructions;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::DeveloperInstructions;
 use codex_protocol::models::ResponseInputItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ModelsResponse;
+use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::ConversationAudioParams;
 use codex_protocol::protocol::RealtimeAudioFrame;
 use codex_protocol::protocol::Submission;
 use codex_protocol::protocol::W3cTraceContext;
+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::ev_completed;
+use core_test_support::responses::ev_response_created;
+use core_test_support::responses::mount_sse_once;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::test_codex::test_codex;
 use core_test_support::tracing::install_test_tracing;
+use core_test_support::wait_for_event;
 use opentelemetry::trace::TraceContextExt;
 use opentelemetry::trace::TraceId;
 use std::path::Path;
@@ -1115,6 +1129,111 @@ async fn record_initial_history_reconstructs_forked_transcript() {
     assert_eq!(expected, history.raw_items());
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn fork_startup_context_then_first_turn_diff_snapshot() -> anyhow::Result<()> {
+    let server = start_mock_server().await;
+    mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp-1"), ev_completed("resp-1")]),
+    )
+    .await;
+    let first_forked_request = mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp-2"), ev_completed("resp-2")]),
+    )
+    .await;
+
+    let mut builder = test_codex().with_config(|config| {
+        config.permissions.approval_policy =
+            codex_config::Constrained::allow_any(AskForApproval::OnRequest);
+    });
+    let initial = builder.build(&server).await?;
+    let rollout_path = initial
+        .session_configured
+        .rollout_path
+        .clone()
+        .expect("rollout path");
+
+    initial
+        .codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "fork seed".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&initial.codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
+
+    let mut fork_config = initial.config.clone();
+    fork_config.permissions.approval_policy =
+        codex_config::Constrained::allow_any(AskForApproval::UnlessTrusted);
+    let forked = initial
+        .thread_manager
+        .fork_thread(usize::MAX, fork_config, rollout_path, false, None)
+        .await?;
+
+    let collaboration_mode = CollaborationMode {
+        mode: ModeKind::Plan,
+        settings: Settings {
+            model: forked.session_configured.model.clone(),
+            reasoning_effort: None,
+            developer_instructions: Some("Fork turn collaboration instructions.".to_string()),
+        },
+    };
+    forked
+        .thread
+        .submit(Op::OverrideTurnContext {
+            cwd: None,
+            approval_policy: Some(AskForApproval::Never),
+            approvals_reviewer: None,
+            sandbox_policy: None,
+            windows_sandbox_level: None,
+            model: None,
+            effort: None,
+            summary: None,
+            service_tier: None,
+            collaboration_mode: Some(collaboration_mode),
+            personality: None,
+        })
+        .await?;
+
+    forked
+        .thread
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "after fork".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await?;
+    wait_for_event(&forked.thread, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
+
+    let request = first_forked_request.single_request();
+    let snapshot = context_snapshot::format_labeled_requests_snapshot(
+        "First request after fork when fork startup changes approval policy and the first forked turn changes approval policy again and enters plan mode.",
+        &[("First Forked Turn Request", &request)],
+        &ContextSnapshotOptions::default()
+            .render_mode(ContextSnapshotRenderMode::KindWithTextPrefix { max_chars: 96 })
+            .strip_capability_instructions()
+            .strip_agents_md_user_context(),
+    );
+
+    let mut settings = insta::Settings::clone_current();
+    settings.set_snapshot_path("snapshots");
+    settings.set_prepend_module_to_snapshot(false);
+    settings.bind(|| {
+        insta::assert_snapshot!(
+            "codex_core__codex_tests__fork_startup_context_then_first_turn_diff",
+            snapshot
+        );
+    });
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn record_initial_history_forked_hydrates_previous_turn_settings() {
     let (session, turn_context) = make_session_and_context().await;

codex-rs/core/src/snapshots/codex_core__codex_tests__fork_startup_context_then_first_turn_diff.snap

@@ -0,0 +1,17 @@
+---
+source: core/src/codex_tests.rs
+assertion_line: 1282
+expression: snapshot
+---
+Scenario: First request after fork when fork startup changes approval policy and the first forked turn changes approval policy again and enters plan mode.
+
+## First Forked Turn Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/user:fork seed
+03:message/developer:<PERMISSIONS_INSTRUCTIONS>
+04:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+05:message/developer[2]:
+    [01] <PERMISSIONS_INSTRUCTIONS>
+    [02] <collaboration_mode>Fork turn collaboration instructions.</collaboration_mode>
+06:message/user:after fork