Avoid hard-coded environment context shell (#21390)

## Summary
- make resolved turn environment shell metadata optional instead of
hard-coding bash
- render environment context shells from explicit environment metadata
when present, falling back to the existing session shell
- update environment context tests for inherited PowerShell-style
fallback and explicit per-environment shell override

## Testing
- Not run (not requested; formatted with `just fmt`).

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

codex-rs/core/src/context/environment_context.rs

@@ -1,4 +1,6 @@
 use crate::session::turn_context::TurnContext;
+use crate::session::turn_context::TurnEnvironment;
+use crate::shell::Shell;
 use codex_protocol::protocol::TurnContextItem;
 use codex_protocol::protocol::TurnContextNetworkItem;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -30,15 +32,16 @@ impl EnvironmentContextEnvironment {
         }
     }
 
-    fn from_turn_environments(
-        environments: &[crate::session::turn_context::TurnEnvironment],
-    ) -> Vec<Self> {
+    fn from_turn_environments(environments: &[TurnEnvironment], shell: &Shell) -> Vec<Self> {
         environments
             .iter()
             .map(|environment| Self {
                 id: environment.environment_id.clone(),
                 cwd: environment.cwd.clone(),
-                shell: environment.shell.clone(),
+                shell: environment
+                    .shell
+                    .clone()
+                    .unwrap_or_else(|| shell.name().to_string()),
             })
             .collect()
     }
@@ -174,10 +177,11 @@ impl EnvironmentContext {
         )
     }
 
-    pub(crate) fn from_turn_context(turn_context: &TurnContext) -> Self {
+    pub(crate) fn from_turn_context(turn_context: &TurnContext, shell: &Shell) -> Self {
         Self::new(
             EnvironmentContextEnvironment::from_turn_environments(
                 &turn_context.environments.turn_environments,
+                shell,
             ),
             turn_context.current_date.clone(),
             turn_context.timezone.clone(),

codex-rs/core/src/context/environment_context_tests.rs

@@ -259,3 +259,46 @@ fn serialize_environment_context_with_multiple_selected_environments() {
 
     assert_eq!(context.render(), expected);
 }
+
+#[test]
+fn serialize_environment_context_prefers_environment_shell_when_present() {
+    let local_cwd = test_path_buf("/repo/local");
+    let remote_cwd = test_path_buf("/repo/remote");
+    let context = EnvironmentContext::new(
+        vec![
+            EnvironmentContextEnvironment {
+                id: "local".to_string(),
+                cwd: local_cwd.abs(),
+                shell: "powershell".to_string(),
+            },
+            EnvironmentContextEnvironment {
+                id: "remote".to_string(),
+                cwd: remote_cwd.abs(),
+                shell: "cmd".to_string(),
+            },
+        ],
+        /*current_date*/ None,
+        /*timezone*/ None,
+        /*network*/ None,
+        /*subagents*/ None,
+    );
+
+    let expected = format!(
+        r#"<environment_context>
+  <environments>
+    <environment id="local">
+      <cwd>{}</cwd>
+      <shell>powershell</shell>
+    </environment>
+    <environment id="remote">
+      <cwd>{}</cwd>
+      <shell>cmd</shell>
+    </environment>
+  </environments>
+</environment_context>"#,
+        local_cwd.display(),
+        remote_cwd.display()
+    );
+
+    assert_eq!(context.render(), expected);
+}

codex-rs/core/src/context_manager/updates.rs

@@ -29,7 +29,7 @@ fn build_environment_update_item(
 
     let prev = previous?;
     let prev_context = EnvironmentContext::from_turn_context_item(prev, shell.name().to_string());
-    let next_context = EnvironmentContext::from_turn_context(next);
+    let next_context = EnvironmentContext::from_turn_context(next, shell);
     if prev_context.equals_except_shell(&next_context) {
         return None;
     }

codex-rs/core/src/environment_selection.rs

@@ -75,9 +75,7 @@ pub(crate) fn resolve_environment_selections(
             environment_id,
             environment,
             cwd: selected_environment.cwd.clone(),
-            // TODO(starr): Resolve shell metadata per environment instead of
-            // hardcoding bash.
-            shell: "bash".to_string(),
+            shell: None,
         });
     }
 
@@ -176,5 +174,6 @@ mod tests {
                 .environment_id,
             "local"
         );
+        assert_eq!(resolved.primary().expect("primary environment").shell, None);
     }
 }

codex-rs/core/src/session/mod.rs

@@ -2703,13 +2703,14 @@ impl Session {
             );
         }
         if turn_context.config.include_environment_context {
+            let shell = self.user_shell();
             let subagents = self
                 .services
                 .agent_control
                 .format_environment_context_subagents(self.conversation_id)
                 .await;
             contextual_user_sections.push(
-                crate::context::EnvironmentContext::from_turn_context(turn_context)
+                crate::context::EnvironmentContext::from_turn_context(turn_context, shell.as_ref())
                     .with_subagents(subagents)
                     .render(),
             );

codex-rs/core/src/session/tests.rs

@@ -3057,7 +3057,7 @@ fn turn_environments_for_tests(
         environment_id: codex_exec_server::LOCAL_ENVIRONMENT_ID.to_string(),
         environment: Arc::clone(environment),
         cwd: cwd.clone(),
-        shell: "bash".to_string(),
+        shell: None,
     }]
 }
 
@@ -4803,7 +4803,7 @@ async fn primary_environment_uses_first_turn_environment() {
             environment_id: "second".to_string(),
             environment: Arc::clone(&first_environment.environment),
             cwd: second_cwd.clone(),
-            shell: "bash".to_string(),
+            shell: None,
         });
 
     assert_eq!(
@@ -5749,6 +5749,47 @@ async fn build_settings_update_items_emits_environment_item_for_network_changes(
     assert!(environment_update.contains("<denied>blocked.example.com</denied>"));
 }
 
+#[tokio::test]
+async fn environment_context_uses_session_shell_when_environment_shell_is_absent() {
+    let (mut session, mut turn_context) = make_session_and_context().await;
+    session.services.user_shell = Arc::new(crate::shell::Shell {
+        shell_type: crate::shell::ShellType::PowerShell,
+        shell_path: PathBuf::from("powershell"),
+        shell_snapshot: crate::shell::empty_shell_snapshot_receiver(),
+    });
+    for environment in &mut turn_context.environments.turn_environments {
+        environment.shell = None;
+    }
+
+    let session_shell = session.user_shell();
+    let environment_context = crate::context::EnvironmentContext::from_turn_context(
+        &turn_context,
+        session_shell.as_ref(),
+    )
+    .render();
+    assert!(
+        environment_context.contains("<shell>powershell</shell>"),
+        "{environment_context}"
+    );
+
+    let primary_environment = turn_context
+        .environments
+        .turn_environments
+        .first_mut()
+        .expect("primary environment");
+    primary_environment.shell = Some("cmd".to_string());
+
+    let environment_context = crate::context::EnvironmentContext::from_turn_context(
+        &turn_context,
+        session_shell.as_ref(),
+    )
+    .render();
+    assert!(
+        environment_context.contains("<shell>cmd</shell>"),
+        "{environment_context}"
+    );
+}
+
 #[tokio::test]
 async fn build_settings_update_items_emits_environment_item_for_time_changes() {
     let (session, previous_context) = make_session_and_context().await;

codex-rs/core/src/session/turn_context.rs

@@ -39,7 +39,7 @@ pub(crate) struct TurnEnvironment {
     pub(crate) environment_id: String,
     pub(crate) environment: Arc<Environment>,
     pub(crate) cwd: AbsolutePathBuf,
-    pub(crate) shell: String,
+    pub(crate) shell: Option<String>,
 }
 
 impl TurnEnvironment {

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

@@ -1,6 +1,7 @@
 #![allow(clippy::unwrap_used)]
 
 use codex_apply_patch::APPLY_PATCH_TOOL_INSTRUCTIONS;
+use codex_core::shell::default_user_shell;
 use codex_features::Feature;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::ModeKind;
@@ -54,7 +55,7 @@ fn assert_default_env_context(text: &str, cwd: &str) {
         "expected cwd in environment context: {text}"
     );
     assert!(
-        text.contains("<shell>bash</shell>"),
+        text.contains(&format!("<shell>{}</shell>", default_user_shell().name())),
         "expected shell in environment context: {text}"
     );
     assert!(