Fall back to configured instruction files if AGENTS.md isn't available (#4544)

Allow users to configure an agents.md alternative to consume, but warn
the user it may degrade model performance.

Fixes #4376

codex-rs/core/src/config.rs

@@ -141,6 +141,9 @@ pub struct Config {
     /// Maximum number of bytes to include from an AGENTS.md project doc file.
     pub project_doc_max_bytes: usize,
 
+    /// Additional filenames to try when looking for project-level docs.
+    pub project_doc_fallback_filenames: Vec<String>,
+
     /// Directory containing all Codex state (defaults to `~/.codex` but can be
     /// overridden by the `CODEX_HOME` environment variable).
     pub codex_home: PathBuf,
@@ -670,6 +673,9 @@ pub struct ConfigToml {
     /// Maximum number of bytes to include from an AGENTS.md project doc file.
     pub project_doc_max_bytes: Option<usize>,
 
+    /// Ordered list of fallback filenames to look for when AGENTS.md is missing.
+    pub project_doc_fallback_filenames: Option<Vec<String>>,
+
     /// Profile to use from the `profiles` map.
     pub profile: Option<String>,
 
@@ -1038,6 +1044,19 @@ impl Config {
             mcp_servers: cfg.mcp_servers,
             model_providers,
             project_doc_max_bytes: cfg.project_doc_max_bytes.unwrap_or(PROJECT_DOC_MAX_BYTES),
+            project_doc_fallback_filenames: cfg
+                .project_doc_fallback_filenames
+                .unwrap_or_default()
+                .into_iter()
+                .filter_map(|name| {
+                    let trimmed = name.trim();
+                    if trimmed.is_empty() {
+                        None
+                    } else {
+                        Some(trimmed.to_string())
+                    }
+                })
+                .collect(),
             codex_home,
             history,
             file_opener: cfg.file_opener.unwrap_or(UriBasedFileOpener::VsCode),
@@ -1811,6 +1830,7 @@ model_verbosity = "high"
                 mcp_servers: HashMap::new(),
                 model_providers: fixture.model_provider_map.clone(),
                 project_doc_max_bytes: PROJECT_DOC_MAX_BYTES,
+                project_doc_fallback_filenames: Vec::new(),
                 codex_home: fixture.codex_home(),
                 history: History::default(),
                 file_opener: UriBasedFileOpener::VsCode,
@@ -1871,6 +1891,7 @@ model_verbosity = "high"
             mcp_servers: HashMap::new(),
             model_providers: fixture.model_provider_map.clone(),
             project_doc_max_bytes: PROJECT_DOC_MAX_BYTES,
+            project_doc_fallback_filenames: Vec::new(),
             codex_home: fixture.codex_home(),
             history: History::default(),
             file_opener: UriBasedFileOpener::VsCode,
@@ -1946,6 +1967,7 @@ model_verbosity = "high"
             mcp_servers: HashMap::new(),
             model_providers: fixture.model_provider_map.clone(),
             project_doc_max_bytes: PROJECT_DOC_MAX_BYTES,
+            project_doc_fallback_filenames: Vec::new(),
             codex_home: fixture.codex_home(),
             history: History::default(),
             file_opener: UriBasedFileOpener::VsCode,
@@ -2007,6 +2029,7 @@ model_verbosity = "high"
             mcp_servers: HashMap::new(),
             model_providers: fixture.model_provider_map.clone(),
             project_doc_max_bytes: PROJECT_DOC_MAX_BYTES,
+            project_doc_fallback_filenames: Vec::new(),
             codex_home: fixture.codex_home(),
             history: History::default(),
             file_opener: UriBasedFileOpener::VsCode,

codex-rs/core/src/project_doc.rs

@@ -1,6 +1,7 @@
 //! Project-level documentation discovery.
 //!
-//! Project-level documentation can be stored in files named `AGENTS.md`.
+//! Project-level documentation is primarily stored in files named `AGENTS.md`.
+//! Additional fallback filenames can be configured via `project_doc_fallback_filenames`.
 //! We include the concatenation of all files found along the path from the
 //! repository root to the current working directory as follows:
 //!
@@ -17,8 +18,8 @@ use std::path::PathBuf;
 use tokio::io::AsyncReadExt;
 use tracing::error;
 
-/// Currently, we only match the filename `AGENTS.md` exactly.
+/// Default filename scanned for project-level docs.
-const CANDIDATE_FILENAMES: &[&str] = &["AGENTS.md"];
+pub const DEFAULT_PROJECT_DOC_FILENAME: &str = "AGENTS.md";
 
 /// When both `Config::instructions` and the project doc are present, they will
 /// be concatenated with the following separator.
@@ -152,8 +153,9 @@ pub fn discover_project_doc_paths(config: &Config) -> std::io::Result<Vec<PathBu
     };
 
     let mut found: Vec<PathBuf> = Vec::new();
+    let candidate_filenames = candidate_filenames(config);
     for d in search_dirs {
-        for name in CANDIDATE_FILENAMES {
+        for name in &candidate_filenames {
             let candidate = d.join(name);
             match std::fs::symlink_metadata(&candidate) {
                 Ok(md) => {
@@ -173,6 +175,22 @@ pub fn discover_project_doc_paths(config: &Config) -> std::io::Result<Vec<PathBu
     Ok(found)
 }
 
+fn candidate_filenames<'a>(config: &'a Config) -> Vec<&'a str> {
+    let mut names: Vec<&'a str> =
+        Vec::with_capacity(1 + config.project_doc_fallback_filenames.len());
+    names.push(DEFAULT_PROJECT_DOC_FILENAME);
+    for candidate in &config.project_doc_fallback_filenames {
+        let candidate = candidate.as_str();
+        if candidate.is_empty() {
+            continue;
+        }
+        if !names.contains(&candidate) {
+            names.push(candidate);
+        }
+    }
+    names
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -202,6 +220,20 @@ mod tests {
         config
     }
 
+    fn make_config_with_fallback(
+        root: &TempDir,
+        limit: usize,
+        instructions: Option<&str>,
+        fallbacks: &[&str],
+    ) -> Config {
+        let mut config = make_config(root, limit, instructions);
+        config.project_doc_fallback_filenames = fallbacks
+            .iter()
+            .map(std::string::ToString::to_string)
+            .collect();
+        config
+    }
+
     /// AGENTS.md missing – should yield `None`.
     #[tokio::test]
     async fn no_doc_file_returns_none() {
@@ -347,4 +379,45 @@ mod tests {
         let res = get_user_instructions(&cfg).await.expect("doc expected");
         assert_eq!(res, "root doc\n\ncrate doc");
     }
+
+    /// When AGENTS.md is absent but a configured fallback exists, the fallback is used.
+    #[tokio::test]
+    async fn uses_configured_fallback_when_agents_missing() {
+        let tmp = tempfile::tempdir().expect("tempdir");
+        fs::write(tmp.path().join("EXAMPLE.md"), "example instructions").unwrap();
+
+        let cfg = make_config_with_fallback(&tmp, 4096, None, &["EXAMPLE.md"]);
+
+        let res = get_user_instructions(&cfg)
+            .await
+            .expect("fallback doc expected");
+
+        assert_eq!(res, "example instructions");
+    }
+
+    /// AGENTS.md remains preferred when both AGENTS.md and fallbacks are present.
+    #[tokio::test]
+    async fn agents_md_preferred_over_fallbacks() {
+        let tmp = tempfile::tempdir().expect("tempdir");
+        fs::write(tmp.path().join("AGENTS.md"), "primary").unwrap();
+        fs::write(tmp.path().join("EXAMPLE.md"), "secondary").unwrap();
+
+        let cfg = make_config_with_fallback(&tmp, 4096, None, &["EXAMPLE.md", ".example.md"]);
+
+        let res = get_user_instructions(&cfg)
+            .await
+            .expect("AGENTS.md should win");
+
+        assert_eq!(res, "primary");
+
+        let discovery = discover_project_doc_paths(&cfg).expect("discover paths");
+        assert_eq!(discovery.len(), 1);
+        assert!(
+            discovery[0]
+                .file_name()
+                .unwrap()
+                .to_string_lossy()
+                .eq(DEFAULT_PROJECT_DOC_FILENAME)
+        );
+    }
 }

codex-rs/tui/src/chatwidget/tests.rs

@@ -2,6 +2,7 @@ use super::*;
 use crate::app_event::AppEvent;
 use crate::app_event_sender::AppEventSender;
 use crate::test_backend::VT100Backend;
+use crate::tui::FrameRequester;
 use codex_core::AuthManager;
 use codex_core::CodexAuth;
 use codex_core::config::Config;

codex-rs/tui/src/status/helpers.rs

@@ -36,9 +36,13 @@ pub(crate) fn compose_agents_summary(config: &Config) -> String {
         Ok(paths) => {
             let mut rels: Vec<String> = Vec::new();
             for p in paths {
+                let file_name = p
+                    .file_name()
+                    .map(|name| name.to_string_lossy().to_string())
+                    .unwrap_or_else(|| "<unknown>".to_string());
                 let display = if let Some(parent) = p.parent() {
                     if parent == config.cwd {
-                        "AGENTS.md".to_string()
+                        file_name.clone()
                     } else {
                         let mut cur = config.cwd.as_path();
                         let mut ups = 0usize;
@@ -53,7 +57,7 @@ pub(crate) fn compose_agents_summary(config: &Config) -> String {
                         }
                         if reached {
                             let up = format!("..{}", std::path::MAIN_SEPARATOR);
-                            format!("{}AGENTS.md", up.repeat(ups))
+                            format!("{}{}", up.repeat(ups), file_name)
                         } else if let Ok(stripped) = p.strip_prefix(&config.cwd) {
                             stripped.display().to_string()
                         } else {

docs/config.md

@@ -705,6 +705,16 @@ This is analogous to `model_context_window`, but for the maximum number of outpu
 
 Maximum number of bytes to read from an `AGENTS.md` file to include in the instructions sent with the first turn of a session. Defaults to 32 KiB.
 
+## project_doc_fallback_filenames
+
+Ordered list of additional filenames to look for when `AGENTS.md` is missing at a given directory level. The CLI always checks `AGENTS.md` first; the configured fallbacks are tried in the order provided. This lets monorepos that already use alternate instruction files (for example, `CLAUDE.md`) work out of the box while you migrate to `AGENTS.md` over time.
+
+```toml
+project_doc_fallback_filenames = ["CLAUDE.md", ".exampleagentrules.md"]
+```
+
+We recommend migrating instructions to AGENTS.md; other filenames may reduce model performance.
+
 ## tui
 
 Options that are specific to the TUI.