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,