extension: move git attribution into an extension (#21738)

## Why

Git commit attribution is prompt policy, not session orchestration.
After #21737 adds the extension-registry seam, this moves that
prompt-only behavior out of `codex-core` so `Session` can consume
extension-contributed prompt fragments instead of owning a one-off
policy path itself.

Before this PR, `Session` injected the trailer instruction directly from
`codex-core` ([session
assembly](a57a747eb6/codex-rs/core/src/session/mod.rs (L2733-L2739)),
[helper
module](a57a747eb6/codex-rs/core/src/commit_attribution.rs (L1-L33))).
This branch moves that same responsibility into
[`codex-git-attribution`](b5029a6736/codex-rs/ext/git-attribution/src/lib.rs (L14-L100)).

## What changed

- Added the `codex-git-attribution` extension crate.
- Snapshot `CodexGitCommit` plus `commit_attribution` at thread start,
then contribute the developer-policy fragment through the extension
registry.
- Register the extension in app-server thread extensions.
- Remove the old `codex-core` helper module and direct `Session`
injection path.

This keeps the existing behavior intact: the prompt is only contributed
when `CodexGitCommit` is enabled, blank attribution still disables the
trailer, and the default remains `Codex <noreply@openai.com>`.

## Stack

- Stacked on #21737.

codex-rs/Cargo.lock

@@ -1902,6 +1902,7 @@ dependencies = [
  "codex-feedback",
  "codex-file-search",
  "codex-file-watcher",
+ "codex-git-attribution",
  "codex-git-utils",
  "codex-hooks",
  "codex-login",
@@ -2918,6 +2919,16 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "codex-git-attribution"
+version = "0.0.0"
+dependencies = [
+ "codex-core",
+ "codex-extension-api",
+ "codex-features",
+ "pretty_assertions",
+]
+
 [[package]]
 name = "codex-git-utils"
 version = "0.0.0"

codex-rs/Cargo.toml

@@ -46,6 +46,7 @@ members = [
     "execpolicy",
     "execpolicy-legacy",
     "ext/extension-api",
+    "ext/git-attribution",
     "external-agent-migration",
     "external-agent-sessions",
     "keyring-store",
@@ -162,6 +163,7 @@ codex-file-system = { path = "file-system" }
 codex-exec-server = { path = "exec-server" }
 codex-execpolicy = { path = "execpolicy" }
 codex-extension-api = { path = "ext/extension-api" }
+codex-git-attribution = { path = "ext/git-attribution" }
 codex-external-agent-migration = { path = "external-agent-migration" }
 codex-external-agent-sessions = { path = "external-agent-sessions" }
 codex-experimental-api-macros = { path = "codex-experimental-api-macros" }

codex-rs/app-server/Cargo.toml

@@ -41,6 +41,7 @@ codex-extension-api = { workspace = true }
 codex-external-agent-migration = { workspace = true }
 codex-external-agent-sessions = { workspace = true }
 codex-features = { workspace = true }
+codex-git-attribution = { workspace = true }
 codex-git-utils = { workspace = true }
 codex-file-watcher = { workspace = true }
 codex-hooks = { workspace = true }

codex-rs/app-server/src/extensions.rs

@@ -5,5 +5,9 @@ use codex_extension_api::ExtensionRegistry;
 use codex_extension_api::ExtensionRegistryBuilder;
 
 pub(crate) fn thread_extensions() -> Arc<ExtensionRegistry<Config>> {
-    Arc::new(ExtensionRegistryBuilder::<Config>::new().build())
+    Arc::new(
+        ExtensionRegistryBuilder::<Config>::new()
+            .with_extension(codex_git_attribution::extension())
+            .build(),
+    )
 }

codex-rs/core/src/commit_attribution.rs

@@ -1,33 +0,0 @@
-const DEFAULT_ATTRIBUTION_VALUE: &str = "Codex <noreply@openai.com>";
-
-fn build_commit_message_trailer(config_attribution: Option<&str>) -> Option<String> {
-    let value = resolve_attribution_value(config_attribution)?;
-    Some(format!("Co-authored-by: {value}"))
-}
-
-pub(crate) fn commit_message_trailer_instruction(
-    config_attribution: Option<&str>,
-) -> Option<String> {
-    let trailer = build_commit_message_trailer(config_attribution)?;
-    Some(format!(
-        "When you write or edit a git commit message, ensure the message ends with this trailer exactly once:\n{trailer}\n\nRules:\n- Keep existing trailers and append this trailer at the end if missing.\n- Do not duplicate this trailer if it already exists.\n- Keep one blank line between the commit body and trailer block."
-    ))
-}
-
-fn resolve_attribution_value(config_attribution: Option<&str>) -> Option<String> {
-    match config_attribution {
-        Some(value) => {
-            let trimmed = value.trim();
-            if trimmed.is_empty() {
-                None
-            } else {
-                Some(trimmed.to_string())
-            }
-        }
-        None => Some(DEFAULT_ATTRIBUTION_VALUE.to_string()),
-    }
-}
-
-#[cfg(test)]
-#[path = "commit_attribution_tests.rs"]
-mod tests;

codex-rs/core/src/commit_attribution_tests.rs

@@ -1,43 +0,0 @@
-use super::build_commit_message_trailer;
-use super::commit_message_trailer_instruction;
-use super::resolve_attribution_value;
-
-#[test]
-fn blank_attribution_disables_trailer_prompt() {
-    assert_eq!(build_commit_message_trailer(Some("")), None);
-    assert_eq!(commit_message_trailer_instruction(Some("   ")), None);
-}
-
-#[test]
-fn default_attribution_uses_codex_trailer() {
-    assert_eq!(
-        build_commit_message_trailer(/*config_attribution*/ None).as_deref(),
-        Some("Co-authored-by: Codex <noreply@openai.com>")
-    );
-}
-
-#[test]
-fn resolve_value_handles_default_custom_and_blank() {
-    assert_eq!(
-        resolve_attribution_value(/*config_attribution*/ None),
-        Some("Codex <noreply@openai.com>".to_string())
-    );
-    assert_eq!(
-        resolve_attribution_value(Some("MyAgent <me@example.com>")),
-        Some("MyAgent <me@example.com>".to_string())
-    );
-    assert_eq!(
-        resolve_attribution_value(Some("MyAgent")),
-        Some("MyAgent".to_string())
-    );
-    assert_eq!(resolve_attribution_value(Some("   ")), None);
-}
-
-#[test]
-fn instruction_mentions_trailer_and_omits_generated_with() {
-    let instruction = commit_message_trailer_instruction(Some("AgentX <agent@example.com>"))
-        .expect("instruction expected");
-    assert!(instruction.contains("Co-authored-by: AgentX <agent@example.com>"));
-    assert!(instruction.contains("exactly once"));
-    assert!(!instruction.contains("Generated-with"));
-}

codex-rs/core/src/lib.rs

@@ -27,7 +27,6 @@ mod agent;
 mod attestation;
 mod codex_delegate;
 mod command_canonicalization;
-mod commit_attribution;
 pub mod config;
 pub mod connectors;
 pub mod context;

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

@@ -16,7 +16,6 @@ use crate::agent::agent_status_from_event;
 use crate::agent::status::is_final;
 use crate::attestation::AttestationProvider;
 use crate::build_available_skills;
-use crate::commit_attribution::commit_message_trailer_instruction;
 use crate::compact;
 use crate::config::ManagedFeatures;
 use crate::config::resolve_tool_suggest_config_from_layer_stack;
@@ -2712,13 +2711,6 @@ impl Session {
         {
             developer_sections.push(plugin_instructions.render());
         }
-        if turn_context.features.enabled(Feature::CodexGitCommit)
-            && let Some(commit_message_instruction) = commit_message_trailer_instruction(
-                turn_context.config.commit_attribution.as_deref(),
-            )
-        {
-            developer_sections.push(commit_message_instruction);
-        }
         for contributor in self.services.extensions.context_contributors() {
             for fragment in contributor.contribute(
                 &self.services.session_extension_data,

codex-rs/ext/git-attribution/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "git-attribution",
+    crate_name = "codex_git_attribution",
+)

codex-rs/ext/git-attribution/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-git-attribution"
+version.workspace = true
+
+[lib]
+name = "codex_git_attribution"
+path = "src/lib.rs"
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-core = { workspace = true }
+codex-extension-api = { workspace = true }
+codex-features = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }

codex-rs/ext/git-attribution/src/lib.rs

@@ -0,0 +1,149 @@
+use std::sync::Arc;
+
+use codex_core::config::Config;
+use codex_extension_api::CodexExtension;
+use codex_extension_api::ContextContributor;
+use codex_extension_api::ExtensionData;
+use codex_extension_api::ExtensionRegistryBuilder;
+use codex_extension_api::PromptFragment;
+use codex_extension_api::ThreadStartContributor;
+use codex_features::Feature;
+
+const DEFAULT_ATTRIBUTION_VALUE: &str = "Codex <noreply@openai.com>";
+
+/// Prompt-only extension that contributes the configured git-attribution instruction.
+#[derive(Clone, Copy, Debug, Default)]
+pub struct GitAttributionExtension;
+
+impl GitAttributionExtension {
+    /// Creates an extension instance.
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+impl ContextContributor for GitAttributionExtension {
+    fn contribute(
+        &self,
+        _session_store: &ExtensionData,
+        thread_store: &ExtensionData,
+    ) -> Vec<PromptFragment> {
+        let Some(config_store) = thread_store.get::<GitAttributionConfig>() else {
+            return Vec::new();
+        };
+        if !config_store.enabled {
+            return Vec::new();
+        }
+        build_instruction(config_store.prompt.as_deref())
+            .map(PromptFragment::developer_policy)
+            .into_iter()
+            .collect()
+    }
+}
+
+#[derive(Clone, Debug, Default)]
+struct GitAttributionConfig {
+    enabled: bool,
+    prompt: Option<String>,
+}
+
+impl ThreadStartContributor<Config> for GitAttributionExtension {
+    fn contribute(
+        &self,
+        config: &Config,
+        _session_store: &ExtensionData,
+        thread_store: &ExtensionData,
+    ) {
+        thread_store.insert(GitAttributionConfig {
+            enabled: config.features.enabled(Feature::CodexGitCommit),
+            prompt: config.commit_attribution.clone(),
+        });
+    }
+}
+
+impl CodexExtension<Config> for GitAttributionExtension {
+    fn install(self: Arc<Self>, registry: &mut ExtensionRegistryBuilder<Config>) {
+        registry.thread_start_contributor(self.clone());
+        registry.prompt_contributor(self);
+    }
+}
+
+/// Creates a shared git-attribution extension instance.
+pub fn extension() -> Arc<GitAttributionExtension> {
+    Arc::new(GitAttributionExtension::new())
+}
+
+fn build_commit_message_trailer(config_attribution: Option<&str>) -> Option<String> {
+    let value = resolve_attribution_value(config_attribution)?;
+    Some(format!("Co-authored-by: {value}"))
+}
+
+fn build_instruction(config_attribution: Option<&str>) -> Option<String> {
+    let trailer = build_commit_message_trailer(config_attribution)?;
+    Some(format!(
+        "When you write or edit a git commit message, ensure the message ends with this trailer exactly once:\n{trailer}\n\nRules:\n- Keep existing trailers and append this trailer at the end if missing.\n- Do not duplicate this trailer if it already exists.\n- Keep one blank line between the commit body and trailer block."
+    ))
+}
+
+fn resolve_attribution_value(config_attribution: Option<&str>) -> Option<String> {
+    match config_attribution {
+        Some(value) => {
+            let trimmed = value.trim();
+            if trimmed.is_empty() {
+                None
+            } else {
+                Some(trimmed.to_string())
+            }
+        }
+        None => Some(DEFAULT_ATTRIBUTION_VALUE.to_string()),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+
+    use super::build_commit_message_trailer;
+    use super::build_instruction;
+    use super::resolve_attribution_value;
+
+    #[test]
+    fn blank_attribution_disables_trailer_prompt() {
+        assert_eq!(build_commit_message_trailer(Some("")), None);
+        assert_eq!(build_instruction(Some("   ")), None);
+    }
+
+    #[test]
+    fn default_attribution_uses_codex_trailer() {
+        assert_eq!(
+            build_commit_message_trailer(/*config_attribution*/ None).as_deref(),
+            Some("Co-authored-by: Codex <noreply@openai.com>")
+        );
+    }
+
+    #[test]
+    fn resolve_value_handles_default_custom_and_blank() {
+        assert_eq!(
+            resolve_attribution_value(/*config_attribution*/ None),
+            Some("Codex <noreply@openai.com>".to_string())
+        );
+        assert_eq!(
+            resolve_attribution_value(Some("MyAgent <me@example.com>")),
+            Some("MyAgent <me@example.com>".to_string())
+        );
+        assert_eq!(
+            resolve_attribution_value(Some("MyAgent")),
+            Some("MyAgent".to_string())
+        );
+        assert_eq!(resolve_attribution_value(Some("   ")), None);
+    }
+
+    #[test]
+    fn instruction_mentions_trailer_and_omits_generated_with() {
+        let instruction =
+            build_instruction(Some("AgentX <agent@example.com>")).expect("instruction expected");
+        assert!(instruction.contains("Co-authored-by: AgentX <agent@example.com>"));
+        assert!(instruction.contains("exactly once"));
+        assert!(!instruction.contains("Generated-with"));
+    }
+}