feat: Compress skill paths with root aliases (#19098)

Add skill root tracking so model-visible skill lists can use short path
aliases when absolute paths would exceed the metadata budget.

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

@@ -1,4 +1,5 @@
 use codex_core_skills::AvailableSkills;
+use codex_core_skills::render_available_skills_body;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_CLOSE_TAG;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
 
@@ -6,12 +7,14 @@ use super::ContextualUserFragment;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct AvailableSkillsInstructions {
+    skill_root_lines: Vec<String>,
     skill_lines: Vec<String>,
 }
 
 impl From<AvailableSkills> for AvailableSkillsInstructions {
     fn from(available_skills: AvailableSkills) -> Self {
         Self {
+            skill_root_lines: available_skills.skill_root_lines,
             skill_lines: available_skills.skill_lines,
         }
     }
@@ -23,34 +26,6 @@ impl ContextualUserFragment for AvailableSkillsInstructions {
     const END_MARKER: &'static str = SKILLS_INSTRUCTIONS_CLOSE_TAG;
 
     fn body(&self) -> String {
-        let mut lines: Vec<String> = Vec::new();
-        lines.push("## Skills".to_string());
-        lines.push("A skill is a set of local instructions to follow that is stored in a `SKILL.md` file. Below is the list of skills that can be used. Each entry includes a name, description, and file path so you can open the source for full instructions when using a specific skill.".to_string());
-        lines.push("### Available skills".to_string());
-        lines.extend(self.skill_lines.iter().cloned());
-
-        lines.push("### How to use skills".to_string());
-        lines.push(
-            r###"- Discovery: The list above is the skills available in this session (name + description + file path). Skill bodies live on disk at the listed paths.
-- Trigger rules: If the user names a skill (with `$SkillName` or plain text) OR the task clearly matches a skill's description shown above, you must use that skill for that turn. Multiple mentions mean use them all. Do not carry skills across turns unless re-mentioned.
-- Missing/blocked: If a named skill isn't in the list or the path can't be read, say so briefly and continue with the best fallback.
-- How to use a skill (progressive disclosure):
-  1) After deciding to use a skill, open its `SKILL.md`. Read only enough to follow the workflow.
-  2) When `SKILL.md` references relative paths (e.g., `scripts/foo.py`), resolve them relative to the skill directory listed above first, and only consider other paths if needed.
-  3) If `SKILL.md` points to extra folders such as `references/`, load only the specific files needed for the request; don't bulk-load everything.
-  4) If `scripts/` exist, prefer running or patching them instead of retyping large code blocks.
-  5) If `assets/` or templates exist, reuse them instead of recreating from scratch.
-- Coordination and sequencing:
-  - If multiple skills apply, choose the minimal set that covers the request and state the order you'll use them.
-  - Announce which skill(s) you're using and why (one short line). If you skip an obvious skill, say why.
-- Context hygiene:
-  - Keep context small: summarize long sections instead of pasting them; only load extra files when needed.
-  - Avoid deep reference-chasing: prefer opening only files directly linked from `SKILL.md` unless you're blocked.
-  - When variants exist (frameworks, providers, domains), pick only the relevant reference file(s) and note that choice.
-- Safety and fallback: If a skill can't be applied cleanly (missing files, unclear instructions), state the issue, pick the next-best approach, and continue."###
-                .to_string(),
-        );
-
-        format!("\n{}\n", lines.join("\n"))
+        render_available_skills_body(&self.skill_root_lines, &self.skill_lines)
     }
 }

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

@@ -2627,12 +2627,8 @@ impl Session {
             }
         }
         if turn_context.config.include_skill_instructions {
-            let implicit_skills = turn_context
-                .turn_skills
-                .outcome
-                .allowed_skills_for_implicit_invocation();
             let available_skills = build_available_skills(
-                &implicit_skills,
+                &turn_context.turn_skills.outcome,
                 default_skill_metadata_budget(turn_context.model_info.context_window),
                 SkillRenderSideEffects::ThreadStart {
                     session_telemetry: &self.services.session_telemetry,

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

@@ -5185,17 +5185,19 @@ async fn build_initial_context_trims_skill_metadata_from_context_window_budget()
 #[test]
 fn emit_thread_start_skill_metrics_records_enabled_kept_and_truncated_values() {
     let session_telemetry = test_session_telemetry_without_metadata();
+    let mut outcome = SkillLoadOutcome::default();
+    outcome.skills = vec![SkillMetadata {
+        name: "repo-skill".to_string(),
+        description: "desc".to_string(),
+        short_description: None,
+        interface: None,
+        dependencies: None,
+        policy: None,
+        path_to_skills_md: test_path_buf("/tmp/repo-skill/SKILL.md").abs(),
+        scope: SkillScope::Repo,
+    }];
     let rendered = build_available_skills(
-        &[SkillMetadata {
-            name: "repo-skill".to_string(),
-            description: "desc".to_string(),
-            short_description: None,
-            interface: None,
-            dependencies: None,
-            policy: None,
-            path_to_skills_md: test_path_buf("/tmp/repo-skill/SKILL.md").abs(),
-            scope: SkillScope::Repo,
-        }],
+        &outcome,
         SkillMetadataBudget::Characters(1),
         SkillRenderSideEffects::ThreadStart {
             session_telemetry: &session_telemetry,
@@ -5255,9 +5257,11 @@ fn emit_thread_start_skill_metrics_records_description_truncated_chars_without_o
             .count()
     };
     let minimum_budget = minimum_skill_line_cost(&alpha) + minimum_skill_line_cost(&beta);
+    let mut outcome = SkillLoadOutcome::default();
+    outcome.skills = vec![alpha, beta];
 
     let rendered = build_available_skills(
-        &[alpha, beta],
+        &outcome,
         SkillMetadataBudget::Characters(minimum_budget + 6),
         SkillRenderSideEffects::ThreadStart {
             session_telemetry: &session_telemetry,