diff --git a/.github/skills/evaluate-skills/SKILL.md b/.github/skills/evaluate-skills/SKILL.md
new file mode 100644
index 0000000..6e694ea
--- /dev/null
+++ b/.github/skills/evaluate-skills/SKILL.md
@@ -0,0 +1,66 @@
+---
+name: evaluate-skills
+description: Use when creating, updating, or reviewing Vally evals for plugin skills. Covers eval.yaml, fixtures, graders, expect_skills, suites, tags, and eval coverage for new or changed skills.
+---
+
+# Evaluate plugin skills
+
+Use the [Vally reference docs](https://microsoft.github.io/vally/) for schema and grader details.
+
+This repo keeps evals under `evals/` and currently uses `.vally.yaml` to define the `pr`
+suite from evals tagged with `priority: p0`.
+
+## Layout
+
+```text
+.vally.yaml
+evals/
+├── <plugin-name>/
+│   ├── eval.yaml
+│   └── fixtures/
+│       └── <scenario>/
+│           └── ...
+└── ...
+```
+
+- Each plugin keeps its eval spec in `evals/<plugin-name>/eval.yaml`.
+- Put sample inputs under `evals/<plugin-name>/fixtures/`.
+- Seed fixture files into the eval environment with `environment.files`.
+
+## When to use this skill
+
+Use this skill when the request involves:
+
+- adding evals for a new plugin skill
+- updating evals after changing an existing plugin skill
+- writing or editing `evals/<plugin-name>/eval.yaml`
+- defining fixtures, graders, `expect_skills`, tags, or suites
+- checking whether plugin-skill coverage is missing
+
+## Writing evals
+
+1. Add or update `evals/<plugin-name>/eval.yaml` with `name`, `version`,
+   `description`, `tags`, `defaults`, `scoring`, and `stimuli`.
+2. Define one or more `stimuli` with a realistic user `prompt`, the skill path in
+   `environment.skills`, and any seeded files in `environment.files`.
+3. Use skill paths relative to the eval file, for example
+   `../../plugins/linting/skills/check-spelling`.
+4. Add `constraints.expect_skills` so the eval asserts the intended skill was used.
+5. Prefer outcome-based `graders` such as `file-matches`, `file-not-matches`, and
+   `file-exists`.
+6. Keep fixtures small and assertions specific to the behavior the skill should change.
+
+## Configuring evals
+
+1. Keep `.vally.yaml` in sync if you add new suites or change how evals are grouped.
+2. Use eval `tags` for suite filters such as the current `priority: p0` pull request
+   suite.
+3. Lint specs with `npx -y @microsoft/vally-cli@0.6.0 lint --eval-spec evals`.
+4. Run the pull request suite with
+   `COPILOT_GITHUB_TOKEN=... npx -y @microsoft/vally-cli@0.6.0 eval --suite pr --output-dir vally-results --junit`.
+
+## Coverage rule
+
+When adding a new plugin skill, or materially changing an existing plugin skill's
+behavior, add or update eval coverage in the corresponding `evals/<plugin-name>/`
+directory as part of the same change.
diff --git a/AGENTS.md b/AGENTS.md
index e68063b..e6dbf9e 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -7,6 +7,7 @@ Plugin marketplace for [GitHub Copilot CLI] and [Claude Code]. Defined in `.clau
 | File | Purpose |
 | --- | --- |
 | `.claude-plugin/marketplace.json` | Marketplace manifest with plugin versions and sources |
+| `.github/skills/<skill-name>/SKILL.md` | Repo-local skill for repository-specific guidance that should not always live in `AGENTS.md` |
 | `plugins/<name>/.claude-plugin/plugin.json` | Plugin details: name, description, version, author, skills |
 | `plugins/<name>/skills/<skill-name>/SKILL.md` | Skill — YAML frontmatter (`name`, `description`, `compatibility`) + instructions |
 | `plugins/<name>/skills/<skill-name>/scripts/` | Python scripts and `requirements.txt` for the skill |
@@ -23,6 +24,7 @@ Each skill folder follows the [Agent Skills](https://agentskills.io) layout:
 ```
 
 Skill paths in `plugin.json` are relative to the plugin directory (e.g., `"./skills/check-spelling"` resolves from `plugins/linting/`).
+Repo-local skills under `.github/skills/` use the same skill-folder layout.
 
 ## Versioning
 
@@ -58,6 +60,12 @@ Keep the `README.md` **Plugins** section in sync with the marketplace:
 2. **Remove** the subsection when a plugin is deleted from the marketplace.
 3. **Update** description, skill names, and skill descriptions when they change in `plugin.json` or `SKILL.md` frontmatter.
 
+## Evals
+
+Add or update associated eval coverage under `evals/<plugin-name>/` when adding a new plugin skill or making substantial changes to an existing one.
+
+Use `.github/skills/evaluate-skills/SKILL.md` for eval work: use when creating, updating, or reviewing Vally evals for plugin skills. Covers `eval.yaml`, fixtures, graders, `expect_skills`, suites, tags, and eval coverage for new or changed skills.
+
 ## Pre-commit checklist
 
 1. Plugin files changed → bump plugin `version` in both `marketplace.json` and `plugin.json`.