Capture the repo's hard-won skill-authoring lessons as guidance

.agents/skills/create-skill/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: create-skill
-description: Scaffolds new agent skills for the dotnet/skills repository. Use when creating a new skill, generating SKILL.md files, or setting up skill directory structures. Handles frontmatter generation, section templates, and validation guidance.
+description: Scaffolds new agent skills for the dotnet/skills repository. Use when creating a new skill, generating SKILL.md files, writing a skill description that the runtime will actually route to, or setting up skill directory structures. Handles frontmatter generation, section templates, and validation guidance. Do not use for fixing a skill that already fails its evaluation (use improve-skill-quality) or for writing eval.yaml (use create-skill-test).
 ---
 
 # Create Skill
@@ -17,6 +17,8 @@ This skill helps you scaffold new agent skills that conform to the Agent Skills
 ## When Not to Use
 
 - Modifying existing skills (edit directly instead)
+- Diagnosing or fixing a skill that fails its evaluation (use `improve-skill-quality`)
+- Writing the skill's `eval.yaml` (use `create-skill-test`)
 - Creating custom agents (use the agents/ directory pattern)
 
 ## Inputs
@@ -38,25 +40,66 @@ Ensure the name:
 - Does not contain consecutive hyphens
 - Is between 1-64 characters
 
-### Step 2: Create the skill directory
+### Step 2: Write the description — it is the router
 
-```
-skills/<skill-name>/
-└── SKILL.md
-```
-
-### Step 3: Generate SKILL.md with frontmatter
-
-Create the file with required YAML frontmatter:
+The `description` is the **only** text the runtime sees when deciding whether to load the skill.
+A perfect body behind a weak description never runs.
 
 ```yaml
 ---
 name: <skill-name>
-description: <description of what the skill does and when to use it>
+description: <what it does>. USE FOR: <symptoms, error codes, artifact names, quoted user requests>. DO NOT USE FOR: <nearby-but-wrong intents, with the skill that owns them>.
 ---
 ```
 
-### Step 4: Add body content sections
+- Lead with an action verb and use the user's own words: symptoms, error codes (`CS1501`,
+  `MSTEST0014`), artifact names (`.testsettings`, `binlog`), and requests phrased as a developer
+  would type them.
+- Partition against sibling skills on the **real discriminator**, not the topic. "Does the
+  abstraction already exist?" separates two skills; "testing" does not. Add the matching exclusion
+  to **both** siblings.
+- Claim the ambiguous words that would otherwise route to a sibling. If prompts say "review my
+  tests" and a sibling owns "review", say so explicitly.
+- Check every `DO NOT USE FOR` clause against the scenarios the skill exists to serve — an
+  exclusion like "already on v3" can lock out the post-upgrade fixes that are the skill's purpose.
+- Budget: 1,024 characters per description, and the whole plugin's rendered skill menu is also
+  budgeted. A helper skill users should never invoke directly can set
+  `disable-model-invocation: true` to free menu space while staying invocable by name.
+
+### Step 3: Write for delta over the baseline model
+
+Every skill is scored head-to-head against the same model with **no skill loaded**. Content the
+model already produces unaided is worth zero; content that makes it slower or more hedged is worth
+less than zero. See
+[improve-skill-quality/references/writing-for-baseline-delta.md](../improve-skill-quality/references/writing-for-baseline-delta.md)
+for the full evidence.
+
+| Do | Instead of |
+|----|------------|
+| Encode the decision the model would otherwise get wrong | Restating API signatures it already reproduces |
+| "When A, do B, never C, verify D" tables | Lists of plausible alternatives |
+| A concrete output contract (exact command, verdict line, findings table) | "Consider…", "you may want to…" |
+| Scale output structure to input size | A 12-section dashboard for an 8-test suite |
+| Stop-conditions that prevent over-applying | Acting before measuring, rewriting working code |
+| Instructing the agent to discover repo paths | Marking discoverable paths as required inputs |
+| Reporting restore/build/test failures truthfully | Claiming success after a failed command |
+| Verifying load-bearing API claims by compiling or probing | Trusting a source read |
+| Gating rare or expensive paths behind `references/` | One large SKILL.md carrying every path |
+
+Do not over-correct: a skilled answer shorter and less actionable than the baseline's still loses.
+
+### Step 4: Create the skill directory
+
+```
+plugins/<plugin>/skills/<skill-name>/
+└── SKILL.md
+```
+
+### Step 5: Generate SKILL.md with frontmatter
+
+Create the file with the frontmatter drafted in Step 2.
+
+### Step 6: Add body content sections
 
 Include these recommended sections:
 
@@ -68,17 +111,17 @@ Include these recommended sections:
 6. **Validation**: How to confirm the skill worked correctly
 7. **Common Pitfalls**: Known traps and how to avoid them
 
-### Step 5: Add optional directories (if needed)
+### Step 7: Add optional directories (if needed)
 
 ```
-skills/<skill-name>/
+plugins/<plugin>/skills/<skill-name>/
 ├── SKILL.md
 ├── scripts/       # Executable code agents can run
 ├── references/    # Additional documentation loaded on demand
 └── assets/        # Templates, images, data files
 ```
 
-### Step 6: Update CODEOWNERS
+### Step 8: Update CODEOWNERS
 
 Add entries in `.github/CODEOWNERS` for the new skill and its test directory:
 
@@ -89,12 +132,23 @@ Add entries in `.github/CODEOWNERS` for the new skill and its test directory:
 
 Match the owner pattern used by sibling skills in the same plugin.
 
-### Step 7: Validate the skill
+### Step 9: Validate the skill
 
 - Confirm frontmatter fields are valid
 - Ensure SKILL.md is under 500 lines
 - Check that file references use relative paths
 - Verify instructions are actionable and specific
+- Run `dotnet run --project eng/skill-validator/src/SkillValidator.csproj -- check --plugin ./plugins/<plugin>`
+
+### Step 10: Add the eval
+
+A skill without an `eval.yaml` has no evidence that it improves on the baseline. Use
+`create-skill-test` to add one in the same pull request, and size it for statistical power — an eval
+below five counted trials can never return a passing verdict.
+
+The exception is a helper skill with `disable-model-invocation: true`: the model cannot
+self-activate it, so an activation-graded eval compares two identical arms. Cover it through the
+evals of the skills that load it instead.
 
 ## SKILL.md Template
 
@@ -161,6 +215,10 @@ After creating a skill, verify:
 - [ ] Validation section exists with observable success criteria
 - [ ] No secrets, tokens, or internal URLs included
 - [ ] `.github/CODEOWNERS` has entries for the new skill and its test directory
+- [ ] The description names concrete triggers and excludes the nearest sibling skills
+- [ ] Every section changes a decision the unskilled model would otherwise get wrong
+- [ ] The skill states when **not** to act, and what a truthful failure report looks like
+- [ ] An `eval.yaml` exists and clears the trial floor (or the skill is `disable-model-invocation: true` and covered through its consumers)
 
 ## Common Pitfalls
 
@@ -173,9 +231,16 @@ After creating a skill, verify:
 | SKILL.md too long | Move detailed content to `references/` files |
 | Hardcoded environment assumptions | Document requirements in `compatibility` field |
 | Missing CODEOWNERS entry | Add entries for both `/plugins/<plugin>/skills/<skill-name>/` and `/tests/<plugin>/<skill-name>/` matching sibling skills' owner pattern |
+| Skill restates what the model already knows | Cut it; a skill is scored as a delta over the unskilled model |
+| Discoverable paths listed as required inputs | Tell the agent to discover them, or it will stop and ask the user |
+| Description partitioned by topic against a sibling | Partition on the real discriminator and exclude on both sides |
+| Exclusion clause blocks the skill's own use cases | Re-read every "do not use for" clause against real workflow phases |
+| Skill added without an eval | Add `eval.yaml` in the same PR; unevaluated skills carry no evidence |
 
 ## References
 
 - [Agent Skills Specification](https://agentskills.io/specification)
-- [Repository README](../../README.md)
-- [Contributing Guidelines](../../CONTRIBUTING.md)
+- [Repository README](../../../README.md)
+- [Contributing Guidelines](../../../CONTRIBUTING.md)
+- [create-skill-test](../create-skill-test/SKILL.md) — authoring the skill's `eval.yaml`
+- [improve-skill-quality](../improve-skill-quality/SKILL.md) — fixing a skill that loses to its baseline