diff --git a/CHANGELOG.md b/CHANGELOG.md
index ac78d56..09bbbcb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Changed
+- `allowed-tools` SKILL.md field now uses space-separated values per agentskills.io spec (was comma-separated) — **breaking change** for skills using comma-delimited allowed-tools (#686)
+
 ### Added
+- Nested `metadata:` block support in SKILL.md frontmatter: indented key-value pairs under `metadata:` are parsed as structured metadata (#686)
+- Field length validation in SKILL.md loader: `description` capped at 1024 characters, `compatibility` capped at 500 characters (#686)
+- Warning log in `load_skill_body()` when body exceeds 20,000 bytes (~5000 tokens) per spec recommendation (#686)
+- Empty value normalization for `compatibility` and `license` frontmatter fields: bare `compatibility:` now produces `None` instead of `Some("")` (#686)
 - `SkillManager` in zeph-skills — install skills from git URLs or local paths, remove, verify blake3 integrity, list with trust metadata
 - CLI subcommands: `zeph skill {install, remove, list, verify, trust, block, unblock}` — runs without agent loop
 - In-session `/skill install <url|path>` and `/skill remove <name>` with hot reload
diff --git a/crates/zeph-skills/src/loader.rs b/crates/zeph-skills/src/loader.rs
index 92349f0..307936b 100644
--- a/crates/zeph-skills/src/loader.rs
+++ b/crates/zeph-skills/src/loader.rs
@@ -83,8 +83,23 @@ fn parse_frontmatter(yaml_str: &str) -> RawFrontmatter {
     let mut metadata = Vec::new();
     let mut allowed_tools = Vec::new();
     let mut requires_secrets = Vec::new();
+    let mut in_metadata = false;
 
     for line in yaml_str.lines() {
+        if in_metadata {
+            if line.starts_with("  ") || line.starts_with('\t') {
+                let trimmed = line.trim();
+                if let Some((k, v)) = trimmed.split_once(':') {
+                    let v = v.trim();
+                    if !v.is_empty() {
+                        metadata.push((k.trim().to_string(), v.to_string()));
+                    }
+                }
+                continue;
+            }
+            in_metadata = false;
+        }
+
         let line = line.trim();
         if line.is_empty() {
             continue;
@@ -95,14 +110,18 @@ fn parse_frontmatter(yaml_str: &str) -> RawFrontmatter {
             match key {
                 "name" => name = Some(value),
                 "description" => description = Some(value),
-                "compatibility" => compatibility = Some(value),
-                "license" => license = Some(value),
+                "compatibility" => {
+                    if !value.is_empty() {
+                        compatibility = Some(value);
+                    }
+                }
+                "license" => {
+                    if !value.is_empty() {
+                        license = Some(value);
+                    }
+                }
                 "allowed-tools" => {
-                    allowed_tools = value
-                        .split(',')
-                        .map(|s| s.trim().to_string())
-                        .filter(|s| !s.is_empty())
-                        .collect();
+                    allowed_tools = value.split_whitespace().map(ToString::to_string).collect();
                 }
                 "requires-secrets" => {
                     requires_secrets = value
@@ -111,6 +130,9 @@ fn parse_frontmatter(yaml_str: &str) -> RawFrontmatter {
                         .filter(|s| !s.is_empty())
                         .collect();
                 }
+                "metadata" if value.is_empty() => {
+                    in_metadata = true;
+                }
                 _ => {
                     if !value.is_empty() {
                         metadata.push((key.to_string(), value));
@@ -203,6 +225,24 @@ pub fn load_skill_meta(path: &Path) -> Result<SkillMeta, SkillError> {
         ))
     })?;
 
+    if description.len() > 1024 {
+        return Err(SkillError::Invalid(format!(
+            "description exceeds 1024 characters ({}) in {}",
+            description.len(),
+            path.display()
+        )));
+    }
+
+    if let Some(ref c) = raw.compatibility
+        && c.len() > 500
+    {
+        return Err(SkillError::Invalid(format!(
+            "compatibility exceeds 500 characters ({}) in {}",
+            c.len(),
+            path.display()
+        )));
+    }
+
     let skill_dir = path.parent().map(Path::to_path_buf).unwrap_or_default();
 
     let dir_name = skill_dir.file_name().and_then(|n| n.to_str()).unwrap_or("");
@@ -235,6 +275,14 @@ pub fn load_skill_body(meta: &SkillMeta) -> Result<String, SkillError> {
     let (_yaml_str, body) = split_frontmatter(&content)
         .map_err(|e| SkillError::Other(format!("in {}: {e}", path.display())))?;
 
+    if body.len() > 20_000 {
+        tracing::warn!(
+            skill = %meta.name,
+            bytes = body.len(),
+            "skill body exceeds 20000 bytes; consider trimming to stay within ~5000 token budget"
+        );
+    }
+
     Ok(body.to_string())
 }
 
@@ -354,7 +402,7 @@ mod tests {
         let path = write_skill(
             dir.path(),
             "my-skill",
-            "---\nname: my-skill\ndescription: desc\ncompatibility: linux\nlicense: MIT\nallowed-tools: bash, python\ncustom-key: custom-value\n---\nbody",
+            "---\nname: my-skill\ndescription: desc\ncompatibility: linux\nlicense: MIT\nallowed-tools: bash python\ncustom-key: custom-value\n---\nbody",
         );
 
         let meta = load_skill_meta(&path).unwrap();
@@ -367,6 +415,88 @@ mod tests {
         );
     }
 
+    #[test]
+    fn allowed_tools_with_parens() {
+        let raw = parse_frontmatter("allowed-tools: Bash(git:*) Bash(jq:*) Read\n");
+        assert_eq!(raw.allowed_tools, vec!["Bash(git:*)", "Bash(jq:*)", "Read"]);
+    }
+
+    #[test]
+    fn allowed_tools_empty() {
+        let raw = parse_frontmatter("allowed-tools:\n");
+        assert!(raw.allowed_tools.is_empty());
+    }
+
+    #[test]
+    fn metadata_nested_block() {
+        let yaml = "metadata:\n  author: example-org\n  version: \"1.0\"\n";
+        let raw = parse_frontmatter(yaml);
+        assert_eq!(
+            raw.metadata,
+            vec![
+                ("author".into(), "example-org".into()),
+                ("version".into(), "\"1.0\"".into()),
+            ]
+        );
+    }
+
+    #[test]
+    fn metadata_nested_with_other_fields() {
+        let yaml = "name: my-skill\nmetadata:\n  author: example-org\nlicense: MIT\n";
+        let raw = parse_frontmatter(yaml);
+        assert_eq!(raw.name.as_deref(), Some("my-skill"));
+        assert_eq!(raw.license.as_deref(), Some("MIT"));
+        assert_eq!(raw.metadata, vec![("author".into(), "example-org".into())]);
+    }
+
+    #[test]
+    fn metadata_flat_still_works() {
+        let yaml = "custom-key: custom-value\n";
+        let raw = parse_frontmatter(yaml);
+        assert_eq!(
+            raw.metadata,
+            vec![("custom-key".into(), "custom-value".into())]
+        );
+    }
+
+    #[test]
+    fn description_exceeds_max_length() {
+        let dir = tempfile::tempdir().unwrap();
+        let desc = "a".repeat(1025);
+        let path = write_skill(
+            dir.path(),
+            "my-skill",
+            &format!("---\nname: my-skill\ndescription: {desc}\n---\nbody"),
+        );
+        let err = load_skill_meta(&path).unwrap_err();
+        assert!(format!("{err:#}").contains("description exceeds 1024 characters"));
+    }
+
+    #[test]
+    fn description_at_max_length() {
+        let dir = tempfile::tempdir().unwrap();
+        let desc = "a".repeat(1024);
+        let path = write_skill(
+            dir.path(),
+            "my-skill",
+            &format!("---\nname: my-skill\ndescription: {desc}\n---\nbody"),
+        );
+        assert!(load_skill_meta(&path).is_ok());
+    }
+
+    #[test]
+    fn compatibility_exceeds_max_length() {
+        let dir = tempfile::tempdir().unwrap();
+        let compat = "a".repeat(501);
+        let path = write_skill(
+            dir.path(),
+            "my-skill",
+            &format!("---\nname: my-skill\ndescription: desc\ncompatibility: {compat}\n---\nbody"),
+        );
+        let err = load_skill_meta(&path).unwrap_err();
+        assert!(format!("{err:#}").contains("compatibility exceeds 500 characters"));
+    }
+
     #[test]
     fn name_validation_rejects_uppercase() {
         let dir = tempfile::tempdir().unwrap();
@@ -524,4 +654,40 @@ mod tests {
         let meta = load_skill_meta(&path).unwrap();
         assert_eq!(meta.requires_secrets, vec!["my_api_key", "another_token"]);
     }
+
+    #[test]
+    fn empty_compatibility_produces_none() {
+        let raw = parse_frontmatter("compatibility:\n");
+        assert!(raw.compatibility.is_none());
+    }
+
+    #[test]
+    fn empty_license_produces_none() {
+        let raw = parse_frontmatter("license:\n");
+        assert!(raw.license.is_none());
+    }
+
+    #[test]
+    fn nonempty_compatibility_produces_some() {
+        let raw = parse_frontmatter("compatibility: linux\n");
+        assert_eq!(raw.compatibility.as_deref(), Some("linux"));
+    }
+
+    #[test]
+    fn metadata_value_with_colon() {
+        let yaml = "metadata:\n  url: https://example.com\n";
+        let raw = parse_frontmatter(yaml);
+        assert_eq!(
+            raw.metadata,
+            vec![("url".into(), "https://example.com".into())]
+        );
+    }
+
+    #[test]
+    fn metadata_empty_block() {
+        let yaml = "metadata:\nname: my-skill\n";
+        let raw = parse_frontmatter(yaml);
+        assert!(raw.metadata.is_empty());
+        assert_eq!(raw.name.as_deref(), Some("my-skill"));
+    }
 }
diff --git a/docs/src/guides/custom-skills.md b/docs/src/guides/custom-skills.md
index e59eaeb..934914f 100644
--- a/docs/src/guides/custom-skills.md
+++ b/docs/src/guides/custom-skills.md
@@ -32,7 +32,7 @@ into the LLM context when the skill is matched.
 | `name` | Yes | Unique identifier (1-64 chars, lowercase, hyphens allowed) |
 | `description` | Yes | Used for embedding-based matching against user queries |
 | `compatibility` | No | Runtime requirements (e.g., "requires curl") |
-| `allowed-tools` | No | Comma-separated tool names this skill can use |
+| `allowed-tools` | No | Space-separated tool names this skill can use |
 | `requires-secrets` | No | Comma-separated secret names the skill needs (see below) |
 
 ### Secret-Gated Skills