fix: prevent extension command shadowing

extensions/EXTENSION-API-REFERENCE.md

@@ -44,7 +44,7 @@ provides:
     - name: string       # Required, pattern: ^speckit\.[a-z0-9-]+\.[a-z0-9-]+$
       file: string       # Required, relative path to command file
       description: string # Required
-      aliases: [string]  # Optional, array of alternate names
+      aliases: [string]  # Optional, same pattern as name; must not shadow core commands
 
   config:                # Optional, array of config files
     - name: string       # Config file name

extensions/EXTENSION-DEVELOPMENT-GUIDE.md

@@ -41,7 +41,7 @@ provides:
     - name: "speckit.my-ext.hello"      # Must follow pattern: speckit.{ext-id}.{cmd}
       file: "commands/hello.md"
       description: "Say hello"
-      aliases: ["speckit.hello"]        # Optional aliases
+      aliases: ["speckit.my-ext.hi"]    # Optional aliases, same pattern
 
   config:                               # Optional: Config files
     - name: "my-ext-config.yml"

extensions/EXTENSION-USER-GUIDE.md

@@ -214,8 +214,8 @@ Extensions add commands that appear in your AI agent (Claude Code):
 # In Claude Code
 > /speckit.jira.specstoissues
 
-# Or use short alias (if provided)
-> /speckit.specstoissues
+# Or use a namespaced alias (if provided)
+> /speckit.jira.sync
 ```
 
 ### Extension Configuration

extensions/RFC-EXTENSION-SYSTEM.md

@@ -223,7 +223,7 @@ provides:
     - name: "speckit.jira.specstoissues"
       file: "commands/specstoissues.md"
       description: "Create Jira hierarchy from spec and tasks"
-      aliases: ["speckit.specstoissues"]  # Alternate names
+      aliases: ["speckit.jira.sync"]  # Alternate names
 
     - name: "speckit.jira.discover-fields"
       file: "commands/discover-fields.md"
@@ -1525,7 +1525,7 @@ provides:
   commands:
     - name: "speckit.github.taskstoissues"
       file: "commands/taskstoissues.md"
-      aliases: ["speckit.taskstoissues"]  # Backward compatibility
+      aliases: ["speckit.github.sync-taskstoissues"]  # Backward compatibility
 ```
 
 AI agent registers both names, so old scripts work.

extensions/template/extension.yml

@@ -47,8 +47,8 @@ provides:
     - name: "speckit.my-extension.example"
       file: "commands/example.md"
       description: "Example command that demonstrates functionality"
-      # Optional: Add aliases for shorter command names
-      aliases: ["speckit.example"]
+      # Optional: Add aliases in the same namespaced format
+      aliases: ["speckit.my-extension.example-short"]
 
     # ADD MORE COMMANDS: Copy this block for each command
     # - name: "speckit.my-extension.another-command"

src/specify_cli/extensions.py

@@ -25,6 +25,19 @@
 from packaging import version as pkg_version
 from packaging.specifiers import SpecifierSet, InvalidSpecifier
 
+CORE_COMMAND_NAMES = frozenset({
+    "analyze",
+    "checklist",
+    "clarify",
+    "constitution",
+    "implement",
+    "plan",
+    "specify",
+    "tasks",
+    "taskstoissues",
+})
+EXTENSION_COMMAND_NAME_PATTERN = re.compile(r"^speckit\.([a-z0-9-]+)\.([a-z0-9-]+)$")
+
 
 class ExtensionError(Exception):
     """Base exception for extension-related errors."""
@@ -446,6 +459,121 @@ def __init__(self, project_root: Path):
         self.extensions_dir = project_root / ".specify" / "extensions"
         self.registry = ExtensionRegistry(self.extensions_dir)
 
+    @staticmethod
+    def _collect_manifest_command_names(manifest: ExtensionManifest) -> Dict[str, str]:
+        """Collect command and alias names declared by a manifest.
+
+        Performs install-time validation for extension-specific constraints that
+        should not invalidate already-installed legacy manifests:
+        - aliases must use the canonical `speckit.{extension}.{command}` shape
+        - command namespaces must not shadow core commands
+        - duplicate command/alias names inside one manifest are rejected
+
+        Args:
+            manifest: Parsed extension manifest
+
+        Returns:
+            Mapping of declared command/alias name -> kind ("command"/"alias")
+
+        Raises:
+            ValidationError: If any declared name is invalid
+        """
+        if manifest.id in CORE_COMMAND_NAMES:
+            raise ValidationError(
+                f"Extension ID '{manifest.id}' conflicts with core command namespace '{manifest.id}'"
+            )
+
+        declared_names: Dict[str, str] = {}
+
+        for cmd in manifest.commands:
+            primary_name = cmd["name"]
+            aliases = cmd.get("aliases", [])
+
+            if aliases is None:
+                aliases = []
+            if not isinstance(aliases, list):
+                raise ValidationError(
+                    f"Aliases for command '{primary_name}' must be a list"
+                )
+
+            for kind, name in [("command", primary_name)] + [
+                ("alias", alias) for alias in aliases
+            ]:
+                if not isinstance(name, str):
+                    raise ValidationError(
+                        f"{kind.capitalize()} for command '{primary_name}' must be a string"
+                    )
+
+                match = EXTENSION_COMMAND_NAME_PATTERN.match(name)
+                if match is None:
+                    raise ValidationError(
+                        f"Invalid {kind} '{name}': "
+                        "must follow pattern 'speckit.{extension}.{command}'"
+                    )
+
+                namespace = match.group(1)
+                if namespace in CORE_COMMAND_NAMES:
+                    raise ValidationError(
+                        f"{kind.capitalize()} '{name}' conflicts with core command namespace '{namespace}'"
+                    )
+
+                if name in declared_names:
+                    raise ValidationError(
+                        f"Duplicate command or alias '{name}' in extension manifest"
+                    )
+
+                declared_names[name] = kind
+
+        return declared_names
+
+    def _get_installed_command_name_map(
+        self,
+        exclude_extension_id: Optional[str] = None,
+    ) -> Dict[str, str]:
+        """Return registered command and alias names for installed extensions."""
+        installed_names: Dict[str, str] = {}
+
+        for ext_id in self.registry.keys():
+            if ext_id == exclude_extension_id:
+                continue
+
+            manifest = self.get_extension(ext_id)
+            if manifest is None:
+                continue
+
+            for cmd in manifest.commands:
+                cmd_name = cmd.get("name")
+                if isinstance(cmd_name, str):
+                    installed_names.setdefault(cmd_name, ext_id)
+
+                aliases = cmd.get("aliases", [])
+                if not isinstance(aliases, list):
+                    continue
+
+                for alias in aliases:
+                    if isinstance(alias, str):
+                        installed_names.setdefault(alias, ext_id)
+
+        return installed_names
+
+    def _validate_install_conflicts(self, manifest: ExtensionManifest) -> None:
+        """Reject installs that would shadow core or installed extension commands."""
+        declared_names = self._collect_manifest_command_names(manifest)
+        installed_names = self._get_installed_command_name_map(
+            exclude_extension_id=manifest.id
+        )
+
+        collisions = [
+            f"{name} (already provided by extension '{installed_names[name]}')"
+            for name in sorted(declared_names)
+            if name in installed_names
+        ]
+        if collisions:
+            raise ValidationError(
+                "Extension commands conflict with installed extensions:\n- "
+                + "\n- ".join(collisions)
+            )
+
     @staticmethod
     def _load_extensionignore(source_dir: Path) -> Optional[Callable[[str, List[str]], Set[str]]]:
         """Load .extensionignore and return an ignore function for shutil.copytree.
@@ -861,6 +989,9 @@ def install_from_directory(
                 f"Use 'specify extension remove {manifest.id}' first."
             )
 
+        # Reject manifests that would shadow core commands or installed extensions.
+        self._validate_install_conflicts(manifest)
+
         # Install extension
         dest_dir = self.extensions_dir / manifest.id
         if dest_dir.exists():