diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3153ea45..092afefa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Preset catalog files (`presets/catalog.json`, `presets/catalog.community.json`)
 - Preset scaffold directory (`presets/scaffold/`)
 - Scripts updated to use template resolution instead of hardcoded paths
+- feat(extensions): support `.extensionignore` to exclude files/folders during `specify extension add` (#1781)
 
 ## [0.2.0] - 2026-03-09
 
@@ -61,7 +62,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - fix: release-trigger uses release branch + PR instead of direct push to main (#1733)
 - fix: Split release process to sync pyproject.toml version with git tags (#1732)
 
-
 ## [0.1.14] - 2026-03-09
 
 ### Added
diff --git a/extensions/EXTENSION-DEVELOPMENT-GUIDE.md b/extensions/EXTENSION-DEVELOPMENT-GUIDE.md
index f86beb62..feea7b27 100644
--- a/extensions/EXTENSION-DEVELOPMENT-GUIDE.md
+++ b/extensions/EXTENSION-DEVELOPMENT-GUIDE.md
@@ -332,6 +332,67 @@ echo "$config"
 
 ---
 
+## Excluding Files with `.extensionignore`
+
+Extension authors can create a `.extensionignore` file in the extension root to exclude files and folders from being copied when a user installs the extension with `specify extension add`. This is useful for keeping development-only files (tests, CI configs, docs source, etc.) out of the installed copy.
+
+### Format
+
+The file uses `.gitignore`-compatible patterns (one per line), powered by the [`pathspec`](https://pypi.org/project/pathspec/) library:
+
+- Blank lines are ignored
+- Lines starting with `#` are comments
+- `*` matches anything **except** `/` (does not cross directory boundaries)
+- `**` matches zero or more directories (e.g., `docs/**/*.draft.md`)
+- `?` matches any single character except `/`
+- A trailing `/` restricts a pattern to directories only
+- Patterns containing `/` (other than a trailing slash) are anchored to the extension root
+- Patterns without `/` match at any depth in the tree
+- `!` negates a previously excluded pattern (re-includes a file)
+- Backslashes in patterns are normalised to forward slashes for cross-platform compatibility
+- The `.extensionignore` file itself is always excluded automatically
+
+### Example
+
+```gitignore
+# .extensionignore
+
+# Development files
+tests/
+.github/
+.gitignore
+
+# Build artifacts
+__pycache__/
+*.pyc
+dist/
+
+# Documentation source (keep only the built README)
+docs/
+CONTRIBUTING.md
+```
+
+### Pattern Matching
+
+| Pattern | Matches | Does NOT match |
+|---------|---------|----------------|
+| `*.pyc` | Any `.pyc` file in any directory | — |
+| `tests/` | The `tests` directory (and all its contents) | A file named `tests` |
+| `docs/*.draft.md` | `docs/api.draft.md` (directly inside `docs/`) | `docs/sub/api.draft.md` (nested) |
+| `.env` | The `.env` file at any level | — |
+| `!README.md` | Re-includes `README.md` even if matched by an earlier pattern | — |
+| `docs/**/*.draft.md` | `docs/api.draft.md`, `docs/sub/api.draft.md` | — |
+
+### Unsupported Features
+
+The following `.gitignore` features are **not applicable** in this context:
+
+- **Multiple `.extensionignore` files**: Only a single file at the extension root is supported (`.gitignore` supports files in subdirectories)
+- **`$GIT_DIR/info/exclude` and `core.excludesFile`**: These are Git-specific and have no equivalent here
+- **Negation inside excluded directories**: Because file copying uses `shutil.copytree`, excluding a directory prevents recursion into it entirely. A negation pattern cannot re-include a file inside a directory that was itself excluded. For example, the combination `tests/` followed by `!tests/important.py` will **not** preserve `tests/important.py` — the `tests/` directory is skipped at the root level and its contents are never evaluated. To work around this, exclude the directory's contents individually instead of the directory itself (e.g., `tests/*.pyc` and `tests/.cache/` rather than `tests/`).
+
+---
+
 ## Validation Rules
 
 ### Extension ID
diff --git a/pyproject.toml b/pyproject.toml
index 04b0a338..04a6791a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,7 @@ dependencies = [
     "truststore>=0.10.4",
     "pyyaml>=6.0",
     "packaging>=23.0",
+    "pathspec>=0.12.0",
 ]
 
 [project.scripts]
diff --git a/src/specify_cli/agents.py b/src/specify_cli/agents.py
index 9b61f424..5184f0f6 100644
--- a/src/specify_cli/agents.py
+++ b/src/specify_cli/agents.py
@@ -58,6 +58,12 @@ class CommandRegistrar:
             "args": "$ARGUMENTS",
             "extension": ".md"
         },
+        "codex": {
+            "dir": ".codex/prompts",
+            "format": "markdown",
+            "args": "$ARGUMENTS",
+            "extension": ".md"
+        },
         "windsurf": {
             "dir": ".windsurf/workflows",
             "format": "markdown",
diff --git a/src/specify_cli/extensions.py b/src/specify_cli/extensions.py
index 3d1a53c1..03cb3afa 100644
--- a/src/specify_cli/extensions.py
+++ b/src/specify_cli/extensions.py
@@ -14,10 +14,12 @@ import zipfile
 import shutil
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Optional, Dict, List, Any
+from typing import Optional, Dict, List, Any, Callable, Set
 from datetime import datetime, timezone
 import re
 
+import pathspec
+
 import yaml
 from packaging import version as pkg_version
 from packaging.specifiers import SpecifierSet, InvalidSpecifier
@@ -280,6 +282,70 @@ class ExtensionManager:
         self.extensions_dir = project_root / ".specify" / "extensions"
         self.registry = ExtensionRegistry(self.extensions_dir)
 
+    @staticmethod
+    def _load_extensionignore(source_dir: Path) -> Optional[Callable[[str, List[str]], Set[str]]]:
+        """Load .extensionignore and return an ignore function for shutil.copytree.
+
+        The .extensionignore file uses .gitignore-compatible patterns (one per line).
+        Lines starting with '#' are comments. Blank lines are ignored.
+        The .extensionignore file itself is always excluded.
+
+        Pattern semantics mirror .gitignore:
+        - '*' matches anything except '/'
+        - '**' matches zero or more directories
+        - '?' matches any single character except '/'
+        - Trailing '/' restricts a pattern to directories only
+        - Patterns with '/' (other than trailing) are anchored to the root
+        - '!' negates a previously excluded pattern
+
+        Args:
+            source_dir: Path to the extension source directory
+
+        Returns:
+            An ignore function compatible with shutil.copytree, or None
+            if no .extensionignore file exists.
+        """
+        ignore_file = source_dir / ".extensionignore"
+        if not ignore_file.exists():
+            return None
+
+        lines: List[str] = ignore_file.read_text().splitlines()
+
+        # Normalise backslashes in patterns so Windows-authored files work
+        normalised: List[str] = []
+        for line in lines:
+            stripped = line.strip()
+            if stripped and not stripped.startswith("#"):
+                normalised.append(stripped.replace("\\", "/"))
+            else:
+                # Preserve blanks/comments so pathspec line numbers stay stable
+                normalised.append(line)
+
+        # Always ignore the .extensionignore file itself
+        normalised.append(".extensionignore")
+
+        spec = pathspec.GitIgnoreSpec.from_lines(normalised)
+
+        def _ignore(directory: str, entries: List[str]) -> Set[str]:
+            ignored: Set[str] = set()
+            rel_dir = Path(directory).relative_to(source_dir)
+            for entry in entries:
+                rel_path = str(rel_dir / entry) if str(rel_dir) != "." else entry
+                # Normalise to forward slashes for consistent matching
+                rel_path_fwd = rel_path.replace("\\", "/")
+
+                entry_full = Path(directory) / entry
+                if entry_full.is_dir():
+                    # Append '/' so directory-only patterns (e.g. tests/) match
+                    if spec.match_file(rel_path_fwd + "/"):
+                        ignored.add(entry)
+                else:
+                    if spec.match_file(rel_path_fwd):
+                        ignored.add(entry)
+            return ignored
+
+        return _ignore
+
     def check_compatibility(
         self,
         manifest: ExtensionManifest,
@@ -353,7 +419,8 @@ class ExtensionManager:
         if dest_dir.exists():
             shutil.rmtree(dest_dir)
 
-        shutil.copytree(source_dir, dest_dir)
+        ignore_fn = self._load_extensionignore(source_dir)
+        shutil.copytree(source_dir, dest_dir, ignore=ignore_fn)
 
         # Register commands with AI agents
         registered_commands = {}
diff --git a/tests/test_agent_config_consistency.py b/tests/test_agent_config_consistency.py
index 607d491d..3615dbc7 100644
--- a/tests/test_agent_config_consistency.py
+++ b/tests/test_agent_config_consistency.py
@@ -28,6 +28,13 @@ class TestAgentConfigConsistency:
         assert cfg["kiro-cli"]["dir"] == ".kiro/prompts"
         assert "q" not in cfg
 
+    def test_extension_registrar_includes_codex(self):
+        """Extension command registrar should include codex targeting .codex/prompts."""
+        cfg = CommandRegistrar.AGENT_CONFIGS
+
+        assert "codex" in cfg
+        assert cfg["codex"]["dir"] == ".codex/prompts"
+
     def test_release_agent_lists_include_kiro_cli_and_exclude_q(self):
         """Bash and PowerShell release scripts should agree on agent key set for Kiro."""
         sh_text = (REPO_ROOT / ".github" / "workflows" / "scripts" / "create-release-packages.sh").read_text(encoding="utf-8")
diff --git a/tests/test_extensions.py b/tests/test_extensions.py
index 9ef9cb73..ba52d034 100644
--- a/tests/test_extensions.py
+++ b/tests/test_extensions.py
@@ -407,6 +407,11 @@ class TestCommandRegistrar:
         assert CommandRegistrar.AGENT_CONFIGS["kiro-cli"]["dir"] == ".kiro/prompts"
         assert "q" not in CommandRegistrar.AGENT_CONFIGS
 
+    def test_codex_agent_config_present(self):
+        """Codex should be mapped to .codex/prompts."""
+        assert "codex" in CommandRegistrar.AGENT_CONFIGS
+        assert CommandRegistrar.AGENT_CONFIGS["codex"]["dir"] == ".codex/prompts"
+
     def test_parse_frontmatter_valid(self):
         """Test parsing valid YAML frontmatter."""
         content = """---
@@ -1598,3 +1603,343 @@ class TestCatalogStack:
         assert len(results) == 1
         assert results[0]["_catalog_name"] == "org"
         assert results[0]["_install_allowed"] is True
+
+
+class TestExtensionIgnore:
+    """Test .extensionignore support during extension installation."""
+
+    def _make_extension(self, temp_dir, valid_manifest_data, extra_files=None, ignore_content=None):
+        """Helper to create an extension directory with optional extra files and .extensionignore."""
+        import yaml
+
+        ext_dir = temp_dir / "ignored-ext"
+        ext_dir.mkdir()
+
+        # Write manifest
+        with open(ext_dir / "extension.yml", "w") as f:
+            yaml.dump(valid_manifest_data, f)
+
+        # Create commands directory with a command file
+        commands_dir = ext_dir / "commands"
+        commands_dir.mkdir()
+        (commands_dir / "hello.md").write_text(
+            "---\ndescription: \"Test hello command\"\n---\n\n# Hello\n\n$ARGUMENTS\n"
+        )
+
+        # Create any extra files/dirs
+        if extra_files:
+            for rel_path, content in extra_files.items():
+                p = ext_dir / rel_path
+                p.parent.mkdir(parents=True, exist_ok=True)
+                if content is None:
+                    # Create directory
+                    p.mkdir(parents=True, exist_ok=True)
+                else:
+                    p.write_text(content)
+
+        # Write .extensionignore
+        if ignore_content is not None:
+            (ext_dir / ".extensionignore").write_text(ignore_content)
+
+        return ext_dir
+
+    def test_no_extensionignore(self, temp_dir, valid_manifest_data):
+        """Without .extensionignore, all files are copied."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={"README.md": "# Hello", "tests/test_foo.py": "pass"},
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "README.md").exists()
+        assert (dest / "tests" / "test_foo.py").exists()
+
+    def test_extensionignore_excludes_files(self, temp_dir, valid_manifest_data):
+        """Files matching .extensionignore patterns are excluded."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "README.md": "# Hello",
+                "tests/test_foo.py": "pass",
+                "tests/test_bar.py": "pass",
+                ".github/workflows/ci.yml": "on: push",
+            },
+            ignore_content="tests/\n.github/\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        # Included
+        assert (dest / "README.md").exists()
+        assert (dest / "extension.yml").exists()
+        assert (dest / "commands" / "hello.md").exists()
+        # Excluded
+        assert not (dest / "tests").exists()
+        assert not (dest / ".github").exists()
+
+    def test_extensionignore_glob_patterns(self, temp_dir, valid_manifest_data):
+        """Glob patterns like *.pyc are respected."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "README.md": "# Hello",
+                "helpers.pyc": b"\x00".decode("latin-1"),
+                "commands/cache.pyc": b"\x00".decode("latin-1"),
+            },
+            ignore_content="*.pyc\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "README.md").exists()
+        assert not (dest / "helpers.pyc").exists()
+        assert not (dest / "commands" / "cache.pyc").exists()
+
+    def test_extensionignore_comments_and_blanks(self, temp_dir, valid_manifest_data):
+        """Comments and blank lines in .extensionignore are ignored."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={"README.md": "# Hello", "notes.txt": "some notes"},
+            ignore_content="# This is a comment\n\nnotes.txt\n\n# Another comment\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "README.md").exists()
+        assert not (dest / "notes.txt").exists()
+
+    def test_extensionignore_itself_excluded(self, temp_dir, valid_manifest_data):
+        """.extensionignore is never copied to the destination."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            ignore_content="# nothing special here\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "extension.yml").exists()
+        assert not (dest / ".extensionignore").exists()
+
+    def test_extensionignore_relative_path_match(self, temp_dir, valid_manifest_data):
+        """Patterns matching relative paths work correctly."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "docs/guide.md": "# Guide",
+                "docs/internal/draft.md": "draft",
+                "README.md": "# Hello",
+            },
+            ignore_content="docs/internal/draft.md\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "docs" / "guide.md").exists()
+        assert not (dest / "docs" / "internal" / "draft.md").exists()
+
+    def test_extensionignore_dotdot_pattern_is_noop(self, temp_dir, valid_manifest_data):
+        """Patterns with '..' should not escape the extension root."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={"README.md": "# Hello"},
+            ignore_content="../sibling/\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        # Everything should still be copied — the '..' pattern matches nothing inside
+        assert (dest / "README.md").exists()
+        assert (dest / "extension.yml").exists()
+        assert (dest / "commands" / "hello.md").exists()
+
+    def test_extensionignore_absolute_path_pattern_is_noop(self, temp_dir, valid_manifest_data):
+        """Absolute path patterns should not match anything."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={"README.md": "# Hello", "passwd": "sensitive"},
+            ignore_content="/etc/passwd\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        # Nothing matches — /etc/passwd is anchored to root and there's no 'etc' dir
+        assert (dest / "README.md").exists()
+        assert (dest / "passwd").exists()
+
+    def test_extensionignore_empty_file(self, temp_dir, valid_manifest_data):
+        """An empty .extensionignore should exclude only itself."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={"README.md": "# Hello", "notes.txt": "notes"},
+            ignore_content="",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "README.md").exists()
+        assert (dest / "notes.txt").exists()
+        assert (dest / "extension.yml").exists()
+        # .extensionignore itself is still excluded
+        assert not (dest / ".extensionignore").exists()
+
+    def test_extensionignore_windows_backslash_patterns(self, temp_dir, valid_manifest_data):
+        """Backslash patterns (Windows-style) are normalised to forward slashes."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "docs/internal/draft.md": "draft",
+                "docs/guide.md": "# Guide",
+            },
+            ignore_content="docs\\internal\\draft.md\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert (dest / "docs" / "guide.md").exists()
+        assert not (dest / "docs" / "internal" / "draft.md").exists()
+
+    def test_extensionignore_star_does_not_cross_directories(self, temp_dir, valid_manifest_data):
+        """'*' should NOT match across directory boundaries (gitignore semantics)."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "docs/api.draft.md": "draft",
+                "docs/sub/api.draft.md": "nested draft",
+            },
+            ignore_content="docs/*.draft.md\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        # docs/*.draft.md should only match directly inside docs/, NOT subdirs
+        assert not (dest / "docs" / "api.draft.md").exists()
+        assert (dest / "docs" / "sub" / "api.draft.md").exists()
+
+    def test_extensionignore_doublestar_crosses_directories(self, temp_dir, valid_manifest_data):
+        """'**' should match across directory boundaries."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "docs/api.draft.md": "draft",
+                "docs/sub/api.draft.md": "nested draft",
+                "docs/guide.md": "guide",
+            },
+            ignore_content="docs/**/*.draft.md\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        assert not (dest / "docs" / "api.draft.md").exists()
+        assert not (dest / "docs" / "sub" / "api.draft.md").exists()
+        assert (dest / "docs" / "guide.md").exists()
+
+    def test_extensionignore_negation_pattern(self, temp_dir, valid_manifest_data):
+        """'!' negation re-includes a previously excluded file."""
+        ext_dir = self._make_extension(
+            temp_dir,
+            valid_manifest_data,
+            extra_files={
+                "docs/guide.md": "# Guide",
+                "docs/internal.md": "internal",
+                "docs/api.md": "api",
+            },
+            ignore_content="docs/*.md\n!docs/api.md\n",
+        )
+
+        proj_dir = temp_dir / "project"
+        proj_dir.mkdir()
+        (proj_dir / ".specify").mkdir()
+
+        manager = ExtensionManager(proj_dir)
+        manager.install_from_directory(ext_dir, "0.1.0", register_commands=False)
+
+        dest = proj_dir / ".specify" / "extensions" / "test-ext"
+        # docs/*.md excludes all .md in docs, but !docs/api.md re-includes it
+        assert not (dest / "docs" / "guide.md").exists()
+        assert not (dest / "docs" / "internal.md").exists()
+        assert (dest / "docs" / "api.md").exists()