diff --git a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
index 833cba4c..3e86a4a1 100644
--- a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
+++ b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
@@ -241,10 +241,16 @@ def _build_command(self) -> list[str]:
         if self._options.tools is not None:
             tools = self._options.tools
             if isinstance(tools, list):
-                if len(tools) == 0:
+                # When skills are enabled, ensure "Skill" is in the tools list
+                # so the skill tool is available for the agent to invoke
+                tools_list = list(tools)  # Copy to avoid mutating the original
+                if self._options.skills is not None and "Skill" not in tools_list:
+                    tools_list.append("Skill")
+
+                if len(tools_list) == 0:
                     cmd.extend(["--tools", ""])
                 else:
-                    cmd.extend(["--tools", ",".join(tools)])
+                    cmd.extend(["--tools", ",".join(tools_list)])
             else:
                 # Preset object - 'claude_code' preset maps to 'default'
                 cmd.extend(["--tools", "default"])
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 1e61e9ad..def729e9 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -627,6 +627,57 @@ def test_build_command_skills_does_not_duplicate_entries(self):
         cmd = transport._build_command()
         assert cmd[cmd.index("--allowedTools") + 1] == "Skill(pdf)"
 
+    def test_build_command_skills_all_with_explicit_tools_adds_skill(self):
+        """When skills='all' is set with explicit tools list, 'Skill' is added to tools."""
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(
+                tools=["WebSearch", "WebFetch", "Read"],
+                skills="all",
+            ),
+        )
+        cmd = transport._build_command()
+        # Skill should be added to the tools list
+        assert "--tools" in cmd
+        tools_idx = cmd.index("--tools")
+        assert cmd[tools_idx + 1] == "WebSearch,WebFetch,Read,Skill"
+        # Skill should also be in allowed_tools
+        assert "--allowedTools" in cmd
+        assert cmd[cmd.index("--allowedTools") + 1] == "Skill"
+
+    def test_build_command_skills_list_with_explicit_tools_adds_skill(self):
+        """When skills list is set with explicit tools list, 'Skill' is added to tools."""
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(
+                tools=["WebSearch", "WebFetch", "Read"],
+                skills=["pdf", "docx"],
+            ),
+        )
+        cmd = transport._build_command()
+        # Skill should be added to the tools list
+        assert "--tools" in cmd
+        tools_idx = cmd.index("--tools")
+        assert cmd[tools_idx + 1] == "WebSearch,WebFetch,Read,Skill"
+        # Skill patterns should be in allowed_tools
+        assert "--allowedTools" in cmd
+        assert cmd[cmd.index("--allowedTools") + 1] == "Skill(pdf),Skill(docx)"
+
+    def test_build_command_skills_with_explicit_tools_already_has_skill(self):
+        """When skills='all' is set and 'Skill' already in tools list, it's not duplicated."""
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(
+                tools=["WebSearch", "Skill", "Read"],
+                skills="all",
+            ),
+        )
+        cmd = transport._build_command()
+        # Skill should not be duplicated
+        assert "--tools" in cmd
+        tools_idx = cmd.index("--tools")
+        assert cmd[tools_idx + 1] == "WebSearch,Skill,Read"
+
     @pytest.mark.parametrize(
         ("skills", "extra", "want_tools", "want_sources", "want_init_skills"),
         [