feat(skills): Allow opt-in session state injection in skill instructions

Agent instructions support injecting dynamic session state via
`{var_name}`, `{artifact.file_name}`, and `{optional?}` templates, but
skill instructions returned by `load_skill` were emitted verbatim, so a
skill body could not reference per-session state.

Add an opt-in frontmatter flag `adk_inject_session_state` (mirroring the
existing `adk_additional_tools` metadata convention). When set to true,
`LoadSkillTool` routes the skill instructions through the existing
`inject_session_state` utility before returning them. The flag defaults
to disabled so that literal braces in skill markdown (code samples,
JSON, templates) are preserved and existing skills are unaffected.

A missing required variable surfaces a `STATE_INJECTION_ERROR` tool
result rather than raising, consistent with the toolset's other error
responses.

Author: lwangverizon

src/google/adk/skills/models.py

@@ -50,7 +50,11 @@ class Frontmatter(BaseModel):
         https://agentskills.io/specification#allowed-tools-field.
       metadata: Key-value pairs for client-specific properties (defaults to
         empty dict). For example, to include additional tools, use the
-        ``adk_additional_tools`` key with a list of tools.
+        ``adk_additional_tools`` key with a list of tools. To enable session
+        state injection (e.g. ``{var_name}`` or ``{artifact.file_name}``) into
+        the skill instructions when the skill is loaded, set the
+        ``adk_inject_session_state`` key to ``True`` (defaults to disabled so
+        that literal braces in skill content are preserved).
   """
 
   model_config = ConfigDict(
@@ -76,6 +80,9 @@ def _validate_metadata(cls, v: dict[str, Any]) -> dict[str, Any]:
       tools = v["adk_additional_tools"]
       if not isinstance(tools, list):
         raise ValueError("adk_additional_tools must be a list of strings")
+    if "adk_inject_session_state" in v:
+      if not isinstance(v["adk_inject_session_state"], bool):
+        raise ValueError("adk_inject_session_state must be a boolean")
     return v
 
   @field_validator("name")

src/google/adk/tools/skill_toolset.py

@@ -36,6 +36,7 @@
 from ..skills import models
 from ..skills import prompt
 from ..skills import SkillRegistry
+from ..utils.instructions_utils import inject_session_state
 from .base_tool import BaseTool
 from .base_toolset import BaseToolset
 from .base_toolset import ToolPredicate
@@ -248,9 +249,25 @@ async def run_async(
       activated_skills.append(skill_name)
       tool_context.state[state_key] = activated_skills
 
+    # Optionally inject session state (e.g. {var_name}, {artifact.file_name})
+    # into the instructions. Disabled by default so that literal braces in
+    # skill content are preserved unless the skill opts in via frontmatter.
+    instructions = skill.instructions
+    if skill.frontmatter.metadata.get("adk_inject_session_state"):
+      try:
+        instructions = await inject_session_state(instructions, tool_context)
+      except (KeyError, ValueError) as e:
+        return {
+            "error": (
+                f"Failed to inject session state into skill '{skill_name}'"
+                f" instructions: {e}"
+            ),
+            "error_code": "STATE_INJECTION_ERROR",
+        }
+
     return {
         "skill_name": skill_name,
-        "instructions": skill.instructions,
+        "instructions": instructions,
         "frontmatter": skill.frontmatter.model_dump(),
     }
 

tests/unittests/skills/test_models.py

@@ -232,3 +232,23 @@ def test_metadata_adk_additional_tools_invalid_type():
         "description": "desc",
         "metadata": {"adk_additional_tools": 123},
     })
+
+
+def test_metadata_adk_inject_session_state_bool():
+  fm = models.Frontmatter.model_validate({
+      "name": "my-skill",
+      "description": "desc",
+      "metadata": {"adk_inject_session_state": True},
+  })
+  assert fm.metadata["adk_inject_session_state"] is True
+
+
+def test_metadata_adk_inject_session_state_invalid_type():
+  with pytest.raises(
+      ValidationError, match="adk_inject_session_state must be a boolean"
+  ):
+    models.Frontmatter.model_validate({
+        "name": "my-skill",
+        "description": "desc",
+        "metadata": {"adk_inject_session_state": "yes"},
+    })

tests/unittests/tools/test_skill_toolset.py

@@ -38,6 +38,7 @@ def _mock_skill1_frontmatter():
   frontmatter.name = "skill1"
   frontmatter.description = "Skill 1 description"
   frontmatter.allowed_tools = ["test_tool"]
+  frontmatter.metadata = {}
   frontmatter.model_dump.return_value = {
       "name": "skill1",
       "description": "Skill 1 description",
@@ -107,6 +108,7 @@ def _mock_skill2_frontmatter():
   frontmatter.name = "skill2"
   frontmatter.description = "Skill 2 description"
   frontmatter.allowed_tools = []
+  frontmatter.metadata = {}
   frontmatter.model_dump.return_value = {
       "name": "skill2",
       "description": "Skill 2 description",
@@ -276,6 +278,82 @@ async def test_load_skill_run_async_state_none(
   )
 
 
+@pytest.mark.asyncio
+async def test_load_skill_no_injection_by_default(
+    mock_skill1, tool_context_instance
+):
+  """Without the opt-in flag, braces in instructions are preserved verbatim."""
+  mock_skill1.instructions = "Greet {user_name} warmly."
+  tool_context_instance._invocation_context.session.state = {
+      "user_name": "Alice"
+  }
+  toolset = skill_toolset.SkillToolset([mock_skill1])
+  tool = skill_toolset.LoadSkillTool(toolset)
+
+  result = await tool.run_async(
+      args={"skill_name": "skill1"}, tool_context=tool_context_instance
+  )
+
+  assert result["instructions"] == "Greet {user_name} warmly."
+
+
+@pytest.mark.asyncio
+async def test_load_skill_injects_session_state_when_enabled(
+    mock_skill1, tool_context_instance
+):
+  """With the opt-in flag set, state variables are substituted."""
+  mock_skill1.instructions = "Greet {user_name} warmly."
+  mock_skill1.frontmatter.metadata = {"adk_inject_session_state": True}
+  tool_context_instance._invocation_context.session.state = {
+      "user_name": "Alice"
+  }
+  toolset = skill_toolset.SkillToolset([mock_skill1])
+  tool = skill_toolset.LoadSkillTool(toolset)
+
+  result = await tool.run_async(
+      args={"skill_name": "skill1"}, tool_context=tool_context_instance
+  )
+
+  assert result["instructions"] == "Greet Alice warmly."
+
+
+@pytest.mark.asyncio
+async def test_load_skill_injection_optional_var_empty(
+    mock_skill1, tool_context_instance
+):
+  """An optional ({var?}) missing variable is replaced with empty string."""
+  mock_skill1.instructions = "Hello{nickname?}."
+  mock_skill1.frontmatter.metadata = {"adk_inject_session_state": True}
+  tool_context_instance._invocation_context.session.state = {}
+  toolset = skill_toolset.SkillToolset([mock_skill1])
+  tool = skill_toolset.LoadSkillTool(toolset)
+
+  result = await tool.run_async(
+      args={"skill_name": "skill1"}, tool_context=tool_context_instance
+  )
+
+  assert result["instructions"] == "Hello."
+
+
+@pytest.mark.asyncio
+async def test_load_skill_injection_missing_required_var_returns_error(
+    mock_skill1, tool_context_instance
+):
+  """A missing required variable returns a STATE_INJECTION_ERROR, not a crash."""
+  mock_skill1.instructions = "Greet {user_name} warmly."
+  mock_skill1.frontmatter.metadata = {"adk_inject_session_state": True}
+  tool_context_instance._invocation_context.session.state = {}
+  toolset = skill_toolset.SkillToolset([mock_skill1])
+  tool = skill_toolset.LoadSkillTool(toolset)
+
+  result = await tool.run_async(
+      args={"skill_name": "skill1"}, tool_context=tool_context_instance
+  )
+
+  assert result["error_code"] == "STATE_INJECTION_ERROR"
+  assert "skill1" in result["error"]
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "args, expected_result",