perf: remove state injection when instruction has no placeholders

inject_session_state runs an async regex substitution over the
instruction on every LLM call to fill {state} and {artifact.x}
placeholders. The pattern ({+[^{}]*}+) requires a '{', so a template
that contains no '{' can never match and is returned unchanged.

Static instructions with no placeholders are the common case, so
short-circuit before the scan and skip the regex pass and the
per-call async substitution setup. Behavior is unchanged.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 940535088

Author: GWeale

src/google/adk/utils/instructions_utils.py

@@ -65,6 +65,12 @@ async def build_instruction(
     The instruction template with values populated.
   """
 
+  # The substitution pattern requires a '{', so a template without one can
+  # never match. Return it as-is to avoid the regex scan on every LLM call,
+  # which is the common case for static instructions.
+  if '{' not in template:
+    return template
+
   invocation_context = readonly_context._invocation_context
 
   async def _async_sub(pattern, repl_async_fn, string) -> str:

tests/unittests/utils/test_instructions_utils.py

@@ -72,6 +72,19 @@ async def test_inject_session_state():
   assert populated_instruction == "Hello Foo, you are in active state."
 
 
+@pytest.mark.asyncio
+async def test_inject_session_state_without_placeholders_returns_template():
+  instruction_template = "A static instruction with no placeholders."
+  invocation_context = await _create_test_readonly_context(
+      state={"user_name": "Foo"}
+  )
+
+  populated_instruction = await instructions_utils.inject_session_state(
+      instruction_template, invocation_context
+  )
+  assert populated_instruction == instruction_template
+
+
 @pytest.mark.asyncio
 async def test_inject_session_state_with_artifact():
   instruction_template = "The artifact content is: {artifact.my_file}"