fix: Fix regression in instructions_utils placeholder matching

Co-authored-by: Bo Yang <ybo@google.com>
PiperOrigin-RevId: 934602575

Author: boyangsvl

src/google/adk/utils/instructions_utils.py

@@ -181,24 +181,29 @@ async def _replace_match(match) -> str:
 
     if leading_count == trailing_count:
       n = leading_count
+      is_valid = _is_valid_path(full_path) or full_path.startswith('artifact.')
       if n % 2 == 0:
         # Even: Escaped, no evaluation
-        half_n = n // 2
-        return '{' * half_n + full_path + '}' * half_n
+        if is_valid:
+          half_n = n // 2
+          return '{' * half_n + full_path + '}' * half_n
+        else:
+          return raw_match
       else:
         # Odd: Evaluate and wrap
-        if not _is_valid_path(full_path) and not full_path.startswith('artifact.'):
+        if not is_valid:
           return raw_match
         evaluated_value = await _evaluate_path(full_path)
         wrap_braces = (n - 1) // 2
         return '{' * wrap_braces + evaluated_value + '}' * wrap_braces
     else:
       # Asymmetric: fallback to old behavior (treat as N=1 if valid path)
-      if not _is_valid_path(full_path) and not full_path.startswith('artifact.'):
+      if not _is_valid_path(full_path) and not full_path.startswith(
+          'artifact.'
+      ):
         return raw_match
       return await _evaluate_path(full_path)
 
-
   return await _async_sub(r'{+[^{}]*}+', _replace_match, template)
 
 

tests/unittests/utils/test_instructions_utils.py

@@ -516,14 +516,19 @@ async def test_inject_session_state_with_invalid_nested_path_returns_original():
 
 @pytest.mark.asyncio
 async def test_inject_session_state_escaped_braces():
-  instruction_template = "This is a literal {{placeholder}} and this is {value}."
+  instruction_template = (
+      "This is a literal {{placeholder}} and this is {value}."
+  )
   invocation_context = await _create_test_readonly_context(
       state={"value": "real_value"}
   )
   populated_instruction = await instructions_utils.inject_session_state(
       instruction_template, invocation_context
   )
-  assert populated_instruction == "This is a literal {placeholder} and this is real_value."
+  assert (
+      populated_instruction
+      == "This is a literal {placeholder} and this is real_value."
+  )
 
 
 @pytest.mark.asyncio
@@ -556,7 +561,9 @@ async def test_inject_session_state_quadruple_braces():
   populated_instruction = await instructions_utils.inject_session_state(
       instruction_template, invocation_context
   )
-  assert populated_instruction == "This is literal double braces: {{placeholder}}."
+  assert (
+      populated_instruction == "This is literal double braces: {{placeholder}}."
+  )
 
 
 @pytest.mark.asyncio
@@ -581,4 +588,3 @@ async def test_inject_session_state_asymmetric_braces_right():
       instruction_template, invocation_context
   )
   assert populated_instruction == "Asymmetric right: real_value."
-