diff --git a/src/google/adk/agents/llm_agent.py b/src/google/adk/agents/llm_agent.py
index b063233f..68a94b58 100644
--- a/src/google/adk/agents/llm_agent.py
+++ b/src/google/adk/agents/llm_agent.py
@@ -134,7 +134,18 @@ class LlmAgent(BaseAgent):
   """The config type for this agent."""
 
   instruction: Union[str, InstructionProvider] = ''
-  """Instructions for the LLM model, guiding the agent's behavior."""
+  """Dynamic instructions for the LLM model, guiding the agent's behavior.
+
+  These instructions can contain placeholders like {variable_name} that will be
+  resolved at runtime using session state and context.
+
+  **Behavior depends on static_instruction:**
+  - If static_instruction is None: instruction goes to system_instruction
+  - If static_instruction is set: instruction goes to user content in the request
+
+  This allows for context caching optimization where static content (static_instruction)
+  comes first in the prompt, followed by dynamic content (instruction).
+  """
 
   global_instruction: Union[str, InstructionProvider] = ''
   """Instructions for all the agents in the entire agent tree.
@@ -145,6 +156,48 @@ class LlmAgent(BaseAgent):
   or personality.
   """
 
+  static_instruction: Optional[types.Content] = None
+  """Static instruction content sent literally as system instruction at the beginning.
+
+  This field is for content that never changes and doesn't contain placeholders.
+  It's sent directly to the model without any processing or variable substitution.
+
+  This field is primarily for context caching optimization. Static instructions
+  are sent as system instruction at the beginning of the request, allowing
+  for improved performance when the static portion remains unchanged. Live API
+  has its own cache mechanism, thus this field doesn't work with Live API.
+
+  **Impact on instruction field:**
+  - When static_instruction is None: instruction → system_instruction
+  - When static_instruction is set: instruction → user content (after static content)
+
+  **Context Caching:**
+  - **Implicit Cache**: Automatic caching by model providers (no config needed)
+  - **Explicit Cache**: Cache explicitly created by user for instructions, tools and contents
+
+  See below for more information of Implicit Cache and Explicit Cache
+  Gemini API: https://ai.google.dev/gemini-api/docs/caching?lang=python
+  Vertex API: https://cloud.google.com/vertex-ai/generative-ai/docs/context-cache/context-cache-overview
+
+  Setting static_instruction alone does NOT enable caching automatically.
+  For explicit caching control, configure context_cache_config at App level.
+
+  **Content Support:**
+  Can contain text, files, binaries, or any combination as types.Content
+  supports multiple part types (text, inline_data, file_data, etc.).
+
+  **Example:**
+  ```python
+  static_instruction = types.Content(
+      role='user',
+      parts=[
+          types.Part(text='You are a helpful assistant.'),
+          types.Part(file_data=types.FileData(...))
+      ]
+  )
+  ```
+  """
+
   tools: list[ToolUnion] = Field(default_factory=list)
   """Tools available to this agent."""
 
@@ -462,9 +515,7 @@ class LlmAgent(BaseAgent):
     ):
 
       result = ''.join(
-          part.text
-          for part in event.content.parts
-          if part.text and not part.thought
+          [part.text if part.text else '' for part in event.content.parts]
       )
       if self.output_schema:
         # If the result from the final chunk is just whitespace or empty,
@@ -600,6 +651,8 @@ class LlmAgent(BaseAgent):
       kwargs['model'] = config.model
     if config.instruction:
       kwargs['instruction'] = config.instruction
+    if config.static_instruction:
+      kwargs['static_instruction'] = config.static_instruction
     if config.disallow_transfer_to_parent:
       kwargs['disallow_transfer_to_parent'] = config.disallow_transfer_to_parent
     if config.disallow_transfer_to_peers:
diff --git a/src/google/adk/agents/llm_agent_config.py b/src/google/adk/agents/llm_agent_config.py
index 1aa935d9..4cb5dc2c 100644
--- a/src/google/adk/agents/llm_agent_config.py
+++ b/src/google/adk/agents/llm_agent_config.py
@@ -53,7 +53,25 @@ class LlmAgentConfig(BaseAgentConfig):
       ),
   )
 
-  instruction: str = Field(description='Required. LlmAgent.instruction.')
+  instruction: str = Field(
+      description=(
+          'Required. LlmAgent.instruction. Dynamic instructions with'
+          ' placeholder support. Behavior: if static_instruction is None, goes'
+          ' to system_instruction; if static_instruction is set, goes to user'
+          ' content after static content.'
+      )
+  )
+
+  static_instruction: Optional[types.Content] = Field(
+      default=None,
+      description=(
+          'Optional. LlmAgent.static_instruction. Static content sent literally'
+          ' at position 0 without placeholder processing. When set, changes'
+          ' instruction behavior to go to user content instead of'
+          ' system_instruction. Supports context caching and rich content'
+          ' (text, files, binaries).'
+      ),
+  )
 
   disallow_transfer_to_parent: Optional[bool] = Field(
       default=None,
diff --git a/src/google/adk/flows/llm_flows/contents.py b/src/google/adk/flows/llm_flows/contents.py
index 93d9a332..da52e0c6 100644
--- a/src/google/adk/flows/llm_flows/contents.py
+++ b/src/google/adk/flows/llm_flows/contents.py
@@ -58,6 +58,11 @@ class _ContentLlmRequestProcessor(BaseLlmRequestProcessor):
           agent.name,
       )
 
+    # Add dynamic instructions to the last user content if static instructions exist
+    await _add_dynamic_instructions_to_user_content(
+        invocation_context, llm_request
+    )
+
     # Maintain async generator behavior
     if False:  # Ensures it behaves as a generator
       yield  # This is a no-op but maintains generator structure
@@ -557,3 +562,49 @@ def _is_live_model_audio_event(event: Event) -> bool:
     if part.file_data and part.file_data.mime_type == 'audio/pcm':
       return True
   return False
+
+
+async def _add_dynamic_instructions_to_user_content(
+    invocation_context: InvocationContext, llm_request: LlmRequest
+) -> None:
+  """Add dynamic instructions to the last user content when static instructions exist."""
+  from ...agents.readonly_context import ReadonlyContext
+  from ...utils import instructions_utils
+
+  agent = invocation_context.agent
+
+  dynamic_instructions = []
+
+  # Handle agent dynamic instructions if static instruction exists
+  if agent.static_instruction and agent.instruction:
+    # Static instruction exists, so add dynamic instruction to content
+    raw_si, bypass_state_injection = await agent.canonical_instruction(
+        ReadonlyContext(invocation_context)
+    )
+    si = raw_si
+    if not bypass_state_injection:
+      si = await instructions_utils.inject_session_state(
+          raw_si, ReadonlyContext(invocation_context)
+      )
+    if si:  # Only add if not empty
+      dynamic_instructions.append(si)
+
+  if not dynamic_instructions:
+    return
+
+  # Find the start of the last continuous batch of user content
+  # Walk backwards to find the first non-user content, then insert before next user content
+  insert_index = len(llm_request.contents)
+  for i in range(len(llm_request.contents) - 1, -1, -1):
+    if llm_request.contents[i].role != 'user':
+      insert_index = i + 1
+      break
+    elif i == 0:
+      # All content from start is user content
+      insert_index = 0
+      break
+
+  # Create new user content with dynamic instructions
+  instruction_parts = [types.Part(text=instr) for instr in dynamic_instructions]
+  new_content = types.Content(role='user', parts=instruction_parts)
+  llm_request.contents.insert(insert_index, new_content)
diff --git a/src/google/adk/flows/llm_flows/instructions.py b/src/google/adk/flows/llm_flows/instructions.py
index 77a1afe2..bf71fa8e 100644
--- a/src/google/adk/flows/llm_flows/instructions.py
+++ b/src/google/adk/flows/llm_flows/instructions.py
@@ -16,16 +16,13 @@
 
 from __future__ import annotations
 
-import re
 from typing import AsyncGenerator
-from typing import Generator
 from typing import TYPE_CHECKING
 
 from typing_extensions import override
 
 from ...agents.readonly_context import ReadonlyContext
 from ...events.event import Event
-from ...sessions.state import State
 from ...utils import instructions_utils
 from ._base_llm_processor import BaseLlmRequestProcessor
 
@@ -50,10 +47,8 @@ class _InstructionsLlmRequestProcessor(BaseLlmRequestProcessor):
 
     root_agent: BaseAgent = agent.root_agent
 
-    # Appends global instructions if set.
-    if (
-        isinstance(root_agent, LlmAgent) and root_agent.global_instruction
-    ):  # not empty str
+    # Handle global instructions
+    if isinstance(root_agent, LlmAgent) and root_agent.global_instruction:
       raw_si, bypass_state_injection = (
           await root_agent.canonical_global_instruction(
               ReadonlyContext(invocation_context)
@@ -66,8 +61,14 @@ class _InstructionsLlmRequestProcessor(BaseLlmRequestProcessor):
         )
       llm_request.append_instructions([si])
 
-    # Appends agent instructions if set.
-    if agent.instruction:  # not empty str
+    # Handle static_instruction - add via append_instructions
+    if agent.static_instruction:
+      llm_request.append_instructions(agent.static_instruction)
+
+    # Handle instruction based on whether static_instruction exists
+    if agent.instruction and not agent.static_instruction:
+      # Only add to system instructions if no static instruction exists
+      # If static instruction exists, content processor will handle it
       raw_si, bypass_state_injection = await agent.canonical_instruction(
           ReadonlyContext(invocation_context)
       )
@@ -79,8 +80,8 @@ class _InstructionsLlmRequestProcessor(BaseLlmRequestProcessor):
       llm_request.append_instructions([si])
 
     # Maintain async generator behavior
-    if False:  # Ensures it behaves as a generator
-      yield  # This is a no-op but maintains generator structure
+    return
+    yield  # This line ensures it behaves as a generator but is never reached
 
 
 request_processor = _InstructionsLlmRequestProcessor()
diff --git a/src/google/adk/models/llm_request.py b/src/google/adk/models/llm_request.py
index beb2decf..aa898ef7 100644
--- a/src/google/adk/models/llm_request.py
+++ b/src/google/adk/models/llm_request.py
@@ -14,7 +14,9 @@
 
 from __future__ import annotations
 
+import logging
 from typing import Optional
+from typing import Union
 
 from google.genai import types
 from pydantic import BaseModel
@@ -86,17 +88,73 @@ class LlmRequest(BaseModel):
   cache_metadata: Optional[CacheMetadata] = None
   """Cache metadata from previous requests, used for cache management."""
 
-  def append_instructions(self, instructions: list[str]) -> None:
+  def append_instructions(
+      self, instructions: Union[list[str], types.Content]
+  ) -> None:
     """Appends instructions to the system instruction.
 
     Args:
-      instructions: The instructions to append.
+      instructions: The instructions to append. Can be:
+        - list[str]: Strings to append/concatenate to system instruction
+        - types.Content: Content object to append to system instruction
+
+    Note: Only text content is supported. Model API requires system_instruction
+    to be a string. Non-text parts in Content will be handled differently.
+
+    Behavior:
+      - list[str]: concatenates with existing system_instruction using \\n\\n
+      - types.Content: extracts text from parts and concatenates
     """
 
-    if self.config.system_instruction:
-      self.config.system_instruction += '\n\n' + '\n\n'.join(instructions)
-    else:
-      self.config.system_instruction = '\n\n'.join(instructions)
+    # Handle Content object - extract only text parts
+    if isinstance(instructions, types.Content):
+      # TODO: Handle non-text contents in instruction by putting non-text parts
+      # into llm_request.contents and adding a reference in the system instruction
+      # that references the contents.
+
+      # Extract text from all text parts
+      text_parts = [part.text for part in instructions.parts if part.text]
+
+      if not text_parts:
+        return  # No text content to append
+
+      new_text = "\n\n".join(text_parts)
+      if not self.config.system_instruction:
+        self.config.system_instruction = new_text
+      elif isinstance(self.config.system_instruction, str):
+        self.config.system_instruction += "\n\n" + new_text
+      else:
+        # Log warning for unsupported system_instruction types
+        logging.warning(
+            "Cannot append to system_instruction of unsupported type: %s. "
+            "Only string system_instruction is supported.",
+            type(self.config.system_instruction),
+        )
+      return
+
+    # Handle list of strings
+    if isinstance(instructions, list) and all(
+        isinstance(inst, str) for inst in instructions
+    ):
+      if not instructions:  # Handle empty list
+        return
+
+      new_text = "\n\n".join(instructions)
+      if not self.config.system_instruction:
+        self.config.system_instruction = new_text
+      elif isinstance(self.config.system_instruction, str):
+        self.config.system_instruction += "\n\n" + new_text
+      else:
+        # Log warning for unsupported system_instruction types
+        logging.warning(
+            "Cannot append to system_instruction of unsupported type: %s. "
+            "Only string system_instruction is supported.",
+            type(self.config.system_instruction),
+        )
+      return
+
+    # Invalid input
+    raise TypeError("instructions must be list[str] or types.Content")
 
   def append_tools(self, tools: list[BaseTool]) -> None:
     """Appends tools to the request.
@@ -138,4 +196,4 @@ class LlmRequest(BaseModel):
     """
 
     self.config.response_schema = base_model
-    self.config.response_mime_type = 'application/json'
+    self.config.response_mime_type = "application/json"
diff --git a/tests/unittests/agents/test_static_instructions.py b/tests/unittests/agents/test_static_instructions.py
new file mode 100644
index 00000000..912a062b
--- /dev/null
+++ b/tests/unittests/agents/test_static_instructions.py
@@ -0,0 +1,272 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for static instruction functionality."""
+
+from google.adk.agents.invocation_context import InvocationContext
+from google.adk.agents.llm_agent import LlmAgent
+from google.adk.agents.run_config import RunConfig
+from google.adk.flows.llm_flows.contents import _add_dynamic_instructions_to_user_content
+from google.adk.flows.llm_flows.instructions import request_processor
+from google.adk.models.llm_request import LlmRequest
+from google.adk.sessions.in_memory_session_service import InMemorySessionService
+from google.genai import types
+import pytest
+
+
+async def _create_invocation_context(agent: LlmAgent) -> InvocationContext:
+  """Helper to create InvocationContext with session."""
+  session_service = InMemorySessionService()
+  session = await session_service.create_session(
+      app_name='test_app', user_id='test_user'
+  )
+  return InvocationContext(
+      invocation_id='test_invocation_id',
+      agent=agent,
+      session=session,
+      session_service=session_service,
+      run_config=RunConfig(),
+      branch='main',
+  )
+
+
+@pytest.mark.parametrize('llm_backend', ['GOOGLE_AI', 'VERTEX'])
+class TestStaticInstructions:
+
+  def test_static_instruction_field_exists(self, llm_backend):
+    """Test that static_instruction field exists and works with types.Content."""
+    static_content = types.Content(
+        role='user', parts=[types.Part(text='This is a static instruction')]
+    )
+    agent = LlmAgent(name='test_agent', static_instruction=static_content)
+    assert agent.static_instruction == static_content
+
+  def test_static_instruction_supports_multiple_parts(self, llm_backend):
+    """Test that static_instruction supports multiple parts including files."""
+    static_content = types.Content(
+        role='user',
+        parts=[
+            types.Part(text='Here is the document:'),
+            types.Part(
+                inline_data=types.Blob(
+                    data=b'fake_file_content', mime_type='text/plain'
+                )
+            ),
+            types.Part(text='Please analyze this document.'),
+        ],
+    )
+    agent = LlmAgent(name='test_agent', static_instruction=static_content)
+    assert agent.static_instruction == static_content
+    assert len(agent.static_instruction.parts) == 3
+
+  def test_static_instruction_outputs_placeholders_literally(self, llm_backend):
+    """Test that static instructions output placeholders literally without processing."""
+    static_content = types.Content(
+        role='user',
+        parts=[
+            types.Part(text='Hello {name}, you have {count} messages'),
+        ],
+    )
+    agent = LlmAgent(name='test_agent', static_instruction=static_content)
+    assert '{name}' in agent.static_instruction.parts[0].text
+    assert '{count}' in agent.static_instruction.parts[0].text
+
+  @pytest.mark.asyncio
+  async def test_static_instruction_added_to_contents(self, llm_backend):
+    """Test that static instructions are added to llm_request.config.system_instruction."""
+    static_content = types.Content(
+        role='user', parts=[types.Part(text='Static instruction content')]
+    )
+    agent = LlmAgent(name='test_agent', static_instruction=static_content)
+
+    invocation_context = await _create_invocation_context(agent)
+
+    llm_request = LlmRequest()
+
+    # Run the instruction processor
+    async for _ in request_processor.run_async(invocation_context, llm_request):
+      pass
+
+    # Static instruction should be added to system instructions, not contents
+    assert len(llm_request.contents) == 0
+    assert llm_request.config.system_instruction == 'Static instruction content'
+
+  @pytest.mark.asyncio
+  async def test_dynamic_instruction_without_static_goes_to_system(
+      self, llm_backend
+  ):
+    """Test that dynamic instructions go to system when no static instruction exists."""
+    agent = LlmAgent(
+        name='test_agent', instruction='Dynamic instruction content'
+    )
+
+    invocation_context = await _create_invocation_context(agent)
+
+    llm_request = LlmRequest()
+
+    # Run the instruction processor
+    async for _ in request_processor.run_async(invocation_context, llm_request):
+      pass
+
+    # Dynamic instruction should be added to system instructions
+    assert (
+        llm_request.config.system_instruction == 'Dynamic instruction content'
+    )
+    assert len(llm_request.contents) == 0
+
+  @pytest.mark.asyncio
+  async def test_dynamic_instruction_with_static_not_in_system(
+      self, llm_backend
+  ):
+    """Test that dynamic instructions don't go to system when static instruction exists."""
+    static_content = types.Content(
+        role='user', parts=[types.Part(text='Static instruction content')]
+    )
+    agent = LlmAgent(
+        name='test_agent',
+        instruction='Dynamic instruction content',
+        static_instruction=static_content,
+    )
+
+    invocation_context = await _create_invocation_context(agent)
+
+    llm_request = LlmRequest()
+
+    # Run the instruction processor
+    async for _ in request_processor.run_async(invocation_context, llm_request):
+      pass
+
+    # Static instruction should be in system instructions
+    assert len(llm_request.contents) == 0
+    assert llm_request.config.system_instruction == 'Static instruction content'
+
+  @pytest.mark.asyncio
+  async def test_dynamic_instructions_added_to_user_content(self, llm_backend):
+    """Test that dynamic instructions are added to user content when static exists."""
+    static_content = types.Content(
+        role='user', parts=[types.Part(text='Static instruction')]
+    )
+    agent = LlmAgent(
+        name='test_agent',
+        instruction='Dynamic instruction',
+        static_instruction=static_content,
+    )
+
+    invocation_context = await _create_invocation_context(agent)
+
+    llm_request = LlmRequest()
+    # Add some existing user content
+    llm_request.contents = [
+        types.Content(role='user', parts=[types.Part(text='Hello world')])
+    ]
+
+    # Run the content processor function
+    await _add_dynamic_instructions_to_user_content(
+        invocation_context, llm_request
+    )
+
+    # Dynamic instruction should be inserted before the last continuous batch of user content
+    assert len(llm_request.contents) == 2
+    assert llm_request.contents[0].role == 'user'
+    assert len(llm_request.contents[0].parts) == 1
+    assert llm_request.contents[0].parts[0].text == 'Dynamic instruction'
+    assert llm_request.contents[1].role == 'user'
+    assert len(llm_request.contents[1].parts) == 1
+    assert llm_request.contents[1].parts[0].text == 'Hello world'
+
+  @pytest.mark.asyncio
+  async def test_dynamic_instructions_create_user_content_when_none_exists(
+      self, llm_backend
+  ):
+    """Test that dynamic instructions create user content when none exists."""
+    static_content = types.Content(
+        role='user', parts=[types.Part(text='Static instruction')]
+    )
+    agent = LlmAgent(
+        name='test_agent',
+        instruction='Dynamic instruction',
+        static_instruction=static_content,
+    )
+
+    invocation_context = await _create_invocation_context(agent)
+
+    llm_request = LlmRequest()
+    # No existing content
+
+    # Run the content processor function
+    await _add_dynamic_instructions_to_user_content(
+        invocation_context, llm_request
+    )
+
+    # Dynamic instruction should create new user content
+    assert len(llm_request.contents) == 1
+    assert llm_request.contents[0].role == 'user'
+    assert len(llm_request.contents[0].parts) == 1
+    assert llm_request.contents[0].parts[0].text == 'Dynamic instruction'
+
+  @pytest.mark.asyncio
+  async def test_no_dynamic_instructions_when_no_static(self, llm_backend):
+    """Test that no dynamic instructions are added to content when no static instructions exist."""
+    agent = LlmAgent(name='test_agent', instruction='Dynamic instruction only')
+
+    invocation_context = await _create_invocation_context(agent)
+
+    llm_request = LlmRequest()
+    # Add some existing user content
+    original_content = types.Content(
+        role='user', parts=[types.Part(text='Hello world')]
+    )
+    llm_request.contents = [original_content]
+
+    # Run the content processor function
+    await _add_dynamic_instructions_to_user_content(
+        invocation_context, llm_request
+    )
+
+    # Content should remain unchanged
+    assert len(llm_request.contents) == 1
+    assert llm_request.contents[0].role == 'user'
+    assert len(llm_request.contents[0].parts) == 1
+    assert llm_request.contents[0].parts[0].text == 'Hello world'
+
+  @pytest.mark.asyncio
+  async def test_static_instruction_with_files_and_text(self, llm_backend):
+    """Test that static instruction can contain files and text together."""
+    static_content = types.Content(
+        role='user',
+        parts=[
+            types.Part(text='Analyze this image:'),
+            types.Part(
+                inline_data=types.Blob(
+                    data=b'fake_image_data', mime_type='image/png'
+                )
+            ),
+            types.Part(text='Focus on the key elements.'),
+        ],
+    )
+    agent = LlmAgent(name='test_agent', static_instruction=static_content)
+
+    invocation_context = await _create_invocation_context(agent)
+    llm_request = LlmRequest()
+
+    # Run the instruction processor
+    async for _ in request_processor.run_async(invocation_context, llm_request):
+      pass
+
+    # Static instruction should extract only text parts and concatenate them
+    assert len(llm_request.contents) == 0
+    assert (
+        llm_request.config.system_instruction
+        == 'Analyze this image:\n\nFocus on the key elements.'
+    )
diff --git a/tests/unittests/models/test_llm_request.py b/tests/unittests/models/test_llm_request.py
index 78942296..b0722291 100644
--- a/tests/unittests/models/test_llm_request.py
+++ b/tests/unittests/models/test_llm_request.py
@@ -156,6 +156,194 @@ def test_append_tools_consolidates_declarations_in_single_tool():
   assert 'third_tool' in request.tools_dict
 
 
+def test_append_instructions_with_string_list():
+  """Test that append_instructions works with list of strings (existing behavior)."""
+  request = LlmRequest()
+
+  # Initially system_instruction should be None
+  assert request.config.system_instruction is None
+
+  # Append first set of instructions
+  request.append_instructions(['First instruction', 'Second instruction'])
+
+  # Should be joined with double newlines
+  expected = 'First instruction\n\nSecond instruction'
+  assert request.config.system_instruction == expected
+  assert len(request.contents) == 0
+
+
+def test_append_instructions_with_string_list_multiple_calls():
+  """Test multiple calls to append_instructions with string lists."""
+  request = LlmRequest()
+
+  # First call
+  request.append_instructions(['First instruction'])
+  assert request.config.system_instruction == 'First instruction'
+
+  # Second call should append with double newlines
+  request.append_instructions(['Second instruction', 'Third instruction'])
+  expected = 'First instruction\n\nSecond instruction\n\nThird instruction'
+  assert request.config.system_instruction == expected
+
+
+def test_append_instructions_with_content():
+  """Test that append_instructions works with types.Content (new behavior)."""
+  request = LlmRequest()
+
+  # Create a Content object
+  content = types.Content(
+      role='user', parts=[types.Part(text='This is content-based instruction')]
+  )
+
+  # Append content
+  request.append_instructions(content)
+
+  # Should be set as system_instruction
+  assert len(request.contents) == 0
+  assert request.config.system_instruction == content
+
+
+def test_append_instructions_with_content_multiple_calls():
+  """Test multiple calls to append_instructions with Content objects."""
+  request = LlmRequest()
+
+  # Add some existing content first
+  existing_content = types.Content(
+      role='user', parts=[types.Part(text='Existing content')]
+  )
+  request.contents.append(existing_content)
+
+  # First Content instruction
+  content1 = types.Content(
+      role='user', parts=[types.Part(text='First instruction')]
+  )
+  request.append_instructions(content1)
+
+  # Should be set as system_instruction, existing content unchanged
+  assert len(request.contents) == 1
+  assert request.contents[0] == existing_content
+  assert request.config.system_instruction == content1
+
+  # Second Content instruction
+  content2 = types.Content(
+      role='user', parts=[types.Part(text='Second instruction')]
+  )
+  request.append_instructions(content2)
+
+  # Second Content should be merged with first in system_instruction
+  assert len(request.contents) == 1
+  assert request.contents[0] == existing_content
+  assert isinstance(request.config.system_instruction, types.Content)
+  assert len(request.config.system_instruction.parts) == 2
+  assert request.config.system_instruction.parts[0].text == 'First instruction'
+  assert request.config.system_instruction.parts[1].text == 'Second instruction'
+
+
+def test_append_instructions_with_content_multipart():
+  """Test append_instructions with Content containing multiple parts."""
+  request = LlmRequest()
+
+  # Create Content with multiple parts (text and potentially files)
+  content = types.Content(
+      role='user',
+      parts=[
+          types.Part(text='Text instruction'),
+          types.Part(text='Additional text part'),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  assert len(request.contents) == 0
+  assert request.config.system_instruction == content
+  assert len(request.config.system_instruction.parts) == 2
+  assert request.config.system_instruction.parts[0].text == 'Text instruction'
+  assert (
+      request.config.system_instruction.parts[1].text == 'Additional text part'
+  )
+
+
+def test_append_instructions_mixed_string_and_content():
+  """Test mixing string list and Content instructions."""
+  request = LlmRequest()
+
+  # First add string instructions
+  request.append_instructions(['String instruction'])
+  assert request.config.system_instruction == 'String instruction'
+
+  # Then add Content instruction
+  content = types.Content(
+      role='user', parts=[types.Part(text='Content instruction')]
+  )
+  request.append_instructions(content)
+
+  # String and Content should be merged in system_instruction
+  assert len(request.contents) == 0
+  assert isinstance(request.config.system_instruction, types.Content)
+  assert len(request.config.system_instruction.parts) == 2
+  assert request.config.system_instruction.parts[0].text == 'String instruction'
+  assert (
+      request.config.system_instruction.parts[1].text == 'Content instruction'
+  )
+
+
+def test_append_instructions_empty_string_list():
+  """Test append_instructions with empty list of strings."""
+  request = LlmRequest()
+
+  # Empty list should not modify anything
+  request.append_instructions([])
+
+  assert request.config.system_instruction is None
+  assert len(request.contents) == 0
+
+
+def test_append_instructions_invalid_input():
+  """Test append_instructions with invalid input types."""
+  request = LlmRequest()
+
+  # Test with invalid types
+  with pytest.raises(
+      TypeError, match='instructions must be list\\[str\\] or types.Content'
+  ):
+    request.append_instructions('single string')  # Should be list[str]
+
+  with pytest.raises(
+      TypeError, match='instructions must be list\\[str\\] or types.Content'
+  ):
+    request.append_instructions(123)  # Invalid type
+
+  with pytest.raises(
+      TypeError, match='instructions must be list\\[str\\] or types.Content'
+  ):
+    request.append_instructions(
+        ['valid string', 123]
+    )  # Mixed valid/invalid in list
+
+
+def test_append_instructions_content_preserves_role_and_parts():
+  """Test that Content objects have text extracted regardless of role or parts."""
+  request = LlmRequest()
+
+  # Create Content with specific role and parts
+  content = types.Content(
+      role='system',  # Different role
+      parts=[
+          types.Part(text='System instruction'),
+          types.Part(text='Additional system part'),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  # Text should be extracted and concatenated to system_instruction string
+  assert len(request.contents) == 0
+  assert (
+      request.config.system_instruction
+      == 'System instruction\n\nAdditional system part'
+  )
+
+
 async def _create_tool_context() -> ToolContext:
   """Helper to create a ToolContext for testing."""
   session_service = InMemorySessionService()
@@ -308,3 +496,232 @@ def test_multiple_append_tools_calls_consolidate():
   assert 'dummy_tool' in request.tools_dict
   assert 'another_tool' in request.tools_dict
   assert 'third_tool' in request.tools_dict
+
+
+# Updated tests for simplified string-only append_instructions behavior
+
+
+def test_append_instructions_with_content():
+  """Test that append_instructions extracts text from types.Content."""
+  request = LlmRequest()
+
+  # Create a Content object
+  content = types.Content(
+      role='user', parts=[types.Part(text='This is content-based instruction')]
+  )
+
+  # Append content
+  request.append_instructions(content)
+
+  # Should extract text and set as system_instruction string
+  assert len(request.contents) == 0
+  assert (
+      request.config.system_instruction == 'This is content-based instruction'
+  )
+
+
+def test_append_instructions_with_content_multiple_calls():
+  """Test multiple calls to append_instructions with Content objects."""
+  request = LlmRequest()
+
+  # Add some existing content first
+  existing_content = types.Content(
+      role='user', parts=[types.Part(text='Existing content')]
+  )
+  request.contents.append(existing_content)
+
+  # First Content instruction
+  content1 = types.Content(
+      role='user', parts=[types.Part(text='First instruction')]
+  )
+  request.append_instructions(content1)
+
+  # Should extract text and set as system_instruction, existing content unchanged
+  assert len(request.contents) == 1
+  assert request.contents[0] == existing_content
+  assert request.config.system_instruction == 'First instruction'
+
+  # Second Content instruction
+  content2 = types.Content(
+      role='user', parts=[types.Part(text='Second instruction')]
+  )
+  request.append_instructions(content2)
+
+  # Second Content text should be appended to existing string
+  assert len(request.contents) == 1
+  assert request.contents[0] == existing_content
+  assert (
+      request.config.system_instruction
+      == 'First instruction\n\nSecond instruction'
+  )
+
+
+def test_append_instructions_with_content_multipart():
+  """Test append_instructions with Content containing multiple text parts."""
+  request = LlmRequest()
+
+  # Create Content with multiple text parts
+  content = types.Content(
+      role='user',
+      parts=[
+          types.Part(text='Text instruction'),
+          types.Part(text='Additional text part'),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  # Should extract and join all text parts
+  assert len(request.contents) == 0
+  assert (
+      request.config.system_instruction
+      == 'Text instruction\n\nAdditional text part'
+  )
+
+
+def test_append_instructions_mixed_string_and_content():
+  """Test mixing string list and Content instructions."""
+  request = LlmRequest()
+
+  # First add string instructions
+  request.append_instructions(['String instruction'])
+  assert request.config.system_instruction == 'String instruction'
+
+  # Then add Content instruction
+  content = types.Content(
+      role='user', parts=[types.Part(text='Content instruction')]
+  )
+  request.append_instructions(content)
+
+  # Content text should be appended to existing string
+  assert len(request.contents) == 0
+  assert (
+      request.config.system_instruction
+      == 'String instruction\n\nContent instruction'
+  )
+
+
+def test_append_instructions_content_extracts_text_only():
+  """Test that Content objects have text extracted regardless of role."""
+  request = LlmRequest()
+
+  # Create Content with specific role and parts
+  content = types.Content(
+      role='system',  # Different role
+      parts=[
+          types.Part(text='System instruction'),
+          types.Part(text='Additional system part'),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  # Only text should be extracted and concatenated
+  assert len(request.contents) == 0
+  assert (
+      request.config.system_instruction
+      == 'System instruction\n\nAdditional system part'
+  )
+
+
+def test_append_instructions_content_with_non_text_parts():
+  """Test that non-text parts in Content are ignored."""
+  request = LlmRequest()
+
+  # Create Content with text and non-text parts
+  content = types.Content(
+      role='user',
+      parts=[
+          types.Part(text='Text instruction'),
+          types.Part(
+              inline_data=types.Blob(data=b'file_data', mime_type='text/plain')
+          ),
+          types.Part(text='More text'),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  # Only text parts should be extracted
+  assert request.config.system_instruction == 'Text instruction\n\nMore text'
+
+
+def test_append_instructions_content_no_text_parts():
+  """Test that Content with no text parts does nothing."""
+  request = LlmRequest()
+
+  # Set initial system instruction
+  request.config.system_instruction = 'Initial'
+
+  # Create Content with only non-text parts
+  content = types.Content(
+      role='user',
+      parts=[
+          types.Part(
+              inline_data=types.Blob(data=b'file_data', mime_type='text/plain')
+          ),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  # Should remain unchanged since no text to extract
+  assert request.config.system_instruction == 'Initial'
+
+
+def test_append_instructions_content_empty_text_parts():
+  """Test that Content with empty text parts are skipped."""
+  request = LlmRequest()
+
+  # Create Content with empty and non-empty text parts
+  content = types.Content(
+      role='user',
+      parts=[
+          types.Part(text='Valid text'),
+          types.Part(text=''),  # Empty text
+          types.Part(text=None),  # None text
+          types.Part(text='More valid text'),
+      ],
+  )
+
+  request.append_instructions(content)
+
+  # Only non-empty text should be extracted
+  assert request.config.system_instruction == 'Valid text\n\nMore valid text'
+
+
+def test_append_instructions_warning_unsupported_system_instruction_type(
+    caplog,
+):
+  """Test that warnings are logged for unsupported system_instruction types."""
+  import logging
+
+  request = LlmRequest()
+
+  # Set unsupported type as system_instruction
+  request.config.system_instruction = {'unsupported': 'dict'}
+
+  with caplog.at_level(logging.WARNING):
+    # Try appending Content - should log warning and skip
+    content = types.Content(role='user', parts=[types.Part(text='Test')])
+    request.append_instructions(content)
+
+    # Should remain unchanged
+    assert request.config.system_instruction == {'unsupported': 'dict'}
+
+    # Try appending strings - should also log warning and skip
+    request.append_instructions(['Test string'])
+
+    # Should remain unchanged
+    assert request.config.system_instruction == {'unsupported': 'dict'}
+
+  # Check that warnings were logged
+  assert (
+      len(
+          [record for record in caplog.records if record.levelname == 'WARNING']
+      )
+      >= 1
+  )
+  assert (
+      'Cannot append to system_instruction of unsupported type' in caplog.text
+  )