diff --git a/pydantic_ai_slim/pydantic_ai/agent/__init__.py b/pydantic_ai_slim/pydantic_ai/agent/__init__.py
index ed1c5ed649..72c256e9c4 100644
--- a/pydantic_ai_slim/pydantic_ai/agent/__init__.py
+++ b/pydantic_ai_slim/pydantic_ai/agent/__init__.py
@@ -66,7 +66,7 @@
 from ..toolsets.combined import CombinedToolset
 from ..toolsets.function import FunctionToolset
 from ..toolsets.prepared import PreparedToolset
-from .abstract import AbstractAgent, EventStreamHandler, RunOutputDataT
+from .abstract import AbstractAgent, EventStreamHandler, Instructions, RunOutputDataT
 from .wrapper import WrapperAgent
 
 if TYPE_CHECKING:
@@ -137,8 +137,7 @@ class Agent(AbstractAgent[AgentDepsT, OutputDataT]):
     _deps_type: type[AgentDepsT] = dataclasses.field(repr=False)
     _output_schema: _output.BaseOutputSchema[OutputDataT] = dataclasses.field(repr=False)
     _output_validators: list[_output.OutputValidator[AgentDepsT, OutputDataT]] = dataclasses.field(repr=False)
-    _instructions: str | None = dataclasses.field(repr=False)
-    _instructions_functions: list[_system_prompt.SystemPromptRunner[AgentDepsT]] = dataclasses.field(repr=False)
+    _instructions: list[str | _system_prompt.SystemPromptFunc[AgentDepsT]] = dataclasses.field(repr=False)
     _system_prompts: tuple[str, ...] = dataclasses.field(repr=False)
     _system_prompt_functions: list[_system_prompt.SystemPromptRunner[AgentDepsT]] = dataclasses.field(repr=False)
     _system_prompt_dynamic_functions: dict[str, _system_prompt.SystemPromptRunner[AgentDepsT]] = dataclasses.field(
@@ -164,10 +163,7 @@ def __init__(
         model: models.Model | models.KnownModelName | str | None = None,
         *,
         output_type: OutputSpec[OutputDataT] = str,
-        instructions: str
-        | _system_prompt.SystemPromptFunc[AgentDepsT]
-        | Sequence[str | _system_prompt.SystemPromptFunc[AgentDepsT]]
-        | None = None,
+        instructions: Instructions[AgentDepsT] = None,
         system_prompt: str | Sequence[str] = (),
         deps_type: type[AgentDepsT] = NoneType,
         name: str | None = None,
@@ -193,10 +189,7 @@ def __init__(
         model: models.Model | models.KnownModelName | str | None = None,
         *,
         output_type: OutputSpec[OutputDataT] = str,
-        instructions: str
-        | _system_prompt.SystemPromptFunc[AgentDepsT]
-        | Sequence[str | _system_prompt.SystemPromptFunc[AgentDepsT]]
-        | None = None,
+        instructions: Instructions[AgentDepsT] = None,
         system_prompt: str | Sequence[str] = (),
         deps_type: type[AgentDepsT] = NoneType,
         name: str | None = None,
@@ -220,10 +213,7 @@ def __init__(
         model: models.Model | models.KnownModelName | str | None = None,
         *,
         output_type: OutputSpec[OutputDataT] = str,
-        instructions: str
-        | _system_prompt.SystemPromptFunc[AgentDepsT]
-        | Sequence[str | _system_prompt.SystemPromptFunc[AgentDepsT]]
-        | None = None,
+        instructions: Instructions[AgentDepsT] = None,
         system_prompt: str | Sequence[str] = (),
         deps_type: type[AgentDepsT] = NoneType,
         name: str | None = None,
@@ -322,16 +312,7 @@ def __init__(
         self._output_schema = _output.OutputSchema[OutputDataT].build(output_type, default_mode=default_output_mode)
         self._output_validators = []
 
-        self._instructions = ''
-        self._instructions_functions = []
-        if isinstance(instructions, str | Callable):
-            instructions = [instructions]
-        for instruction in instructions or []:
-            if isinstance(instruction, str):
-                self._instructions += instruction + '\n'
-            else:
-                self._instructions_functions.append(_system_prompt.SystemPromptRunner(instruction))
-        self._instructions = self._instructions.strip() or None
+        self._instructions = self._normalize_instructions(instructions)
 
         self._system_prompts = (system_prompt,) if isinstance(system_prompt, str) else tuple(system_prompt)
         self._system_prompt_functions = []
@@ -371,6 +352,9 @@ def __init__(
         self._override_tools: ContextVar[
             _utils.Option[Sequence[Tool[AgentDepsT] | ToolFuncEither[AgentDepsT, ...]]]
         ] = ContextVar('_override_tools', default=None)
+        self._override_instructions: ContextVar[
+            _utils.Option[list[str | _system_prompt.SystemPromptFunc[AgentDepsT]]]
+        ] = ContextVar('_override_instructions', default=None)
 
         self._enter_lock = Lock()
         self._entered_count = 0
@@ -593,10 +577,12 @@ async def main():
         model_settings = merge_model_settings(merged_settings, model_settings)
         usage_limits = usage_limits or _usage.UsageLimits()
 
+        instructions_literal, instructions_functions = self._get_instructions()
+
         async def get_instructions(run_context: RunContext[AgentDepsT]) -> str | None:
             parts = [
-                self._instructions,
-                *[await func.run(run_context) for func in self._instructions_functions],
+                instructions_literal,
+                *[await func.run(run_context) for func in instructions_functions],
             ]
 
             model_profile = model_used.profile
@@ -634,11 +620,12 @@ async def get_instructions(run_context: RunContext[AgentDepsT]) -> str | None:
             get_instructions=get_instructions,
             instrumentation_settings=instrumentation_settings,
         )
+
         start_node = _agent_graph.UserPromptNode[AgentDepsT](
             user_prompt=user_prompt,
             deferred_tool_results=deferred_tool_results,
-            instructions=self._instructions,
-            instructions_functions=self._instructions_functions,
+            instructions=instructions_literal,
+            instructions_functions=instructions_functions,
             system_prompts=self._system_prompts,
             system_prompt_functions=self._system_prompt_functions,
             system_prompt_dynamic_functions=self._system_prompt_dynamic_functions,
@@ -690,6 +677,8 @@ async def get_instructions(run_context: RunContext[AgentDepsT]) -> str | None:
     def _run_span_end_attributes(
         self, state: _agent_graph.GraphAgentState, usage: _usage.RunUsage, settings: InstrumentationSettings
     ):
+        literal_instructions, _ = self._get_instructions()
+
         if settings.version == 1:
             attrs = {
                 'all_messages_events': json.dumps(
@@ -702,7 +691,7 @@ def _run_span_end_attributes(
         else:
             attrs = {
                 'pydantic_ai.all_messages': json.dumps(settings.messages_to_otel_messages(state.message_history)),
-                **settings.system_instructions_attributes(self._instructions),
+                **settings.system_instructions_attributes(literal_instructions),
             }
 
         return {
@@ -727,8 +716,9 @@ def override(
         model: models.Model | models.KnownModelName | str | _utils.Unset = _utils.UNSET,
         toolsets: Sequence[AbstractToolset[AgentDepsT]] | _utils.Unset = _utils.UNSET,
         tools: Sequence[Tool[AgentDepsT] | ToolFuncEither[AgentDepsT, ...]] | _utils.Unset = _utils.UNSET,
+        instructions: Instructions[AgentDepsT] | _utils.Unset = _utils.UNSET,
     ) -> Iterator[None]:
-        """Context manager to temporarily override agent dependencies, model, toolsets, or tools.
+        """Context manager to temporarily override agent dependencies, model, toolsets, tools, or instructions.
 
         This is particularly useful when testing.
         You can find an example of this [here](../testing.md#overriding-model-via-pytest-fixtures).
@@ -738,6 +728,7 @@ def override(
             model: The model to use instead of the model passed to the agent run.
             toolsets: The toolsets to use instead of the toolsets passed to the agent constructor and agent run.
             tools: The tools to use instead of the tools registered with the agent.
+            instructions: The instructions to use instead of the instructions registered with the agent.
         """
         if _utils.is_set(deps):
             deps_token = self._override_deps.set(_utils.Some(deps))
@@ -759,6 +750,12 @@ def override(
         else:
             tools_token = None
 
+        if _utils.is_set(instructions):
+            normalized_instructions = self._normalize_instructions(instructions)
+            instructions_token = self._override_instructions.set(_utils.Some(normalized_instructions))
+        else:
+            instructions_token = None
+
         try:
             yield
         finally:
@@ -770,6 +767,8 @@ def override(
                 self._override_toolsets.reset(toolsets_token)
             if tools_token is not None:
                 self._override_tools.reset(tools_token)
+            if instructions_token is not None:
+                self._override_instructions.reset(instructions_token)
 
     @overload
     def instructions(
@@ -830,12 +829,12 @@ async def async_instructions(ctx: RunContext[str]) -> str:
             def decorator(
                 func_: _system_prompt.SystemPromptFunc[AgentDepsT],
             ) -> _system_prompt.SystemPromptFunc[AgentDepsT]:
-                self._instructions_functions.append(_system_prompt.SystemPromptRunner(func_))
+                self._instructions.append(func_)
                 return func_
 
             return decorator
         else:
-            self._instructions_functions.append(_system_prompt.SystemPromptRunner(func))
+            self._instructions.append(func)
             return func
 
     @overload
@@ -1276,6 +1275,34 @@ def _get_deps(self: Agent[T, OutputDataT], deps: T) -> T:
         else:
             return deps
 
+    def _normalize_instructions(
+        self,
+        instructions: Instructions[AgentDepsT],
+    ) -> list[str | _system_prompt.SystemPromptFunc[AgentDepsT]]:
+        if instructions is None:
+            return []
+        if isinstance(instructions, str) or callable(instructions):
+            return [instructions]
+        return list(instructions)
+
+    def _get_instructions(
+        self,
+    ) -> tuple[str | None, list[_system_prompt.SystemPromptRunner[AgentDepsT]]]:
+        override_instructions = self._override_instructions.get()
+        instructions = override_instructions.value if override_instructions else self._instructions
+
+        literal_parts: list[str] = []
+        functions: list[_system_prompt.SystemPromptRunner[AgentDepsT]] = []
+
+        for instruction in instructions:
+            if isinstance(instruction, str):
+                literal_parts.append(instruction)
+            else:
+                functions.append(_system_prompt.SystemPromptRunner[AgentDepsT](instruction))
+
+        literal = '\n'.join(literal_parts).strip() or None
+        return literal, functions
+
     def _get_toolset(
         self,
         output_toolset: AbstractToolset[AgentDepsT] | None | _utils.Unset = _utils.UNSET,
diff --git a/pydantic_ai_slim/pydantic_ai/agent/abstract.py b/pydantic_ai_slim/pydantic_ai/agent/abstract.py
index 8d6c9ff293..fdd21b8065 100644
--- a/pydantic_ai_slim/pydantic_ai/agent/abstract.py
+++ b/pydantic_ai_slim/pydantic_ai/agent/abstract.py
@@ -14,6 +14,7 @@
 
 from .. import (
     _agent_graph,
+    _system_prompt,
     _utils,
     exceptions,
     messages as _messages,
@@ -60,6 +61,14 @@
 """A function that receives agent [`RunContext`][pydantic_ai.tools.RunContext] and an async iterable of events from the model's streaming response and the agent's execution of tools."""
 
 
+Instructions = (
+    str
+    | _system_prompt.SystemPromptFunc[AgentDepsT]
+    | Sequence[str | _system_prompt.SystemPromptFunc[AgentDepsT]]
+    | None
+)
+
+
 class AbstractAgent(Generic[AgentDepsT, OutputDataT], ABC):
     """Abstract superclass for [`Agent`][pydantic_ai.agent.Agent], [`WrapperAgent`][pydantic_ai.agent.WrapperAgent], and your own custom agent implementations."""
 
@@ -681,8 +690,9 @@ def override(
         model: models.Model | models.KnownModelName | str | _utils.Unset = _utils.UNSET,
         toolsets: Sequence[AbstractToolset[AgentDepsT]] | _utils.Unset = _utils.UNSET,
         tools: Sequence[Tool[AgentDepsT] | ToolFuncEither[AgentDepsT, ...]] | _utils.Unset = _utils.UNSET,
+        instructions: Instructions[AgentDepsT] | _utils.Unset = _utils.UNSET,
     ) -> Iterator[None]:
-        """Context manager to temporarily override agent dependencies, model, toolsets, or tools.
+        """Context manager to temporarily override agent dependencies, model, toolsets, tools, or instructions.
 
         This is particularly useful when testing.
         You can find an example of this [here](../testing.md#overriding-model-via-pytest-fixtures).
@@ -692,6 +702,7 @@ def override(
             model: The model to use instead of the model passed to the agent run.
             toolsets: The toolsets to use instead of the toolsets passed to the agent constructor and agent run.
             tools: The tools to use instead of the tools registered with the agent.
+            instructions: The instructions to use instead of the instructions registered with the agent.
         """
         raise NotImplementedError
         yield
diff --git a/pydantic_ai_slim/pydantic_ai/agent/wrapper.py b/pydantic_ai_slim/pydantic_ai/agent/wrapper.py
index 36f7969323..ba735f0907 100644
--- a/pydantic_ai_slim/pydantic_ai/agent/wrapper.py
+++ b/pydantic_ai_slim/pydantic_ai/agent/wrapper.py
@@ -20,7 +20,7 @@
     ToolFuncEither,
 )
 from ..toolsets import AbstractToolset
-from .abstract import AbstractAgent, EventStreamHandler, RunOutputDataT
+from .abstract import AbstractAgent, EventStreamHandler, Instructions, RunOutputDataT
 
 
 class WrapperAgent(AbstractAgent[AgentDepsT, OutputDataT]):
@@ -214,8 +214,9 @@ def override(
         model: models.Model | models.KnownModelName | str | _utils.Unset = _utils.UNSET,
         toolsets: Sequence[AbstractToolset[AgentDepsT]] | _utils.Unset = _utils.UNSET,
         tools: Sequence[Tool[AgentDepsT] | ToolFuncEither[AgentDepsT, ...]] | _utils.Unset = _utils.UNSET,
+        instructions: Instructions[AgentDepsT] | _utils.Unset = _utils.UNSET,
     ) -> Iterator[None]:
-        """Context manager to temporarily override agent dependencies, model, toolsets, or tools.
+        """Context manager to temporarily override agent dependencies, model, toolsets, tools, or instructions.
 
         This is particularly useful when testing.
         You can find an example of this [here](../testing.md#overriding-model-via-pytest-fixtures).
@@ -225,6 +226,13 @@ def override(
             model: The model to use instead of the model passed to the agent run.
             toolsets: The toolsets to use instead of the toolsets passed to the agent constructor and agent run.
             tools: The tools to use instead of the tools registered with the agent.
+            instructions: The instructions to use instead of the instructions registered with the agent.
         """
-        with self.wrapped.override(deps=deps, model=model, toolsets=toolsets, tools=tools):
+        with self.wrapped.override(
+            deps=deps,
+            model=model,
+            toolsets=toolsets,
+            tools=tools,
+            instructions=instructions,
+        ):
             yield
diff --git a/pydantic_ai_slim/pydantic_ai/durable_exec/dbos/_agent.py b/pydantic_ai_slim/pydantic_ai/durable_exec/dbos/_agent.py
index 10b3184fd4..d7d4987d8f 100644
--- a/pydantic_ai_slim/pydantic_ai/durable_exec/dbos/_agent.py
+++ b/pydantic_ai_slim/pydantic_ai/durable_exec/dbos/_agent.py
@@ -15,6 +15,7 @@
     usage as _usage,
 )
 from pydantic_ai.agent import AbstractAgent, AgentRun, AgentRunResult, EventStreamHandler, RunOutputDataT, WrapperAgent
+from pydantic_ai.agent.abstract import Instructions
 from pydantic_ai.exceptions import UserError
 from pydantic_ai.models import Model
 from pydantic_ai.output import OutputDataT, OutputSpec
@@ -704,8 +705,9 @@ def override(
         model: models.Model | models.KnownModelName | str | _utils.Unset = _utils.UNSET,
         toolsets: Sequence[AbstractToolset[AgentDepsT]] | _utils.Unset = _utils.UNSET,
         tools: Sequence[Tool[AgentDepsT] | ToolFuncEither[AgentDepsT, ...]] | _utils.Unset = _utils.UNSET,
+        instructions: Instructions[AgentDepsT] | _utils.Unset = _utils.UNSET,
     ) -> Iterator[None]:
-        """Context manager to temporarily override agent dependencies, model, toolsets, or tools.
+        """Context manager to temporarily override agent dependencies, model, toolsets, tools, or instructions.
 
         This is particularly useful when testing.
         You can find an example of this [here](../testing.md#overriding-model-via-pytest-fixtures).
@@ -715,11 +717,18 @@ def override(
             model: The model to use instead of the model passed to the agent run.
             toolsets: The toolsets to use instead of the toolsets passed to the agent constructor and agent run.
             tools: The tools to use instead of the tools registered with the agent.
+            instructions: The instructions to use instead of the instructions registered with the agent.
         """
         if _utils.is_set(model) and not isinstance(model, (DBOSModel)):
             raise UserError(
                 'Non-DBOS model cannot be contextually overridden inside a DBOS workflow, it must be set at agent creation time.'
             )
 
-        with super().override(deps=deps, model=model, toolsets=toolsets, tools=tools):
+        with super().override(
+            deps=deps,
+            model=model,
+            toolsets=toolsets,
+            tools=tools,
+            instructions=instructions,
+        ):
             yield
diff --git a/pydantic_ai_slim/pydantic_ai/durable_exec/temporal/_agent.py b/pydantic_ai_slim/pydantic_ai/durable_exec/temporal/_agent.py
index 4f2161ebbf..a87b5195a9 100644
--- a/pydantic_ai_slim/pydantic_ai/durable_exec/temporal/_agent.py
+++ b/pydantic_ai_slim/pydantic_ai/durable_exec/temporal/_agent.py
@@ -22,7 +22,8 @@
     models,
     usage as _usage,
 )
-from pydantic_ai.agent import AbstractAgent, AgentRun, AgentRunResult, EventStreamHandler, RunOutputDataT, WrapperAgent
+from pydantic_ai.agent import AbstractAgent, AgentRun, AgentRunResult, EventStreamHandler, WrapperAgent
+from pydantic_ai.agent.abstract import Instructions, RunOutputDataT
 from pydantic_ai.exceptions import UserError
 from pydantic_ai.models import Model
 from pydantic_ai.output import OutputDataT, OutputSpec
@@ -748,8 +749,9 @@ def override(
         model: models.Model | models.KnownModelName | str | _utils.Unset = _utils.UNSET,
         toolsets: Sequence[AbstractToolset[AgentDepsT]] | _utils.Unset = _utils.UNSET,
         tools: Sequence[Tool[AgentDepsT] | ToolFuncEither[AgentDepsT, ...]] | _utils.Unset = _utils.UNSET,
+        instructions: Instructions[AgentDepsT] | _utils.Unset = _utils.UNSET,
     ) -> Iterator[None]:
-        """Context manager to temporarily override agent dependencies, model, toolsets, or tools.
+        """Context manager to temporarily override agent dependencies, model, toolsets, tools, or instructions.
 
         This is particularly useful when testing.
         You can find an example of this [here](../testing.md#overriding-model-via-pytest-fixtures).
@@ -759,6 +761,7 @@ def override(
             model: The model to use instead of the model passed to the agent run.
             toolsets: The toolsets to use instead of the toolsets passed to the agent constructor and agent run.
             tools: The tools to use instead of the tools registered with the agent.
+            instructions: The instructions to use instead of the instructions registered with the agent.
         """
         if workflow.in_workflow():
             if _utils.is_set(model):
@@ -774,5 +777,11 @@ def override(
                     'Tools cannot be contextually overridden inside a Temporal workflow, they must be set at agent creation time.'
                 )
 
-        with super().override(deps=deps, model=model, toolsets=toolsets, tools=tools):
+        with super().override(
+            deps=deps,
+            model=model,
+            toolsets=toolsets,
+            tools=tools,
+            instructions=instructions,
+        ):
             yield
diff --git a/tests/test_agent.py b/tests/test_agent.py
index fcffd2b846..f11ba9aadb 100644
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -1,3 +1,4 @@
+import asyncio
 import json
 import re
 import sys
@@ -5148,3 +5149,231 @@ def llm(messages: list[ModelMessage], info: AgentInfo) -> ModelResponse:
             ),
         ]
     )
+
+
+def test_override_instructions_basic():
+    """Test that override can override instructions."""
+    agent = Agent('test')
+
+    @agent.instructions
+    def instr_fn() -> str:
+        return 'SHOULD_BE_IGNORED'
+
+    with capture_run_messages() as base_messages:
+        agent.run_sync('Hello', model=TestModel(custom_output_text='baseline'))
+
+    base_req = base_messages[0]
+    assert isinstance(base_req, ModelRequest)
+    assert base_req.instructions == 'SHOULD_BE_IGNORED'
+
+    with agent.override(instructions='OVERRIDE'):
+        with capture_run_messages() as messages:
+            agent.run_sync('Hello', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'OVERRIDE'
+
+
+def test_override_reset_after_context():
+    """Test that instructions are reset after exiting the override context."""
+    agent = Agent('test', instructions='ORIG')
+
+    with agent.override(instructions='NEW'):
+        with capture_run_messages() as messages_new:
+            agent.run_sync('Hi', model=TestModel(custom_output_text='ok'))
+
+    with capture_run_messages() as messages_orig:
+        agent.run_sync('Hi', model=TestModel(custom_output_text='ok'))
+
+    req_new = messages_new[0]
+    assert isinstance(req_new, ModelRequest)
+    req_orig = messages_orig[0]
+    assert isinstance(req_orig, ModelRequest)
+    assert req_new.instructions == 'NEW'
+    assert req_orig.instructions == 'ORIG'
+
+
+def test_override_none_clears_instructions():
+    """Test that passing None for instructions clears all instructions."""
+    agent = Agent('test', instructions='BASE')
+
+    @agent.instructions
+    def instr_fn() -> str:  # pragma: no cover - ignored under override
+        return 'ALSO_BASE'
+
+    with agent.override(instructions=None):
+        with capture_run_messages() as messages:
+            agent.run_sync('Hello', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions is None
+
+
+def test_override_instructions_callable_replaces_functions():
+    """Override with a callable should replace existing instruction functions."""
+    agent = Agent('test')
+
+    @agent.instructions
+    def base_fn() -> str:
+        return 'BASE_FN'
+
+    def override_fn() -> str:
+        return 'OVERRIDE_FN'
+
+    with capture_run_messages() as base_messages:
+        agent.run_sync('Hello', model=TestModel(custom_output_text='baseline'))
+
+    base_req = base_messages[0]
+    assert isinstance(base_req, ModelRequest)
+    assert base_req.instructions is not None
+    assert 'BASE_FN' in base_req.instructions
+
+    with agent.override(instructions=override_fn):
+        with capture_run_messages() as messages:
+            agent.run_sync('Hello', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'OVERRIDE_FN'
+    assert 'BASE_FN' not in req.instructions
+
+
+async def test_override_instructions_async_callable():
+    """Override with an async callable should be awaited."""
+    agent = Agent('test')
+
+    async def override_fn() -> str:
+        await asyncio.sleep(0)
+        return 'ASYNC_FN'
+
+    with agent.override(instructions=override_fn):
+        with capture_run_messages() as messages:
+            await agent.run('Hi', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'ASYNC_FN'
+
+
+def test_override_instructions_sequence_mixed_types():
+    """Override can mix literal strings and functions."""
+    agent = Agent('test', instructions='BASE')
+
+    def override_fn() -> str:
+        return 'FUNC_PART'
+
+    def override_fn_2() -> str:
+        return 'FUNC_PART_2'
+
+    with agent.override(instructions=['OVERRIDE1', override_fn, 'OVERRIDE2', override_fn_2]):
+        with capture_run_messages() as messages:
+            agent.run_sync('Hello', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'OVERRIDE1\nOVERRIDE2\n\nFUNC_PART\n\nFUNC_PART_2'
+    assert 'BASE' not in req.instructions
+
+
+async def test_override_concurrent_isolation():
+    """Test that concurrent overrides are isolated from each other."""
+    agent = Agent('test', instructions='ORIG')
+
+    async def run_with(instr: str) -> str | None:
+        with agent.override(instructions=instr):
+            with capture_run_messages() as messages:
+                await agent.run('Hi', model=TestModel(custom_output_text='ok'))
+            req = messages[0]
+            assert isinstance(req, ModelRequest)
+            return req.instructions
+
+    a, b = await asyncio.gather(
+        run_with('A'),
+        run_with('B'),
+    )
+
+    assert a == 'A'
+    assert b == 'B'
+
+
+def test_override_replaces_instructions():
+    """Test overriding instructions replaces the base instructions."""
+    agent = Agent('test', instructions='ORIG_INSTR')
+
+    with agent.override(instructions='NEW_INSTR'):
+        with capture_run_messages() as messages:
+            agent.run_sync('Hi', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'NEW_INSTR'
+
+
+def test_override_nested_contexts():
+    """Test nested override contexts."""
+    agent = Agent('test', instructions='ORIG')
+
+    with agent.override(instructions='OUTER'):
+        with capture_run_messages() as outer_messages:
+            agent.run_sync('Hi', model=TestModel(custom_output_text='ok'))
+
+        with agent.override(instructions='INNER'):
+            with capture_run_messages() as inner_messages:
+                agent.run_sync('Hi', model=TestModel(custom_output_text='ok'))
+
+    outer_req = outer_messages[0]
+    assert isinstance(outer_req, ModelRequest)
+    inner_req = inner_messages[0]
+    assert isinstance(inner_req, ModelRequest)
+
+    assert outer_req.instructions == 'OUTER'
+    assert inner_req.instructions == 'INNER'
+
+
+async def test_override_async_run():
+    """Test override with async run method."""
+    agent = Agent('test', instructions='ORIG')
+
+    with agent.override(instructions='ASYNC_OVERRIDE'):
+        with capture_run_messages() as messages:
+            await agent.run('Hi', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'ASYNC_OVERRIDE'
+
+
+def test_override_with_dynamic_prompts():
+    """Test override interacting with dynamic prompts."""
+    agent = Agent('test')
+
+    dynamic_value = 'DYNAMIC'
+
+    @agent.system_prompt
+    def dynamic_sys() -> str:
+        return dynamic_value
+
+    @agent.instructions
+    def dynamic_instr() -> str:
+        return 'DYNAMIC_INSTR'
+
+    with capture_run_messages() as base_messages:
+        agent.run_sync('Hi', model=TestModel(custom_output_text='baseline'))
+
+    base_req = base_messages[0]
+    assert isinstance(base_req, ModelRequest)
+    assert base_req.instructions == 'DYNAMIC_INSTR'
+
+    # Override should take precedence over dynamic instructions but leave system prompts intact
+    with agent.override(instructions='OVERRIDE_INSTR'):
+        with capture_run_messages() as messages:
+            agent.run_sync('Hi', model=TestModel(custom_output_text='ok'))
+
+    req = messages[0]
+    assert isinstance(req, ModelRequest)
+    assert req.instructions == 'OVERRIDE_INSTR'
+    sys_texts = [p.content for p in req.parts if isinstance(p, SystemPromptPart)]
+    # The dynamic system prompt should still be present since overrides target instructions only
+    assert dynamic_value in sys_texts