diff --git a/docs/examples/instruct_validate_repair/sampling_with_prefix_strategy.py b/docs/examples/instruct_validate_repair/sampling_with_prefix_strategy.py
new file mode 100644
index 00000000..dd56a9c7
--- /dev/null
+++ b/docs/examples/instruct_validate_repair/sampling_with_prefix_strategy.py
@@ -0,0 +1,30 @@
+from mellea.backends import ModelOption
+from mellea.stdlib.requirement import check, req, simple_validate
+from mellea.stdlib.sampling.prefix_cached import RejectionSamplingStrategyWithPrefix
+
+requirements = [
+    req("The email should have a salutation"),  # == r1
+    req(
+        "Use only lower-case letters",
+        validation_fn=simple_validate(lambda x: x.lower() == x),
+    ),  # == r2
+    check("Do not mention purple elephants."),  # == r3
+    req("The email should be funny."),
+]
+
+import mellea  # noqa: E402
+
+
+def write_email(m: mellea.MelleaSession, name: str, notes: str) -> str:
+    email_candidate = m.instruct(
+        "Write an email to {{name}} using the notes following: {{notes}}.",
+        requirements=requirements,
+        strategy=RejectionSamplingStrategyWithPrefix(loop_budget=5),
+        user_variables={"name": name, "notes": notes},
+        return_sampling_results=True,
+    )
+
+    if email_candidate.success:
+        return str(email_candidate.result)
+    else:
+        return email_candidate.sample_generations[0].value
diff --git a/docs/examples/mcp/README.md b/docs/examples/mcp/README.md
index 8202f789..48161c33 100644
--- a/docs/examples/mcp/README.md
+++ b/docs/examples/mcp/README.md
@@ -14,7 +14,7 @@ uv pip install "mcp[cli]"
 
 and run the example in MCP debug UI:
 ```bash
-uv run mcp dev docs/examples/tutorial/mcp_example.py
+uv run mcp dev docs/examples/mcp/mcp_example.py
 ```
 
 
diff --git a/mellea/stdlib/funcs.py b/mellea/stdlib/funcs.py
index 5990edb2..b0a6cbb1 100644
--- a/mellea/stdlib/funcs.py
+++ b/mellea/stdlib/funcs.py
@@ -263,9 +263,9 @@ def chat(
 
 def validate(
     reqs: Requirement | list[Requirement],
-    context: Context,
     backend: Backend,
     *,
+    context: Context | None = None,
     output: CBlock | None = None,
     format: type[BaseModelSubclass] | None = None,
     model_options: dict | None = None,
@@ -701,9 +701,9 @@ async def achat(
 
 async def avalidate(
     reqs: Requirement | list[Requirement],
-    context: Context,
     backend: Backend,
     *,
+    context: Context | None = None,
     output: CBlock | None = None,
     format: type[BaseModelSubclass] | None = None,
     model_options: dict | None = None,
@@ -715,7 +715,12 @@ async def avalidate(
     reqs = [reqs] if not isinstance(reqs, list) else reqs
     reqs = [Requirement(req) if type(req) is str else req for req in reqs]
 
+    assert (context is not None) != (output is not None), (
+        "Either context or output must be provided. Not both."
+    )
+
     if output is None:
+        assert context is not None
         validation_target_ctx = context
     else:
         validation_target_ctx = SimpleContext()
diff --git a/mellea/stdlib/reqlib/md.py b/mellea/stdlib/reqlib/md.py
index 3cee2770..d5d2539f 100644
--- a/mellea/stdlib/reqlib/md.py
+++ b/mellea/stdlib/reqlib/md.py
@@ -3,7 +3,7 @@
 import mistletoe
 
 from mellea.stdlib.base import Context
-from mellea.stdlib.requirement import Requirement
+from mellea.stdlib.requirement import Requirement, ValidationResult, simple_validate
 
 # region lists
 
@@ -25,8 +25,9 @@ def as_markdown_list(ctx: Context) -> list[str] | None:
         return None
 
 
-def _md_list(ctx: Context):
-    return as_markdown_list(ctx) is not None
+async def _md_list(ctx: Context) -> ValidationResult:
+    re = as_markdown_list(ctx) is not None
+    return ValidationResult(re)
 
 
 is_markdown_list = Requirement(
@@ -40,11 +41,10 @@ def _md_list(ctx: Context):
 # region tables
 
 
-def _md_table(ctx: Context):
-    raw_output = ctx.last_output()
+def _md_table(raw_output: str) -> bool:
     assert raw_output is not None
     try:
-        parsed = mistletoe.Document(raw_output.value)
+        parsed = mistletoe.Document(raw_output)
         if len(parsed.children) != 1:
             return False
         return type(parsed.children[0]) is mistletoe.block_token.Table
@@ -54,8 +54,7 @@ def _md_table(ctx: Context):
 
 is_markdown_table = Requirement(
     description="The output should be formatted as a Markdown table.",
-    validation_fn=_md_table,
+    validation_fn=simple_validate(_md_table),
 )
 
-
 # endregion
diff --git a/mellea/stdlib/requirement.py b/mellea/stdlib/requirement.py
index f10a3aaf..1679b675 100644
--- a/mellea/stdlib/requirement.py
+++ b/mellea/stdlib/requirement.py
@@ -2,7 +2,7 @@
 
 import inspect
 import re
-from collections.abc import Callable
+from collections.abc import Awaitable, Callable
 from copy import copy
 from typing import Any, overload
 
@@ -93,7 +93,7 @@ class Requirement(Component):
     def __init__(
         self,
         description: str | None = None,
-        validation_fn: Callable[[Context], ValidationResult] | None = None,
+        validation_fn: Callable[[Context], Awaitable[ValidationResult]] | None = None,
         *,
         output_to_bool: Callable[[CBlock | str], bool] | None = default_output_to_bool,
         check_only: bool = False,
@@ -127,7 +127,7 @@ async def validate(
         """Chooses the appropriate validation strategy and applies that strategy."""
         if self.validation_fn is not None:
             # Python validation strategy
-            return self.validation_fn(ctx)
+            return await self.validation_fn(ctx)
         else:
             # LLMaJ validation strategy. This includes ALora because the backend generate call will appropriately dispatch.
             assert self.output_to_bool is not None
@@ -197,7 +197,7 @@ class ScorerRequirement(Requirement):
     def __init__(
         self,
         description: str | None = None,
-        validation_fn: Callable[[Context], ValidationResult] | None = None,
+        validation_fn: Callable[[Context], Awaitable[ValidationResult]] | None = None,
         preference_ordering: str = "max",
         *,
         output_to_bool: Callable[[CBlock | str], bool] | None = default_output_to_bool,
@@ -234,7 +234,7 @@ async def validate(
         """Chooses the appropriate validation strategy and applies that strategy. Asserts that the returned ValidationResult has a valid score."""
         if self.validation_fn is not None:
             # Python validation strategy
-            validation_result = self.validation_fn(ctx)
+            validation_result = await self.validation_fn(ctx)
             assert validation_result._score is not None, (
                 "ScorerRequirement must have a score that is not None"
             )
@@ -292,18 +292,18 @@ def check(*args, **kwargs) -> Requirement:
 @overload
 def simple_validate(
     fn: Callable[[str], tuple[bool, str]],
-) -> Callable[[Context], ValidationResult]: ...
+) -> Callable[[Context], Awaitable[ValidationResult]]: ...
 
 
 @overload
 def simple_validate(
     fn: Callable[[str], bool], *, reason: str | None = None
-) -> Callable[[Context], ValidationResult]: ...
+) -> Callable[[Context], Awaitable[ValidationResult]]: ...
 
 
 def simple_validate(
     fn: Callable[[str], Any], *, reason: str | None = None
-) -> Callable[[Context], ValidationResult]:
+) -> Callable[[Context], Awaitable[ValidationResult]]:
     """Syntactic sugar for writing validation functions that only operate over the last output from the model (interpreted as a string).
 
     This is useful when your validation logic only depends upon the most recent model output. For example:
@@ -321,7 +321,7 @@ def simple_validate(
         reason: only used if the provided function returns a bool; if the validation function fails, a static reason for that failure to give to the llm when repairing
     """
 
-    def validate(ctx: Context) -> ValidationResult:
+    async def validate(ctx: Context) -> ValidationResult:
         o = ctx.last_output()
         if o is None or o.value is None:
             FancyLogger.get_logger().warn(
diff --git a/mellea/stdlib/sampling/base.py b/mellea/stdlib/sampling/base.py
index 6520a7da..e0d51925 100644
--- a/mellea/stdlib/sampling/base.py
+++ b/mellea/stdlib/sampling/base.py
@@ -165,7 +165,6 @@ async def sample(
                 reqs=reqs,
                 context=result_ctx,
                 backend=backend,
-                output=result,
                 format=format,
                 model_options=model_options,
                 # tool_calls=tool_calls  # Don't support using tool calls in validation strategies.
diff --git a/mellea/stdlib/sampling/best_of_n.py b/mellea/stdlib/sampling/best_of_n.py
index 59c402b2..5dbb7e1d 100644
--- a/mellea/stdlib/sampling/best_of_n.py
+++ b/mellea/stdlib/sampling/best_of_n.py
@@ -131,7 +131,6 @@ async def sample(
                 reqs=reqs,
                 context=result_ctx,
                 backend=backend,
-                output=result,
                 format=format,
                 model_options=model_options,
                 input=next_action._description,  # type: ignore
diff --git a/mellea/stdlib/sampling/prefix_cached.py b/mellea/stdlib/sampling/prefix_cached.py
new file mode 100644
index 00000000..3317c1b5
--- /dev/null
+++ b/mellea/stdlib/sampling/prefix_cached.py
@@ -0,0 +1,85 @@
+"""Sampling Strategy that uses prefix caching idea based on two turn chats."""
+
+from collections.abc import Awaitable, Callable
+
+from mellea.backends import Backend, BaseModelSubclass, ModelOption
+from mellea.stdlib.base import ChatContext, Component, Context, ContextTurn
+from mellea.stdlib.chat import Message
+from mellea.stdlib.requirement import Requirement, ValidationResult
+from mellea.stdlib.sampling import RejectionSamplingStrategy, SamplingResult
+
+
+class RejectionSamplingStrategyWithPrefix(RejectionSamplingStrategy):
+    """Rejection Sampling class that uses the last turn as prefix cache for requirement checking."""
+
+    async def sample(
+        self,
+        action: Component,
+        context: Context,
+        backend: Backend,
+        requirements: list[Requirement] | None,
+        *,
+        validation_ctx: Context | None = None,
+        format: type[BaseModelSubclass] | None = None,
+        model_options: dict | None = None,
+        tool_calls: bool = False,
+        show_progress: bool = True,
+    ) -> SamplingResult:
+        """Sample method inherited from RejectionSamplingStrategy."""
+        reqs: list[Requirement] = []
+        if self.requirements is not None:
+            reqs += self.requirements
+        elif requirements is not None:
+            reqs += requirements
+        reqs = list(set(reqs))
+
+        def make_val(
+            req_string: str,
+        ) -> Callable[[Context], Awaitable[ValidationResult]]:
+            async def validate_agentic(ctx: Context) -> ValidationResult:
+                lt = ctx.last_turn()
+                assert isinstance(lt, ContextTurn)
+                assert lt.model_input is not None
+                assert lt.output is not None
+
+                chat_ctx = ChatContext()
+                chat_ctx = chat_ctx.add(lt.model_input)
+                chat_ctx = chat_ctx.add(lt.output)
+
+                action = Message(
+                    role="user",
+                    content=f"Does the output fulfill the requirement? Answer only with yes or no.  Requirement: '{req_string}'",
+                )
+
+                llm_as_a_judge_result, _ = backend.generate_from_context(
+                    action,
+                    chat_ctx,
+                    format=format,
+                    model_options={ModelOption.MAX_NEW_TOKENS: 10},
+                )
+                value = await llm_as_a_judge_result.avalue()
+
+                return ValidationResult(
+                    result=value.lower().startswith("yes"),
+                    reason=value,
+                    thunk=llm_as_a_judge_result,
+                )
+
+            return validate_agentic
+
+        for req in reqs:
+            if req.validation_fn is None:
+                req.validation_fn = make_val(str(req.description))
+
+        res = await super().sample(
+            action=action,
+            context=context,
+            backend=backend,
+            requirements=reqs,
+            validation_ctx=validation_ctx,
+            format=format,
+            model_options=model_options,
+            tool_calls=tool_calls,
+            show_progress=show_progress,
+        )
+        return res
diff --git a/mellea/stdlib/session.py b/mellea/stdlib/session.py
index 2a63a71a..665d4f2b 100644
--- a/mellea/stdlib/session.py
+++ b/mellea/stdlib/session.py
@@ -446,8 +446,8 @@ def validate(
         """Validates a set of requirements over the output (if provided) or the current context (if the output is not provided)."""
         return mfuncs.validate(
             reqs=reqs,
-            context=self.ctx,
             backend=self.backend,
+            context=self.ctx if output is None else None,
             output=output,
             format=format,
             model_options=model_options,
@@ -730,7 +730,7 @@ async def avalidate(
         """Validates a set of requirements over the output (if provided) or the current context (if the output is not provided)."""
         return await mfuncs.avalidate(
             reqs=reqs,
-            context=self.ctx,
+            context=self.ctx if output is None else None,
             backend=self.backend,
             output=output,
             format=format,
diff --git a/test/stdlib_basics/test_funcs.py b/test/stdlib_basics/test_funcs.py
index 4e99afc5..61763ed2 100644
--- a/test/stdlib_basics/test_funcs.py
+++ b/test/stdlib_basics/test_funcs.py
@@ -62,12 +62,10 @@ async def test_ainstruct(m_session):
     assert ctx._data is out
 
 async def test_avalidate(m_session):
-    initial_ctx = m_session.ctx
     backend = m_session.backend
 
     val_result = await avalidate(
         reqs=[req("Be formal."), req("Avoid telling jokes.")],
-        context=initial_ctx,
         backend=backend,
         output=ModelOutputThunk("Here is an output.")
     )
@@ -77,4 +75,4 @@ async def test_avalidate(m_session):
 
 
 if __name__ == "__main__":
-    pytest.main([__file__])
\ No newline at end of file
+    pytest.main([__file__])