diff --git a/mellea/core/requirement.py b/mellea/core/requirement.py
index 0d3381f16..e8c55564b 100644
--- a/mellea/core/requirement.py
+++ b/mellea/core/requirement.py
@@ -283,6 +283,51 @@ async def validate(
                 context=val_ctx,
             )
 
+    async def stream_validate(
+        self, chunk: str, *, backend: Backend, ctx: Context
+    ) -> PartialValidationResult:
+        """Hook for per-chunk streaming validation.
+
+        The default implementation returns ``PartialValidationResult("unknown")``
+        — meaning insufficient data to decide yet. Subclasses override this method
+        to inspect the current chunk and return ``"pass"`` or ``"fail"`` early.
+
+        Implementations may accumulate state on ``self`` across calls within a
+        single attempt. The orchestrator clones the requirement (``copy(req)``)
+        before each attempt, so state does not bleed across retries.
+
+        Shallow-copy caveat: mutable container fields (e.g. ``self._buffer = []``)
+        are shared by reference under ``copy()``. Reassign rather than mutate in
+        place (``self._buffer = self._buffer + [chunk]``, not
+        ``self._buffer.append(chunk)``), or override ``__copy__`` for proper
+        isolation.
+
+        Implementations must not call ``mot.astream()`` or otherwise read the
+        underlying stream; the orchestrator is the single consumer of the MOT
+        stream (see ``ModelOutputThunk.astream``). Requirements that need access
+        to the text seen so far should accumulate it themselves from the
+        ``chunk`` values they receive.
+
+        Args:
+            chunk: A single complete semantic chunk produced by the chunking
+                strategy (e.g. one sentence for ``SentenceChunker``). This is
+                the delta since the previous ``stream_validate`` call for this
+                attempt, not the accumulated output. Requirements that need
+                earlier context should retain it on ``self`` across calls.
+            backend: The inference backend, available for backend-assisted checks.
+            ctx: The current generation context. During streaming the MOT is
+                not yet computed, so ``ctx`` does not contain the generated
+                output; use ``chunk`` (and any state accumulated on ``self``)
+                instead.
+
+        Returns:
+            PartialValidationResult: ``"unknown"`` by default. Subclasses may return
+            ``"pass"`` (constraint satisfied so far) or ``"fail"`` (constraint violated,
+            streaming should be aborted). ``"pass"`` does not short-circuit the final
+            ``validate()`` call; the orchestrator decides whether to skip it.
+        """
+        return PartialValidationResult("unknown")
+
     def parts(self) -> list[Component | CBlock]:
         """Returns all of the constituent parts of a Requirement.
 
diff --git a/test/core/test_stream_validate.py b/test/core/test_stream_validate.py
new file mode 100644
index 000000000..4973608c8
--- /dev/null
+++ b/test/core/test_stream_validate.py
@@ -0,0 +1,164 @@
+"""Unit tests for Requirement.stream_validate() hook."""
+
+import inspect
+from copy import copy
+
+import pytest
+
+from mellea.core import Backend, Context, PartialValidationResult, Requirement
+
+
+@pytest.mark.asyncio
+async def test_default_returns_unknown():
+    req = Requirement(description="some requirement")
+    result = await req.stream_validate("some chunk", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert result.success == "unknown"
+
+
+@pytest.mark.asyncio
+async def test_default_returns_partial_validation_result_instance():
+    req = Requirement()
+    result = await req.stream_validate("chunk", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert isinstance(result, PartialValidationResult)
+
+
+def test_stream_validate_is_coroutine():
+    req = Requirement()
+    assert inspect.iscoroutinefunction(req.stream_validate)
+
+
+@pytest.mark.asyncio
+async def test_subclass_can_return_pass():
+    class PassRequirement(Requirement):
+        async def stream_validate(
+            self, chunk: str, *, backend: Backend, ctx: Context
+        ) -> PartialValidationResult:
+            return PartialValidationResult("pass")
+
+    req = PassRequirement(description="always passes")
+    result = await req.stream_validate("any chunk", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert result.success == "pass"
+
+
+@pytest.mark.asyncio
+async def test_subclass_can_return_fail():
+    class FailRequirement(Requirement):
+        async def stream_validate(
+            self, chunk: str, *, backend: Backend, ctx: Context
+        ) -> PartialValidationResult:
+            if "bad" in chunk:
+                return PartialValidationResult("fail", reason="bad word detected")
+            return PartialValidationResult("unknown")
+
+    req = FailRequirement(description="no bad words")
+    result = await req.stream_validate("this is bad content", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert result.success == "fail"
+    assert result.reason == "bad word detected"
+
+    result_unknown = await req.stream_validate("good content", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert result_unknown.success == "unknown"
+
+
+@pytest.mark.asyncio
+async def test_does_not_mutate_requirement():
+    req = Requirement(description="original description")
+    original_description = req.description
+    original_output = req._output
+    original_validation_fn = req.validation_fn
+
+    await req.stream_validate("some chunk", backend=None, ctx=None)  # type: ignore[arg-type]
+
+    assert req.description == original_description
+    assert req._output == original_output
+    assert req.validation_fn == original_validation_fn
+
+
+@pytest.mark.asyncio
+async def test_stream_validate_idempotent():
+    req = Requirement(description="repeated calls")
+    result1 = await req.stream_validate("chunk one", backend=None, ctx=None)  # type: ignore[arg-type]
+    result2 = await req.stream_validate("chunk two", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert result1.success == "unknown"
+    assert result2.success == "unknown"
+    assert req._output is None
+
+
+@pytest.mark.asyncio
+async def test_stateful_subclass_accumulates_state():
+    """Stateful subclass correctly accumulates state across stream_validate calls.
+
+    Each call receives a single chunk (the delta produced by the chunking
+    strategy). Requirements maintain their own running state across calls
+    rather than re-scanning accumulated text.
+    """
+
+    class BulletCounter(Requirement):
+        def __init__(self) -> None:
+            super().__init__(description="no more than 3 bullets")
+            self._bullet_count = 0
+
+        async def stream_validate(
+            self, chunk: str, *, backend: Backend, ctx: Context
+        ) -> PartialValidationResult:
+            self._bullet_count += chunk.count("\n-")
+            if self._bullet_count > 3:
+                return PartialValidationResult(
+                    "fail", reason=f"{self._bullet_count} bullets exceeds limit"
+                )
+            return PartialValidationResult("unknown")
+
+    req = BulletCounter()
+    assert req._bullet_count == 0
+
+    await req.stream_validate("intro text", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert req._bullet_count == 0
+
+    await req.stream_validate("\n- one\n- two", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert req._bullet_count == 2
+
+    result = await req.stream_validate(
+        "\n- three\n- four",
+        backend=None,  # type: ignore[arg-type]
+        ctx=None,  # type: ignore[arg-type]
+    )
+    assert req._bullet_count == 4
+    assert result.success == "fail"
+    assert result.reason is not None and "4" in result.reason
+
+
+@pytest.mark.asyncio
+async def test_stateful_subclass_clone_isolation():
+    """Orchestrator clone pattern: copy() before each attempt gives a fresh independent clone.
+
+    The orchestrator holds the original requirement and never calls stream_validate on it
+    directly. Before each attempt it clones the original; each clone starts from the
+    original's (zero) state and advances independently.
+    """
+
+    class CallCounter(Requirement):
+        def __init__(self) -> None:
+            super().__init__(description="call counter")
+            self._calls = 0
+
+        async def stream_validate(
+            self, chunk: str, *, backend: Backend, ctx: Context
+        ) -> PartialValidationResult:
+            self._calls += 1
+            return PartialValidationResult("unknown")
+
+    req = CallCounter()  # original — never used directly by the orchestrator
+
+    # Attempt 1
+    attempt1 = copy(req)
+    assert attempt1._calls == 0
+    await attempt1.stream_validate("a", backend=None, ctx=None)  # type: ignore[arg-type]
+    await attempt1.stream_validate("b", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert attempt1._calls == 2
+
+    # Attempt 2 (retry) — fresh clone from the same original
+    attempt2 = copy(req)
+    assert attempt2._calls == 0  # starts clean, not carrying attempt1's state
+    await attempt2.stream_validate("c", backend=None, ctx=None)  # type: ignore[arg-type]
+    assert attempt2._calls == 1
+
+    assert req._calls == 0  # original never mutated