diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..6eaf346
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,107 @@
+"""Shared e2e fixtures (Step 13): a scripted chat client that emits a SYNTHETIC UsageDetails
+(so token accounting is real-shaped without an LLM), plus store + docs-dir fixtures.
+
+The synthetic ``UsageDetails`` is what lets the budget meter / provenance ``token_usage`` be a
+positive, UsageDetails-sourced number in CI — the REAL-provider populated-usage assertion is
+the gated live arm (Step 14).
+"""
+
+from __future__ import annotations
+
+from collections.abc import Callable, Sequence
+from typing import Any
+
+import pytest
+from agent_framework import (
+    BaseChatClient,
+    ChatResponse,
+    ChatResponseUpdate,
+    Message,
+    UsageDetails,
+)
+
+from portfolio_optimiser.verdicts import VerdictStore, seed_store
+
+
+class SyntheticUsageChatClient(BaseChatClient):
+    """Network-free chat client returning scripted/default replies WITH a synthetic
+    ``UsageDetails`` (``total_token_count``), so strict usage accounting does not hard-fail."""
+
+    OTEL_PROVIDER_NAME = "synthetic"
+
+    def __init__(
+        self,
+        scripted: Sequence[str] | None = None,
+        *,
+        default_reply: str = "ok",
+        tokens_per_reply: int = 8,
+    ) -> None:
+        super().__init__()
+        self._scripted = list(scripted or [])
+        self._idx = 0
+        self._default = default_reply
+        self._tokens = tokens_per_reply
+        self.call_count = 0
+
+    def _next_reply(self) -> str:
+        reply = self._scripted[self._idx] if self._idx < len(self._scripted) else self._default
+        self._idx += 1
+        self.call_count += 1
+        return reply
+
+    def _inner_get_response(
+        self, *, messages: Sequence[Message], stream: bool, options: Any, **kwargs: Any
+    ) -> Any:
+        reply = self._next_reply()
+        usage = UsageDetails(total_token_count=self._tokens)
+        if stream:
+
+            async def _agen() -> Any:
+                yield ChatResponseUpdate(
+                    role="assistant", contents=[{"type": "text", "text": reply}]
+                )
+
+            return self._build_response_stream(_agen())
+
+        async def _coro() -> ChatResponse:
+            return ChatResponse(
+                messages=[Message(role="assistant", contents=[reply])],
+                response_id="synthetic",
+                usage_details=usage,
+            )
+
+        return _coro()
+
+
+@pytest.fixture()
+def make_client_factory() -> Callable[..., Callable[[str], BaseChatClient]]:
+    """Return a maker that builds a per-role client factory emitting synthetic usage."""
+
+    def _make(default_reply: str, *, tokens: int = 8) -> Callable[[str], BaseChatClient]:
+        def factory(role: str) -> BaseChatClient:
+            return SyntheticUsageChatClient(default_reply=default_reply, tokens_per_reply=tokens)
+
+        return factory
+
+    return _make
+
+
+@pytest.fixture()
+def fresh_store() -> VerdictStore:
+    return VerdictStore(verdicts=[])
+
+
+@pytest.fixture()
+def seeded_store() -> VerdictStore:
+    return seed_store()
+
+
+@pytest.fixture()
+def docs_dir(tmp_path) -> str:
+    d = tmp_path / "docs"
+    d.mkdir()
+    (d / "cost.txt").write_text(
+        "Asphalt Ab11 unit rate renegotiation reduced the paving cost on the school stretch.",
+        encoding="utf-8",
+    )
+    return str(d)
diff --git a/tests/test_vertical_slice_e2e.py b/tests/test_vertical_slice_e2e.py
new file mode 100644
index 0000000..bc74d05
--- /dev/null
+++ b/tests/test_vertical_slice_e2e.py
@@ -0,0 +1,100 @@
+"""Step 13 — end-to-end vertical-slice integration (deterministic, synthetic usage, no LLM).
+
+Covers the six success criteria: (a) one valid proposal -> ValidatedProposal with populated
+provenance (token usage > 0 from synthetic UsageDetails); (b) out-of-range -> Rejection with
+reason; (c) Layer-2 verdict persisted; (d) a second structurally-similar run retrieves the
+prior verdict via ExpeL (exercising two-arg extend_instructions); (e) tiny budget halts via
+the meter/round-cap (cap not exceeded); (f) malformed contract raises before any chat call.
+Pattern: tests/spikes/test_harness.py + test_c_validator.py + test_d_verdictstore.py.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from portfolio_optimiser.budget import BudgetExceeded
+from portfolio_optimiser.run import run_project
+from portfolio_optimiser.validator import Rejection, ValidatedProposal
+
+_VALID = (
+    '{"project_id":"FV42-GSV-E1","measure":"Reduce scope",'
+    '"affected_items":[{"code":"05.2","quantity":4300,"unit_cost":215},'
+    '{"code":"03.1","quantity":1800,"unit_cost":310}],"claimed_saving_nok":200000}'
+)
+_OUT_OF_RANGE = _VALID.replace("200000", "800000")  # parses fine, exceeds the ~445k P90
+
+_VI = {"decision": "approved", "rationale": "feasible within range"}
+
+
+async def test_a_valid_proposal_end_to_end(docs_dir, make_client_factory, fresh_store) -> None:
+    result = await run_project(
+        "FV42-GSV-E1", "local", docs_dir=docs_dir, verdict_input=_VI,
+        client_factory=make_client_factory(_VALID), store=fresh_store,
+    )
+    assert isinstance(result.outcome, ValidatedProposal)
+    stamp = result.provenance
+    assert len(stamp.citations) >= 1 and stamp.model and stamp.role
+    assert stamp.validator_decision == "validated"
+    assert stamp.token_usage > 0  # sourced from the synthetic UsageDetails
+
+
+async def test_b_out_of_range_is_rejected(docs_dir, make_client_factory, fresh_store) -> None:
+    result = await run_project(
+        "FV42-GSV-E1", "local", docs_dir=docs_dir,
+        verdict_input={"decision": "rejected", "rationale": "claim too high"},
+        client_factory=make_client_factory(_OUT_OF_RANGE), store=fresh_store,
+    )
+    assert isinstance(result.outcome, Rejection)
+    assert result.outcome.reason
+
+
+async def test_c_layer2_verdict_is_persisted(docs_dir, make_client_factory, fresh_store) -> None:
+    result = await run_project(
+        "FV42-GSV-E1", "local", docs_dir=docs_dir, verdict_input=_VI,
+        client_factory=make_client_factory(_VALID), store=fresh_store,
+    )
+    assert any(v.id == result.verdict.id for v in result.store.verdicts)
+
+
+async def test_d_second_run_retrieves_prior_verdict(
+    docs_dir, make_client_factory, fresh_store
+) -> None:
+    factory = make_client_factory(_VALID)
+    r1 = await run_project(
+        "FV42-GSV-E1", "local", docs_dir=docs_dir, verdict_input=_VI,
+        client_factory=factory, store=fresh_store,
+    )
+    r2 = await run_project(
+        "FV42-GSV-E1", "local", docs_dir=docs_dir, verdict_input=_VI,
+        client_factory=factory, store=fresh_store,
+    )
+    assert r2.retrieved  # ExpeL surfaced prior verdicts on the second run
+    assert r2.retrieved[0].id == r1.verdict.id  # structurally identical -> stable id
+
+
+async def test_e_tiny_budget_halts_without_exceeding(
+    docs_dir, make_client_factory, fresh_store
+) -> None:
+    with pytest.raises(BudgetExceeded):
+        await run_project(
+            "FV42-GSV-E1", "local", docs_dir=docs_dir, verdict_input=_VI,
+            client_factory=make_client_factory(_VALID, tokens=100),
+            store=fresh_store, max_tokens=10,  # synthetic 100 tokens > cap 10 -> halt
+        )
+
+
+async def test_f_malformed_contract_raises_before_any_chat(
+    docs_dir, make_client_factory
+) -> None:
+    calls = {"n": 0}
+    base = make_client_factory(_VALID)
+
+    def spy(role: str):
+        calls["n"] += 1
+        return base(role)
+
+    with pytest.raises(ValidationError):
+        await run_project(
+            "FV42-GSV-E1", "local", docs_dir=docs_dir, verdict_input=_VI,
+            client_factory=spy, top_k=0,  # malformed data-source contract
+        )
+    assert calls["n"] == 0  # validation fails before any chat client is touched