feat: add cross-encoder reranking to Document-RAG with two-limit control (#878) (#1011)

Wire the FlashRank reranker subsystem from #1005 into Document-RAG: after
vector retrieval, over-fetch a wider candidate pool, rerank with the
cross-encoder, and keep the top doc_limit chunks for synthesis.

Per maintainer review, the fetch and select sizes are two caller-controlled
limits rather than one internal heuristic:

- doc_limit:   chunks selected into the synthesis prompt (unchanged meaning).
- fetch_limit: candidate pool pulled from the vector store before reranking.
  0 = derive (OVERFETCH_FACTOR x doc_limit); values below doc_limit are
  raised to it. Lets the caller control how hard the reranker has to work.

Details:
- schema: DocumentRagQuery.fetch_limit (additive, backward compatible).
- document_rag.py / rag.py: fetch_limit resolved in the processor (mirrors
  doc_limit); the core applies the heuristic default and derives synthesis
  provenance from the chunk-selection focus when reranking ran.
- provenance: tg:ChunkSelection focus stage (mirrors tg:EdgeSelection).
- request translator + client SDKs + CLI: fetch-limit / --fetch-limit,
  threaded exactly like doc_limit and the GraphRAG limits.
- tests: no-op identity, over-fetch/narrow, explicit fetch_limit, heuristic
  default, floor-at-doc_limit, provenance lineage, cross-repo topic wiring.

Reranking is skipped byte-identically when no reranker role is wired.
Requires the companion trustgraph-templates change wiring the reranker
topics into the document-rag flow (mirrors #279 for GraphRAG).

tests/unit/test_retrieval/test_document_rag.py

@@ -101,27 +101,27 @@ class TestQuery:
         assert query.rag == mock_rag
         assert query.collection == "test_collection"
         assert query.verbose is False
-        assert query.doc_limit == 20  # Default value
+        assert query.fetch_limit == 20  # Default value
 
-    def test_query_initialization_with_custom_doc_limit(self):
+    def test_query_initialization_with_custom_fetch_limit(self):
-        """Test Query initialization with custom doc_limit"""
+        """Test Query initialization with custom fetch_limit"""
         # Create mock DocumentRag
         mock_rag = MagicMock()
 
-        # Initialize Query with custom doc_limit
+        # Initialize Query with custom fetch_limit
         query = Query(
             rag=mock_rag,
             workspace="test_workspace",
             collection="custom_collection",
             verbose=True,
-            doc_limit=50
+            fetch_limit=50
         )
 
         # Verify initialization
         assert query.rag == mock_rag
         assert query.collection == "custom_collection"
         assert query.verbose is True
-        assert query.doc_limit == 50
+        assert query.fetch_limit == 50
 
     @pytest.mark.asyncio
     async def test_extract_concepts(self):
@@ -224,7 +224,7 @@ class TestQuery:
             workspace="test_workspace",
             collection="test_collection",
             verbose=False,
-            doc_limit=15
+            fetch_limit=15
         )
 
         # Call get_docs with concepts list
@@ -377,7 +377,7 @@ class TestQuery:
             workspace="test_workspace",
             collection="test_collection",
             verbose=True,
-            doc_limit=5
+            fetch_limit=5
         )
 
         # Call get_docs with concepts
@@ -615,7 +615,7 @@ class TestQuery:
             workspace="test_workspace",
             collection="test_collection",
             verbose=False,
-            doc_limit=10
+            fetch_limit=10
         )
 
         docs, chunk_ids = await query.get_docs(["concept A", "concept B"])

tests/unit/test_retrieval/test_document_rag_rerank.py

@@ -0,0 +1,478 @@
+"""
+Tests for the optional cross-encoder reranking pass in DocumentRag.query().
+
+Two behaviours are covered:
+
+  1. No-op: when no reranker_client is wired (the default), query() must feed
+     the LLM the exact same chunks, in the same order, that retrieval produced
+     - byte-identical to the pre-reranker behaviour - and must NOT emit a
+     chunk-selection provenance event.
+
+  2. Rerank: when a reranker_client is wired, the retrieved chunks are reordered
+     and truncated according to the reranker's results, the LLM receives the
+     reranked top-N, and a tg:ChunkSelection (focus) provenance event is emitted
+     carrying the per-surviving-chunk scores and chunk references.
+
+These are pure orchestration tests - the reranker is a stub, so there is no
+torch / network dependency.
+"""
+
+import pytest
+from unittest.mock import AsyncMock
+from dataclasses import dataclass
+
+from trustgraph.retrieval.document_rag.document_rag import DocumentRag
+from trustgraph.base import PromptResult
+from trustgraph.schema import RerankerResult
+
+from trustgraph.provenance.namespaces import (
+    RDF_TYPE, PROV_WAS_DERIVED_FROM,
+    TG_DOC_RAG_QUESTION, TG_GROUNDING, TG_EXPLORATION,
+    TG_FOCUS, TG_SYNTHESIS,
+    TG_CHUNK_SELECTION, TG_SELECTED_CHUNK, TG_SCORE, TG_DOCUMENT,
+)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def find_triple(triples, predicate, subject=None):
+    for t in triples:
+        if t.p.iri == predicate:
+            if subject is None or t.s.iri == subject:
+                return t
+    return None
+
+
+def find_triples(triples, predicate, subject=None):
+    return [
+        t for t in triples
+        if t.p.iri == predicate
+        and (subject is None or t.s.iri == subject)
+    ]
+
+
+def has_type(triples, subject, rdf_type):
+    return any(
+        t.s.iri == subject and t.p.iri == RDF_TYPE and t.o.iri == rdf_type
+        for t in triples
+    )
+
+
+def derived_from(triples, subject):
+    t = find_triple(triples, PROV_WAS_DERIVED_FROM, subject)
+    return t.o.iri if t else None
+
+
+@dataclass
+class ChunkMatch:
+    """Mimics the result from doc_embeddings_client.query()."""
+    chunk_id: str
+
+
+# ---------------------------------------------------------------------------
+# Fixtures: three retrievable chunks
+# ---------------------------------------------------------------------------
+
+CHUNK_A = "urn:chunk:policy-doc-1:chunk-0"
+CHUNK_B = "urn:chunk:policy-doc-1:chunk-1"
+CHUNK_C = "urn:chunk:policy-doc-1:chunk-2"
+
+CHUNK_A_CONTENT = "Customers may return items within 30 days of purchase."
+CHUNK_B_CONTENT = "Our stores are open from 9am to 5pm on weekdays."
+CHUNK_C_CONTENT = "Refunds are processed to the original payment method."
+
+# Retrieval (post-dedupe) order is A, B, C.
+ORDERED_CONTENT = [CHUNK_A_CONTENT, CHUNK_B_CONTENT, CHUNK_C_CONTENT]
+ORDERED_CHUNK_IDS = [CHUNK_A, CHUNK_B, CHUNK_C]
+
+
+def build_mock_clients():
+    """
+    Build mock subsidiary clients for a document-rag query returning three
+    distinct chunks (A, B, C) in that order.
+    """
+    prompt_client = AsyncMock()
+    embeddings_client = AsyncMock()
+    doc_embeddings_client = AsyncMock()
+    fetch_chunk = AsyncMock()
+
+    async def mock_prompt(template_id, variables=None, **kwargs):
+        if template_id == "extract-concepts":
+            return PromptResult(response_type="text", text="return policy\nrefund")
+        return PromptResult(response_type="text", text="")
+
+    prompt_client.prompt.side_effect = mock_prompt
+
+    embeddings_client.embed.return_value = [[0.1, 0.2], [0.3, 0.4]]
+
+    # Each concept query returns the same three chunks; dedupe keeps A, B, C.
+    doc_embeddings_client.query.return_value = [
+        ChunkMatch(chunk_id=CHUNK_A),
+        ChunkMatch(chunk_id=CHUNK_B),
+        ChunkMatch(chunk_id=CHUNK_C),
+    ]
+
+    async def mock_fetch(chunk_id):
+        return {
+            CHUNK_A: CHUNK_A_CONTENT,
+            CHUNK_B: CHUNK_B_CONTENT,
+            CHUNK_C: CHUNK_C_CONTENT,
+        }[chunk_id]
+
+    fetch_chunk.side_effect = mock_fetch
+
+    prompt_client.document_prompt.return_value = PromptResult(
+        response_type="text",
+        text="Items can be returned within 30 days for a full refund.",
+    )
+
+    return prompt_client, embeddings_client, doc_embeddings_client, fetch_chunk
+
+
+class StubReranker:
+    """
+    Stub reranker_client mirroring RerankerClient.rerank(): returns a fixed,
+    pre-sorted, truncated list of RerankerResult - exactly the contract the
+    flashrank service guarantees (sorted desc by score, truncated to limit).
+    """
+
+    def __init__(self, results):
+        self._results = results
+        self.calls = []
+
+    async def rerank(self, queries, documents, limit=10, timeout=300):
+        self.calls.append(
+            {"queries": queries, "documents": documents, "limit": limit}
+        )
+        return self._results
+
+
+# ---------------------------------------------------------------------------
+# 1. No-op: reranker_client=None must not change anything
+# ---------------------------------------------------------------------------
+
+class TestRerankNoOp:
+
+    @pytest.mark.asyncio
+    async def test_documents_passed_to_llm_are_unchanged(self):
+        """
+        With no reranker wired, document_prompt must receive the retrieved
+        chunks in the original order and length.
+        """
+        clients = build_mock_clients()
+        rag = DocumentRag(*clients)  # reranker_client defaults to None
+
+        await rag.query(query="What is the return policy?")
+
+        call = rag.prompt_client.document_prompt.call_args
+        passed_docs = call.kwargs["documents"]
+        assert passed_docs == ORDERED_CONTENT
+
+    @pytest.mark.asyncio
+    async def test_no_chunk_selection_event_emitted(self):
+        """
+        Without a reranker, the provenance chain is the original 4 stages:
+        question, grounding, exploration, synthesis - no focus stage.
+        """
+        clients = build_mock_clients()
+        rag = DocumentRag(*clients)
+
+        events = []
+
+        async def explain_callback(triples, explain_id):
+            events.append({"triples": triples, "explain_id": explain_id})
+
+        await rag.query(
+            query="What is the return policy?",
+            explain_callback=explain_callback,
+        )
+
+        assert len(events) == 4
+        types = [
+            TG_DOC_RAG_QUESTION, TG_GROUNDING, TG_EXPLORATION, TG_SYNTHESIS,
+        ]
+        for i, expected in enumerate(types):
+            assert has_type(events[i]["triples"], events[i]["explain_id"], expected)
+
+        # No chunk-selection entity anywhere.
+        for e in events:
+            assert not any(
+                t.o.iri == TG_CHUNK_SELECTION
+                for t in e["triples"]
+                if t.p.iri == RDF_TYPE
+            )
+
+    @pytest.mark.asyncio
+    async def test_synthesis_derives_from_exploration_when_no_rerank(self):
+        """
+        No-op lineage is unchanged: synthesis derives from exploration
+        (there is no focus stage). Guards the conditional synthesis parent.
+        """
+        clients = build_mock_clients()
+        rag = DocumentRag(*clients)
+
+        events = []
+
+        async def explain_callback(triples, explain_id):
+            events.append({"triples": triples, "explain_id": explain_id})
+
+        await rag.query(
+            query="What is the return policy?",
+            explain_callback=explain_callback,
+        )
+
+        # events: question, grounding, exploration, synthesis
+        exp_uri = events[2]["explain_id"]
+        syn_event = events[3]
+        assert derived_from(syn_event["triples"], syn_event["explain_id"]) == exp_uri
+
+
+# ---------------------------------------------------------------------------
+# 2. Rerank: reorder + truncate + provenance
+# ---------------------------------------------------------------------------
+
+class TestRerankActive:
+
+    def _reranker_keeping_C_then_A(self):
+        # Reranker says chunk index 2 (C) is best, then index 0 (A); B dropped.
+        # Pre-sorted desc by score and truncated to limit, per the contract.
+        return StubReranker([
+            RerankerResult(document_id="2", query_id="0", score=0.95),
+            RerankerResult(document_id="0", query_id="0", score=0.42),
+        ])
+
+    @pytest.mark.asyncio
+    async def test_documents_reordered_and_truncated(self):
+        clients = build_mock_clients()
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        await rag.query(query="What is the return policy?")
+
+        call = rag.prompt_client.document_prompt.call_args
+        passed_docs = call.kwargs["documents"]
+        assert passed_docs == [CHUNK_C_CONTENT, CHUNK_A_CONTENT]
+
+    @pytest.mark.asyncio
+    async def test_reranker_called_with_single_query_and_all_docs(self):
+        clients = build_mock_clients()
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        await rag.query(query="What is the return policy?", doc_limit=2)
+
+        assert len(reranker.calls) == 1
+        c = reranker.calls[0]
+        assert c["queries"] == [{"id": "0", "text": "What is the return policy?"}]
+        assert c["documents"] == [
+            {"id": "0", "text": CHUNK_A_CONTENT},
+            {"id": "1", "text": CHUNK_B_CONTENT},
+            {"id": "2", "text": CHUNK_C_CONTENT},
+        ]
+        # The rerank narrows down to the final doc_limit, NOT fetch_limit
+        # (fetch_limit is the over-fetched candidate pool size).
+        assert c["limit"] == 2
+
+    @pytest.mark.asyncio
+    async def test_explicit_fetch_limit_over_fetches_then_narrows(self):
+        """
+        Semantic guard for the value of reranking AND the maintainer's two-limit
+        contract: an explicit fetch_limit makes retrieval OVER-FETCH a wider
+        candidate pool so the cross-encoder can surface chunks the bi-encoder
+        ranked outside the final doc_limit, then the rerank narrows the pool back
+        down to doc_limit. The fetch_limit is honoured directly (caller controls
+        how hard the reranker works), not overridden by any heuristic.
+        """
+        clients = build_mock_clients()
+        prompt_client, embeddings_client, doc_embeddings_client, fetch_chunk = clients
+        reranker = self._reranker_keeping_C_then_A()
+        # Candidate pool (fetch_limit=60) >> final doc_limit (6).
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        await rag.query(
+            query="What is the return policy?", doc_limit=6, fetch_limit=60,
+        )
+
+        # Over-fetch: the embeddings store is queried with the fetch_limit
+        # budget (60 // 2 concept-vectors = 30 per concept), NOT the doc_limit
+        # budget (6 // 2 = 3). This is the bug guard.
+        q_limit = doc_embeddings_client.query.call_args.kwargs["limit"]
+        assert q_limit == 30
+
+        # Narrow: the rerank keeps the final doc_limit (6), not fetch_limit.
+        assert reranker.calls[0]["limit"] == 6
+
+    @pytest.mark.asyncio
+    async def test_default_fetch_limit_derives_overfetch_from_doc_limit(self):
+        """
+        With no fetch_limit passed to query(), the candidate pool falls back to
+        the OVERFETCH_FACTOR x doc_limit heuristic, so over-fetch scales with
+        doc_limit and reranking keeps its recall benefit out of the box.
+        """
+        clients = build_mock_clients()
+        prompt_client, embeddings_client, doc_embeddings_client, fetch_chunk = clients
+        reranker = self._reranker_keeping_C_then_A()
+        # No fetch_limit -> heuristic default.
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        await rag.query(query="What is the return policy?", doc_limit=20)
+
+        # fetch = 3 x 20 = 60 -> 60 // 2 concept-vectors = 30 per concept.
+        q_limit = doc_embeddings_client.query.call_args.kwargs["limit"]
+        assert q_limit == 30
+        # Rerank narrows to the final doc_limit (20).
+        assert reranker.calls[0]["limit"] == 20
+
+    @pytest.mark.asyncio
+    async def test_fetch_limit_floored_at_doc_limit(self):
+        """
+        A fetch_limit below doc_limit is floored up to doc_limit: retrieval must
+        never fetch fewer candidates than the rerank is asked to keep, else the
+        prompt could not be filled.
+        """
+        clients = build_mock_clients()
+        prompt_client, embeddings_client, doc_embeddings_client, fetch_chunk = clients
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        await rag.query(
+            query="What is the return policy?", doc_limit=10, fetch_limit=4,
+        )
+
+        # fetch = max(4, 10) = 10 -> 10 // 2 concept-vectors = 5 per concept.
+        q_limit = doc_embeddings_client.query.call_args.kwargs["limit"]
+        assert q_limit == 5
+        assert reranker.calls[0]["limit"] == 10
+
+    @pytest.mark.asyncio
+    async def test_chunk_selection_event_emitted(self):
+        clients = build_mock_clients()
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        events = []
+
+        async def explain_callback(triples, explain_id):
+            events.append({"triples": triples, "explain_id": explain_id})
+
+        await rag.query(
+            query="What is the return policy?",
+            explain_callback=explain_callback,
+        )
+
+        # Now 5 stages: question, grounding, exploration, focus, synthesis.
+        assert len(events) == 5
+        ordered_types = [
+            TG_DOC_RAG_QUESTION, TG_GROUNDING, TG_EXPLORATION,
+            TG_FOCUS, TG_SYNTHESIS,
+        ]
+        for i, expected in enumerate(ordered_types):
+            assert has_type(events[i]["triples"], events[i]["explain_id"], expected)
+
+    @pytest.mark.asyncio
+    async def test_chunk_selection_carries_scores_and_chunk_refs(self):
+        clients = build_mock_clients()
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        events = []
+
+        async def explain_callback(triples, explain_id):
+            events.append({"triples": triples, "explain_id": explain_id})
+
+        await rag.query(
+            query="What is the return policy?",
+            explain_callback=explain_callback,
+        )
+
+        focus_event = events[3]
+        foc_uri = focus_event["explain_id"]
+        triples = focus_event["triples"]
+
+        # focus is derived from exploration
+        exp_uri = events[2]["explain_id"]
+        assert derived_from(triples, foc_uri) == exp_uri
+
+        # Two ChunkSelection sub-entities, linked from focus.
+        sel_links = find_triples(triples, TG_SELECTED_CHUNK, foc_uri)
+        assert len(sel_links) == 2
+
+        # Each selection has a ChunkSelection type, a chunk document ref and a score.
+        chunk_refs = set()
+        scores = set()
+        for link in sel_links:
+            sel_uri = link.o.iri
+            assert has_type(triples, sel_uri, TG_CHUNK_SELECTION)
+            doc_ref = find_triple(triples, TG_DOCUMENT, sel_uri)
+            assert doc_ref is not None
+            chunk_refs.add(doc_ref.o.iri)
+            score_t = find_triple(triples, TG_SCORE, sel_uri)
+            assert score_t is not None
+            scores.add(score_t.o.value)
+
+        # Surviving chunks are C and A (B dropped), with the reranker scores.
+        assert chunk_refs == {CHUNK_C, CHUNK_A}
+        assert scores == {"0.95", "0.42"}
+
+    @pytest.mark.asyncio
+    async def test_all_focus_triples_in_retrieval_graph(self):
+        clients = build_mock_clients()
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        events = []
+
+        async def explain_callback(triples, explain_id):
+            events.append({"triples": triples, "explain_id": explain_id})
+
+        await rag.query(
+            query="What is the return policy?",
+            explain_callback=explain_callback,
+        )
+
+        for t in events[3]["triples"]:
+            assert t.g == "urn:graph:retrieval"
+
+    @pytest.mark.asyncio
+    async def test_synthesis_derives_from_focus_when_reranking(self):
+        """
+        When reranking runs, synthesis must derive from the focus node (the
+        reranked chunks actually fed to the LLM), mirroring GraphRAG - not from
+        exploration, which would leave focus as a dangling branch and
+        misrepresent what fed the answer.
+        """
+        clients = build_mock_clients()
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        events = []
+
+        async def explain_callback(triples, explain_id):
+            events.append({"triples": triples, "explain_id": explain_id})
+
+        await rag.query(
+            query="What is the return policy?",
+            doc_limit=2,
+            explain_callback=explain_callback,
+        )
+
+        # events: question, grounding, exploration, focus, synthesis
+        foc_uri = events[3]["explain_id"]
+        syn_event = events[4]
+        assert derived_from(syn_event["triples"], syn_event["explain_id"]) == foc_uri
+
+    @pytest.mark.asyncio
+    async def test_empty_docs_skips_reranker(self):
+        """If retrieval returns no chunks, the reranker is never called."""
+        clients = build_mock_clients()
+        prompt_client, embeddings_client, doc_embeddings_client, fetch_chunk = clients
+        doc_embeddings_client.query.return_value = []  # no matches
+
+        reranker = self._reranker_keeping_C_then_A()
+        rag = DocumentRag(*clients, reranker_client=reranker)
+
+        await rag.query(query="What is the return policy?")
+
+        assert reranker.calls == []

tests/unit/test_retrieval/test_document_rag_reranker_wiring.py

@@ -0,0 +1,89 @@
+"""
+Cross-layer wiring contract for the Document-RAG reranker (issue #878).
+
+The Document-RAG processor registers a ``RerankerClientSpec`` for the
+``reranker-request`` / ``reranker-response`` roles (see
+``retrieval/document_rag/rag.py``). At flow construction every spec runs
+``spec.add(flow, processor, definition)``, and ``RequestResponseSpec.add``
+resolves its topics via ``definition["topics"][name]`` - which raises
+``KeyError`` if the flow blueprint does not provide those topics.
+
+This means the monorepo code change is only safe to deploy together with the
+companion ``trustgraph-templates`` change that wires ``reranker-request`` /
+``reranker-response`` into the Document-RAG flow (mirroring what templates
+PR #279 did for GraphRAG via ``graph-store.jsonnet``). These tests pin that
+contract from the monorepo side:
+
+  * with the reranker topics present (as the updated templates compile them),
+    the spec binds cleanly and registers the client;
+  * without them (the pre-companion blueprint), construction fails fast with a
+    KeyError naming the missing role - documenting exactly why the templates
+    change is required.
+
+No broker/network: the pub/sub backend is mocked (topics are bound at add()
+time, connections happen later at start()).
+"""
+
+import pytest
+from unittest.mock import MagicMock
+
+from trustgraph.base import RerankerClientSpec
+
+
+def _flow():
+    f = MagicMock()
+    f.workspace = "ws"
+    f.name = "document-rag"
+    f.id = "proc1"
+    f.consumer = {}
+    return f
+
+
+def _processor():
+    p = MagicMock()
+    p.pubsub = MagicMock()
+    p.id = "proc1"
+    p.taskgroup = MagicMock()
+    return p
+
+
+def _spec():
+    return RerankerClientSpec(
+        request_name="reranker-request",
+        response_name="reranker-response",
+    )
+
+
+# Topics dict as the UPDATED document-store.jsonnet compiles them
+# (verified by compiling the template: reranker-request -> request:tg:reranker:{workspace}:{id}).
+DEFINITION_WITH_RERANKER = {
+    "topics": {
+        "request": "request:tg:document-rag:ws:id",
+        "response": "response:tg:document-rag:ws:id",
+        "reranker-request": "request:tg:reranker:ws:id",
+        "reranker-response": "response:tg:reranker:ws:id",
+    }
+}
+
+# Pre-companion blueprint: no reranker topics (document-rag before the templates change).
+DEFINITION_WITHOUT_RERANKER = {
+    "topics": {
+        "request": "request:tg:document-rag:ws:id",
+        "response": "response:tg:document-rag:ws:id",
+    }
+}
+
+
+def test_reranker_client_binds_when_flow_provides_topics():
+    flow = _flow()
+    _spec().add(flow, _processor(), DEFINITION_WITH_RERANKER)
+    # The client consumer is registered against the reranker role.
+    assert "reranker-request" in flow.consumer
+
+
+def test_reranker_client_keyerrors_without_companion_template_topics():
+    with pytest.raises(KeyError) as exc:
+        _spec().add(_flow(), _processor(), DEFINITION_WITHOUT_RERANKER)
+    # Fails fast naming the missing role -> the trustgraph-templates companion
+    # change (wire reranker-request/response into the document-rag flow) is required.
+    assert "reranker-request" in str(exc.value)

tests/unit/test_retrieval/test_document_rag_service.py

@@ -66,6 +66,7 @@ class TestDocumentRagService:
             workspace=ANY,            # Workspace comes from flow.workspace (mock)
             collection="test_coll_1", # Must be from message, not hardcoded default
             doc_limit=5,
+            fetch_limit=0,            # Unset -> core derives the candidate pool
             explain_callback=ANY,     # Explainability callback is always passed
             save_answer_callback=ANY, # Librarian save callback is always passed
         )

trustgraph-base/trustgraph/api/async_flow.py

@@ -527,7 +527,8 @@ class AsyncFlowInstance:
         return result.get("response", "")
 
     async def document_rag(self, query: str, collection: str,
-                           doc_limit: int = 10, **kwargs: Any) -> str:
+                           doc_limit: int = 10, fetch_limit: int = 0,
+                           **kwargs: Any) -> str:
         """
         Execute document-based RAG query (non-streaming).
 
@@ -541,7 +542,9 @@ class AsyncFlowInstance:
         Args:
             query: User query text
             collection: Collection identifier containing documents
-            doc_limit: Maximum number of document chunks to retrieve (default: 10)
+            doc_limit: Document chunks selected into the prompt (default: 10)
+            fetch_limit: Candidate chunks fetched from the vector store before
+                reranking (default: 0 = derive from doc_limit)
             **kwargs: Additional service-specific parameters
 
         Returns:
@@ -564,6 +567,7 @@ class AsyncFlowInstance:
             "query": query,
             "collection": collection,
             "doc-limit": doc_limit,
+            "fetch-limit": fetch_limit,
             "streaming": False
         }
         request_data.update(kwargs)

trustgraph-base/trustgraph/api/async_socket_client.py

@@ -379,12 +379,14 @@ class AsyncSocketFlowInstance:
                 yield chunk.content
 
     async def document_rag(self, query: str, collection: str,
-                           doc_limit: int = 10, streaming: bool = False, **kwargs):
+                           doc_limit: int = 10, fetch_limit: int = 0,
+                           streaming: bool = False, **kwargs):
         """Document RAG with optional streaming"""
         request = {
             "query": query,
             "collection": collection,
             "doc-limit": doc_limit,
+            "fetch-limit": fetch_limit,
             "streaming": streaming
         }
         request.update(kwargs)

trustgraph-base/trustgraph/api/flow.py

@@ -415,7 +415,7 @@ class FlowInstance:
 
     def document_rag(
             self, query,collection="default",
-            doc_limit=10,
+            doc_limit=10, fetch_limit=0,
     ):
         """
         Execute document-based Retrieval-Augmented Generation (RAG) query.
@@ -426,7 +426,9 @@ class FlowInstance:
         Args:
             query: Natural language query
             collection: Collection identifier (default: "default")
-            doc_limit: Maximum document chunks to retrieve (default: 10)
+            doc_limit: Document chunks selected into the prompt (default: 10)
+            fetch_limit: Candidate chunks fetched from the vector store before
+                reranking (default: 0 = derive from doc_limit)
 
         Returns:
             str: Generated response incorporating document context
@@ -447,6 +449,7 @@ class FlowInstance:
             "query": query,
             "collection": collection,
             "doc-limit": doc_limit,
+            "fetch-limit": fetch_limit,
         }
 
         result = self.request(

trustgraph-base/trustgraph/api/socket_client.py

@@ -752,6 +752,7 @@ class SocketFlowInstance:
         query: str,
         collection: str,
         doc_limit: int = 10,
+        fetch_limit: int = 0,
         streaming: bool = False,
         **kwargs: Any
     ) -> Union[TextCompletionResult, Iterator[RAGChunk]]:
@@ -764,6 +765,7 @@ class SocketFlowInstance:
             "query": query,
             "collection": collection,
             "doc-limit": doc_limit,
+            "fetch-limit": fetch_limit,
             "streaming": streaming
         }
         request.update(kwargs)
@@ -785,6 +787,7 @@ class SocketFlowInstance:
         query: str,
         collection: str,
         doc_limit: int = 10,
+        fetch_limit: int = 0,
         **kwargs: Any
     ) -> Iterator[Union[RAGChunk, ProvenanceEvent]]:
         """Execute document-based RAG query with explainability support."""
@@ -792,6 +795,7 @@ class SocketFlowInstance:
             "query": query,
             "collection": collection,
             "doc-limit": doc_limit,
+            "fetch-limit": fetch_limit,
             "streaming": True,
             "explainable": True,
         }

trustgraph-base/trustgraph/messaging/translators/retrieval.py

@@ -12,6 +12,7 @@ class DocumentRagRequestTranslator(MessageTranslator):
             query=data["query"],
             collection=data.get("collection", "default"),
             doc_limit=int(data.get("doc-limit", 20)),
+            fetch_limit=int(data.get("fetch-limit", 0)),
             streaming=data.get("streaming", False)
         )
 
@@ -20,6 +21,7 @@ class DocumentRagRequestTranslator(MessageTranslator):
             "query": obj.query,
             "collection": obj.collection,
             "doc-limit": obj.doc_limit,
+            "fetch-limit": obj.fetch_limit,
             "streaming": getattr(obj, "streaming", False)
         }
 

trustgraph-base/trustgraph/provenance/__init__.py

@@ -64,6 +64,8 @@ from . uris import (
     docrag_question_uri,
     docrag_grounding_uri,
     docrag_exploration_uri,
+    docrag_focus_uri,
+    chunk_selection_uri,
     docrag_synthesis_uri,
 )
 
@@ -94,6 +96,8 @@ from . namespaces import (
     TG_EDGE_SELECTION,
     # Query-time provenance predicates (DocumentRAG)
     TG_CHUNK_COUNT, TG_SELECTED_CHUNK,
+    # Chunk selection entity type
+    TG_CHUNK_SELECTION,
     # Explainability entity types
     TG_QUESTION, TG_GROUNDING, TG_EXPLORATION, TG_FOCUS, TG_SYNTHESIS,
     TG_ANALYSIS, TG_CONCLUSION,
@@ -132,6 +136,7 @@ from . triples import (
     # Query-time provenance triple builders (DocumentRAG)
     docrag_question_triples,
     docrag_exploration_triples,
+    docrag_chunk_selection_triples,
     docrag_synthesis_triples,
     # Utility
     set_graph,
@@ -196,6 +201,8 @@ __all__ = [
     "docrag_question_uri",
     "docrag_grounding_uri",
     "docrag_exploration_uri",
+    "docrag_focus_uri",
+    "chunk_selection_uri",
     "docrag_synthesis_uri",
     # Namespaces
     "PROV", "PROV_ENTITY", "PROV_ACTIVITY", "PROV_AGENT",
@@ -219,6 +226,8 @@ __all__ = [
     "TG_EDGE_SELECTION",
     # Query-time provenance predicates (DocumentRAG)
     "TG_CHUNK_COUNT", "TG_SELECTED_CHUNK",
+    # Chunk selection entity type
+    "TG_CHUNK_SELECTION",
     # Explainability entity types
     "TG_QUESTION", "TG_GROUNDING", "TG_EXPLORATION", "TG_FOCUS", "TG_SYNTHESIS",
     "TG_ANALYSIS", "TG_CONCLUSION",
@@ -254,6 +263,7 @@ __all__ = [
     # Query-time provenance triple builders (DocumentRAG)
     "docrag_question_triples",
     "docrag_exploration_triples",
+    "docrag_chunk_selection_triples",
     "docrag_synthesis_triples",
     # Agent provenance triple builders
     "agent_session_triples",

trustgraph-base/trustgraph/provenance/namespaces.py

@@ -76,6 +76,9 @@ TG_EDGE_SELECTION = TG + "EdgeSelection"
 TG_CHUNK_COUNT = TG + "chunkCount"
 TG_SELECTED_CHUNK = TG + "selectedChunk"
 
+# Chunk selection entity type (cross-encoder reranked chunk in Focus)
+TG_CHUNK_SELECTION = TG + "ChunkSelection"
+
 # Extraction provenance entity types
 TG_DOCUMENT_TYPE = TG + "Document"
 TG_PAGE_TYPE = TG + "Page"

trustgraph-base/trustgraph/provenance/triples.py

@@ -30,6 +30,8 @@ from . namespaces import (
     TG_EDGE_SELECTION,
     # Query-time provenance predicates (DocumentRAG)
     TG_CHUNK_COUNT, TG_SELECTED_CHUNK,
+    # Chunk selection entity type
+    TG_CHUNK_SELECTION,
     # Explainability entity types
     TG_QUESTION, TG_GROUNDING, TG_EXPLORATION, TG_FOCUS, TG_SYNTHESIS,
     # Unifying types
@@ -40,7 +42,10 @@ from . namespaces import (
     TG_IN_TOKEN, TG_OUT_TOKEN,
 )
 
-from . uris import activity_uri, agent_uri, subgraph_uri, edge_selection_uri
+from . uris import (
+    activity_uri, agent_uri, subgraph_uri, edge_selection_uri,
+    chunk_selection_uri,
+)
 
 
 def set_graph(triples: List[Triple], graph: str) -> List[Triple]:
@@ -718,6 +723,75 @@ def docrag_exploration_triples(
     return triples
 
 
+def docrag_chunk_selection_triples(
+    focus_uri: str,
+    exploration_uri: str,
+    selected_chunks_with_scores: List[dict],
+    session_id: str,
+) -> List[Triple]:
+    """
+    Build triples for a document RAG focus entity (chunks selected by the
+    cross-encoder reranker).
+
+    Mirrors GraphRAG's focus_triples / tg:EdgeSelection pattern: a Focus entity
+    derived from exploration, with one ChunkSelection sub-entity per surviving
+    chunk carrying the chunk reference and the reranker score.
+
+    Structure:
+        <focus> a tg:Focus ; prov:wasDerivedFrom <exploration> .
+        <focus> tg:selectedChunk <chunk_sel_0> .
+        <chunk_sel_0> a tg:ChunkSelection .
+        <chunk_sel_0> tg:document <chunk_id> .
+        <chunk_sel_0> tg:score "0.97" .
+
+    Args:
+        focus_uri: URI of the focus entity (from docrag_focus_uri)
+        exploration_uri: URI of the parent exploration entity
+        selected_chunks_with_scores: List of dicts with 'chunk_id' and 'score'
+        session_id: Session UUID for generating chunk selection URIs
+
+    Returns:
+        List of Triple objects
+    """
+    triples = [
+        _triple(focus_uri, RDF_TYPE, _iri(PROV_ENTITY)),
+        _triple(focus_uri, RDF_TYPE, _iri(TG_FOCUS)),
+        _triple(focus_uri, RDFS_LABEL, _literal("Chunk Selection")),
+        _triple(focus_uri, PROV_WAS_DERIVED_FROM, _iri(exploration_uri)),
+    ]
+
+    for idx, chunk_info in enumerate(selected_chunks_with_scores):
+        chunk_id = chunk_info.get("chunk_id")
+        if not chunk_id:
+            continue
+
+        chunk_sel_uri = chunk_selection_uri(session_id, idx)
+
+        # Link focus to chunk selection entity
+        triples.append(
+            _triple(focus_uri, TG_SELECTED_CHUNK, _iri(chunk_sel_uri))
+        )
+
+        # Type the chunk selection entity
+        triples.append(
+            _triple(chunk_sel_uri, RDF_TYPE, _iri(TG_CHUNK_SELECTION))
+        )
+
+        # Reference the actual chunk (in librarian)
+        triples.append(
+            _triple(chunk_sel_uri, TG_DOCUMENT, _iri(chunk_id))
+        )
+
+        # Cross-encoder score
+        score = chunk_info.get("score")
+        if score is not None:
+            triples.append(
+                _triple(chunk_sel_uri, TG_SCORE, _literal(str(score)))
+            )
+
+    return triples
+
+
 def docrag_synthesis_triples(
     synthesis_uri: str,
     exploration_uri: str,

trustgraph-base/trustgraph/provenance/uris.py

@@ -309,6 +309,35 @@ def docrag_exploration_uri(session_id: str) -> str:
     return f"urn:trustgraph:docrag:{session_id}/exploration"
 
 
+def docrag_focus_uri(session_id: str) -> str:
+    """
+    Generate URI for a document RAG focus entity (chunks selected by the
+    cross-encoder reranker).
+
+    Args:
+        session_id: The session UUID.
+
+    Returns:
+        URN in format: urn:trustgraph:docrag:{uuid}/focus
+    """
+    return f"urn:trustgraph:docrag:{session_id}/focus"
+
+
+def chunk_selection_uri(session_id: str, chunk_index: int) -> str:
+    """
+    Generate URI for a chunk selection item (links a reranked chunk to its
+    score). Mirrors edge_selection_uri for GraphRAG.
+
+    Args:
+        session_id: The session UUID.
+        chunk_index: Index of this chunk in the selection (0-based).
+
+    Returns:
+        URN in format: urn:trustgraph:prov:chunk:{uuid}:{index}
+    """
+    return f"urn:trustgraph:prov:chunk:{session_id}:{chunk_index}"
+
+
 def docrag_synthesis_uri(session_id: str) -> str:
     """
     Generate URI for a document RAG synthesis entity (final answer).

trustgraph-base/trustgraph/provenance/vocabulary.py

@@ -30,6 +30,7 @@ from . namespaces import (
     TG_DECOMPOSITION, TG_FINDING, TG_PLAN_TYPE, TG_STEP_RESULT,
     TG_SUBAGENT_GOAL, TG_PLAN_STEP,
     TG_EDGE_SELECTION, TG_SCORE,
+    TG_CHUNK_SELECTION,
 )
 
 
@@ -95,6 +96,7 @@ TG_CLASS_LABELS = [
     _label_triple(TG_PLAN_TYPE, "Plan"),
     _label_triple(TG_STEP_RESULT, "Step Result"),
     _label_triple(TG_EDGE_SELECTION, "Edge Selection"),
+    _label_triple(TG_CHUNK_SELECTION, "Chunk Selection"),
 ]
 
 # TrustGraph predicate labels

trustgraph-base/trustgraph/schema/services/retrieval.py

@@ -40,7 +40,10 @@ class GraphRagResponse:
 class DocumentRagQuery:
     query: str = ""
     collection: str = ""
-    doc_limit: int = 0
+    doc_limit: int = 0        # docs selected into the synthesis prompt
+    fetch_limit: int = 0      # candidate pool fetched from the vector store
+                              # before reranking (0 = derive from doc_limit;
+                              # values below doc_limit are raised to it)
     streaming: bool = False
 
 @dataclass

trustgraph-cli/trustgraph/cli/invoke_document_rag.py

@@ -21,10 +21,12 @@ default_token = os.getenv("TRUSTGRAPH_TOKEN", None)
 default_workspace = os.getenv("TRUSTGRAPH_WORKSPACE", "default")
 default_collection = 'default'
 default_doc_limit = 10
+default_fetch_limit = 0
 
 
 def question_explainable(
-    url, flow_id, question_text, collection, doc_limit, token=None, debug=False,
+    url, flow_id, question_text, collection, doc_limit, fetch_limit=0,
+    token=None, debug=False,
     workspace="default",
 ):
     """Execute document RAG with explainability - shows provenance events inline."""
@@ -39,6 +41,7 @@ def question_explainable(
             query=question_text,
                         collection=collection,
             doc_limit=doc_limit,
+            fetch_limit=fetch_limit,
         ):
             if isinstance(item, RAGChunk):
                 # Print response content
@@ -97,7 +100,7 @@ def question_explainable(
 
 
 def question(
-    url, flow_id, question_text, collection, doc_limit,
+    url, flow_id, question_text, collection, doc_limit, fetch_limit=0,
     streaming=True, token=None, explainable=False, debug=False,
     show_usage=False, workspace="default",
 ):
@@ -109,6 +112,7 @@ def question(
             question_text=question_text,
                         collection=collection,
             doc_limit=doc_limit,
+            fetch_limit=fetch_limit,
             token=token,
             debug=debug,
             workspace=workspace,
@@ -128,6 +132,7 @@ def question(
                 query=question_text,
                                 collection=collection,
                 doc_limit=doc_limit,
+                fetch_limit=fetch_limit,
                 streaming=True
             )
 
@@ -155,6 +160,7 @@ def question(
             query=question_text,
                         collection=collection,
             doc_limit=doc_limit,
+            fetch_limit=fetch_limit,
         )
         print(result.text)
 
@@ -214,7 +220,15 @@ def main():
         '-d', '--doc-limit',
         type=int,
         default=default_doc_limit,
-        help=f'Document limit (default: {default_doc_limit})'
+        help=f'Documents selected into the prompt (default: {default_doc_limit})'
+    )
+
+    parser.add_argument(
+        '--fetch-limit',
+        type=int,
+        default=default_fetch_limit,
+        help='Candidate documents fetched from the vector store before '
+             'reranking (default: derive from doc-limit)'
     )
 
     parser.add_argument(
@@ -251,6 +265,7 @@ def main():
             question_text=args.question,
             collection=args.collection,
             doc_limit=args.doc_limit,
+            fetch_limit=args.fetch_limit,
             streaming=not args.no_streaming,
             token=args.token,
             explainable=args.explainable,

trustgraph-flow/trustgraph/retrieval/document_rag/document_rag.py

@@ -9,10 +9,12 @@ from trustgraph.provenance import (
     docrag_question_uri,
     docrag_grounding_uri,
     docrag_exploration_uri,
+    docrag_focus_uri,
     docrag_synthesis_uri,
     docrag_question_triples,
     grounding_triples,
     docrag_exploration_triples,
+    docrag_chunk_selection_triples,
     docrag_synthesis_triples,
     set_graph,
     GRAPH_RETRIEVAL,
@@ -21,19 +23,25 @@ from trustgraph.provenance import (
 # Module logger
 logger = logging.getLogger(__name__)
 
+# When the caller does not specify a fetch_limit, reranking over-fetches this
+# many times the final doc_limit as the candidate pool, so the cross-encoder can
+# recover relevant chunks the bi-encoder ranked just outside the top doc_limit.
+# This is only the fallback default: an explicit fetch_limit overrides it.
+OVERFETCH_FACTOR = 3
+
 LABEL="http://www.w3.org/2000/01/rdf-schema#label"
 
 class Query:
 
     def __init__(
             self, rag, workspace, collection, verbose,
-            doc_limit=20, track_usage=None,
+            fetch_limit=20, track_usage=None,
     ):
         self.rag = rag
         self.workspace = workspace
         self.collection = collection
         self.verbose = verbose
-        self.doc_limit = doc_limit
+        self.fetch_limit = fetch_limit
         self.track_usage = track_usage
 
     async def extract_concepts(self, query):
@@ -91,7 +99,7 @@ class Query:
 
         # Query chunk matches for each concept concurrently
         per_concept_limit = max(
-            1, self.doc_limit // len(vectors)
+            1, self.fetch_limit // len(vectors)
         )
 
         async def query_concept(vec):
@@ -140,6 +148,7 @@ class DocumentRag:
     def __init__(
             self, prompt_client, embeddings_client, doc_embeddings_client,
             fetch_chunk,
+            reranker_client=None,
             verbose=False,
     ):
 
@@ -150,12 +159,16 @@ class DocumentRag:
         self.doc_embeddings_client = doc_embeddings_client
         self.fetch_chunk = fetch_chunk
 
+        # Optional cross-encoder reranker. When None, the retrieval path is
+        # byte-identical to the pre-reranker behaviour.
+        self.reranker_client = reranker_client
+
         if self.verbose:
             logger.debug("DocumentRag initialized")
 
     async def query(
             self, query, workspace="default", collection="default",
-            doc_limit=20, streaming=False, chunk_callback=None,
+            doc_limit=20, fetch_limit=0, streaming=False, chunk_callback=None,
             explain_callback=None, save_answer_callback=None,
     ):
         """
@@ -165,7 +178,10 @@ class DocumentRag:
             query: The query string
             workspace: Workspace for isolation (also scopes chunk lookup)
             collection: Collection identifier
-            doc_limit: Max chunks to retrieve
+            doc_limit: Chunks selected into the synthesis prompt (after rerank)
+            fetch_limit: Candidate pool fetched from the vector store before
+                reranking. 0 = derive (OVERFETCH_FACTOR x doc_limit when a
+                reranker is wired, else doc_limit).
             streaming: Enable streaming LLM response
             chunk_callback: async def callback(chunk, end_of_stream) for streaming
             explain_callback: async def callback(triples, explain_id) for explainability
@@ -197,6 +213,7 @@ class DocumentRag:
         q_uri = docrag_question_uri(session_id)
         gnd_uri = docrag_grounding_uri(session_id)
         exp_uri = docrag_exploration_uri(session_id)
+        foc_uri = docrag_focus_uri(session_id)
         syn_uri = docrag_synthesis_uri(session_id)
 
         timestamp = datetime.now(timezone.utc).isoformat().replace("+00:00", "Z")
@@ -209,10 +226,21 @@ class DocumentRag:
             )
             await explain_callback(q_triples, q_uri)
 
+        # Resolve the candidate-pool size fetched from the vector store. When a
+        # reranker is wired, honour an explicit fetch_limit; if unset, fall back
+        # to the OVERFETCH_FACTOR heuristic. Never fetch fewer than doc_limit,
+        # else the rerank could not fill the prompt. Without a reranker, fetch
+        # doc_limit as before (byte-identical behaviour).
+        if self.reranker_client is not None:
+            fl = fetch_limit or (OVERFETCH_FACTOR * doc_limit)
+            fetch_count = max(fl, doc_limit)
+        else:
+            fetch_count = doc_limit
+
         q = Query(
             rag=self, workspace=workspace, collection=collection,
             verbose=self.verbose,
-            doc_limit=doc_limit, track_usage=track_usage,
+            fetch_limit=fetch_count, track_usage=track_usage,
         )
 
         # Extract concepts from query (grounding step)
@@ -235,6 +263,7 @@ class DocumentRag:
         docs, chunk_ids = await q.get_docs(concepts)
 
         # Emit exploration explainability after chunks retrieved
+        # (full candidate set, before any reranking)
         if explain_callback:
             exp_triples = set_graph(
                 docrag_exploration_triples(exp_uri, gnd_uri, len(chunk_ids), chunk_ids),
@@ -242,6 +271,45 @@ class DocumentRag:
             )
             await explain_callback(exp_triples, exp_uri)
 
+        # Optional cross-encoder reranking pass between retrieval and
+        # synthesis. Mirrors GraphRAG's reranker usage but with a single
+        # query (the question). When no reranker is wired, this block is
+        # skipped entirely and behaviour is byte-identical to before.
+        reranked = False
+        if self.reranker_client is not None and docs:
+            results = await self.reranker_client.rerank(
+                queries=[{"id": "0", "text": query}],
+                documents=[
+                    {"id": str(i), "text": d} for i, d in enumerate(docs)
+                ],
+                # Narrow the over-fetched candidate pool down to the final
+                # doc_limit requested for synthesis.
+                limit=doc_limit,
+            )
+
+            # results are sorted desc by score and truncated to limit by the
+            # reranker service, so order gives the surviving top-N directly.
+            order = [int(r.document_id) for r in results]
+            docs = [docs[i] for i in order]
+            chunk_ids = [chunk_ids[i] for i in order]
+            reranked = True
+
+            # Emit chunk-selection (focus) explainability: surviving chunks
+            # with their cross-encoder scores, derived from exploration.
+            if explain_callback:
+                selected_chunks_with_scores = [
+                    {"chunk_id": chunk_ids[i], "score": r.score}
+                    for i, r in enumerate(results)
+                ]
+                foc_triples = set_graph(
+                    docrag_chunk_selection_triples(
+                        foc_uri, exp_uri,
+                        selected_chunks_with_scores, session_id,
+                    ),
+                    GRAPH_RETRIEVAL
+                )
+                await explain_callback(foc_triples, foc_uri)
+
         if self.verbose:
             logger.debug("Invoking LLM...")
             logger.debug(f"Documents: {docs}")
@@ -291,9 +359,15 @@ class DocumentRag:
                     logger.warning(f"Failed to save answer to librarian: {e}")
                     synthesis_doc_id = None
 
+            # When reranking ran, synthesis derives from the focus (the
+            # reranked chunks actually fed to the LLM), as GraphRAG always does.
+            # When no reranker is wired, there is no focus stage, so synthesis
+            # derives from exploration (the unchanged no-op lineage) - a
+            # deliberate divergence from GraphRAG's always-on focus.
+            syn_parent = foc_uri if reranked else exp_uri
             syn_triples = set_graph(
                 docrag_synthesis_triples(
-                    syn_uri, exp_uri,
+                    syn_uri, syn_parent,
                     document_id=synthesis_doc_id,
                     in_token=synthesis_result.in_token if synthesis_result else None,
                     out_token=synthesis_result.out_token if synthesis_result else None,

trustgraph-flow/trustgraph/retrieval/document_rag/rag.py

@@ -13,6 +13,7 @@ from . document_rag import DocumentRag
 from ... base import FlowProcessor, ConsumerSpec, ProducerSpec
 from ... base import PromptClientSpec, EmbeddingsClientSpec
 from ... base import DocumentEmbeddingsClientSpec
+from ... base import RerankerClientSpec
 from ... base import LibrarianSpec
 
 # Module logger
@@ -28,14 +29,21 @@ class Processor(FlowProcessor):
 
         doc_limit = params.get("doc_limit", 5)
 
+        # Instance-default candidate-pool size fetched before cross-encoder
+        # reranking; the rerank step narrows it back down to doc_limit for the
+        # LLM. 0 means the core derives it (OVERFETCH_FACTOR x doc_limit).
+        fetch_limit = params.get("fetch_limit", 0)
+
         super(Processor, self).__init__(
             **params | {
                 "id": id,
                 "doc_limit": doc_limit,
+                "fetch_limit": fetch_limit,
             }
         )
 
         self.doc_limit = doc_limit
+        self.fetch_limit = fetch_limit
 
         self.register_specification(
             ConsumerSpec(
@@ -66,6 +74,13 @@ class Processor(FlowProcessor):
             )
         )
 
+        self.register_specification(
+            RerankerClientSpec(
+                request_name = "reranker-request",
+                response_name = "reranker-response",
+            )
+        )
+
         self.register_specification(
             ProducerSpec(
                 name = "response",
@@ -105,6 +120,7 @@ class Processor(FlowProcessor):
                 doc_embeddings_client = flow("document-embeddings-request"),
                 prompt_client = flow("prompt-request"),
                 fetch_chunk = fetch_chunk,
+                reranker_client = flow("reranker-request"),
                 verbose=True,
             )
 
@@ -113,6 +129,13 @@ class Processor(FlowProcessor):
             else:
                 doc_limit = self.doc_limit
 
+            # Candidate-pool size: per-request override, else the instance
+            # default; 0 lets the core derive it from doc_limit.
+            if v.fetch_limit:
+                fetch_limit = v.fetch_limit
+            else:
+                fetch_limit = self.fetch_limit
+
             async def send_explainability(triples, explain_id):
                 await flow("explainability").send(Triples(
                     metadata=Metadata(
@@ -163,6 +186,7 @@ class Processor(FlowProcessor):
                     workspace=flow.workspace,
                     collection=v.collection,
                     doc_limit=doc_limit,
+                    fetch_limit=fetch_limit,
                     streaming=True,
                     chunk_callback=send_chunk,
                     explain_callback=send_explainability,
@@ -188,6 +212,7 @@ class Processor(FlowProcessor):
                     workspace=flow.workspace,
                     collection=v.collection,
                     doc_limit=doc_limit,
+                    fetch_limit=fetch_limit,
                     explain_callback=send_explainability,
                     save_answer_callback=save_answer,
                 )
@@ -243,6 +268,15 @@ class Processor(FlowProcessor):
             help=f'Default document fetch limit (default: 10)'
         )
 
+        parser.add_argument(
+            '--fetch-limit',
+            type=int,
+            default=0,
+            help='Candidate chunks to fetch from the vector store and rerank '
+                 'before keeping the top doc-limit for the LLM '
+                 '(default: derive from doc-limit)'
+        )
+
 def run():
 
     Processor.launch(default_ident, __doc__)