diff --git a/app/api/chat.py b/app/api/chat.py
index 8260801..1ec6547 100644
--- a/app/api/chat.py
+++ b/app/api/chat.py
@@ -1,5 +1,126 @@
 # app/api/chat.py
-"""RAG chat API — streaming LLM responses with page citations (Task 6)."""
-from fastapi import APIRouter
+"""
+RAG chat endpoint — retrieves relevant page chunks and synthesizes an answer.
 
+BSL 1.1 — BYOK gate: requires PAGEPIPER_OLLAMA_URL or a Paid tier license.
+Returns 402 with clear upgrade message if neither is configured.
+"""
+from __future__ import annotations
+
+import logging
+import os
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+from app.services.retriever import Retriever
+from app.services.synthesizer import Synthesizer
+
+logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/api/chat", tags=["chat"])
+
+
+class ChatTurn(BaseModel):
+    role: str  # "user" | "assistant"
+    content: str
+
+
+class ChatRequest(BaseModel):
+    message: str
+    history: list[ChatTurn] = []
+    doc_ids: list[str] | None = None
+    top_k: int = 5
+
+
+class ChatResponse(BaseModel):
+    answer: str
+    citations: list[dict]
+
+
+def _get_llm_router():
+    """Return LLMRouter if Ollama configured, else None."""
+    from app.config import get_llm_config
+
+    cfg = get_llm_config()
+    if cfg is None:
+        return None
+    from circuitforge_core.llm import LLMRouter
+
+    return LLMRouter(cfg)
+
+
+def _get_db_path() -> str:
+    """Read lazily so test fixtures take effect."""
+    import pathlib
+
+    data_dir = pathlib.Path(os.environ.get("PAGEPIPER_DATA_DIR", "data"))
+    return str(data_dir / "pagepiper.db")
+
+
+def _get_vec_db_path() -> str:
+    import pathlib
+
+    data_dir = pathlib.Path(os.environ.get("PAGEPIPER_DATA_DIR", "data"))
+    return str(data_dir / "pagepiper_vecs.db")
+
+
+def _require_llm():
+    """Return LLMRouter or raise 402."""
+    llm = _get_llm_router()
+    if llm is None:
+        raise HTTPException(
+            status_code=402,
+            detail={
+                "error": "ollama_required",
+                "message": (
+                    "RAG chat requires Ollama. Set PAGEPIPER_OLLAMA_URL in your .env file, "
+                    "then restart. Run: ollama pull nomic-embed-text && ollama pull mistral:7b"
+                ),
+            },
+        )
+    return llm
+
+
+@router.post("")
+def chat(req: ChatRequest) -> ChatResponse:
+    llm = _require_llm()
+
+    from app.main import _bm25
+
+    retriever = Retriever(_bm25)
+    chunks = retriever.hybrid_search(
+        query=req.message,
+        top_k=req.top_k,
+        doc_ids=req.doc_ids,
+        db_path=_get_db_path(),
+        vec_db_path=_get_vec_db_path(),
+        llm=llm,
+    )
+
+    if not chunks:
+        return ChatResponse(
+            answer=(
+                "I couldn't find any relevant passages. "
+                "Try a different query or check which documents are indexed."
+            ),
+            citations=[],
+        )
+
+    synth = Synthesizer(llm)
+    result = synth.synthesize(
+        message=req.message,
+        history=[t.model_dump() for t in req.history],
+        chunks=chunks,
+    )
+
+    return ChatResponse(
+        answer=result.answer,
+        citations=[
+            {
+                "doc_id": c.doc_id,
+                "page_number": c.page_number,
+                "snippet": c.snippet,
+            }
+            for c in result.citations
+        ],
+    )
diff --git a/app/services/retriever.py b/app/services/retriever.py
new file mode 100644
index 0000000..b2d09a5
--- /dev/null
+++ b/app/services/retriever.py
@@ -0,0 +1,121 @@
+# app/services/retriever.py
+"""
+Hybrid BM25 + semantic retriever.
+
+BSL 1.1 — semantic path requires PAGEPIPER_OLLAMA_URL (BYOK gate).
+BM25-only path is MIT and has no gate.
+"""
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass
+
+from app.services.bm25_index import BM25Index
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass(frozen=True)
+class RetrievedChunk:
+    """A chunk returned by the retriever, with source scores."""
+
+    chunk_id: str
+    doc_id: str
+    page_number: int
+    text: str
+    bm25_score: float
+    vector_score: float | None
+
+
+class Retriever:
+    def __init__(self, bm25: BM25Index) -> None:
+        self._bm25 = bm25
+
+    def hybrid_search(
+        self,
+        query: str,
+        top_k: int,
+        doc_ids: list[str] | None,
+        db_path: str,
+        vec_db_path: str,
+        llm,  # LLMRouter | None — caller must pass
+    ) -> list[RetrievedChunk]:
+        """
+        Merge BM25 and semantic results.
+        Falls back to BM25-only if llm is None.
+        """
+        if llm is None:
+            return self._bm25_only(query, top_k, doc_ids, db_path)
+
+        from circuitforge_core.vector.sqlite_vec import LocalSQLiteVecStore
+
+        self._bm25.ensure_fresh(db_path)
+        bm25_hits = {
+            r.chunk_id: r
+            for r in self._bm25.query(query, top_k=top_k * 2, doc_ids=doc_ids)
+        }
+
+        vec = llm.embed([query])[0]
+        store = LocalSQLiteVecStore(db_path=vec_db_path, table="page_vecs", dimensions=768)
+        filter_meta = {"doc_id": doc_ids[0]} if doc_ids and len(doc_ids) == 1 else None
+        vec_hits = store.query(vec, top_k=top_k * 2, filter_metadata=filter_meta)
+
+        if doc_ids and len(doc_ids) > 1:
+            vec_hits = [h for h in vec_hits if h.metadata.get("doc_id") in doc_ids]
+
+        # Merge: BM25 hits take priority; vector hits fill in additional results
+        merged: dict[str, RetrievedChunk] = {}
+        for cid, r in bm25_hits.items():
+            merged[cid] = RetrievedChunk(
+                chunk_id=cid,
+                doc_id=r.doc_id,
+                page_number=r.page_number,
+                text=r.text,
+                bm25_score=r.score,
+                vector_score=None,
+            )
+        for vh in vec_hits:
+            text = next((c["text"] for c in self._bm25._chunks if c["id"] == vh.id), "")
+            if vh.id in merged:
+                existing = merged[vh.id]
+                merged[vh.id] = RetrievedChunk(
+                    chunk_id=existing.chunk_id,
+                    doc_id=existing.doc_id,
+                    page_number=existing.page_number,
+                    text=existing.text,
+                    bm25_score=existing.bm25_score,
+                    vector_score=vh.score,
+                )
+            else:
+                merged[vh.id] = RetrievedChunk(
+                    chunk_id=vh.id,
+                    doc_id=vh.metadata.get("doc_id", ""),
+                    page_number=int(vh.metadata.get("page_number", 0)),
+                    text=text,
+                    bm25_score=0.0,
+                    vector_score=vh.score,
+                )
+
+        def _combined(r: RetrievedChunk) -> float:
+            bm25 = r.bm25_score
+            vec = (1.0 / (1.0 + r.vector_score)) if r.vector_score is not None else 0.0
+            return bm25 * 0.5 + vec * 0.5
+
+        ranked = sorted(merged.values(), key=_combined, reverse=True)
+        return ranked[:top_k]
+
+    def _bm25_only(
+        self, query: str, top_k: int, doc_ids: list[str] | None, db_path: str
+    ) -> list[RetrievedChunk]:
+        self._bm25.ensure_fresh(db_path)
+        return [
+            RetrievedChunk(
+                chunk_id=r.chunk_id,
+                doc_id=r.doc_id,
+                page_number=r.page_number,
+                text=r.text,
+                bm25_score=r.score,
+                vector_score=None,
+            )
+            for r in self._bm25.query(query, top_k=top_k, doc_ids=doc_ids)
+        ]
diff --git a/app/services/synthesizer.py b/app/services/synthesizer.py
new file mode 100644
index 0000000..d11640b
--- /dev/null
+++ b/app/services/synthesizer.py
@@ -0,0 +1,58 @@
+# app/services/synthesizer.py
+"""
+LLM answer synthesis over retrieved chunks.
+
+BSL 1.1 — requires LLMRouter (Ollama BYOK or cloud tier).
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from app.services.retriever import RetrievedChunk
+
+_SYSTEM_PROMPT = (
+    "You are a helpful document assistant. "
+    "Answer the user's question using ONLY the provided document excerpts. "
+    "For each claim, cite the source page as [p.N]. "
+    "If the excerpts are insufficient, say so. Do not invent information."
+)
+
+
+@dataclass(frozen=True)
+class Citation:
+    doc_id: str
+    page_number: int
+    snippet: str
+
+
+@dataclass(frozen=True)
+class SynthesisResult:
+    answer: str
+    citations: list[Citation]
+
+
+class Synthesizer:
+    def __init__(self, llm) -> None:  # LLMRouter
+        self._llm = llm
+
+    def synthesize(
+        self,
+        message: str,
+        history: list[dict],
+        chunks: list[RetrievedChunk],
+    ) -> SynthesisResult:
+        context_parts = [f"[p.{c.page_number}]\n{c.text[:500]}" for c in chunks]
+        context = "\n\n---\n\n".join(context_parts)
+        prompt = f"Document excerpts:\n\n{context}\n\nQuestion: {message}"
+
+        answer = self._llm.complete(prompt, system=_SYSTEM_PROMPT)
+
+        citations = [
+            Citation(
+                doc_id=c.doc_id,
+                page_number=c.page_number,
+                snippet=c.text[:200],
+            )
+            for c in chunks
+        ]
+        return SynthesisResult(answer=answer, citations=citations)
diff --git a/tests/test_chat_api.py b/tests/test_chat_api.py
new file mode 100644
index 0000000..476d5b4
--- /dev/null
+++ b/tests/test_chat_api.py
@@ -0,0 +1,59 @@
+# tests/test_chat_api.py
+"""Tests for POST /api/chat — RAG chat (BSL, BYOK gate)."""
+from __future__ import annotations
+
+import sqlite3
+from unittest.mock import MagicMock, patch
+
+from app.services.retriever import RetrievedChunk
+
+
+def test_chat_returns_402_without_ollama(client, monkeypatch):
+    monkeypatch.delenv("PAGEPIPER_OLLAMA_URL", raising=False)
+    resp = client.post("/api/chat", json={"message": "How does Fireball work?", "history": []})
+    assert resp.status_code == 402
+    body = resp.json()
+    assert "detail" in body
+    assert "Ollama" in body["detail"]["message"]
+
+
+def test_chat_returns_answer_with_mocked_ollama(client, test_db, monkeypatch):
+    monkeypatch.setenv("PAGEPIPER_OLLAMA_URL", "http://localhost:11434")
+
+    conn = sqlite3.connect(test_db)
+    conn.execute(
+        "INSERT OR IGNORE INTO documents(id, title, file_path, status) VALUES ('b1','PHB','phb.pdf','ready')"
+    )
+    conn.execute(
+        "INSERT INTO page_chunks(doc_id, page_number, text, source, word_count) "
+        "VALUES ('b1',15,'Fireball deals 8d6 fire damage.','text_layer',6)"
+    )
+    conn.commit()
+    conn.close()
+
+    mock_llm = MagicMock()
+    mock_llm.complete.return_value = "Fireball deals 8d6 fire damage [p.15]."
+
+    mock_chunks = [
+        RetrievedChunk(
+            chunk_id="c1",
+            doc_id="b1",
+            page_number=15,
+            text="Fireball deals 8d6 fire damage.",
+            bm25_score=1.0,
+            vector_score=None,
+        )
+    ]
+
+    with patch("app.api.chat.Retriever.hybrid_search", return_value=mock_chunks):
+        with patch("app.api.chat._get_llm_router", return_value=mock_llm):
+            resp = client.post(
+                "/api/chat",
+                json={"message": "How does Fireball work?", "history": [], "doc_ids": ["b1"]},
+            )
+
+    assert resp.status_code == 200
+    body = resp.json()
+    assert "answer" in body
+    assert "citations" in body
+    assert "Fireball" in body["answer"]
diff --git a/tests/test_synthesizer.py b/tests/test_synthesizer.py
new file mode 100644
index 0000000..b173960
--- /dev/null
+++ b/tests/test_synthesizer.py
@@ -0,0 +1,54 @@
+# tests/test_synthesizer.py
+"""Tests for Synthesizer — mocked LLM, citation assembly."""
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+from app.services.retriever import RetrievedChunk
+from app.services.synthesizer import Synthesizer, SynthesisResult
+
+
+def _chunk(doc_id: str = "book-a", page: int = 5, text: str = "Fireball rules") -> RetrievedChunk:
+    return RetrievedChunk(
+        chunk_id="c1", doc_id=doc_id, page_number=page, text=text,
+        bm25_score=1.0, vector_score=None,
+    )
+
+
+def test_synthesizer_returns_answer_and_citations():
+    mock_llm = MagicMock()
+    mock_llm.complete.return_value = "Fireball deals 8d6 damage [p.5]."
+
+    synth = Synthesizer(mock_llm)
+    result = synth.synthesize(
+        message="How does Fireball work?",
+        history=[],
+        chunks=[_chunk()],
+    )
+
+    assert isinstance(result, SynthesisResult)
+    assert "Fireball" in result.answer
+    assert len(result.citations) == 1
+    assert result.citations[0].page_number == 5
+    assert result.citations[0].doc_id == "book-a"
+
+
+def test_synthesizer_builds_context_from_chunks():
+    mock_llm = MagicMock()
+    mock_llm.complete.return_value = "Answer."
+
+    synth = Synthesizer(mock_llm)
+    synth.synthesize("Q?", [], [_chunk(text="Detailed rule text here.")])
+
+    call_args = mock_llm.complete.call_args
+    assert "Detailed rule text here." in call_args[0][0] or "Detailed rule text here." in str(call_args)
+
+
+def test_synthesizer_uses_system_prompt():
+    mock_llm = MagicMock()
+    mock_llm.complete.return_value = "Answer."
+    synth = Synthesizer(mock_llm)
+    synth.synthesize("Q?", [], [_chunk()])
+
+    call_kwargs = mock_llm.complete.call_args
+    assert call_kwargs.kwargs.get("system") or call_kwargs[1].get("system")