diff --git a/codex/discover.py b/codex/discover.py
new file mode 100644
index 0000000..8f1e0f2
--- /dev/null
+++ b/codex/discover.py
@@ -0,0 +1,64 @@
+"""Discovery queries over the citation graph stored in PostgreSQL."""
+
+from __future__ import annotations
+
+from codex.db import get_conn
+
+
+def discovery_leads(limit: int = 20) -> list[dict[str, int | str]]:
+    """Return papers referenced by already-ingested papers but not yet collected.
+
+    Returns list of dicts with keys:
+        cited_id (str)   — arXiv ID, DOI, or OpenAlex W-ID of the target
+        pull     (int)   — how many already-ingested papers cite this target
+
+    Ordered by pull DESC, limited to `limit` rows.
+    """
+    sql = """
+        SELECT cited_id, count(*) AS pull
+        FROM   citations
+        WHERE  cited_id NOT IN (SELECT id FROM papers)
+        GROUP  BY cited_id
+        ORDER  BY pull DESC
+        LIMIT  %(limit)s
+    """
+    with get_conn() as conn:
+        rows = conn.execute(sql, {"limit": limit}).fetchall()
+    return [{"cited_id": row["cited_id"], "pull": row["pull"]} for row in rows]
+
+
+def citing_papers(paper_id: str) -> list[str]:
+    """Return IDs of all papers that cite `paper_id` (in-graph reverse citations)."""
+    sql = "SELECT citing_id FROM citations WHERE cited_id = %(paper_id)s"
+    with get_conn() as conn:
+        rows = conn.execute(sql, {"paper_id": paper_id}).fetchall()
+    return [row["citing_id"] for row in rows]
+
+
+def cited_by(paper_id: str) -> list[str]:
+    """Return IDs of all papers that `paper_id` cites (forward citations)."""
+    sql = "SELECT cited_id FROM citations WHERE citing_id = %(paper_id)s"
+    with get_conn() as conn:
+        rows = conn.execute(sql, {"paper_id": paper_id}).fetchall()
+    return [row["cited_id"] for row in rows]
+
+
+def cocited_papers(paper_id: str, limit: int = 10) -> list[dict[str, int | str]]:
+    """Return papers frequently co-cited with `paper_id`.
+
+    A paper X is co-cited with `paper_id` if some paper cites both.
+    Returns list of dicts: {paper_id: str, co_citations: int}, ordered DESC.
+    """
+    sql = """
+        SELECT c2.cited_id AS paper_id, count(*) AS co_citations
+        FROM   citations c1
+        JOIN   citations c2 ON c1.citing_id = c2.citing_id
+        WHERE  c1.cited_id = %(paper_id)s
+          AND  c2.cited_id != %(paper_id)s
+        GROUP  BY c2.cited_id
+        ORDER  BY co_citations DESC
+        LIMIT  %(limit)s
+    """
+    with get_conn() as conn:
+        rows = conn.execute(sql, {"paper_id": paper_id, "limit": limit}).fetchall()
+    return [{"paper_id": row["paper_id"], "co_citations": row["co_citations"]} for row in rows]
diff --git a/codex/provenance.py b/codex/provenance.py
new file mode 100644
index 0000000..2a56c5a
--- /dev/null
+++ b/codex/provenance.py
@@ -0,0 +1,184 @@
+"""Provenance tracking: @cite-scan, code_links CRUD, references.bib export."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+from codex.db import get_conn
+from codex.models import CodeLink
+
+# ---------------------------------------------------------------------------
+# @cite scanning
+# ---------------------------------------------------------------------------
+
+_CITE_RE = re.compile(r"@cite\s+(\S+)", re.IGNORECASE)
+_FUNC_RE = re.compile(r"(\w+)\s*\(")
+_CLASS_RE = re.compile(r"class\s+(\w+)")
+
+_CPP_SUFFIXES = {".cpp", ".h", ".cxx", ".hpp"}
+
+
+def scan_cite_tags(root_dir: str) -> list[dict[str, str]]:
+    """Recursively scan C++ source files for @cite tags (Doxygen-style).
+
+    Returns one dict per hit:
+        file   (str) — relative path to the source file
+        line   (int as str) — 1-based line number
+        symbol (str) — function or class name (extracted from context, best-effort)
+        bibkey (str) — the @cite argument
+
+    Scans files matching: *.cpp, *.h, *.cxx, *.hpp
+    Pattern matched: ``@cite <bibkey>`` (case-insensitive, optional whitespace)
+    """
+    root = Path(root_dir)
+    results: list[dict[str, str]] = []
+
+    for path in sorted(root.rglob("*")):
+        if path.suffix.lower() not in _CPP_SUFFIXES:
+            continue
+        try:
+            lines = path.read_text(encoding="utf-8", errors="replace").splitlines()
+        except OSError:
+            continue
+
+        rel_path = str(path.relative_to(root))
+
+        for lineno, line in enumerate(lines, start=1):
+            for match in _CITE_RE.finditer(line):
+                bibkey = match.group(1)
+                # Best-effort symbol extraction: scan backwards for func or class
+                symbol = ""
+                context_lines = lines[: lineno - 1]
+                for prev_line in reversed(context_lines):
+                    func_match = _FUNC_RE.search(prev_line)
+                    class_match = _CLASS_RE.search(prev_line)
+                    if func_match:
+                        symbol = func_match.group(1)
+                        break
+                    if class_match:
+                        symbol = class_match.group(1)
+                        break
+
+                results.append(
+                    {
+                        "file": rel_path,
+                        "line": str(lineno),
+                        "symbol": symbol,
+                        "bibkey": bibkey,
+                    }
+                )
+
+    return results
+
+
+# ---------------------------------------------------------------------------
+# code_links CRUD
+# ---------------------------------------------------------------------------
+
+
+def add_code_link(
+    symbol: str,
+    paper_id: str | None = None,
+    role: str | None = None,
+    note: str | None = None,
+) -> CodeLink:
+    """Insert a new code_link row and return the populated CodeLink (with db-assigned id)."""
+    sql = """
+        INSERT INTO code_links (symbol, paper_id, role, note)
+        VALUES (%(symbol)s, %(paper_id)s, %(role)s, %(note)s)
+        RETURNING id, added_at
+    """
+    with get_conn() as conn:
+        row = conn.execute(
+            sql,
+            {"symbol": symbol, "paper_id": paper_id, "role": role, "note": note},
+        ).fetchone()
+        conn.commit()
+    assert row is not None
+    return CodeLink(
+        symbol=symbol,
+        paper_id=paper_id,
+        role=role,
+        note=note,
+        id=row["id"],
+        added_at=row["added_at"],
+    )
+
+
+def list_code_links(paper_id: str | None = None) -> list[CodeLink]:
+    """Return all code_links, optionally filtered by paper_id."""
+    if paper_id is not None:
+        sql = (
+            "SELECT id, symbol, paper_id, role, note, added_at"
+            " FROM code_links WHERE paper_id = %(paper_id)s"
+        )
+        params: dict[str, str] = {"paper_id": paper_id}
+    else:
+        sql = "SELECT id, symbol, paper_id, role, note, added_at FROM code_links"
+        params = {}
+
+    with get_conn() as conn:
+        rows = conn.execute(sql, params).fetchall()
+
+    return [
+        CodeLink(
+            symbol=row["symbol"],
+            paper_id=row["paper_id"],
+            role=row["role"],
+            note=row["note"],
+            id=row["id"],
+            added_at=row["added_at"],
+        )
+        for row in rows
+    ]
+
+
+# ---------------------------------------------------------------------------
+# references.bib export
+# ---------------------------------------------------------------------------
+
+
+def export_bib(paper_ids: list[str] | None = None) -> str:
+    """Export papers as a BibTeX string.
+
+    If paper_ids is None, exports ALL papers with a non-NULL bibkey.
+    Only papers with a non-NULL bibkey are included (bibkey is the BibTeX key).
+
+    Entry format (article):
+        @article{<bibkey>,
+          title  = {<title>},
+          author = {<authors joined with " and ">},
+          year   = {<year>},
+          note   = {<id>},
+        }
+    """
+    if paper_ids is not None:
+        sql = (
+            "SELECT id, bibkey, title, authors, year"
+            " FROM papers"
+            " WHERE bibkey IS NOT NULL AND id = ANY(%(paper_ids)s)"
+        )
+        params: dict[str, list[str]] = {"paper_ids": paper_ids}
+    else:
+        sql = "SELECT id, bibkey, title, authors, year FROM papers WHERE bibkey IS NOT NULL"
+        params = {}
+
+    with get_conn() as conn:
+        rows = conn.execute(sql, params).fetchall()
+
+    entries: list[str] = []
+    for row in rows:
+        authors: list[str] = row["authors"] or []
+        author_str = " and ".join(authors) if authors else ""
+        entry = (
+            f"@article{{{row['bibkey']},\n"
+            f"  title  = {{{row['title']}}},\n"
+            f"  author = {{{author_str}}},\n"
+            f"  year   = {{{row['year']}}},\n"
+            f"  note   = {{{row['id']}}},\n"
+            f"}}"
+        )
+        entries.append(entry)
+
+    return "\n\n".join(entries)
diff --git a/tests/discover/__init__.py b/tests/discover/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/discover/test_discover.py b/tests/discover/test_discover.py
new file mode 100644
index 0000000..2284942
--- /dev/null
+++ b/tests/discover/test_discover.py
@@ -0,0 +1,145 @@
+"""Tests for codex.discover — graph-based discovery queries.
+
+All DB access is mocked; no real PostgreSQL server required.
+"""
+
+from __future__ import annotations
+
+from contextlib import contextmanager
+from typing import Any
+from unittest.mock import MagicMock, patch
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_conn_cm(mock_conn: MagicMock) -> Any:
+    """Return a context-manager factory that yields *mock_conn*."""
+
+    @contextmanager  # type: ignore[arg-type]
+    def _cm() -> Any:
+        yield mock_conn
+
+    return _cm
+
+
+def _dict_row(**kwargs: Any) -> dict[str, Any]:
+    return dict(kwargs)
+
+
+# ---------------------------------------------------------------------------
+# 1. test_discovery_leads_returns_correct_format
+# ---------------------------------------------------------------------------
+
+
+def test_discovery_leads_returns_correct_format() -> None:
+    """Mock conn returns rows; verify output has correct shape and ordering."""
+    from codex.discover import discovery_leads
+
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(cited_id="W111", pull=5),
+        _dict_row(cited_id="W222", pull=3),
+        _dict_row(cited_id="W333", pull=1),
+    ]
+
+    with patch("codex.discover.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        result = discovery_leads(limit=10)
+
+    assert len(result) == 3
+    assert result[0] == {"cited_id": "W111", "pull": 5}
+    assert result[1] == {"cited_id": "W222", "pull": 3}
+    assert result[2] == {"cited_id": "W333", "pull": 1}
+
+    # Verify the SQL and limit param were passed
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    params: dict[str, Any] = call_args[0][1]
+    assert "cited_id NOT IN" in sql
+    assert "pull DESC" in sql
+    assert params["limit"] == 10
+
+
+# ---------------------------------------------------------------------------
+# 2. test_citing_papers
+# ---------------------------------------------------------------------------
+
+
+def test_citing_papers() -> None:
+    """Mock returns citing_id list; verify flat list of IDs is returned."""
+    from codex.discover import citing_papers
+
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(citing_id="2301.00001"),
+        _dict_row(citing_id="2302.00002"),
+    ]
+
+    with patch("codex.discover.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        result = citing_papers("W999")
+
+    assert result == ["2301.00001", "2302.00002"]
+
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    params: dict[str, Any] = call_args[0][1]
+    assert "cited_id" in sql
+    assert params["paper_id"] == "W999"
+
+
+# ---------------------------------------------------------------------------
+# 3. test_cited_by
+# ---------------------------------------------------------------------------
+
+
+def test_cited_by() -> None:
+    """Mock returns cited_id list; verify flat list of IDs is returned."""
+    from codex.discover import cited_by
+
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(cited_id="10.1000/ref_a"),
+        _dict_row(cited_id="10.1000/ref_b"),
+    ]
+
+    with patch("codex.discover.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        result = cited_by("2301.00001")
+
+    assert result == ["10.1000/ref_a", "10.1000/ref_b"]
+
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    params: dict[str, Any] = call_args[0][1]
+    assert "citing_id" in sql
+    assert params["paper_id"] == "2301.00001"
+
+
+# ---------------------------------------------------------------------------
+# 4. test_cocited_papers
+# ---------------------------------------------------------------------------
+
+
+def test_cocited_papers() -> None:
+    """Mock returns co-citation list; verify shape and ordering."""
+    from codex.discover import cocited_papers
+
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(paper_id="W444", co_citations=7),
+        _dict_row(paper_id="W555", co_citations=2),
+    ]
+
+    with patch("codex.discover.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        result = cocited_papers("W111", limit=5)
+
+    assert len(result) == 2
+    assert result[0] == {"paper_id": "W444", "co_citations": 7}
+    assert result[1] == {"paper_id": "W555", "co_citations": 2}
+
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    params: dict[str, Any] = call_args[0][1]
+    assert "co_citations DESC" in sql
+    assert params["paper_id"] == "W111"
+    assert params["limit"] == 5
diff --git a/tests/provenance/__init__.py b/tests/provenance/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/provenance/test_provenance.py b/tests/provenance/test_provenance.py
new file mode 100644
index 0000000..228552a
--- /dev/null
+++ b/tests/provenance/test_provenance.py
@@ -0,0 +1,209 @@
+"""Tests for codex.provenance — @cite scan, code_links CRUD, bib export.
+
+All DB access is mocked; no real PostgreSQL server required.
+"""
+
+from __future__ import annotations
+
+from contextlib import contextmanager
+from datetime import UTC, datetime
+from pathlib import Path
+from typing import Any
+from unittest.mock import MagicMock, patch
+
+from codex.models import CodeLink
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_conn_cm(mock_conn: MagicMock) -> Any:
+    """Return a context-manager factory that yields *mock_conn*."""
+
+    @contextmanager  # type: ignore[arg-type]
+    def _cm() -> Any:
+        yield mock_conn
+
+    return _cm
+
+
+def _dict_row(**kwargs: Any) -> dict[str, Any]:
+    return dict(kwargs)
+
+
+# ---------------------------------------------------------------------------
+# 1. test_scan_cite_tags
+# ---------------------------------------------------------------------------
+
+
+def test_scan_cite_tags(tmp_path: Path) -> None:
+    """A .cpp file with @cite produces correct dict entries."""
+    from codex.provenance import scan_cite_tags
+
+    src_file = tmp_path / "algo.cpp"
+    src_file.write_text(
+        "// Helper function\n"
+        "void compute_hodge() {\n"
+        "    // @cite somepaper2023\n"
+        "    do_stuff();\n"
+        "}\n",
+        encoding="utf-8",
+    )
+
+    results = scan_cite_tags(str(tmp_path))
+
+    assert len(results) == 1
+    hit = results[0]
+    assert hit["bibkey"] == "somepaper2023"
+    assert hit["line"] == "3"
+    assert hit["file"] == "algo.cpp"
+    # symbol extracted from "compute_hodge()" on the line above
+    assert hit["symbol"] == "compute_hodge"
+
+
+# ---------------------------------------------------------------------------
+# 2. test_scan_cite_tags_no_files
+# ---------------------------------------------------------------------------
+
+
+def test_scan_cite_tags_no_files(tmp_path: Path) -> None:
+    """Empty directory returns an empty list."""
+    from codex.provenance import scan_cite_tags
+
+    result = scan_cite_tags(str(tmp_path))
+    assert result == []
+
+
+# ---------------------------------------------------------------------------
+# 3. test_add_code_link
+# ---------------------------------------------------------------------------
+
+
+def test_add_code_link() -> None:
+    """Mock conn, verify INSERT is issued and returned CodeLink has id from RETURNING."""
+    from codex.provenance import add_code_link
+
+    now = datetime(2026, 6, 5, tzinfo=UTC)
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchone.return_value = _dict_row(id=42, added_at=now)
+
+    with patch("codex.provenance.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        link = add_code_link(
+            symbol="dec::hodge_star",
+            paper_id="2301.07041",
+            role="implements Thm 3.2",
+            note="core algorithm",
+        )
+
+    assert isinstance(link, CodeLink)
+    assert link.id == 42
+    assert link.added_at == now
+    assert link.symbol == "dec::hodge_star"
+    assert link.paper_id == "2301.07041"
+    assert link.role == "implements Thm 3.2"
+    assert link.note == "core algorithm"
+
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    assert "INSERT INTO code_links" in sql
+    assert "RETURNING" in sql
+
+    mock_conn.commit.assert_called_once()
+
+
+# ---------------------------------------------------------------------------
+# 4. test_list_code_links_all
+# ---------------------------------------------------------------------------
+
+
+def test_list_code_links_all() -> None:
+    """No filter → all rows returned as CodeLink list."""
+    from codex.provenance import list_code_links
+
+    now = datetime(2026, 6, 5, tzinfo=UTC)
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(id=1, symbol="foo::bar", paper_id="W111", role="uses", note=None, added_at=now),
+        _dict_row(id=2, symbol="baz::qux", paper_id=None, role=None, note="ref", added_at=now),
+    ]
+
+    with patch("codex.provenance.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        links = list_code_links()
+
+    assert len(links) == 2
+    assert all(isinstance(lnk, CodeLink) for lnk in links)
+    assert links[0].symbol == "foo::bar"
+    assert links[0].id == 1
+    assert links[1].symbol == "baz::qux"
+    assert links[1].paper_id is None
+
+    # No WHERE clause in the SQL
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    assert "WHERE" not in sql
+
+
+# ---------------------------------------------------------------------------
+# 5. test_list_code_links_filtered
+# ---------------------------------------------------------------------------
+
+
+def test_list_code_links_filtered() -> None:
+    """Filter by paper_id → WHERE clause applied and only matching rows returned."""
+    from codex.provenance import list_code_links
+
+    now = datetime(2026, 6, 5, tzinfo=UTC)
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(
+            id=3,
+            symbol="some::func",
+            paper_id="2301.07041",
+            role=None,
+            note=None,
+            added_at=now,
+        ),
+    ]
+
+    with patch("codex.provenance.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        links = list_code_links(paper_id="2301.07041")
+
+    assert len(links) == 1
+    assert links[0].paper_id == "2301.07041"
+
+    call_args = mock_conn.execute.call_args
+    sql: str = call_args[0][0]
+    params: dict[str, Any] = call_args[0][1]
+    assert "WHERE paper_id" in sql
+    assert params["paper_id"] == "2301.07041"
+
+
+# ---------------------------------------------------------------------------
+# 6. test_export_bib_format
+# ---------------------------------------------------------------------------
+
+
+def test_export_bib_format() -> None:
+    """Mock returns one paper row; verify valid BibTeX string output."""
+    from codex.provenance import export_bib
+
+    mock_conn = MagicMock()
+    mock_conn.execute.return_value.fetchall.return_value = [
+        _dict_row(
+            id="2301.07041",
+            bibkey="smith2023conformal",
+            title="Conformal Approaches to Laplacian Eigenmaps",
+            authors=["Alice Smith", "Bob Jones"],
+            year=2023,
+        ),
+    ]
+
+    with patch("codex.provenance.get_conn", side_effect=_make_conn_cm(mock_conn)):
+        bib = export_bib()
+
+    assert "@article{smith2023conformal," in bib
+    assert "title  = {Conformal Approaches to Laplacian Eigenmaps}" in bib
+    assert "author = {Alice Smith and Bob Jones}" in bib
+    assert "year   = {2023}" in bib
+    assert "note   = {2301.07041}" in bib