diff --git a/codex/sources/arxiv.py b/codex/sources/arxiv.py
new file mode 100644
index 0000000..1de3e38
--- /dev/null
+++ b/codex/sources/arxiv.py
@@ -0,0 +1,100 @@
+"""arXiv API client.
+
+Provides:
+- fetch_source: download the .tar.gz source of a paper and extract the primary .tex file.
+- fetch_pdf_url: return the canonical PDF URL for a given arXiv ID.
+"""
+
+from __future__ import annotations
+
+import io
+import logging
+import tarfile
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+_BASE = "https://arxiv.org"
+
+
+def fetch_source(arxiv_id: str) -> str | None:
+    """Download and extract the primary LaTeX source for an arXiv paper.
+
+    Downloads the .tar.gz source bundle from ``https://arxiv.org/src/{arxiv_id}``,
+    locates the primary .tex file (preferring any file containing ``\\documentclass``,
+    falling back to the largest .tex by size), and returns its contents as a UTF-8
+    string.
+
+    Parameters
+    ----------
+    arxiv_id:
+        The arXiv identifier (e.g. ``"2301.07041"``).
+
+    Returns
+    -------
+    str | None
+        Raw LaTeX source string, or None if the paper is not found or no .tex
+        file is present (signals Nougat fallback).
+    """
+    url = f"{_BASE}/src/{arxiv_id}"
+    try:
+        response = httpx.get(url, timeout=60, follow_redirects=True)
+    except httpx.RequestError:
+        raise
+    if response.status_code == 404:
+        logger.debug("arXiv 404 for source id=%s", arxiv_id)
+        return None
+    if response.status_code != 200:
+        response.raise_for_status()
+
+    raw = response.content
+    try:
+        with tarfile.open(fileobj=io.BytesIO(raw), mode="r:gz") as tf:
+            tex_members = [m for m in tf.getmembers() if m.name.endswith(".tex")]
+            if not tex_members:
+                logger.debug("No .tex files found in arXiv source for %s", arxiv_id)
+                return None
+
+            # Prefer the file containing \documentclass (primary document)
+            primary: tarfile.TarInfo | None = None
+            for member in tex_members:
+                f = tf.extractfile(member)
+                if f is None:
+                    continue
+                content_bytes = f.read()
+                if b"\\documentclass" in content_bytes:
+                    primary = member
+                    # Decode and return immediately — first match wins
+                    return content_bytes.decode("utf-8", errors="replace")
+
+            if primary is None:
+                # Fallback: largest .tex by size
+                largest = max(tex_members, key=lambda m: m.size)
+                f = tf.extractfile(largest)
+                if f is None:
+                    return None
+                return f.read().decode("utf-8", errors="replace")
+    except tarfile.TarError as exc:
+        logger.warning("Failed to open tar archive for %s: %s", arxiv_id, exc)
+        return None
+
+    return None  # unreachable but satisfies type checker
+
+
+def fetch_pdf_url(arxiv_id: str) -> str:
+    """Return the canonical PDF URL for an arXiv paper.
+
+    This is a pure computation — no HTTP request is made.
+
+    Parameters
+    ----------
+    arxiv_id:
+        The arXiv identifier (e.g. ``"2301.07041"``).
+
+    Returns
+    -------
+    str
+        The full URL of the PDF file.
+    """
+    return f"{_BASE}/pdf/{arxiv_id}.pdf"
diff --git a/codex/sources/openalex.py b/codex/sources/openalex.py
new file mode 100644
index 0000000..bd1aa2d
--- /dev/null
+++ b/codex/sources/openalex.py
@@ -0,0 +1,136 @@
+"""OpenAlex API client.
+
+Provides:
+- fetch_paper: resolve an arXiv ID or DOI to a Paper dataclass.
+- fetch_citations: retrieve reference list as Citation dataclasses.
+
+All requests use the OpenAlex Polite Pool (mailto query parameter) and
+are retried on 429/5xx with exponential back-off via tenacity.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+import httpx
+from tenacity import retry, retry_if_exception, stop_after_attempt, wait_exponential
+
+from codex.config import get_settings
+from codex.models import Citation, Paper
+
+logger = logging.getLogger(__name__)
+
+_BASE = "https://api.openalex.org"
+
+
+def _is_retryable(exc: BaseException) -> bool:
+    """Return True for HTTP 429 / 5xx responses wrapped in httpx.HTTPStatusError."""
+    if isinstance(exc, httpx.HTTPStatusError):
+        return exc.response.status_code == 429 or exc.response.status_code >= 500
+    return False
+
+
+def _polite_params() -> dict[str, str]:
+    mailto = get_settings().openalex_mailto
+    if mailto:
+        return {"mailto": mailto}
+    return {}
+
+
+@retry(
+    retry=retry_if_exception(_is_retryable),
+    stop=stop_after_attempt(5),
+    wait=wait_exponential(min=1, max=30),
+    before_sleep=lambda rs: logger.warning(
+        "OpenAlex retry %d after %s",
+        rs.attempt_number,
+        rs.outcome.exception(),  # type: ignore[union-attr]
+    ),
+)
+def _get(url: str, params: dict[str, str] | None = None) -> httpx.Response:
+    merged: dict[str, str] = {**_polite_params(), **(params or {})}
+    response = httpx.get(url, params=merged, timeout=30)
+    response.raise_for_status()
+    return response
+
+
+def _reconstruct_abstract(inverted_index: dict[str, list[int]] | None) -> str | None:
+    """Convert OpenAlex abstract_inverted_index back to plain text."""
+    if not inverted_index:
+        return None
+    word_positions: list[tuple[int, str]] = []
+    for word, positions in inverted_index.items():
+        for pos in positions:
+            word_positions.append((pos, word))
+    word_positions.sort(key=lambda t: t[0])
+    return " ".join(w for _, w in word_positions)
+
+
+def _map_paper(data: dict[str, Any]) -> Paper:
+    authors: list[str] = [
+        a.get("author", {}).get("display_name") or "" for a in data.get("authorships", [])
+    ]
+    abstract = _reconstruct_abstract(data.get("abstract_inverted_index"))
+    return Paper(
+        id=data.get("doi") or data.get("id") or "",
+        title=data.get("title") or "",
+        openalex_id=data.get("id"),
+        authors=authors,
+        year=data.get("publication_year"),
+        abstract=abstract,
+    )
+
+
+def fetch_paper(id: str) -> Paper | None:
+    """Fetch a single paper by arXiv ID or DOI.
+
+    Parameters
+    ----------
+    id:
+        An arXiv ID (``"2301.07041"``) or a DOI (``"10.1145/…"``).
+
+    Returns
+    -------
+    Paper | None
+        A populated Paper instance, or None on 404.
+    """
+    url = f"{_BASE}/works/{id}"
+    try:
+        response = _get(url)
+    except httpx.HTTPStatusError as exc:
+        if exc.response.status_code == 404:
+            logger.debug("OpenAlex 404 for id=%s", id)
+            return None
+        raise
+    return _map_paper(response.json())
+
+
+def fetch_citations(openalex_id: str) -> list[Citation]:
+    """Fetch the reference list for a paper as Citation objects.
+
+    Parameters
+    ----------
+    openalex_id:
+        The OpenAlex work ID (``"https://openalex.org/W…"`` or short ``"W…"``).
+
+    Returns
+    -------
+    list[Citation]
+        One Citation per referenced work.
+    """
+    url = f"{_BASE}/works/{openalex_id}/references"
+    try:
+        response = _get(url)
+    except httpx.HTTPStatusError as exc:
+        if exc.response.status_code == 404:
+            return []
+        raise
+
+    results: list[dict[str, Any]] = response.json().get("results", [])
+    citations: list[Citation] = []
+    for ref in results:
+        cited_id: str = ref.get("doi") or ref.get("id") or ""
+        if cited_id:
+            citations.append(Citation(citing_id=openalex_id, cited_id=cited_id))
+    return citations
diff --git a/codex/sources/semanticscholar.py b/codex/sources/semanticscholar.py
new file mode 100644
index 0000000..8eb45d8
--- /dev/null
+++ b/codex/sources/semanticscholar.py
@@ -0,0 +1,106 @@
+"""Semantic Scholar API client.
+
+Provides:
+- fetch_references: retrieve references for a paper as Citation dataclasses.
+- fetch_recommendations: retrieve recommended paper IDs.
+
+Requests are rate-limited to 1 per second via tenacity wait_fixed.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+import httpx
+from tenacity import retry, stop_after_attempt, wait_fixed
+
+from codex.models import Citation
+
+logger = logging.getLogger(__name__)
+
+_BASE_GRAPH = "https://api.semanticscholar.org/graph/v1"
+_BASE_RECS = "https://api.semanticscholar.org/recommendations/v1"
+
+
+@retry(
+    stop=stop_after_attempt(5),
+    wait=wait_fixed(1),
+    before_sleep=lambda rs: logger.warning(
+        "SemanticScholar retry %d after %s",
+        rs.attempt_number,
+        rs.outcome.exception(),  # type: ignore[union-attr]
+    ),
+)
+def _get(url: str, params: dict[str, Any] | None = None) -> httpx.Response:
+    response = httpx.get(url, params=params, timeout=30)
+    response.raise_for_status()
+    return response
+
+
+def fetch_references(paper_id: str) -> list[Citation]:
+    """Fetch references for a paper from Semantic Scholar.
+
+    Parameters
+    ----------
+    paper_id:
+        Semantic Scholar paper ID (or ``arXiv:…`` / ``DOI:…`` prefixed ID).
+
+    Returns
+    -------
+    list[Citation]
+        One Citation per reference, with optional context snippet.
+    """
+    url = f"{_BASE_GRAPH}/paper/{paper_id}/references"
+    params: dict[str, Any] = {"fields": "externalIds,contexts"}
+    try:
+        response = _get(url, params=params)
+    except httpx.HTTPStatusError as exc:
+        if exc.response.status_code == 404:
+            return []
+        raise
+
+    data = response.json()
+    raw_refs: list[dict[str, Any]] = data.get("data", [])
+    citations: list[Citation] = []
+    for entry in raw_refs:
+        cited_paper: dict[str, Any] = entry.get("citedPaper", {})
+        external_ids: dict[str, str] = cited_paper.get("externalIds") or {}
+        contexts: list[str] = entry.get("contexts", [])
+        context: str | None = contexts[0] if contexts else None
+
+        cited_id: str = (
+            external_ids.get("DOI") or external_ids.get("ArXiv") or cited_paper.get("paperId") or ""
+        )
+        if cited_id:
+            citations.append(Citation(citing_id=paper_id, cited_id=cited_id, context=context))
+    return citations
+
+
+def fetch_recommendations(paper_id: str, limit: int = 20) -> list[str]:
+    """Fetch recommended paper IDs from Semantic Scholar.
+
+    Parameters
+    ----------
+    paper_id:
+        Semantic Scholar paper ID.
+    limit:
+        Maximum number of recommendations to return.
+
+    Returns
+    -------
+    list[str]
+        List of recommended paper IDs.
+    """
+    url = f"{_BASE_RECS}/papers/forpaper/{paper_id}"
+    params: dict[str, Any] = {"limit": limit}
+    try:
+        response = _get(url, params=params)
+    except httpx.HTTPStatusError as exc:
+        if exc.response.status_code == 404:
+            return []
+        raise
+
+    data = response.json()
+    recommended: list[dict[str, Any]] = data.get("recommendedPapers", [])
+    return [p["paperId"] for p in recommended if p.get("paperId")]
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/sources/__init__.py b/tests/sources/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/sources/test_arxiv.py b/tests/sources/test_arxiv.py
new file mode 100644
index 0000000..1c08500
--- /dev/null
+++ b/tests/sources/test_arxiv.py
@@ -0,0 +1,140 @@
+"""Tests for codex.sources.arxiv."""
+
+from __future__ import annotations
+
+import io
+import tarfile
+
+import httpx
+import pytest
+
+from codex.sources import arxiv
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_tar_gz(files: dict[str, bytes]) -> bytes:
+    """Build an in-memory .tar.gz archive from a dict of {name: content}."""
+    buf = io.BytesIO()
+    with tarfile.open(fileobj=buf, mode="w:gz") as tf:
+        for name, content in files.items():
+            info = tarfile.TarInfo(name=name)
+            info.size = len(content)
+            tf.addfile(info, io.BytesIO(content))
+    return buf.getvalue()
+
+
+# ---------------------------------------------------------------------------
+# fetch_source — success: in-memory tar.gz with one .tex
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_source_returns_latex(monkeypatch: pytest.MonkeyPatch) -> None:
+    """fetch_source extracts and returns the LaTeX content from a .tar.gz."""
+    latex_content = b"\\documentclass{article}\n\\begin{document}\nHello world\n\\end{document}"
+    tar_bytes = _make_tar_gz({"main.tex": latex_content})
+
+    def mock_get(
+        url: str,
+        *,
+        timeout: int = 60,
+        follow_redirects: bool = True,
+    ) -> httpx.Response:
+        return httpx.Response(200, content=tar_bytes)
+
+    monkeypatch.setattr(httpx, "get", mock_get)
+
+    result = arxiv.fetch_source("2301.07041")
+
+    assert result is not None
+    assert "\\documentclass" in result
+    assert "Hello world" in result
+
+
+# ---------------------------------------------------------------------------
+# fetch_source — 404 → None
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_source_404_returns_none(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A 404 response should return None."""
+
+    def mock_get(
+        url: str,
+        *,
+        timeout: int = 60,
+        follow_redirects: bool = True,
+    ) -> httpx.Response:
+        return httpx.Response(404)
+
+    monkeypatch.setattr(httpx, "get", mock_get)
+
+    result = arxiv.fetch_source("nonexistent-id")
+    assert result is None
+
+
+# ---------------------------------------------------------------------------
+# fetch_source — no .tex in archive → None
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_source_no_tex_returns_none(monkeypatch: pytest.MonkeyPatch) -> None:
+    """If the archive contains no .tex files, return None."""
+    tar_bytes = _make_tar_gz({"README.md": b"# Paper\nNo LaTeX here."})
+
+    def mock_get(
+        url: str,
+        *,
+        timeout: int = 60,
+        follow_redirects: bool = True,
+    ) -> httpx.Response:
+        return httpx.Response(200, content=tar_bytes)
+
+    monkeypatch.setattr(httpx, "get", mock_get)
+
+    result = arxiv.fetch_source("2301.07041")
+    assert result is None
+
+
+# ---------------------------------------------------------------------------
+# fetch_source — fallback to largest .tex when no \documentclass
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_source_fallback_to_largest_tex(monkeypatch: pytest.MonkeyPatch) -> None:
+    """When no file has \\documentclass, the largest .tex is returned."""
+    small_tex = b"% small file\n\\section{Intro}"
+    large_tex = b"% large file\n" + b"x" * 500
+
+    tar_bytes = _make_tar_gz({"small.tex": small_tex, "large.tex": large_tex})
+
+    def mock_get(
+        url: str,
+        *,
+        timeout: int = 60,
+        follow_redirects: bool = True,
+    ) -> httpx.Response:
+        return httpx.Response(200, content=tar_bytes)
+
+    monkeypatch.setattr(httpx, "get", mock_get)
+
+    result = arxiv.fetch_source("2301.07041")
+
+    assert result is not None
+    assert "large file" in result
+
+
+# ---------------------------------------------------------------------------
+# fetch_pdf_url — pure computation, no mock needed
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_pdf_url_pure_computation() -> None:
+    """fetch_pdf_url returns the correct URL without making any HTTP request."""
+    url = arxiv.fetch_pdf_url("2301.07041")
+    assert url == "https://arxiv.org/pdf/2301.07041.pdf"
+
+    url2 = arxiv.fetch_pdf_url("1234.56789")
+    assert url2 == "https://arxiv.org/pdf/1234.56789.pdf"
diff --git a/tests/sources/test_openalex.py b/tests/sources/test_openalex.py
new file mode 100644
index 0000000..54b9b8b
--- /dev/null
+++ b/tests/sources/test_openalex.py
@@ -0,0 +1,142 @@
+"""Tests for codex.sources.openalex."""
+
+from __future__ import annotations
+
+import httpx
+import pytest
+
+from codex.models import Citation, Paper
+from codex.sources import openalex
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+_SAMPLE_WORK = {
+    "id": "https://openalex.org/W2741809807",
+    "doi": "10.1145/3592430",
+    "title": "Conformal Prediction: A Review",
+    "publication_year": 2023,
+    "authorships": [
+        {"author": {"display_name": "Alice Smith"}},
+        {"author": {"display_name": "Bob Jones"}},
+    ],
+    "abstract_inverted_index": {
+        "Conformal": [0],
+        "prediction": [1],
+        "is": [2],
+        "great": [3],
+    },
+}
+
+_SAMPLE_REFS = {
+    "results": [
+        {"doi": "10.1000/ref1", "id": "https://openalex.org/W1"},
+        {"doi": "10.1000/ref2", "id": "https://openalex.org/W2"},
+    ]
+}
+
+
+# ---------------------------------------------------------------------------
+# fetch_paper — success
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_paper_success(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Successful fetch maps all fields to Paper correctly."""
+
+    def mock_get(url: str, params: dict[str, str] | None = None) -> httpx.Response:
+        return httpx.Response(200, json=_SAMPLE_WORK)
+
+    monkeypatch.setattr(openalex, "_get", mock_get)
+
+    paper = openalex.fetch_paper("10.1145/3592430")
+
+    assert paper is not None
+    assert isinstance(paper, Paper)
+    assert paper.title == "Conformal Prediction: A Review"
+    assert paper.year == 2023
+    assert paper.authors == ["Alice Smith", "Bob Jones"]
+    assert paper.abstract == "Conformal prediction is great"
+    assert paper.openalex_id == "https://openalex.org/W2741809807"
+
+
+# ---------------------------------------------------------------------------
+# fetch_paper — 404 → None
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_paper_404_returns_none(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A 404 response should return None without raising."""
+
+    def mock_get(url: str, params: dict[str, str] | None = None) -> httpx.Response:
+        raise httpx.HTTPStatusError(
+            "Not Found",
+            request=httpx.Request("GET", url),
+            response=httpx.Response(404, request=httpx.Request("GET", url)),
+        )
+
+    monkeypatch.setattr(openalex, "_get", mock_get)
+
+    result = openalex.fetch_paper("nonexistent-id")
+    assert result is None
+
+
+# ---------------------------------------------------------------------------
+# fetch_paper — 429 retry then success
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_paper_retry_on_429(monkeypatch: pytest.MonkeyPatch) -> None:
+    """On 429 x3 then 200, should return 1 Paper after 3 failed attempts."""
+    attempt = [0]
+
+    def controlled_get(url: str, params: dict[str, str] | None = None) -> httpx.Response:
+        attempt[0] += 1
+        if attempt[0] <= 3:
+            resp = httpx.Response(429, request=httpx.Request("GET", url))
+            raise httpx.HTTPStatusError(
+                "Too Many Requests",
+                request=httpx.Request("GET", url),
+                response=resp,
+            )
+        return httpx.Response(200, json=_SAMPLE_WORK)
+
+    monkeypatch.setattr(openalex, "_get", controlled_get)
+
+    # Call fetch_paper in a loop to simulate retry behaviour
+    result: Paper | None = None
+    errors = 0
+    for _ in range(5):
+        try:
+            result = openalex.fetch_paper("10.1145/3592430")
+            break
+        except httpx.HTTPStatusError:
+            errors += 1
+
+    assert result is not None
+    assert isinstance(result, Paper)
+    assert errors == 3  # 3 failures before success
+
+
+# ---------------------------------------------------------------------------
+# fetch_citations
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_citations(monkeypatch: pytest.MonkeyPatch) -> None:
+    """fetch_citations returns one Citation per reference entry."""
+
+    def mock_get(url: str, params: dict[str, str] | None = None) -> httpx.Response:
+        return httpx.Response(200, json=_SAMPLE_REFS)
+
+    monkeypatch.setattr(openalex, "_get", mock_get)
+
+    oa_id = "https://openalex.org/W2741809807"
+    citations = openalex.fetch_citations(oa_id)
+
+    assert len(citations) == 2
+    assert all(isinstance(c, Citation) for c in citations)
+    assert citations[0].citing_id == oa_id
+    assert citations[0].cited_id == "10.1000/ref1"
+    assert citations[1].cited_id == "10.1000/ref2"
diff --git a/tests/sources/test_semanticscholar.py b/tests/sources/test_semanticscholar.py
new file mode 100644
index 0000000..42066d6
--- /dev/null
+++ b/tests/sources/test_semanticscholar.py
@@ -0,0 +1,126 @@
+"""Tests for codex.sources.semanticscholar."""
+
+from __future__ import annotations
+
+import httpx
+import pytest
+
+from codex.models import Citation
+from codex.sources import semanticscholar
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+_SAMPLE_REFS_RESPONSE = {
+    "data": [
+        {
+            "citedPaper": {
+                "paperId": "abc123",
+                "externalIds": {"DOI": "10.1000/xyz1"},
+            },
+            "contexts": ["This approach was first described in [1]."],
+        },
+        {
+            "citedPaper": {
+                "paperId": "def456",
+                "externalIds": {"ArXiv": "2301.07041"},
+            },
+            "contexts": [],
+        },
+    ]
+}
+
+_SAMPLE_RECS_RESPONSE = {
+    "recommendedPapers": [
+        {"paperId": "rec_id_1"},
+        {"paperId": "rec_id_2"},
+        {"paperId": "rec_id_3"},
+    ]
+}
+
+
+# ---------------------------------------------------------------------------
+# fetch_references
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_references_returns_citations(monkeypatch: pytest.MonkeyPatch) -> None:
+    """fetch_references maps API response to Citation objects correctly."""
+
+    def mock_get(url: str, params: dict[str, object] | None = None) -> httpx.Response:
+        return httpx.Response(200, json=_SAMPLE_REFS_RESPONSE)
+
+    monkeypatch.setattr(semanticscholar, "_get", mock_get)
+
+    citations = semanticscholar.fetch_references("abc123")
+
+    assert len(citations) == 2
+    assert all(isinstance(c, Citation) for c in citations)
+
+    # First citation uses DOI and has context
+    assert citations[0].citing_id == "abc123"
+    assert citations[0].cited_id == "10.1000/xyz1"
+    assert citations[0].context == "This approach was first described in [1]."
+
+    # Second citation falls back to ArXiv ID, no context
+    assert citations[1].citing_id == "abc123"
+    assert citations[1].cited_id == "2301.07041"
+    assert citations[1].context is None
+
+
+def test_fetch_references_404_returns_empty(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A 404 response should return an empty list."""
+
+    def mock_get(url: str, params: dict[str, object] | None = None) -> httpx.Response:
+        raise httpx.HTTPStatusError(
+            "Not Found",
+            request=httpx.Request("GET", url),
+            response=httpx.Response(404, request=httpx.Request("GET", url)),
+        )
+
+    monkeypatch.setattr(semanticscholar, "_get", mock_get)
+
+    result = semanticscholar.fetch_references("nonexistent")
+    assert result == []
+
+
+# ---------------------------------------------------------------------------
+# fetch_recommendations
+# ---------------------------------------------------------------------------
+
+
+def test_fetch_recommendations_returns_list_of_str(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """fetch_recommendations returns a flat list of paperId strings."""
+
+    def mock_get(url: str, params: dict[str, object] | None = None) -> httpx.Response:
+        return httpx.Response(200, json=_SAMPLE_RECS_RESPONSE)
+
+    monkeypatch.setattr(semanticscholar, "_get", mock_get)
+
+    result = semanticscholar.fetch_recommendations("abc123", limit=3)
+
+    assert isinstance(result, list)
+    assert len(result) == 3
+    assert all(isinstance(r, str) for r in result)
+    assert result == ["rec_id_1", "rec_id_2", "rec_id_3"]
+
+
+def test_fetch_recommendations_404_returns_empty(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """A 404 response should return an empty list."""
+
+    def mock_get(url: str, params: dict[str, object] | None = None) -> httpx.Response:
+        raise httpx.HTTPStatusError(
+            "Not Found",
+            request=httpx.Request("GET", url),
+            response=httpx.Response(404, request=httpx.Request("GET", url)),
+        )
+
+    monkeypatch.setattr(semanticscholar, "_get", mock_get)
+
+    result = semanticscholar.fetch_recommendations("nonexistent")
+    assert result == []