diff --git a/src/paperqa/settings.py b/src/paperqa/settings.py
index 91a5fec03..1a58e3211 100644
--- a/src/paperqa/settings.py
+++ b/src/paperqa/settings.py
@@ -7,6 +7,7 @@
 import warnings
 from collections import defaultdict
 from collections.abc import Awaitable, Callable, Mapping, Sequence
+from contextlib import suppress
 from enum import IntEnum, StrEnum
 from itertools import starmap
 from pydoc import locate
@@ -74,7 +75,12 @@
 )
 from paperqa.readers import PDFParserFn
 from paperqa.types import Context, ParsedMedia, ParsedText
-from paperqa.utils import hexdigest, parse_enrichment_irrelevance, pqa_directory
+from paperqa.utils import (
+    get_stable_str,
+    hexdigest,
+    parse_enrichment_irrelevance,
+    pqa_directory,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -316,13 +322,10 @@ def _custom_serializer(
         if isinstance(self.parse_pdf, str):
             # Already JSON-compliant, so let's un-exclude
             data["parse_pdf"] = self.parse_pdf
-        elif (
-            info.mode == "json"
-            and hasattr(self.parse_pdf, "__module__")
-            and hasattr(self.parse_pdf, "__name__")
-        ):
+        elif info.mode == "json":
             # If going to JSON, and we can get a FQN, do so for JSON compliance
-            data["parse_pdf"] = f"{self.parse_pdf.__module__}.{self.parse_pdf.__name__}"
+            with suppress(ValueError):  # Suppress when not serialization-safe
+                data["parse_pdf"] = get_stable_str(self.parse_pdf)
         return data
 
     doc_filters: Sequence[Mapping[str, Any]] | None = Field(
@@ -852,7 +855,7 @@ def get_index_name(self) -> str:
             first_segment,
             str(self.agent.index.use_absolute_paper_directory),
             self.embedding,
-            str(self.parsing.parse_pdf),  # Don't use __name__ as lambda wouldn't differ
+            get_stable_str(self.parsing.parse_pdf, for_hash=True),
             str(self.parsing.reader_config["chunk_chars"]),
             str(self.parsing.reader_config["overlap"]),
             str(self.parsing.reader_config.get("full_page", False)),
diff --git a/src/paperqa/utils.py b/src/paperqa/utils.py
index 2f31a8a4c..2ef93ea72 100644
--- a/src/paperqa/utils.py
+++ b/src/paperqa/utils.py
@@ -10,7 +10,7 @@
 from collections import Counter
 from collections.abc import Awaitable, Callable, Collection, Iterable, Iterator, Mapping
 from datetime import datetime
-from functools import reduce
+from functools import partial, reduce
 from http import HTTPStatus
 from pathlib import Path
 from typing import Any, BinaryIO, ClassVar, TypeVar
@@ -686,3 +686,31 @@ def parse_enrichment_irrelevance(enrichment: str) -> tuple[bool, str]:
         return False, enrichment.strip()
     no_label_enrichment = enrichment[label_match.end() :] if label_match else enrichment
     return is_irrelevant, no_label_enrichment.strip()
+
+
+def get_stable_str(fn: Callable, for_hash: bool = False) -> str:
+    """
+    Get a stable string from a function, for serialization (default) or hashing.
+
+    It avoids common pitfalls such as:
+    - All lambda functions having the same name '<lambda>'.
+    - IDs in normal string representation of functions breaking cache keys.
+    """
+    if (
+        hasattr(fn, "__module__")
+        and hasattr(fn, "__name__")
+        and fn.__name__ != "<lambda>"
+        and not isinstance(fn, partial)
+    ):
+        # Named function: use fully qualified name
+        return f"{fn.__module__}.{fn.__name__}"
+    if not for_hash:
+        raise ValueError(
+            f"Cannot form serialization-safe string representation for {fn}."
+        )
+    # Fallback to lower-quality hash name
+    # NOTE: we could look into `fn.__code__`, but as `fn.__code__.co_code`
+    # is not stable across Python minor versions,
+    # this unfortunately makes the cache key incompatible across Python versions,
+    # which is too brittle
+    return type(fn).__name__
diff --git a/tests/test_paperqa.py b/tests/test_paperqa.py
index 850ff9e26..c176b5502 100644
--- a/tests/test_paperqa.py
+++ b/tests/test_paperqa.py
@@ -13,6 +13,7 @@
 from collections.abc import AsyncIterable, Sequence
 from copy import deepcopy
 from datetime import datetime, timedelta
+from functools import partial
 from io import BytesIO
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, cast
@@ -3291,6 +3292,12 @@ async def test_timeout_resilience() -> None:
     assert not llm_results
 
 
+TEST_STUB_LAMBDA = lambda: 1  # noqa: E731
+
+
+TEST_STUB_PARTIAL = partial(pymupdf_parse_pdf_to_pages, kwarg=2)
+
+
 def test_parse_pdf_string_resolution() -> None:
     # Test with a valid string FQN
     pymupdf_str = Settings(
@@ -3314,7 +3321,7 @@ def test_parse_pdf_string_resolution() -> None:
     )
     assert "parse_pdf" not in pypdf_str.model_dump()["parsing"]
 
-    # Test directly passing a parser
+    # Test directly passing a normal parser
     pymupdf_fn = Settings(parsing=ParsingSettings(parse_pdf=pymupdf_parse_pdf_to_pages))
     assert pymupdf_fn.parsing.parse_pdf == pymupdf_parse_pdf_to_pages
     assert (
@@ -3323,6 +3330,18 @@ def test_parse_pdf_string_resolution() -> None:
     )
     assert "parse_pdf" not in pymupdf_fn.model_dump()["parsing"]
 
+    # Test directly passing a lambda parser
+    lambda_fn = Settings(parsing=ParsingSettings(parse_pdf=TEST_STUB_LAMBDA))
+    assert lambda_fn.parsing.parse_pdf == TEST_STUB_LAMBDA
+    assert "parse_pdf" not in lambda_fn.model_dump(mode="json")["parsing"]
+    assert "parse_pdf" not in lambda_fn.model_dump()["parsing"]
+
+    # Test directly passing a functools partial parser
+    partial_fn = Settings(parsing=ParsingSettings(parse_pdf=TEST_STUB_PARTIAL))
+    assert partial_fn.parsing.parse_pdf == TEST_STUB_PARTIAL
+    assert "parse_pdf" not in partial_fn.model_dump(mode="json")["parsing"]
+    assert "parse_pdf" not in partial_fn.model_dump()["parsing"]
+
     # Test a nonexistent FQN
     with pytest.raises(ValueError, match="Failed to locate"):
         Settings(parsing=ParsingSettings(parse_pdf="nonexistent.module.function"))