Partly: in 4 runs, Claude Fable 5.1's langchain attribution reaches 1.0.0 (2025-10-17), 1.1.0 (2025-11-24) and 1.2.0 (2025-12-15), inside langchain 1, and stops there — langchain was at 1.4.0 when this index last verified against it (2026-09-06). That is ~6 to ~8 months below the training cutoff the subject stated in those runs (2026-06). 2 findings are currently charged against Claude Fable 5.1 on langchain (2x S3 deprecated), each reproduced in a published run and checked against langchain's own release notes. Read the boundary precisely: it is the newest release whose contents the model can correctly attribute to that release, not the newest langchain feature it can use. Past it a model often writes working code with a newer API while naming the wrong release for it.
Answer class inside, decided by one rule applied to every page
of this kind: every measured boundary is at or above the first release of the major named in the question. Every figure below is read from
the dataset at build time; nothing on this page is written by hand.
| Battery | Newest release it can place | Oldest it cannot | Lag vs stated cutoff |
|---|---|---|---|
| v4 2026-09-06 | 1.0.0 2025-10-17 |
1.1.0 2025-11-24 |
~8 months |
| v5-a 2026-09-06 | 1.1.0 2025-11-24 |
1.2.0 2025-12-15 |
~6 months |
| v5-b 2026-09-06 | 1.1.0 2025-11-24 |
1.2.0 2025-12-15 |
~6 months |
| v6-e 2026-09-06 | 1.2.0 2025-12-15 |
1.3.0 2026-05-12 |
~6 months |
4 measurements of this pair, giving 3 different boundaries — 59 days apart, 2 releases apart. No two of them were drawn from an identical prompt, so the spread mixes prompt differences with draw variance.
langchain published 4 minor or major releases in the twelve months before this model’s stated cutoff, which is the scale a spread should be read against.
“Chargeable” means the change was published before this model’s own stated cutoff, so it had the opportunity to know it.
| Severity | Belief | Changed in | Chargeable | Proof |
|---|---|---|---|---|
| S3deprecated | Denies that create_agent can register stream transformers at all, then ships a consumer-side wrapper that rebuilds per-scope identity by hand - the verdict and the artefact wrong in the same direction, four tasks apartcreate_agent(transformers=...) |
1.3.0 2026-05-12 |
yes | run · source |
| S3deprecated | Names v2 as the highest event-stream protocol version an agent accepts, and describes it as the ceiling - v3 shipped for langchain agents in the same release as F1's parameterastream_events(version="v3") on a create_agent agent |
1.3.0 2026-05-12 |
yes | run · source |
The langchain correction pack states what is true now for each corrected fact, with a primary-source citation, as markdown you can paste into a rules file (raw). What a correction pack measurably changed when one was tested — a pre-registered run on zod — is on the benchmark page, including where it changed nothing.
All of them at once: Which Claude model knows langchain 1 best?
Other subjects on langchain:
Claude Fable 5.1 on the other libraries: