--- title: [FATAL] Ansible Release 12.0 Breaks netlab Jinja2... description: Ansible 12.0, released on PyPI on September 9th, requires ansible-core 2.19 which introduced breaking changes to Jinja2 templating. Specifically, access to... canonical: https://daily.dev/posts/fatal-ansible-release-12-0-breaks-netlab-jinja2-templates-ipspace-net-blog-3xgqekoin twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: [FATAL] Ansible Release 12.0 Breaks netlab Jinja2 Templates « ipSpace.net blog | daily.dev og:description: Ansible 12.0, released on PyPI on September 9th, requires ansible-core 2.19 which introduced breaking changes to Jinja2 templating. Specifically, access to... og:url: https://daily.dev/posts/fatal-ansible-release-12-0-breaks-netlab-jinja2-templates-ipspace-net-blog-3xgqekoin og:image: https://api.daily.dev/og/posts/3xgQEKoiN.png og:image:alt: [FATAL] Ansible Release 12.0 Breaks netlab Jinja2 Templates « ipSpace.net blog og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # [FATAL] Ansible Release 12.0 Breaks netlab Jinja2 Templates « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 3 min read · 0 upvotes · 0 comments ## Summary Ansible 12.0, released on PyPI on September 9th, requires ansible-core 2.19 which introduced breaking changes to Jinja2 templating. Specifically, access to underscore-prefixed attributes and methods is no longer permitted. netlab heavily relies on such attributes in its Jinja2 templates and Ansible filters (like ipaddr), causing crashes on any new installation created after that date. The netlab 25.09-post1 release implements a workaround: the netlab command now refuses to run with ansible-core 2.19 or higher, and the install script pins Ansible to version 11.10 or lower. Future plans include removing the dependency on Ansible filters and potentially decoupling netlab's configuration generation from Ansible entirely. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does Ansible 12.0 break Jinja2 templates that use underscore-prefixed attributes? ansible-core 2.19 (required by Ansible 12.0) no longer permits access to underscore-prefixed attributes and methods in Jinja2 templates, as part of increased template environment isolation. Templates that use such attributes — even with the |default() filter — now raise errors. This affects any tool that relies on Ansible's Jinja2 environment and uses _-prefixed internal data attributes. _Teams maintaining network automation templates with Ansible track breaking changes like this on daily.dev before they hit production._ ### How do I fix netlab crashing after upgrading to Ansible 12.0? Run pip3 install --upgrade networklab to get netlab release 25.09-post1, which refuses to start with ansible-core 2.19 or higher and pins Ansible to 11.10 or lower via the netlab install ansible command. Alternatively, manually downgrade Ansible to version 11.10 or lower in your environment. _Developers managing netlab environments find workarounds for dependency conflicts like this on daily.dev._ ## Similar posts on daily.dev - [netlab 26.09: Syslog, More DNS, Netmiko « ipSpace.net blog](https://daily.dev/posts/netlab-26-09-syslog-more-dns-netmiko-ipspace-net-blog-7hzbp4smq) · ipSpace.net Blog · 0 upvotes · 0 comments --- Tags: [#networking](https://daily.dev/tags/networking), [#ansible](https://daily.dev/tags/ansible) [View this post on daily.dev](https://daily.dev/posts/fatal-ansible-release-12-0-breaks-netlab-jinja2-templates-ipspace-net-blog-3xgqekoin) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"[FATAL] Ansible Release 12.0 Breaks netlab Jinja2 Templates « ipSpace.net blog","url":"https://daily.dev/posts/fatal-ansible-release-12-0-breaks-netlab-jinja2-templates-ipspace-net-blog-3xgqekoin","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/fatal-ansible-release-12-0-breaks-netlab-jinja2-templates-ipspace-net-blog-3xgqekoin"},"datePublished":"2026-08-11T08:55:59.246Z","dateModified":"2026-08-11T09:07:46.767Z","description":"Ansible 12.0, released on PyPI on September 9th, requires ansible-core 2.19 which introduced breaking changes to Jinja2 templating. 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