Mistral Vibe: Unified Harness Graduates From Experimental to Default
Mistral VibeMistral Vibe's v2.25.5 release removed the "experimental" label from its Unified Harness, making it the default execution engine for every session. Developers who want the old behavior can still reach it through the newly documented --legacy-harness flag, which now serves as the official escape hatch back to the legacy Python harness. The change marks the point where Mistral Vibe's newer architecture, which has been picking up features release after release, becomes the path every user runs by default rather than an opt-in flag.
Key Takeaways
- Mistral Vibe's Unified Harness is now the default execution engine for every new session, no longer requiring an opt-in flag.
--legacy-harnessis the newly documented flag for reverting to the original Python-based harness if needed.- The change caps a series of releases in which the Unified Harness steadily gained parity features, including workspace-trust recording.
- Because the change is architectural rather than a new UI element, most users will not notice anything different unless they hit a regression that requires falling back.
- This is a strong signal that Mistral Vibe considers the newer harness stable and production-ready.
- No independent third-party coverage exists yet; the only mentions found are Mistral's own CHANGELOG.md, GitHub release page, and PyPI listing.
The Unified Harness Is No Longer Experimental
Mistral Vibe's v2.25.5 release changed the status of its Unified Harness: the CHANGELOG.md entry states plainly that "the Unified Harness is no longer labeled 'experimental'." For a component that previously required developers to opt in with a flag, this is a default-behavior change that affects every user starting a new session, not just those who had been testing the newer harness deliberately.
--legacy-harness Becomes the Documented Fallback
Mistral Vibe paired the change with a clearly named escape hatch: --legacy-harness is now the documented way to fall back to the original Python-based harness if a developer hits a regression or depends on legacy-specific behavior. This gives teams a safety valve during the transition rather than forcing an all-or-nothing cutover.
Why This Matters
The Unified Harness has been accumulating capability over several recent releases, including workspace-trust recording over ACP and, in this same release, todo tracking and a persistent scratchpad. Removing the "experimental" label signals that Mistral Vibe now considers this harness stable enough to be the default, which is likely to be the most consequential single line in the v2.25.5 changelog for anyone running Mistral Vibe day to day, even though most of what changed underneath is invisible unless something breaks.