In brief
Claude Code shipped fifteen prompt-cache repairs and a setting that caps reasoning effort
Roughly fifteen fixes in 2.1.267 are prompt-cache-break repairs: resumed sessions rewriting tool definitions, mid-session MCP tools appended inline, `/model` switching resending every tool definition. If you resume long sessions or attach connectors mid-run you were paying uncached input on turns you believed were cached. Separately, `effort:` frontmatter on custom commands, skills and subagents was being ignored entirely on Opus 4.7, Opus 4.8 and Fable 5. A new `maxEffortLevel` caps effort across Bedrock, Vertex and Foundry.
What it changes Upgrade if you resume sessions over 5 MB or attach MCP tools mid-run, and re-check that any effort: frontmatter you wrote was ever taking effect.
The changelog carries no dates. The 9 September date is the npm registry publish timestamp for 2.1.267, which is the release event rather than a date printed on the page.