Claude Code v2.1.91 Updates: Agent Design Patterns, Memory Rules, and Tool Improvements

Claude Code v2.1.91 introduces updates to agent design patterns, memory management rules, and tool functionality. The release adds a reference guide for building agents on the Claude API and modifies several existing features.

New Agent Design Patterns Guide

The update adds a Skill: Agent Design Patterns reference guide covering decision heuristics for building agents. This includes guidance on tool surface design, context management, caching strategies, and composing tool calls. The guide is referenced in multiple locations including the Build with Claude API skill and tool use concepts documentation.

Memory Management Changes

The "Determine which memory files to attach" agent prompt now uses a simpler rule: do not re-select memories already returned for an earlier query in the same conversation. This replaces the previous rule about skipping memories for recently-used tools. The prompt clarifies that the first message lists available memories and subsequent messages each contain one user query.

Security monitoring for autonomous agent actions now includes a "Memory Poisoning" block rule covering writes to the agent's memory directory that would function as permission grants, BLOCK-rule bypasses, or fabricated user authorization. A corresponding "Memory Directory" allow exception permits routine memory writes (user preferences, project facts, references) that don't constitute poisoning.

Tool Improvements

• Edit Tool: Removed the inline note about edits failing when old_string is not unique; replaced with a slot for additional edit guidelines.
• ReadFile Tool: Added support for relative file paths (preferred for brevity) as a conditional alternative to the absolute-path-only requirement. Made the default line-read limit and additional read notes configurable.
• Write Tool: Replaced the blanket "read first" requirement with a conditional note for new files. Made the "prefer Edit" guidance configurable.

Documentation and Skill Updates

Live documentation sources added WebFetch URLs for six additional tool documentation pages: Bash Tool, Text Editor, Memory Tool, Tool Search, Programmatic Tool Calling, and Skills. Context Editing was added to the Advanced Features section.

Tool use concepts documentation added new sections for Skills (task-specific instruction packages loaded on demand) and Context Editing (pruning stale tool results from the transcript). The Programmatic Tool Calling description was expanded to explain the round-trip cost problem and how scripts run in the code execution container.

The Verify skill now includes a SKIP verdict for changes with no runtime surface (docs-only, types-only, tests-only), distinct from BLOCKED which now strictly means the verifier couldn't reach an observable state. Tests in the diff are considered the author's evidence, not a verification surface.

Removed Features

• Agent Prompt: /pr-comments slash command for fetching and displaying GitHub PR comments
• Agent Prompt: Update Magic Docs

The Build with Claude API skill now recommends medium effort as a favorable balance and max effort when correctness matters more than cost.