Primary Sources for Claude Context Engineering

How to use this source index

Use this page to verify the factual base behind ClaudeContext.com. The source index is intentionally narrow: official Anthropic and MCP sources are preferred over commentary. Third-party writing can be useful for interpretation, but it should not override current primary docs for model limits, API behavior, or Claude Code behavior.

Which source answers which question

Use the model overview for current model comparison and context-window rows, and the Fable 5.1 and Opus 5 model pages when a claim is specific to one of those models. Use the context-window docs for what counts in context and when compaction is the primary strategy. Use token counting for preflight estimates. Use prompt caching for cache mechanics and the pricing page for cache read and write multipliers. Use Claude Code memory and subagent docs for Code-specific behavior. Use the MCP resources spec for resource semantics.

Citation policy

Every article page includes a stable "Cite this page" block and a source list. When a fact is current and likely to drift, the page should include an updated date and point to the exact source used. If a page cannot verify a claim from a primary source, it should present the claim as interpretation or omit it.

FAQ

Sources Used

• Effective context engineering for AI agents, Anthropic Engineering. Accessed 2026-07-06. Primary explanation of context engineering, context pollution, compaction, note-taking, and sub-agent architectures.
• Context windows, Claude Platform Docs. Accessed 2026-09-19. Official context-window behavior, token accumulation, extended thinking and tool-use accounting, and compaction guidance.
• Models overview, Claude Platform Docs. Accessed 2026-09-19. Current model comparison, including context-window and output limits listed by Anthropic for Fable 5.1, Opus 5, Sonnet 5, and Haiku 4.5.
• Claude Fable 5.1 overview, Claude Platform Docs. Accessed 2026-09-19. Fable 5.1 model card: 1M-token context window, cheaper cache reads, and the breaking and additive behaviors relative to Fable 5.
• Claude Opus 5 overview, Claude Platform Docs. Accessed 2026-09-19. Opus 5 model card: 1M-token context window, adaptive thinking, and the beta allowing mid-conversation tool changes without invalidating the prompt cache.
• Claude Fable and Mythos 5.1, Anthropic. Accessed 2026-09-19. September 1, 2026 launch announcement, including the 75 percent cache-read price reduction and its expected effect on agentic workloads.
• Prompting best practices, Claude Platform Docs. Accessed 2026-07-06. Official guidance on clear instructions, examples, XML sections, long-context prompting, and tool-use prompting.
• Token counting, Claude Platform Docs. Accessed 2026-09-19. Official endpoint guidance for estimating input tokens, costs, rate limits, and model routing.
• Prompt caching, Claude Platform Docs. Accessed 2026-09-19. Official prompt-caching behavior, cache lifetime, prefix caching, usage fields, and invalidation patterns.
• Pricing, Claude Platform Docs. Accessed 2026-09-19. Official model pricing and prompt-caching multipliers for base input, cache writes, cache reads, and output tokens.
• Context engineering: memory, compaction, and tool clearing, Claude Cookbook. Accessed 2026-07-06. Cookbook comparison of server-side compaction, context editing, tool-result clearing, and memory.
• How Claude remembers your project, Claude Code Docs. Accessed 2026-07-06. Official explanation of fresh Claude Code sessions, CLAUDE.md, auto memory, and load behavior.
• Claude Code settings, Claude Code Docs. Accessed 2026-07-06. Configuration categories, including memory files, settings, skills, MCP servers, and precedence.
• Create custom subagents, Claude Code Docs. Accessed 2026-07-06. Official subagent context-window behavior, scoped prompts, tool access, and context preservation.
• Resources, Model Context Protocol Specification. Accessed 2026-07-06. Protocol definition for resources, resource templates, subscriptions, contents, and annotations.
• MCP connector, Claude Platform Docs. Accessed 2026-07-06. Official Claude API MCP connector behavior for remote MCP servers, OAuth, multiple servers, and tool access.