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Diagnose prompt caching with the Cache Explorer

When you send a prompt to the AI in Visual Studio Code, the language model provider can reuse the parts of your request that match a previous request. This is called prompt caching. The Cache Explorer view helps you diagnose prompt cache misses by comparing consecutive model requests in a chat session, which helps you reduce token cost and latency.

The Cache Explorer is one of the views in the Agent Debug Logs panel.

Why prompt caching matters

Prompt caching lets a model provider reuse the prefix of a request that matches a previous one. When the beginning of a request is identical to the previous request, the provider serves that portion from cache instead of processing it again. A higher cache hit rate reduces both latency and token cost.

The cache only applies to the matching prefix of a request. As soon as the content of two consecutive requests diverges, everything after that point is a cache miss. Small changes early in the prompt, such as a reordered tool definition or a modified instruction, break the cache for the rest of the request. The Cache Explorer pinpoints exactly where the prompt prefix diverges, which helps you understand and address a low cache hit rate.

Open the Cache Explorer

  1. Open the Agent Debug panel by selecting the ellipsis (...) menu in the Chat view and selecting Show Agent Debug Logs.

  2. Select the session description in the breadcrumb at the top to go to the Summary view.

  3. Select Cache Explorer to open the Cache Explorer view for the selected session.

Screenshot showing the Cache Explorer view in Agent Logs, with a side-by-side diff of two model requests.

Read the Cache Explorer

The Cache Explorer has two panels:

  • The side panel lists all model turns in the session, grouped by user request. Each turn shows the cache hit percentage, duration, model name, and timestamp. Select a turn to compare it against the previous turn.
  • The main content shows a side-by-side prefix diff between the current request and the previous request.

The main content area includes the following information:

  • Cache performance: the cache hit percentage and the number of input tokens reused out of the total.
  • Prompt signature: a visual summary of each component in the request, such as system instructions, tool definitions, and messages, with color-coded status indicators. The first divergence point marks where the prompt cache breaks.
  • Components: expandable sections for system instructions, tool definitions, and individual messages that show the text-level diff between the two requests.

To find the cause of a cache miss, locate the first divergence point in the prompt signature, then expand the corresponding component to see the exact text that changed between the two requests.

Improve your cache hit rate

Prompt caching works best when the early parts of your requests stay stable across turns. Use the following practices to keep the prompt prefix consistent:

  • Lock in settings before you start: Switching the model, reasoning effort, context size, or the enabled tools and MCP servers during a session rebuilds the cache. Choose them upfront, or use auto model selection, which switches models only at cache boundaries.
  • Keep instructions stable: Changing instructions files or custom agent definitions mid-session breaks the cache. For more information, see the context engineering guide.
  • Add volatile context late: Place content that changes often, such as file attachments or terminal output, later in the conversation.
  • Isolate exploration in subagents: Run research in a subagent to keep the parent session prompt stable.
  • Start fresh after a break: Caches expire after inactivity. Start a new session or run /compact to rebuild from a short summary instead of the full history.

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