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Prompt caching

docs/reference/prompt-caching.md

2026.5.515.5 KB
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Prompt caching means the model provider can reuse unchanged prompt prefixes (usually system/developer instructions and other stable context) across turns instead of re-processing them every time. OpenClaw normalizes provider usage into cacheRead and cacheWrite where the upstream API exposes those counters directly.

Status surfaces can also recover cache counters from the most recent transcript usage log when the live session snapshot is missing them, so /status can keep showing a cache line after partial session metadata loss. Existing nonzero live cache values still take precedence over transcript fallback values.

Why this matters: lower token cost, faster responses, and more predictable performance for long-running sessions. Without caching, repeated prompts pay the full prompt cost on every turn even when most input did not change.

The sections below cover every cache-related knob that affects prompt reuse and token cost.

Provider references:

Primary knobs

cacheRetention (global default, model, and per-agent)

Set cache retention as a global default for all models:

yaml
agents:
  defaults:
    params:
      cacheRetention: "long" # none | short | long

Override per-model:

yaml
agents:
  defaults:
    models:
      "anthropic/claude-opus-4-6":
        params:
          cacheRetention: "short" # none | short | long

Per-agent override:

yaml
agents:
  list:
    - id: "alerts"
      params:
        cacheRetention: "none"

Config merge order:

  1. agents.defaults.params (global default — applies to all models)
  2. agents.defaults.models["provider/model"].params (per-model override)
  3. agents.list[].params (matching agent id; overrides by key)

contextPruning.mode: "cache-ttl"

Prunes old tool-result context after cache TTL windows so post-idle requests do not re-cache oversized history.

yaml
agents:
  defaults:
    contextPruning:
      mode: "cache-ttl"
      ttl: "1h"

See Session Pruning for full behavior.

Heartbeat keep-warm

Heartbeat can keep cache windows warm and reduce repeated cache writes after idle gaps.

yaml
agents:
  defaults:
    heartbeat:
      every: "55m"

Per-agent heartbeat is supported at agents.list[].heartbeat.

Provider behavior

Anthropic (direct API)

  • cacheRetention is supported.
  • With Anthropic API-key auth profiles, OpenClaw seeds cacheRetention: "short" for Anthropic model refs when unset.
  • Anthropic native Messages responses expose both cache_read_input_tokens and cache_creation_input_tokens, so OpenClaw can show both cacheRead and cacheWrite.
  • For native Anthropic requests, cacheRetention: "short" maps to the default 5-minute ephemeral cache, and cacheRetention: "long" upgrades to the 1-hour TTL only on direct api.anthropic.com hosts.

OpenAI (direct API)

  • Prompt caching is automatic on supported recent models. OpenClaw does not need to inject block-level cache markers.
  • OpenClaw uses prompt_cache_key to keep cache routing stable across turns and uses prompt_cache_retention: "24h" only when cacheRetention: "long" is selected on direct OpenAI hosts.
  • OpenAI-compatible Completions providers receive prompt_cache_key only when their model config explicitly sets compat.supportsPromptCacheKey: true; cacheRetention: "none" still suppresses it.
  • OpenAI responses expose cached prompt tokens via usage.prompt_tokens_details.cached_tokens (or input_tokens_details.cached_tokens on Responses API events). OpenClaw maps that to cacheRead.
  • OpenAI does not expose a separate cache-write token counter, so cacheWrite stays 0 on OpenAI paths even when the provider is warming a cache.
  • OpenAI returns useful tracing and rate-limit headers such as x-request-id, openai-processing-ms, and x-ratelimit-*, but cache-hit accounting should come from the usage payload, not from headers.
  • In practice, OpenAI often behaves like an initial-prefix cache rather than Anthropic-style moving full-history reuse. Stable long-prefix text turns can land near a 4864 cached-token plateau in current live probes, while tool-heavy or MCP-style transcripts often plateau near 4608 cached tokens even on exact repeats.

Anthropic Vertex

  • Anthropic models on Vertex AI (anthropic-vertex/*) support cacheRetention the same way as direct Anthropic.
  • cacheRetention: "long" maps to the real 1-hour prompt-cache TTL on Vertex AI endpoints.
  • Default cache retention for anthropic-vertex matches direct Anthropic defaults.
  • Vertex requests are routed through boundary-aware cache shaping so cache reuse stays aligned with what providers actually receive.

Amazon Bedrock

  • Anthropic Claude model refs (amazon-bedrock/*anthropic.claude*) support explicit cacheRetention pass-through.
  • Non-Anthropic Bedrock models are forced to cacheRetention: "none" at runtime.

OpenRouter models

For openrouter/anthropic/* model refs, OpenClaw injects Anthropic cache_control on system/developer prompt blocks to improve prompt-cache reuse only when the request is still targeting a verified OpenRouter route (openrouter on its default endpoint, or any provider/base URL that resolves to openrouter.ai).

For openrouter/deepseek/*, openrouter/moonshot*/*, and openrouter/zai/* model refs, contextPruning.mode: "cache-ttl" is allowed because OpenRouter handles provider-side prompt caching automatically. OpenClaw does not inject Anthropic cache_control markers into those requests.

DeepSeek cache construction is best-effort and can take a few seconds. An immediate follow-up may still show cached_tokens: 0; verify with a repeated same-prefix request after a short delay and use usage.prompt_tokens_details.cached_tokens as the cache-hit signal.

If you repoint the model at an arbitrary OpenAI-compatible proxy URL, OpenClaw stops injecting those OpenRouter-specific Anthropic cache markers.

Other providers

If the provider does not support this cache mode, cacheRetention has no effect.

Google Gemini direct API

  • Direct Gemini transport (api: "google-generative-ai") reports cache hits through upstream cachedContentTokenCount; OpenClaw maps that to cacheRead.
  • When cacheRetention is set on a direct Gemini model, OpenClaw automatically creates, reuses, and refreshes cachedContents resources for system prompts on Google AI Studio runs. This means you no longer need to pre-create a cached-content handle manually.
  • You can still pass a pre-existing Gemini cached-content handle through as params.cachedContent (or legacy params.cached_content) on the configured model.
  • This is separate from Anthropic/OpenAI prompt-prefix caching. For Gemini, OpenClaw manages a provider-native cachedContents resource rather than injecting cache markers into the request.

Gemini CLI JSON usage

  • Gemini CLI JSON output can also surface cache hits through stats.cached; OpenClaw maps that to cacheRead.
  • If the CLI omits a direct stats.input value, OpenClaw derives input tokens from stats.input_tokens - stats.cached.
  • This is usage normalization only. It does not mean OpenClaw is creating Anthropic/OpenAI-style prompt-cache markers for Gemini CLI.

System-prompt cache boundary

OpenClaw splits the system prompt into a stable prefix and a volatile suffix separated by an internal cache-prefix boundary. Content above the boundary (tool definitions, skills metadata, workspace files, and other relatively static context) is ordered so it stays byte-identical across turns. Content below the boundary (for example HEARTBEAT.md, runtime timestamps, and other per-turn metadata) is allowed to change without invalidating the cached prefix.

Key design choices:

  • Stable workspace project-context files are ordered before HEARTBEAT.md so heartbeat churn does not bust the stable prefix.
  • The boundary is applied across Anthropic-family, OpenAI-family, Google, and CLI transport shaping so all supported providers benefit from the same prefix stability.
  • Codex Responses and Anthropic Vertex requests are routed through boundary-aware cache shaping so cache reuse stays aligned with what providers actually receive.
  • System-prompt fingerprints are normalized (whitespace, line endings, hook-added context, runtime capability ordering) so semantically unchanged prompts share KV/cache across turns.

If you see unexpected cacheWrite spikes after a config or workspace change, check whether the change lands above or below the cache boundary. Moving volatile content below the boundary (or stabilizing it) often resolves the issue.

OpenClaw cache-stability guards

OpenClaw also keeps several cache-sensitive payload shapes deterministic before the request reaches the provider:

  • Bundle MCP tool catalogs are sorted deterministically before tool registration, so listTools() order changes do not churn the tools block and bust prompt-cache prefixes.
  • Legacy sessions with persisted image blocks keep the 3 most recent completed turns intact; older already-processed image blocks may be replaced with a marker so image-heavy follow-ups do not keep re-sending large stale payloads.

Tuning patterns

Mixed traffic (recommended default)

Keep a long-lived baseline on your main agent, disable caching on bursty notifier agents:

yaml
agents:
  defaults:
    model:
      primary: "anthropic/claude-opus-4-6"
    models:
      "anthropic/claude-opus-4-6":
        params:
          cacheRetention: "long"
  list:
    - id: "research"
      default: true
      heartbeat:
        every: "55m"
    - id: "alerts"
      params:
        cacheRetention: "none"

Cost-first baseline

  • Set baseline cacheRetention: "short".
  • Enable contextPruning.mode: "cache-ttl".
  • Keep heartbeat below your TTL only for agents that benefit from warm caches.

Cache diagnostics

OpenClaw exposes dedicated cache-trace diagnostics for embedded agent runs.

For normal user-facing diagnostics, /status and other usage summaries can use the latest transcript usage entry as a fallback source for cacheRead / cacheWrite when the live session entry does not have those counters.

Live regression tests

OpenClaw keeps one combined live cache regression gate for repeated prefixes, tool turns, image turns, MCP-style tool transcripts, and an Anthropic no-cache control.

  • src/agents/live-cache-regression.live.test.ts
  • src/agents/live-cache-regression-baseline.ts

Run the narrow live gate with:

sh
OPENCLAW_LIVE_TEST=1 OPENCLAW_LIVE_CACHE_TEST=1 pnpm test:live:cache

The baseline file stores the most recent observed live numbers plus the provider-specific regression floors used by the test. The runner also uses fresh per-run session IDs and prompt namespaces so previous cache state does not pollute the current regression sample.

These tests intentionally do not use identical success criteria across providers.

Anthropic live expectations

  • Expect explicit warmup writes via cacheWrite.
  • Expect near-full history reuse on repeated turns because Anthropic cache control advances the cache breakpoint through the conversation.
  • Current live assertions still use high hit-rate thresholds for stable, tool, and image paths.

OpenAI live expectations

  • Expect cacheRead only. cacheWrite remains 0.
  • Treat repeated-turn cache reuse as a provider-specific plateau, not as Anthropic-style moving full-history reuse.
  • Current live assertions use conservative floor checks derived from observed live behavior on gpt-5.4-mini:
    • stable prefix: cacheRead >= 4608, hit rate >= 0.90
    • tool transcript: cacheRead >= 4096, hit rate >= 0.85
    • image transcript: cacheRead >= 3840, hit rate >= 0.82
    • MCP-style transcript: cacheRead >= 4096, hit rate >= 0.85

Fresh combined live verification on 2026-04-04 landed at:

  • stable prefix: cacheRead=4864, hit rate 0.966
  • tool transcript: cacheRead=4608, hit rate 0.896
  • image transcript: cacheRead=4864, hit rate 0.954
  • MCP-style transcript: cacheRead=4608, hit rate 0.891

Recent local wall-clock time for the combined gate was about 88s.

Why the assertions differ:

  • Anthropic exposes explicit cache breakpoints and moving conversation-history reuse.
  • OpenAI prompt caching is still exact-prefix sensitive, but the effective reusable prefix in live Responses traffic can plateau earlier than the full prompt.
  • Because of that, comparing Anthropic and OpenAI by a single cross-provider percentage threshold creates false regressions.

diagnostics.cacheTrace config

yaml
diagnostics:
  cacheTrace:
    enabled: true
    filePath: "~/.openclaw/logs/cache-trace.jsonl" # optional
    includeMessages: false # default true
    includePrompt: false # default true
    includeSystem: false # default true

Defaults:

  • filePath: $OPENCLAW_STATE_DIR/logs/cache-trace.jsonl
  • includeMessages: true
  • includePrompt: true
  • includeSystem: true

Env toggles (one-off debugging)

  • OPENCLAW_CACHE_TRACE=1 enables cache tracing.
  • OPENCLAW_CACHE_TRACE_FILE=/path/to/cache-trace.jsonl overrides output path.
  • OPENCLAW_CACHE_TRACE_MESSAGES=0|1 toggles full message payload capture.
  • OPENCLAW_CACHE_TRACE_PROMPT=0|1 toggles prompt text capture.
  • OPENCLAW_CACHE_TRACE_SYSTEM=0|1 toggles system prompt capture.

What to inspect

  • Cache trace events are JSONL and include staged snapshots like session:loaded, prompt:before, stream:context, and session:after.
  • Per-turn cache token impact is visible in normal usage surfaces via cacheRead and cacheWrite (for example /usage full and session usage summaries).
  • For Anthropic, expect both cacheRead and cacheWrite when caching is active.
  • For OpenAI, expect cacheRead on cache hits and cacheWrite to remain 0; OpenAI does not publish a separate cache-write token field.
  • If you need request tracing, log request IDs and rate-limit headers separately from cache metrics. OpenClaw's current cache-trace output is focused on prompt/session shape and normalized token usage rather than raw provider response headers.

Quick troubleshooting

  • High cacheWrite on most turns: check for volatile system-prompt inputs and verify model/provider supports your cache settings.
  • High cacheWrite on Anthropic: often means the cache breakpoint is landing on content that changes every request.
  • Low OpenAI cacheRead: verify the stable prefix is at the front, the repeated prefix is at least 1024 tokens, and the same prompt_cache_key is reused for turns that should share a cache.
  • No effect from cacheRetention: confirm model key matches agents.defaults.models["provider/model"].
  • Bedrock Nova/Mistral requests with cache settings: expected runtime force to none.

Related docs: