feat(coding-agent): add prompt cache warming (#9668)

packages/coding-agent/docs/models.md

@@ -9,6 +9,7 @@ Add custom providers and models (Ollama, vLLM, LM Studio, proxies) via `~/.pi/ag
 - [Supported APIs](#supported-apis)
 - [Provider Configuration](#provider-configuration)
 - [Model Configuration](#model-configuration)
+- [Prompt Cache Lifetimes](#prompt-cache-lifetimes)
 - [Overriding Built-in Providers](#overriding-built-in-providers)
 - [Per-model Overrides](#per-model-overrides)
 - [Anthropic Messages Compatibility](#anthropic-messages-compatibility)
@@ -208,6 +209,7 @@ If your command is slow, expensive, rate-limited, or should keep using a previou
 | `maxTokens` | No | `16384` | Maximum output tokens |
 | `samplingParams` | No | omitted | Sampling parameters merged verbatim into every request body (see below) |
 | `cost` | No | all zeros | Per-million-token rates with optional request-wide input pricing tiers |
+| `promptCache` | No | omitted | Best-effort prompt cache lifetime in seconds per retention tier (see below) |
 | `compat` | No | provider `compat` | Provider compatibility overrides. Merged with provider-level `compat` when both are set. |
 
 A cost tier supplies a complete alternate rate set and applies to the full request when total input usage (`input + cacheRead + cacheWrite`) exceeds `inputTokensAbove`. When multiple tiers match, the highest threshold wins.
@@ -236,6 +238,19 @@ Current behavior:
 - `/model`, `--list-models`, and the interactive footer display entries by model `id`.
 - The configured `name` is used for model matching and secondary model detail text. It does not replace the footer/status-bar model id.
 
+### Prompt Cache Lifetimes
+
+`promptCache` states how long the provider keeps a prompt cache entry alive for each retention tier pi can request (`short` is the default tier; `long` is used when `PI_CACHE_RETENTION=long`). Values are seconds and are estimates: providers publish ranges, so pick the conservative end.
+
+```json
+{
+  "id": "claude-sonnet-5",
+  "promptCache": { "short": 300, "long": 3600 }
+}
+```
+
+The built-in catalog fills this in for direct Anthropic (5 min / 1 h). Other providers, including direct OpenAI, have no built-in lifetime until their cache-expiry and replay behavior has been validated for warming. A model without a value for the tier a request used is never warmed; custom models and provider overrides can opt in when the backing cache behavior is known. See [Cache Warming](settings.md#cache-warming).
+
 ### Sampling Parameters
 
 `samplingParams` is a free-form object merged verbatim into every request body for the model, after the fields pi sets itself, so its keys win. Use it to send sampling parameters pi does not model — including server-specific ones like llama.cpp's `min_p` or vLLM's `top_k`:
@@ -359,7 +374,23 @@ Use `modelOverrides` to customize built-in models and matching extension-registe
 }
 ```
 
-`modelOverrides` supports these fields per model: `name`, `reasoning`, `thinkingLevelMap`, `input`, `cost` (partial), `contextWindow`, `maxTokens`, `samplingParams` (merged per key), `headers`, `compat`.
+`modelOverrides` supports these fields per model: `name`, `reasoning`, `thinkingLevelMap`, `input`, `cost` (partial), `promptCache` (merged per tier), `contextWindow`, `maxTokens`, `samplingParams` (merged per key), `headers`, `compat`.
+
+Use a `promptCache` override to enable cache warming through a proxy whose backing cache you know, for example OpenRouter routed to Anthropic:
+
+```json
+{
+  "providers": {
+    "openrouter": {
+      "modelOverrides": {
+        "anthropic/claude-sonnet-4": {
+          "promptCache": { "short": 300 }
+        }
+      }
+    }
+  }
+}
+```
 
 Direct OpenAI GPT-5.6 Sol, Terra, and Luna default to a `272000` context window so requests remain within OpenAI's short-context pricing tier. To opt into OpenAI's 1.05M context window, increase it for each model you use: