feat(ai): add input-based pricing tiers

packages/coding-agent/docs/models.md

@@ -205,9 +205,31 @@ If your command is slow, expensive, rate-limited, or should keep using a previou
 | `input` | No | `["text"]` | Input types: `["text"]` or `["text", "image"]` |
 | `contextWindow` | No | `128000` | Context window size in tokens |
 | `maxTokens` | No | `16384` | Maximum output tokens |
-| `cost` | No | all zeros | `{"input": 0, "output": 0, "cacheRead": 0, "cacheWrite": 0}` (per million tokens) |
+| `cost` | No | all zeros | Per-million-token rates with optional request-wide input pricing tiers |
 | `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.
+
+```json
+{
+  "cost": {
+    "input": 5,
+    "output": 30,
+    "cacheRead": 0.5,
+    "cacheWrite": 6.25,
+    "tiers": [
+      {
+        "inputTokensAbove": 272000,
+        "input": 10,
+        "output": 45,
+        "cacheRead": 1,
+        "cacheWrite": 12.5
+      }
+    ]
+  }
+}
+```
+
 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.
@@ -317,6 +339,24 @@ 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`, `headers`, `compat`.
 
+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:
+
+```json
+{
+  "providers": {
+    "openai": {
+      "modelOverrides": {
+        "gpt-5.6-sol": {
+          "contextWindow": 1050000
+        }
+      }
+    }
+  }
+}
+```
+
+The override preserves the built-in pricing metadata. Requests with more than 272K total input tokens use GPT-5.6's long-context rates for the entire request. Apply the same override to `gpt-5.6-terra` or `gpt-5.6-luna` when needed.
+
 Behavior notes:
 - `modelOverrides` are applied to built-in provider models and matching extension-registered provider models.
 - Unknown model IDs are ignored.