fix(provider): improve prompt caching

packages/core/test/provider-cache-usage.test.ts

@@ -0,0 +1,69 @@
+import { createCohere } from "@ai-sdk/cohere"
+import { createGroq } from "@ai-sdk/groq"
+import { createTogetherAI } from "@ai-sdk/togetherai"
+import { describe, expect, test } from "bun:test"
+
+const prompt = [{ role: "user" as const, content: [{ type: "text" as const, text: "Hello" }] }]
+
+describe("provider cache usage", () => {
+  test("Cohere reports cached input tokens", async () => {
+    const model = createCohere({
+      apiKey: "test",
+      fetch: mockFetch({
+        generation_id: "response-1",
+        message: { role: "assistant", content: [{ type: "text", text: "Hello" }] },
+        finish_reason: "COMPLETE",
+        usage: {
+          billed_units: { input_tokens: 500, output_tokens: 1 },
+          tokens: { input_tokens: 500, output_tokens: 1, cached_tokens: 400 },
+        },
+      }),
+    })("command-r")
+
+    const result = await model.doGenerate({ prompt })
+    expect(result.usage.inputTokens).toEqual({ total: 500, noCache: 100, cacheRead: 400, cacheWrite: undefined })
+  })
+
+  test("Groq reports cached input tokens", async () => {
+    const model = createGroq({
+      apiKey: "test",
+      fetch: mockFetch({
+        id: "response-1",
+        created: 0,
+        model: "openai/gpt-oss-20b",
+        object: "chat.completion",
+        choices: [{ index: 0, message: { role: "assistant", content: "Hello" }, finish_reason: "stop" }],
+        usage: {
+          prompt_tokens: 500,
+          completion_tokens: 1,
+          total_tokens: 501,
+          prompt_tokens_details: { cached_tokens: 400 },
+        },
+      }),
+    })("openai/gpt-oss-20b")
+
+    const result = await model.doGenerate({ prompt })
+    expect(result.usage.inputTokens).toEqual({ total: 500, noCache: 100, cacheRead: 400, cacheWrite: undefined })
+  })
+
+  test("Together AI reports flat cached input tokens", async () => {
+    const model = createTogetherAI({
+      apiKey: "test",
+      fetch: mockFetch({
+        id: "response-1",
+        created: 0,
+        model: "moonshotai/Kimi-K2.6",
+        object: "chat.completion",
+        choices: [{ index: 0, message: { role: "assistant", content: "Hello" }, finish_reason: "stop" }],
+        usage: { prompt_tokens: 500, completion_tokens: 1, total_tokens: 501, cached_tokens: 400 },
+      }),
+    })("moonshotai/Kimi-K2.6")
+
+    const result = await model.doGenerate({ prompt })
+    expect(result.usage.inputTokens).toEqual({ total: 500, noCache: 100, cacheRead: 400, cacheWrite: undefined })
+  })
+})
+
+function mockFetch(response: unknown) {
+  return Object.assign(async () => Response.json(response), { preconnect: fetch.preconnect })
+}