chore: import upstream snapshot with attribution

tests/unit/compression/strategySelector-cache-aware.test.ts

@@ -0,0 +1,60 @@
+/**
+ * Tests for #3890: the cache-aware `skipSystemPrompt` flag was computed by
+ * getCacheAwareStrategy() but dropped by selectCompressionStrategy() (which can only
+ * return a mode string). resolveCacheAwareConfig() applies it: in a caching context the
+ * system prompt must stay uncompressed (it is part of the cacheable prefix) even when the
+ * operator disabled preserveSystemPrompt.
+ */
+
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { resolveCacheAwareConfig } from "../../../open-sse/services/compression/strategySelector.ts";
+import type { CompressionConfig } from "../../../open-sse/services/compression/types.ts";
+
+function cfg(overrides: Partial<CompressionConfig> = {}): CompressionConfig {
+  return {
+    enabled: true,
+    defaultMode: "standard",
+    autoTriggerTokens: 0,
+    cacheMinutes: 5,
+    preserveSystemPrompt: true,
+    comboOverrides: {},
+    ...overrides,
+  } as CompressionConfig;
+}
+
+describe("resolveCacheAwareConfig (#3890)", () => {
+  it("forces preserveSystemPrompt on for a caching request that disabled it", () => {
+    const out = resolveCacheAwareConfig(
+      cfg({ preserveSystemPrompt: false }),
+      { messages: [{ role: "system", content: "x", cache_control: { type: "ephemeral" } }] },
+      { provider: "anthropic", targetFormat: "claude" }
+    );
+    assert.equal(out.preserveSystemPrompt, true);
+  });
+
+  it("leaves a non-caching request untouched (preserveSystemPrompt stays false)", () => {
+    // google has no prompt caching, so the prefix-protection guard does not apply.
+    // (openai/codex now count as automatic-cache providers per #3955.)
+    const out = resolveCacheAwareConfig(
+      cfg({ preserveSystemPrompt: false }),
+      { messages: [{ role: "system", content: "x" }] },
+      { provider: "google" }
+    );
+    assert.equal(out.preserveSystemPrompt, false);
+  });
+
+  it("returns the same config object when there is no body", () => {
+    const base = cfg({ preserveSystemPrompt: false });
+    assert.equal(resolveCacheAwareConfig(base), base);
+  });
+
+  it("does not change a config that already preserves the system prompt", () => {
+    const out = resolveCacheAwareConfig(
+      cfg({ preserveSystemPrompt: true }),
+      { messages: [{ role: "system", content: "x", cache_control: { type: "ephemeral" } }] },
+      { provider: "anthropic", targetFormat: "claude" }
+    );
+    assert.equal(out.preserveSystemPrompt, true);
+  });
+});