test: add regression test for command file dedup from $ARGUMENTS

packages/opencode/test/session/prompt.test.ts

@@ -2316,3 +2316,56 @@ noLLMServer.instance(
     }),
   30_000,
 )
+
+it.instance(
+  "command deduplicates file parts from $ARGUMENTS and input parts",
+  () =>
+    Effect.gen(function* () {
+      const { dir, llm } = yield* useServerConfig((url) => ({
+        ...providerCfg(url),
+        command: {
+          testdedup: {
+            template: "Read $ARGUMENTS",
+            description: "Test command for file dedup",
+          },
+        },
+      }))
+      const prompt = yield* SessionPrompt.Service
+      const sessions = yield* Session.Service
+      const session = yield* sessions.create({ title: "File dedup test" })
+
+      const testFile = path.join(dir, "dedup-test.txt")
+      yield* writeText(testFile, "dedup-content")
+
+      yield* llm.text("done")
+
+      const result = yield* prompt.command({
+        sessionID: session.id,
+        command: "testdedup",
+        arguments: "@dedup-test.txt",
+        parts: [
+          {
+            type: "file",
+            url: pathToFileURL(testFile).href,
+            filename: "dedup-test.txt",
+            mime: "text/plain",
+          },
+        ],
+      })
+
+      expect(result.info.role).toBe("assistant")
+
+      const msgs = yield* sessions.messages({ sessionID: session.id })
+      const userMsg = msgs.findLast((msg) => msg.info.role === "user")
+      expect(userMsg?.info.role).toBe("user")
+      if (userMsg?.info.role !== "user") return
+
+      const syntheticTexts = userMsg.parts.filter(
+        (part): part is SessionV1.TextPart => part.type === "text" && "synthetic" in part && part.synthetic === true,
+      )
+      const contentCount = syntheticTexts.filter((part) => part.text.includes("dedup-content")).length
+      expect(contentCount).toBe(1)
+    }),
+  { config: cfg },
+  30_000,
+)