diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index fe8c32f0323..8f3b3c4d523 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -6,17 +6,27 @@ import { Config } from "../config/config"
 import { Instance } from "../project/instance"
 import path from "path"
 import os from "os"
+import * as fs from "fs"
 
 import PROMPT_ANTHROPIC from "./prompt/anthropic.txt"
 import PROMPT_ANTHROPIC_WITHOUT_TODO from "./prompt/qwen.txt"
 import PROMPT_BEAST from "./prompt/beast.txt"
 import PROMPT_GEMINI from "./prompt/gemini.txt"
 import PROMPT_ANTHROPIC_SPOOF from "./prompt/anthropic_spoof.txt"
-
 import PROMPT_CODEX from "./prompt/codex.txt"
 import type { Provider } from "@/provider/provider"
 import { Flag } from "@/flag/flag"
 
+const PROMPT_CACHE = new Map<string, string>()
+
+function getSystemPromptPaths(): string[] {
+  return [
+    process.env.OPENCODE_PROMPTS_DIR,
+    path.join(Instance.directory, ".opencode", "prompt"),
+    path.join(Global.Path.config, "prompt"),
+  ].filter(Boolean) as string[]
+}
+
 export namespace SystemPrompt {
   export function header(providerID: string) {
     if (providerID.includes("anthropic")) return [PROMPT_ANTHROPIC_SPOOF.trim()]
@@ -24,14 +34,44 @@ export namespace SystemPrompt {
   }
 
   export function provider(model: Provider.Model) {
-    if (model.api.id.includes("gpt-5")) return [PROMPT_CODEX]
-    if (model.api.id.includes("gpt-") || model.api.id.includes("o1") || model.api.id.includes("o3"))
+    const providerId = model.api.id
+    const promptFile = getPromptFile(providerId)
+    const customPrompt = findCustomPrompt(promptFile)
+    if (customPrompt) return [customPrompt]
+
+    if (providerId.includes("gpt-5")) return [PROMPT_CODEX]
+    if (providerId.includes("gpt-") || providerId.includes("o1") || providerId.includes("o3"))
       return [PROMPT_BEAST]
-    if (model.api.id.includes("gemini-")) return [PROMPT_GEMINI]
-    if (model.api.id.includes("claude")) return [PROMPT_ANTHROPIC]
+    if (providerId.includes("gemini-")) return [PROMPT_GEMINI]
+    if (providerId.includes("claude")) return [PROMPT_ANTHROPIC]
     return [PROMPT_ANTHROPIC_WITHOUT_TODO]
   }
 
+  function getPromptFile(modelId: string): string {
+    if (modelId.includes("claude")) return "anthropic.txt"
+    if (modelId.includes("gpt-5")) return "codex.txt"
+    if (modelId.includes("gpt-") || modelId.includes("o1") || modelId.includes("o3"))
+      return "beast.txt"
+    if (modelId.includes("gemini-")) return "gemini.txt"
+    return "qwen.txt"
+  }
+
+  function findCustomPrompt(promptFile: string): string | undefined {
+    const searchPaths = getSystemPromptPaths()
+    for (const searchPath of searchPaths) {
+      const fullPath = path.join(searchPath, promptFile)
+      const cached = PROMPT_CACHE.get(fullPath)
+      if (cached) return cached
+
+      if (fs.existsSync(fullPath)) {
+        const content = fs.readFileSync(fullPath, "utf-8")
+        PROMPT_CACHE.set(fullPath, content)
+        return content
+      }
+    }
+    return undefined
+  }
+
   export async function environment() {
     const project = Instance.project
     return [