minimize system prompt

packages/core/src/session/generate-node.ts

@@ -13,7 +13,7 @@ import { SessionHistory } from "./history.js"
 import { SessionModelHeaders } from "./model-headers.js"
 import { SessionPromptCacheKey } from "./prompt-cache-key.js"
 import { SessionRunnerModel } from "./runner/model.js"
-import PROMPT_DEFAULT from "./runner/prompt/base.txt"
+import { SessionSystemPrompt } from "./system-prompt.js"
 import { toLLMMessages } from "./runner/to-llm-message.js"
 
 export const layer = Layer.effect(
@@ -39,7 +39,12 @@ export const layer = Layer.effect(
           sessionID: selection.session.id,
           agent: selection.agent.id,
           model: model.ref,
-          system: [selection.agent.info.system ? selection.agent.info.system : PROMPT_DEFAULT, history.initial]
+          system: [
+            selection.agent.info.system
+              ? selection.agent.info.system
+              : SessionSystemPrompt.make(toolDefinitions.map((tool) => tool.name)),
+            history.initial,
+          ]
             .filter((part) => part.length > 0)
             .map(SystemPart.make),
           messages: [

packages/core/src/session/model-request.ts

@@ -18,7 +18,7 @@ import { SessionModelHttp } from "./model-http.js"
 import { SessionPromptCacheKey } from "./prompt-cache-key.js"
 import { PromptCacheDiagnostics } from "./prompt-cache-diagnostics.js"
 import { MAX_STEPS_PROMPT } from "./runner/max-steps.js"
-import PROMPT_DEFAULT from "./runner/prompt/base.txt"
+import { SessionSystemPrompt } from "./system-prompt.js"
 import { toLLMMessages } from "./runner/to-llm-message.js"
 
 const IMAGE_BYTES_TRIGGER = 25 * 1024 * 1024 // 25 MiB
@@ -182,7 +182,12 @@ export const layer = Layer.effect(
       // The final Step keeps definitions available to protocols with native "none",
       // preserving their prompt cache prefix. Calls are still rejected at execution.
       const tools = input.context.tools
-      const system = [agent.info.system ? agent.info.system : PROMPT_DEFAULT, input.context.initial]
+      const system = [
+        agent.info.system
+          ? agent.info.system
+          : SessionSystemPrompt.make(tools.definitions.map((tool) => tool.name)),
+        input.context.initial,
+      ]
         .filter((part) => part.length > 0)
         .map(SystemPart.make)
       const history = toLLMMessages(input.context.messages, resolved.ref, providerMetadataKey)

packages/core/src/session/runner/prompt/system.txt

@@ -0,0 +1,18 @@
+You are an AI agent powered by OpenCode, a coding agent harness. Help the user accomplish their goals using the available tools.
+
+# Harness
+- Responses are rendered as GitHub-flavored Markdown.
+- Do not use shell commands to communicate thoughts, explanations, or instructions. Communicate directly in your response instead.
+- Prefer using the read tool rather than shell commands like `cat`.
+- `<system-reminder>` blocks are harness instructions, not user-authored content. Read and follow them.
+${OPENCODE_TOOL_INSTRUCTIONS}
+
+# Communication
+- Use clear file paths when referring to files.
+- Keep responses clear and concise, and avoid unnecessary technical jargon.
+
+# Working in codebases
+- Keep changes consistent with the structure, naming, style, and patterns of the surrounding code.
+- Treat unfamiliar files or changes as potential user work and investigate before deleting or overwriting them.
+
+${OPENCODE_INSTRUCTIONS}

packages/core/src/session/system-prompt.ts

@@ -0,0 +1,12 @@
+export * as SessionSystemPrompt from "./system-prompt.js"
+
+import PROMPT from "./runner/prompt/system.txt"
+
+const TOOL_INSTRUCTIONS = new Map<string, string>()
+const OPENCODE_INSTRUCTIONS = ""
+
+export function make(tools: string[]) {
+  return PROMPT.replace("${OPENCODE_TOOL_INSTRUCTIONS}", () =>
+    tools.flatMap((tool) => TOOL_INSTRUCTIONS.get(tool) ?? []).join("\n\n"),
+  ).replace("${OPENCODE_INSTRUCTIONS}", () => OPENCODE_INSTRUCTIONS)
+}

packages/core/test/plugin/system-prompt.test.ts

@@ -7,6 +7,7 @@ import { PluginHooks } from "@opencode-ai/core/plugin/hooks"
 import { PluginHost } from "@opencode-ai/core/plugin/host"
 import { SystemPromptPlugin } from "@opencode-ai/core/plugin/system-prompt"
 import { Session } from "@opencode-ai/core/session"
+import { SessionSystemPrompt } from "@opencode-ai/core/session/system-prompt"
 import type { SessionHooks } from "@opencode-ai/plugin/effect/session"
 import { Model } from "@opencode-ai/schema/model"
 import { Provider } from "@opencode-ai/schema/provider"
@@ -14,10 +15,9 @@ import { Effect } from "effect"
 import { testEffect } from "../lib/effect"
 import { PluginTestLayer } from "./fixture"
 import PROMPT_META from "../../src/plugin/system-prompt/meta.txt"
-import PROMPT_DEFAULT from "../../src/session/runner/prompt/base.txt"
 
 const it = testEffect(PluginTestLayer)
-const fallback = PROMPT_DEFAULT
+const fallback = SessionSystemPrompt.make([])
 const makeHost = Effect.gen(function* () {
   const agents = yield* Agent.Service
   const plugins = yield* Plugin.Service

packages/core/test/session-runner.test.ts

@@ -67,6 +67,7 @@ import { InstructionDiscovery } from "@opencode-ai/core/instruction-discovery"
 import { SkillInstructions } from "@opencode-ai/core/skill/instructions"
 import { ReferenceInstructions } from "@opencode-ai/core/reference/instructions"
 import { McpInstructions } from "@opencode-ai/core/mcp/instructions"
+import { SessionSystemPrompt } from "@opencode-ai/core/session/system-prompt"
 import { ID } from "@opencode-ai/core/model"
 import { Location } from "@opencode-ai/core/location"
 import { Provider } from "@opencode-ai/core/provider"
@@ -76,7 +77,6 @@ import { asc, desc, eq } from "drizzle-orm"
 import { testEffect } from "./lib/effect"
 import { permissionLayer } from "./lib/permission"
 import { agentHost, catalogHost, host } from "./plugin/host"
-import PROMPT_DEFAULT from "../src/session/runner/prompt/base.txt"
 import { CodeModeInstructions } from "@opencode-ai/core/codemode/instructions"
 
 let requests: LLMRequest[] = []
@@ -133,7 +133,7 @@ const testLLM = TestLLM.layer({
 })
 const client = TestLLM.clientLayer
 const model = LanguageModel.make({ id: "fake-model", provider: "fake", route: OpenAIChat.route })
-const defaultSystem = PROMPT_DEFAULT
+const defaultSystem = SessionSystemPrompt.make([])
 const replacementModel = LanguageModel.make({ id: "replacement", provider: "fake", route: OpenAIChat.route })
 const compactModel = LanguageModel.make({
   id: "compact",

packages/core/test/session-system-prompt.test.ts

@@ -0,0 +1,8 @@
+import { expect, test } from "bun:test"
+import { SessionSystemPrompt } from "@opencode-ai/core/session/system-prompt"
+
+test("renders the default system prompt instructions", () => {
+  const prompt = SessionSystemPrompt.make(["read", "shell"])
+  expect(prompt).not.toContain("${OPENCODE_TOOL_INSTRUCTIONS}")
+  expect(prompt).not.toContain("${OPENCODE_INSTRUCTIONS}")
+})