Structure system prompt into composable sections (#5)

* Split system prompt into composable sections

* Test prompt section composition

Author: TheGreatAxios

src/prompts.ts

@@ -1,27 +1,86 @@
-export function buildSystemPrompt(): string {
+const defaultAgentTools = [
+  "read_file",
+  "write_file",
+  "edit_file",
+  "run_shell",
+  "search_files",
+  "grep",
+  "list_dir",
+  "submit_plan",
+  "submit_output",
+];
+
+const defaultChatTools = [
+  "read_file",
+  "write_file",
+  "edit_file",
+  "run_shell",
+  "search_files",
+  "grep",
+  "list_dir",
+];
+
+const joinSections = (sections: string[]) => sections.join("\n\n");
+
+export function buildAgentRole(): string {
+  return "You are an autonomous coding agent operating inside an event-driven loop.";
+}
+
+export function buildToolCallDiscipline(): string {
   return [
-    "You are an autonomous coding agent operating inside an event-driven loop.",
-    "",
-    "Rules:",
+    "Tool-call discipline:",
     "1. Every turn must produce at least one tool_call. Conversational text without tool_calls is not progress.",
     "2. Do not explain what you will do before doing it. Just call the tool.",
-    "4. You MUST call submit_output when the task is fully complete. No other action signals completion.",
-    "5. If tests are failing, you MUST NOT submit. Fix the tests first.",
-    "6. Do not re-read a file you already read. The tool will return an error if you try.",
-    "7. Never write large files in a single write_file call. If a file exceeds ~200 lines, write it in sections using run_shell (printf or cat heredoc) or break the work into edit_file calls on an existing scaffold.",
-    "",
-    "Available tools: read_file, write_file, edit_file, run_shell, search_files, grep, list_dir, submit_plan, submit_output.",
-    "",
-    "When calling submit_output, include a brief summary of what was done.",
   ].join("\n");
 }
 
-export function buildChatSystemPrompt(): string {
+export function buildSubmitRules(): string {
   return [
+    "Submit and completion rules:",
+    "1. You MUST call submit_output when the task is fully complete. No other action signals completion.",
+    "2. If tests are failing, you MUST NOT submit. Fix the tests first.",
+    "3. When calling submit_output, include a brief summary of what was done.",
+  ].join("\n");
+}
+
+export function buildBudgetRules(): string {
+  return [
+    "Budget and read limits:",
+    "1. Do not re-read a file you already read. The tool will return an error if you try.",
+    "2. Never write large files in a single write_file call. If a file exceeds ~200 lines, write it in sections using run_shell (printf or cat heredoc) or break the work into edit_file calls on an existing scaffold.",
+  ].join("\n");
+}
+
+export function buildPlanRules(): string {
+  return [
+    "Plan requirements:",
+    "1. Submit a plan before making changes when the task requires multiple steps.",
+    "2. Keep work aligned with the submitted plan and update it if the approach changes.",
+  ].join("\n");
+}
+
+export function buildAvailableTools(tools = defaultAgentTools): string {
+  return `Available tools: ${tools.join(", ")}.`;
+}
+
+export function buildSystemPrompt(tools = defaultAgentTools): string {
+  return joinSections([
+    buildAgentRole(),
+    buildToolCallDiscipline(),
+    buildSubmitRules(),
+    buildBudgetRules(),
+    buildPlanRules(),
+    buildAvailableTools(tools),
+  ]);
+}
+
+export function buildChatSystemPrompt(): string {
+  return joinSections([
     "You are a helpful coding assistant. You can answer questions, write code, and use tools when needed.",
-    "",
-    "Available tools: read_file, write_file, edit_file, run_shell, search_files, grep, list_dir.",
-    "",
+    buildToolCallDiscipline(),
+    buildBudgetRules(),
+    buildPlanRules(),
+    buildAvailableTools(defaultChatTools),
     "When the user asks you to do something, use the appropriate tools. When you are done, reply with a summary.",
-  ].join("\n");
+  ]);
 }

tests/unit/prompts.test.ts

@@ -0,0 +1,62 @@
+import { expect, test } from "bun:test";
+
+import {
+  buildAgentRole,
+  buildAvailableTools,
+  buildBudgetRules,
+  buildChatSystemPrompt,
+  buildPlanRules,
+  buildSubmitRules,
+  buildSystemPrompt,
+  buildToolCallDiscipline,
+} from "../../src/prompts.js";
+
+const sections = [
+  buildAgentRole,
+  buildToolCallDiscipline,
+  buildSubmitRules,
+  buildBudgetRules,
+  buildPlanRules,
+  buildAvailableTools,
+];
+
+test("prompt sections are non-empty", () => {
+  for (const buildSection of sections) {
+    expect(buildSection().trim().length).toBeGreaterThan(0);
+  }
+});
+
+test("system prompt includes every section", () => {
+  const prompt = buildSystemPrompt();
+
+  expect(prompt).toContain(buildAgentRole());
+  expect(prompt).toContain(buildToolCallDiscipline());
+  expect(prompt).toContain(buildSubmitRules());
+  expect(prompt).toContain(buildBudgetRules());
+  expect(prompt).toContain(buildPlanRules());
+  expect(prompt).toContain(buildAvailableTools());
+});
+
+test("chat system prompt excludes submit rules", () => {
+  const prompt = buildChatSystemPrompt();
+
+  expect(prompt).not.toContain(buildSubmitRules());
+  expect(prompt).not.toContain("submit_output");
+});
+
+test("full system prompt contains provided tool names", () => {
+  const prompt = buildSystemPrompt(["first_tool", "second_tool"]);
+
+  expect(prompt).toContain("first_tool");
+  expect(prompt).toContain("second_tool");
+});
+
+test("system prompt preserves core agent instructions", () => {
+  const prompt = buildSystemPrompt();
+
+  expect(prompt).toContain("autonomous coding agent");
+  expect(prompt).toContain("Every turn must produce at least one tool_call");
+  expect(prompt).toContain("Do not re-read a file you already read");
+  expect(prompt).toContain("MUST call submit_output");
+  expect(prompt).toContain("If tests are failing, you MUST NOT submit");
+});