From 81f978abeb28d34bab3af03d5862511187099d15 Mon Sep 17 00:00:00 2001
From: Christian Gunderman <gundermanc@google.com>
Date: Thu, 19 Mar 2026 16:01:37 -0700
Subject: [PATCH] Basic prompt templating.

---
 .../core/src/prompts/promptTemplating.test.ts | 97 +++++++++++++++++++
 packages/core/src/prompts/promptTemplating.ts | 72 ++++++++++++++
 2 files changed, 169 insertions(+)
 create mode 100644 packages/core/src/prompts/promptTemplating.test.ts
 create mode 100644 packages/core/src/prompts/promptTemplating.ts

diff --git a/packages/core/src/prompts/promptTemplating.test.ts b/packages/core/src/prompts/promptTemplating.test.ts
new file mode 100644
index 0000000000..6d09f604c3
--- /dev/null
+++ b/packages/core/src/prompts/promptTemplating.test.ts
@@ -0,0 +1,97 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect, it } from 'vitest';
+import { promptComponent, toPrompt, enabledWhen, section, xmlSection, each, switchOn } from './promptTemplating.js';
+
+// Sample prompt components.
+type MainPromptOptions = { isInteractive: boolean };
+
+const identity = promptComponent('Your name is Gemini CLI and you are a coding agent');
+
+const interactive = promptComponent<MainPromptOptions>(
+  enabledWhen('isInteractive', 'You are operating in an interactive session')
+);
+
+const mainPrompt = promptComponent<MainPromptOptions>(identity, interactive);
+
+const variadicPrompt = promptComponent('A', 'B', 'C');
+
+const nestedVariadicPrompt = promptComponent('A', ['B', 'C'], 'D');
+
+const enabledWhenPrompt = promptComponent<Record<string, boolean>>(
+  'Always',
+  enabledWhen('show', 'Sometimes')
+);
+
+const xmlPrompt = xmlSection('rules', 'Be helpful');
+
+const emptyXmlPrompt = xmlSection('rules', '');
+
+const markdownPrompt = section('Rules', 'Be helpful');
+
+const emptyMarkdownPrompt = section('Rules', '');
+
+const customHeaderMarkdownPrompt = section('Rules', 'Be helpful', { headerLevel: 3 });
+
+const eachPrompt = each<{ items: string[] }>('items', (item) => `Item: ${item}`);
+
+const switchOnPrompt = switchOn<{ mode: string }>('mode', {
+  fast: 'Speed is key',
+  safe: 'Safety first',
+});
+
+describe('promptTemplating', () => {
+  it('should take variadic arguments', () => {
+    expect(toPrompt({}, variadicPrompt)).toBe('A,B,C');
+  });
+
+  it('should handle nested arrays (variadic with arrays)', () => {
+    expect(toPrompt({}, nestedVariadicPrompt)).toBe('A,B,C,D');
+  });
+
+  it('should handle enabledWhen', () => {
+    expect(toPrompt({ show: true }, enabledWhenPrompt)).toBe('Always,Sometimes');
+    expect(toPrompt({ show: false }, enabledWhenPrompt)).toBe('Always,');
+  });
+
+  it('should handle xmlSection', () => {
+    expect(toPrompt({}, xmlPrompt)).toBe('<rules>\nBe helpful\n</rules>');
+  });
+
+  it('should return empty string for xmlSection if content is empty', () => {
+    expect(toPrompt({}, emptyXmlPrompt)).toBe('');
+  });
+
+  it('should handle markdown section with default header', () => {
+    expect(toPrompt({}, markdownPrompt)).toBe('# Rules\n\nBe helpful');
+  });
+
+  it('should return empty string for markdown section if content is empty', () => {
+    expect(toPrompt({}, emptyMarkdownPrompt)).toBe('');
+  });
+
+  it('should handle markdown section with custom header level', () => {
+    expect(toPrompt({}, customHeaderMarkdownPrompt)).toBe('### Rules\n\nBe helpful');
+  });
+
+  it('should handle each', () => {
+    const options = { items: ['A', 'B'] };
+    expect(toPrompt(options, eachPrompt)).toBe('Item: A\nItem: B');
+  });
+
+  it('should handle switchOn', () => {
+    expect(toPrompt({ mode: 'fast' }, switchOnPrompt)).toBe('Speed is key');
+    expect(toPrompt({ mode: 'safe' }, switchOnPrompt)).toBe('Safety first');
+    expect(toPrompt({ mode: 'unknown' }, switchOnPrompt)).toBe('');
+  });
+
+  it('should output correctly for mainPrompt', () => {
+    const text = toPrompt({ isInteractive: true }, mainPrompt);
+    expect(text).toContain('Your name is Gemini CLI and you are a coding agent');
+    expect(text).toContain('You are operating in an interactive session');
+  });
+});
diff --git a/packages/core/src/prompts/promptTemplating.ts b/packages/core/src/prompts/promptTemplating.ts
new file mode 100644
index 0000000000..6f43a174bb
--- /dev/null
+++ b/packages/core/src/prompts/promptTemplating.ts
@@ -0,0 +1,72 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export type DynamicSnippet<TOption> = (options: TOption) => string;
+
+export type Snippet<TOption> = string | DynamicSnippet<TOption> | Array<Snippet<TOption>>;
+
+export function promptComponent<TOption>(...snippets: Array<Snippet<TOption>>): Snippet<TOption> {
+  return snippets;
+}
+
+export function enabledWhen<TOption>(optionName: keyof TOption, snippet: Snippet<TOption>): Snippet<TOption> {
+  return (option: TOption) => option[optionName] ? toPrompt(option, snippet) : '';
+}
+
+export function xmlSection<TOption>(name: string, snippet: Snippet<TOption>): Snippet<TOption> {
+  return (options: TOption) => {
+    const content = toPrompt(options, snippet);
+    return content ? `<${name}>\n${content}\n</${name}>` : '';
+  };
+}
+
+export interface SectionOptions {
+  headerLevel?: number;
+}
+
+export function section<TOption>(name: string, snippet: Snippet<TOption>, sectionOptions?: SectionOptions): Snippet<TOption> {
+  return (options: TOption) => {
+    const content = toPrompt(options, snippet);
+    const level = sectionOptions?.headerLevel ?? 1;
+    const hashes = '#'.repeat(level);
+    return content ? `${hashes} ${name}\n\n${content}` : '';
+  };
+}
+
+export function each<TOption>(
+  optionName: keyof TOption,
+  snippet: Snippet<TOption>,
+  separator = '\n'
+): Snippet<TOption> {
+  return (options: TOption) => {
+    const items = options[optionName];
+    if (Array.isArray(items)) {
+      return items.map((item: any) => toPrompt(item, snippet)).join(separator);
+    }
+    return '';
+  };
+}
+
+export function switchOn<TOption>(
+  optionName: keyof TOption,
+  cases: Record<string, Snippet<TOption>>
+): Snippet<TOption> {
+  return (options: TOption) => {
+    const value = String(options[optionName]);
+    const caseSnippet = cases[value];
+    return caseSnippet ? toPrompt(options, caseSnippet) : '';
+  };
+}
+
+export function toPrompt<TOption>(options: TOption, snippet: Snippet<TOption>): string {
+  if (typeof snippet === 'string') {
+    return snippet;
+  } else if (Array.isArray(snippet)) {
+    return snippet.map(eachSnippet => toPrompt<TOption>(options, eachSnippet)).join();
+  } else {
+    return snippet(options);
+  }
+}