diff --git a/packages/core/src/prompter/types.ts b/packages/core/src/prompter/types.ts
deleted file mode 100644
index 15521a05ae..0000000000
--- a/packages/core/src/prompter/types.ts
+++ /dev/null
@@ -1,36 +0,0 @@
-/**
- * @license
- * Copyright 2026 Google LLC
- * SPDX-License-Identifier: Apache-2.0
- */
-
-export type ContextResolver<C, O> = O | ((ctx: C) => O | Promise<O>);
-
-export type PromptSlot = { slot: string; content?: never };
-
-export type PromptSection<C> = {
-  /** Add a Markdown heading of appropriate level to this section. */
-  heading?: string;
-  /** If supplied, wrap this section in an XML tag. */
-  tag?: string;
-  /** If supplied, add attributes to the XML section tag. */
-  attrs?: Record<string, string>;
-  /** Formatting of the content inside this section. Defaults to 'block'. */
-  format?: 'inline' | 'block';
-
-  /** Condition that must evaluate to true for the section to be rendered. */
-  condition?: (ctx: C) => boolean | Promise<boolean>;
-  content: PromptContent<C>;
-};
-
-// The core recursive type.
-// It wraps your 3 base node shapes (string, section, or array) in the resolver.
-export type PromptContent<C> = ContextResolver<
-  C,
-  | string
-  | number
-  | boolean
-  | PromptSection<C>
-  | PromptSlot
-  | Array<PromptContent<C>>
->;
diff --git a/packages/core/src/prompter/prompter.test.ts b/packages/core/src/prompts/render-prompt.test.ts
similarity index 86%
rename from packages/core/src/prompter/prompter.test.ts
rename to packages/core/src/prompts/render-prompt.test.ts
index bb49204d1f..d3b74cbcf2 100644
--- a/packages/core/src/prompter/prompter.test.ts
+++ b/packages/core/src/prompts/render-prompt.test.ts
@@ -5,8 +5,8 @@
  */
 
 import { describe, expect, it } from 'vitest';
-import { renderPrompt, p } from './prompter.js';
-import type { PromptContent } from './types.js';
+import { renderPrompt, p } from './render-prompt.js';
+import type { PromptContent } from './render-prompt.js';
 
 type TestContext = { name?: string; shouldRender?: boolean };
 
@@ -215,6 +215,42 @@ const tests: TestCase[] = [
     context: {},
     expect: 'Prefix: FirstSecond',
   },
+  {
+    desc: 'conditionally omits items when null or undefined are present',
+    content: [
+      'Item 1',
+      null,
+      'Item 2',
+      undefined,
+      {
+        heading: 'Optional Section',
+        content: null,
+      },
+      'Item 3',
+    ],
+    context: {},
+    expect: 'Item 1\n\nItem 2\n\nItem 3',
+  },
+  {
+    desc: 'renders lists with dashes',
+    content: {
+      heading: 'My List',
+      format: 'list',
+      content: ['Apple', 'Banana', null, 'Cherry'],
+    },
+    context: {},
+    expect: '# My List\n\n- Apple\n- Banana\n- Cherry',
+  },
+  {
+    desc: 'supports custom format functions',
+    content: {
+      heading: 'Custom Format',
+      format: (parts) => parts.join(' | '),
+      content: ['A', 'B', undefined, 'C'],
+    },
+    context: {},
+    expect: '# Custom Format\n\nA | B | C',
+  },
 ];
 
 describe('renderPrompt', () => {
diff --git a/packages/core/src/prompter/prompter.ts b/packages/core/src/prompts/render-prompt.ts
similarity index 77%
rename from packages/core/src/prompter/prompter.ts
rename to packages/core/src/prompts/render-prompt.ts
index b101a62ad1..67e083b56a 100644
--- a/packages/core/src/prompter/prompter.ts
+++ b/packages/core/src/prompts/render-prompt.ts
@@ -4,15 +4,47 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import type { SystemPromptOptions } from 'src/prompts/snippets.js';
-import type { PromptContent, PromptSlot } from './types.js';
+import type { SystemPromptOptions } from './snippets.js';
+
+export type ContextResolver<C, O> = O | ((ctx: C) => O | Promise<O>);
+
+export type PromptSlot = { slot: string; content?: never };
+
+export type PromptSection<C> = {
+  /** Add a Markdown heading of appropriate level to this section. */
+  heading?: string;
+  /** If supplied, wrap this section in an XML tag. */
+  tag?: string;
+  /** If supplied, add attributes to the XML section tag. */
+  attrs?: Record<string, string>;
+  /** Formatting of the content inside this section. Defaults to 'block'. */
+  format?: 'inline' | 'block' | 'list' | ((parts: string[]) => string);
+
+  /** Condition that must evaluate to true for the section to be rendered. */
+  condition?: (ctx: C) => boolean | Promise<boolean>;
+  content: PromptContent<C>;
+};
+
+// The core recursive type.
+// It wraps your 3 base node shapes (string, section, or array) in the resolver.
+export type PromptContent<C> = ContextResolver<
+  C,
+  | string
+  | number
+  | boolean
+  | null
+  | undefined
+  | PromptSection<C>
+  | PromptSlot
+  | Array<PromptContent<C>>
+>;
 
 type BaseContent = string | BaseSection | PromptSlot | BaseContent[];
 type BaseSection = {
   heading?: string;
   tag?: string;
   attrs?: Record<string, string>;
-  format?: 'inline' | 'block';
+  format?: 'inline' | 'block' | 'list' | ((parts: string[]) => string);
   content: BaseContent;
 };
 
@@ -68,6 +100,7 @@ export async function renderPrompt<C = SystemPromptOptions>({
   const resolveToBasic = async (
     c: PromptContent<C>,
   ): Promise<BaseContent | null> => {
+    if (c === undefined || c === null) return null;
     if (typeof c === 'function') {
       const resolved = await c(context);
       return resolveToBasic(resolved);
@@ -133,14 +166,21 @@ export async function renderPrompt<C = SystemPromptOptions>({
   const formatBasic = (
     c: BaseContent | null,
     depth: number,
-    format: 'inline' | 'block',
+    format: 'inline' | 'block' | 'list' | ((parts: string[]) => string),
   ): string => {
     if (c === null) return '';
     if (typeof c === 'string') return c;
     if (Array.isArray(c)) {
-      return c
+      const parts = c
         .map((item) => formatBasic(item, depth, format))
-        .join(format === 'inline' ? '' : '\n\n');
+        .filter((p) => p !== '');
+      if (typeof format === 'function') {
+        return format(parts);
+      }
+      if (format === 'list') {
+        return parts.map((p) => '- ' + p).join('\n');
+      }
+      return parts.join(format === 'inline' ? '' : '\n\n');
     }
     if ('slot' in c) {
       const slotContributions = resolvedContributions[c.slot];