diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index bdab5d4..72cf565 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -34,6 +34,10 @@ export {
   WebSearchTool,
   AskUserQuestionTool,
   ExitPlanModeTool,
+  makeToolSearchTool,
+  RegistryDeferredStore,
+  type DeferredToolEntry,
+  type DeferredToolStore,
   readTodos,
   TODO_FILE,
   parseDuckDuckGoHtml,
diff --git a/packages/core/src/tools/index.ts b/packages/core/src/tools/index.ts
index 219cd6e..00efa4e 100644
--- a/packages/core/src/tools/index.ts
+++ b/packages/core/src/tools/index.ts
@@ -15,5 +15,11 @@ export { WebSearchTool, parseDuckDuckGoHtml } from './web-search.js';
 export type { SearchHit } from './web-search.js';
 export { AskUserQuestionTool } from './ask-user.js';
 export { ExitPlanModeTool } from './exit-plan.js';
+export {
+  makeToolSearchTool,
+  RegistryDeferredStore,
+  type DeferredToolEntry,
+  type DeferredToolStore,
+} from './tool-search.js';
 export { ToolRegistry, BUILTIN_TOOLS } from './registry.js';
 export type { ToolDefinition, ToolContext, ToolResult, ToolHandler } from './types.js';
diff --git a/packages/core/src/tools/tool-search.test.ts b/packages/core/src/tools/tool-search.test.ts
new file mode 100644
index 0000000..3ae4568
--- /dev/null
+++ b/packages/core/src/tools/tool-search.test.ts
@@ -0,0 +1,104 @@
+import { describe, expect, it } from 'vitest';
+import { ToolRegistry } from './registry.js';
+import {
+  makeToolSearchTool,
+  RegistryDeferredStore,
+  type DeferredToolEntry,
+} from './tool-search.js';
+import type { ToolHandler } from '../types.js';
+
+function fakeHandler(name: string, description = ''): ToolHandler {
+  return {
+    name,
+    definition: { name, description, inputSchema: { type: 'object' } },
+    async execute() {
+      return { content: `ran ${name}` };
+    },
+  };
+}
+
+function entry(name: string, description: string): DeferredToolEntry {
+  return {
+    name,
+    description,
+    expand: () => fakeHandler(name, description),
+  };
+}
+
+describe('ToolSearch keyword query', () => {
+  it('returns sorted matches with a "select:" hint', async () => {
+    const reg = new ToolRegistry([]);
+    const store = new RegistryDeferredStore(reg, [
+      entry('mcp__slack__send', 'Send a message to Slack'),
+      entry('mcp__gmail__draft', 'Draft a Gmail email'),
+      entry('mcp__notion__page', 'Create a Notion page'),
+    ]);
+    const search = makeToolSearchTool(store);
+    const r = await search.execute({ query: 'slack' }, { cwd: '/x' });
+    expect(r.content).toContain('mcp__slack__send');
+    expect(r.content).toContain('select:mcp__slack__send');
+  });
+
+  it('returns "no matched" when nothing scores', async () => {
+    const reg = new ToolRegistry([]);
+    const store = new RegistryDeferredStore(reg, [entry('foo', 'bar')]);
+    const search = makeToolSearchTool(store);
+    const r = await search.execute({ query: 'zzz-no-such' }, { cwd: '/x' });
+    expect(r.content).toMatch(/No deferred tools matched/);
+  });
+
+  it('caps results at max_results', async () => {
+    const reg = new ToolRegistry([]);
+    const entries: DeferredToolEntry[] = [];
+    for (let i = 0; i < 20; i++) entries.push(entry(`tool${i}`, 'common-word common'));
+    const store = new RegistryDeferredStore(reg, entries);
+    const search = makeToolSearchTool(store);
+    const r = await search.execute(
+      { query: 'common', max_results: 3 },
+      { cwd: '/x' },
+    );
+    const data = r.data as { hits: unknown[] };
+    expect(data.hits).toHaveLength(3);
+  });
+});
+
+describe('ToolSearch select: query', () => {
+  it('loads named tools into the registry', async () => {
+    const reg = new ToolRegistry([]);
+    const store = new RegistryDeferredStore(reg, [
+      entry('A', 'desc A'),
+      entry('B', 'desc B'),
+    ]);
+    const search = makeToolSearchTool(store);
+    const r = await search.execute({ query: 'select:A,B' }, { cwd: '/x' });
+    expect(r.content).toMatch(/Loaded: A, B/);
+    expect(reg.get('A')).toBeDefined();
+    expect(reg.get('B')).toBeDefined();
+  });
+
+  it('reports missing tools without failing', async () => {
+    const reg = new ToolRegistry([]);
+    const store = new RegistryDeferredStore(reg, [entry('A', 'a')]);
+    const search = makeToolSearchTool(store);
+    const r = await search.execute({ query: 'select:A,DoesNotExist' }, { cwd: '/x' });
+    expect(r.content).toMatch(/Loaded: A/);
+    expect(r.content).toMatch(/Not found: DoesNotExist/);
+  });
+
+  it('is idempotent — second select: doesnt double-register', async () => {
+    const reg = new ToolRegistry([]);
+    const store = new RegistryDeferredStore(reg, [entry('A', 'a')]);
+    const search = makeToolSearchTool(store);
+    await search.execute({ query: 'select:A' }, { cwd: '/x' });
+    const r2 = await search.execute({ query: 'select:A' }, { cwd: '/x' });
+    expect(r2.content).toMatch(/Loaded: A/);
+  });
+
+  it('errors on empty query', async () => {
+    const reg = new ToolRegistry([]);
+    const store = new RegistryDeferredStore(reg, []);
+    const search = makeToolSearchTool(store);
+    const r = await search.execute({ query: '' }, { cwd: '/x' });
+    expect(r.isError).toBe(true);
+  });
+});
diff --git a/packages/core/src/tools/tool-search.ts b/packages/core/src/tools/tool-search.ts
new file mode 100644
index 0000000..15e9c5c
--- /dev/null
+++ b/packages/core/src/tools/tool-search.ts
@@ -0,0 +1,149 @@
+// ToolSearch tool — deferred-tool loading. Lets the agent discover and
+// "expand" tools that aren't loaded by default (large MCP toolkits, computer-
+// use tools, etc.) without bloating the system prompt with their full schema.
+//
+// Spec: docs/DEVELOPMENT_PLAN.md §3.15.6
+//
+// Wire-up:
+//   · ToolRegistry tracks a `deferred` map of name → { description, expand() }.
+//   · Agent loop exposes ONLY the deferred-tool names (not schemas) until
+//     ToolSearch is called with `select:Name1,Name2,...`.
+//   · The tool returns the schemas and asks the registry to register them.
+
+import type { ToolContext, ToolHandler, ToolResult } from '../types.js';
+
+export interface DeferredToolEntry {
+  name: string;
+  description: string;
+  /** Lazily produce the full ToolHandler when the tool is "expanded". */
+  expand: () => Promise<ToolHandler> | ToolHandler;
+}
+
+export interface DeferredToolStore {
+  /** Returns all deferred entries (for keyword search). */
+  list(): DeferredToolEntry[];
+  /** Expand and register an entry; idempotent on already-registered names. */
+  expand(name: string): Promise<ToolHandler | undefined>;
+}
+
+interface SearchInput {
+  query: string;
+  max_results?: number;
+}
+
+const DEFAULT_MAX_RESULTS = 5;
+
+export function makeToolSearchTool(store: DeferredToolStore): ToolHandler {
+  return {
+    name: 'ToolSearch',
+    definition: {
+      name: 'ToolSearch',
+      description:
+        'Find and load deferred tools by name or keyword. Use "select:Name1,Name2" to load tools by exact name; otherwise the query is matched as a fuzzy keyword search against tool names and descriptions. Once loaded, the tools become callable in subsequent turns.',
+      inputSchema: {
+        type: 'object',
+        properties: {
+          query: {
+            type: 'string',
+            description:
+              'Either "select:<csv of tool names>" or a free-text query matching name+description.',
+          },
+          max_results: {
+            type: 'number',
+            description: 'Cap on results for keyword queries (default 5).',
+          },
+        },
+        required: ['query'],
+      },
+    },
+    async execute(rawInput: Record<string, unknown>, _ctx: ToolContext): Promise<ToolResult> {
+      const input = rawInput as unknown as SearchInput;
+      if (!input?.query || typeof input.query !== 'string') {
+        return { content: 'Error: query is required (string).', isError: true };
+      }
+      const max = Math.max(1, input.max_results ?? DEFAULT_MAX_RESULTS);
+
+      if (input.query.startsWith('select:')) {
+        const names = input.query
+          .slice('select:'.length)
+          .split(',')
+          .map((s) => s.trim())
+          .filter(Boolean);
+        const loaded: string[] = [];
+        const missing: string[] = [];
+        for (const n of names) {
+          const h = await store.expand(n);
+          if (h) loaded.push(h.name);
+          else missing.push(n);
+        }
+        const lines: string[] = [];
+        if (loaded.length > 0) lines.push(`Loaded: ${loaded.join(', ')}`);
+        if (missing.length > 0) lines.push(`Not found: ${missing.join(', ')}`);
+        if (lines.length === 0) lines.push('No tools loaded.');
+        return { content: lines.join('\n'), data: { loaded, missing } };
+      }
+
+      // Keyword search — rank by token overlap of name + description
+      const tokens = input.query
+        .toLowerCase()
+        .split(/\s+/)
+        .filter((t) => t.length > 0);
+      const ranked = store
+        .list()
+        .map((e) => ({ entry: e, score: score(e, tokens) }))
+        .filter((r) => r.score > 0)
+        .sort((a, b) => b.score - a.score)
+        .slice(0, max);
+      if (ranked.length === 0) {
+        return { content: `No deferred tools matched "${input.query}".`, data: { hits: [] } };
+      }
+      const lines = ranked.map(
+        (r) => `${r.entry.name} — ${r.entry.description.slice(0, 120)}`,
+      );
+      lines.push('');
+      lines.push(`Use \`select:${ranked.map((r) => r.entry.name).join(',')}\` to load.`);
+      return {
+        content: lines.join('\n'),
+        data: { hits: ranked.map((r) => ({ name: r.entry.name, score: r.score })) },
+      };
+    },
+  };
+}
+
+function score(entry: DeferredToolEntry, tokens: string[]): number {
+  if (tokens.length === 0) return 0;
+  const text = `${entry.name} ${entry.description}`.toLowerCase();
+  let s = 0;
+  for (const t of tokens) {
+    if (entry.name.toLowerCase() === t) s += 100;
+    else if (entry.name.toLowerCase().includes(t)) s += 10;
+    if (text.includes(t)) s += 1;
+  }
+  return s;
+}
+
+/**
+ * Default DeferredToolStore backed by a ToolRegistry. Builds an internal map
+ * of name → entry on construction; expand() calls registry.register().
+ */
+export class RegistryDeferredStore implements DeferredToolStore {
+  private readonly entries = new Map<string, DeferredToolEntry>();
+  constructor(
+    private readonly registry: { register: (h: ToolHandler) => void; get: (name: string) => ToolHandler | undefined },
+    entries: DeferredToolEntry[],
+  ) {
+    for (const e of entries) this.entries.set(e.name, e);
+  }
+  list(): DeferredToolEntry[] {
+    return [...this.entries.values()];
+  }
+  async expand(name: string): Promise<ToolHandler | undefined> {
+    const existing = this.registry.get(name);
+    if (existing) return existing;
+    const entry = this.entries.get(name);
+    if (!entry) return undefined;
+    const handler = await entry.expand();
+    this.registry.register(handler);
+    return handler;
+  }
+}