diff --git a/lib/ai/anthropic.js b/lib/ai/anthropic.js
new file mode 100644
index 0000000..fe9b834
--- /dev/null
+++ b/lib/ai/anthropic.js
@@ -0,0 +1,80 @@
+import Anthropic from '@anthropic-ai/sdk';
+import { resolveSecret } from './secret.js';
+
+export const DEFAULT_MODEL = process.env.ANTHROPIC_MODEL || 'claude-sonnet-4-6';
+
+/**
+ * Build an Anthropic client, resolving the API key via the secret resolver.
+ * Key reference can be overridden with ANTHROPIC_API_KEY_REF env var.
+ */
+export function getAnthropicClient() {
+  const key = resolveSecret(process.env.ANTHROPIC_API_KEY_REF || 'env:ANTHROPIC_API_KEY');
+  if (!key) {
+    throw new Error(
+      'Anthropic API key not configured (set ANTHROPIC_API_KEY or ANTHROPIC_API_KEY_REF)'
+    );
+  }
+  return new Anthropic({ apiKey: key });
+}
+
+/**
+ * Factory that returns a callModel function bound to a specific client + model.
+ *
+ * @param {object} opts
+ * @param {object}   opts.client    - Anthropic client (or fake for tests)
+ * @param {string}  [opts.model]    - Model ID (default: DEFAULT_MODEL)
+ * @param {number}  [opts.maxTokens] - max_tokens (default: 1024)
+ * @returns {Function} callModel({ system, messages, tools, onTextDelta })
+ *
+ * callModel returns a stable shape:
+ *   { text: string, toolUses: [{id, name, input}], stopReason: string, usage: object }
+ *
+ * Text deltas are streamed via onTextDelta(chunk) as they arrive.
+ * Tool-use blocks are collected from the final assembled message.
+ */
+export function makeCallModel({ client, model = DEFAULT_MODEL, maxTokens = 1024 }) {
+  return async function callModel({ system, messages, tools, onTextDelta }) {
+    // client.messages.stream returns a MessageStream: AsyncIterable<MessageStreamEvent>
+    // with a finalMessage() method that resolves to the assembled Message.
+    const stream = client.messages.stream({
+      model,
+      max_tokens: maxTokens,
+      ...(system !== undefined && { system }),
+      messages,
+      ...(tools !== undefined && tools !== null && { tools }),
+    });
+
+    // Iterate events; fire onTextDelta for text_delta events only.
+    // RawContentBlockDeltaEvent: { type: 'content_block_delta', index, delta }
+    // TextDelta: { type: 'text_delta', text }
+    for await (const event of stream) {
+      if (
+        event.type === 'content_block_delta' &&
+        event.delta?.type === 'text_delta'
+      ) {
+        onTextDelta?.(event.delta.text);
+      }
+    }
+
+    // finalMessage() resolves to the fully assembled Message object.
+    const final = await stream.finalMessage();
+
+    // Collect text from all text blocks (typically just one, but join defensively).
+    const text = final.content
+      .filter((b) => b.type === 'text')
+      .map((b) => b.text)
+      .join('');
+
+    // Collect tool_use blocks, normalising to a stable {id, name, input} shape.
+    const toolUses = final.content
+      .filter((b) => b.type === 'tool_use')
+      .map((b) => ({ id: b.id, name: b.name, input: b.input }));
+
+    return {
+      text,
+      toolUses,
+      stopReason: final.stop_reason,
+      usage: final.usage,
+    };
+  };
+}
diff --git a/tests/ai/anthropic.test.js b/tests/ai/anthropic.test.js
new file mode 100644
index 0000000..98ba733
--- /dev/null
+++ b/tests/ai/anthropic.test.js
@@ -0,0 +1,116 @@
+import { describe, it, expect } from 'vitest';
+import { makeCallModel } from '../../lib/ai/anthropic.js';
+
+// ---------------------------------------------------------------------------
+// Fake client whose messages.stream() returns an async-iterable of real-SDK-
+// shaped RawMessageStreamEvent objects, plus a finalMessage() method that
+// resolves to the assembled Message.  Shape matches @anthropic-ai/sdk@0.40.1.
+// ---------------------------------------------------------------------------
+
+function makeFakeStream({ events, finalMsg }) {
+  // The stream object is both async-iterable and has a finalMessage() method,
+  // exactly as MessageStream from the real SDK.
+  return {
+    [Symbol.asyncIterator]() {
+      let idx = 0;
+      return {
+        async next() {
+          if (idx < events.length) return { value: events[idx++], done: false };
+          return { value: undefined, done: true };
+        },
+      };
+    },
+    async finalMessage() {
+      return finalMsg;
+    },
+  };
+}
+
+const FAKE_FINAL_MESSAGE = {
+  content: [
+    { type: 'text', text: 'Hello', citations: null },
+    { type: 'tool_use', id: 'tu_1', name: 'search', input: { q: 'x' } },
+  ],
+  stop_reason: 'tool_use',
+  usage: { input_tokens: 10, output_tokens: 5 },
+};
+
+// Events emitted during streaming: two text deltas that together form 'Hello'
+const FAKE_EVENTS = [
+  // message_start — no text
+  { type: 'message_start', message: {} },
+  // content_block_start for text block
+  { type: 'content_block_start', index: 0, content_block: { type: 'text', text: '' } },
+  // text deltas
+  { type: 'content_block_delta', index: 0, delta: { type: 'text_delta', text: 'Hel' } },
+  { type: 'content_block_delta', index: 0, delta: { type: 'text_delta', text: 'lo' } },
+  // content_block_stop for text block
+  { type: 'content_block_stop', index: 0 },
+  // content_block_start for tool_use block
+  { type: 'content_block_start', index: 1, content_block: { type: 'tool_use', id: 'tu_1', name: 'search', input: {} } },
+  // input_json delta (not text — should NOT trigger onTextDelta)
+  { type: 'content_block_delta', index: 1, delta: { type: 'input_json_delta', partial_json: '{"q":"x"}' } },
+  { type: 'content_block_stop', index: 1 },
+  // message_stop
+  { type: 'message_stop' },
+];
+
+const fakeClient = {
+  messages: {
+    stream(_params) {
+      return makeFakeStream({ events: FAKE_EVENTS, finalMsg: FAKE_FINAL_MESSAGE });
+    },
+  },
+};
+
+describe('makeCallModel', () => {
+  it('streams text deltas and returns stable shape', async () => {
+    const callModel = makeCallModel({ client: fakeClient, model: 'm', maxTokens: 512 });
+
+    const deltas = [];
+    const out = await callModel({
+      system: 'You are helpful.',
+      messages: [{ role: 'user', content: 'Hi' }],
+      tools: [],
+      onTextDelta: (chunk) => deltas.push(chunk),
+    });
+
+    // Streamed deltas must concatenate to 'Hello'
+    expect(deltas.join('')).toBe('Hello');
+
+    // Stable return shape
+    expect(out.text).toBe('Hello');
+    expect(out.toolUses).toHaveLength(1);
+    expect(out.toolUses[0]).toMatchObject({ id: 'tu_1', name: 'search' });
+    expect(out.toolUses[0].input).toEqual({ q: 'x' });
+    expect(out.stopReason).toBe('tool_use');
+    expect(out.usage).toMatchObject({ input_tokens: 10, output_tokens: 5 });
+  });
+
+  it('works without onTextDelta callback', async () => {
+    const callModel = makeCallModel({ client: fakeClient, model: 'm' });
+    const out = await callModel({
+      messages: [{ role: 'user', content: 'Hi' }],
+    });
+    expect(out.text).toBe('Hello');
+    expect(out.stopReason).toBe('tool_use');
+  });
+
+  it('returns empty toolUses when no tool_use blocks', async () => {
+    const textOnlyMsg = {
+      content: [{ type: 'text', text: 'Just text', citations: null }],
+      stop_reason: 'end_turn',
+      usage: { input_tokens: 5, output_tokens: 3 },
+    };
+    const textOnlyClient = {
+      messages: {
+        stream: () => makeFakeStream({ events: [], finalMsg: textOnlyMsg }),
+      },
+    };
+    const callModel = makeCallModel({ client: textOnlyClient, model: 'm' });
+    const out = await callModel({ messages: [{ role: 'user', content: 'Hi' }] });
+    expect(out.text).toBe('Just text');
+    expect(out.toolUses).toEqual([]);
+    expect(out.stopReason).toBe('end_turn');
+  });
+});