diff --git a/lib/ai/agent/runtime.js b/lib/ai/agent/runtime.js
new file mode 100644
index 0000000..15058fa
--- /dev/null
+++ b/lib/ai/agent/runtime.js
@@ -0,0 +1,58 @@
+// Tool-use loop. callModel + registry injected (no network here).
+// Emits: { type:'tool', tool, args, status } | { type:'delta', text }
+//        | { type:'draft', pending_change_id, summary }
+// Returns: { text, toolTrace, draftIds, usage, stoppedOnGuard }
+export async function runTurn({ callModel, registry, system, messages, ctx, onEvent, maxIterations = 6 }) {
+  const convo = [...messages];
+  const toolTrace = [];
+  const draftIds = [];
+  let usage = {};
+  let stoppedOnGuard = false;
+
+  for (let i = 0; i < maxIterations; i++) {
+    const res = await callModel({
+      system,
+      messages: convo,
+      tools: registry.toAnthropicTools(),
+      onTextDelta: (t) => onEvent?.({ type: 'delta', text: t })
+    });
+    usage = res.usage || usage;
+
+    if (!res.toolUses?.length) {
+      return { text: res.text, toolTrace, draftIds, usage, stoppedOnGuard };
+    }
+
+    // Record the assistant turn (text + tool_use blocks) for the next round.
+    convo.push({
+      role: 'assistant',
+      content: [
+        ...(res.text ? [{ type: 'text', text: res.text }] : []),
+        ...res.toolUses.map(t => ({ type: 'tool_use', id: t.id, name: t.name, input: t.input }))
+      ]
+    });
+
+    const toolResults = [];
+    for (const call of res.toolUses) {
+      const tool = registry.getTool(call.name);
+      let result;
+      try {
+        result = tool ? await tool.handler(call.input, ctx) : { error: `unknown tool ${call.name}` };
+      } catch (e) {
+        result = { error: String(e?.message || e) };
+      }
+      const status = result?.error ? 'error' : 'done';
+      toolTrace.push({ tool: call.name, args: call.input, ok: !result?.error });
+      onEvent?.({ type: 'tool', tool: call.name, args: call.input, status });
+      if (result?.pending_change_id) {
+        draftIds.push(result.pending_change_id);
+        onEvent?.({ type: 'draft', pending_change_id: result.pending_change_id, summary: result.summary });
+      }
+      toolResults.push({ type: 'tool_result', tool_use_id: call.id, content: JSON.stringify(result) });
+    }
+    convo.push({ role: 'user', content: toolResults });
+
+    if (i === maxIterations - 1) stoppedOnGuard = true;
+  }
+
+  return { text: '', toolTrace, draftIds, usage, stoppedOnGuard };
+}
diff --git a/tests/ai/agent/runtime.test.js b/tests/ai/agent/runtime.test.js
new file mode 100644
index 0000000..f0dcc8c
--- /dev/null
+++ b/tests/ai/agent/runtime.test.js
@@ -0,0 +1,68 @@
+import { describe, it, expect } from 'vitest';
+import { createRegistry } from '../../../lib/ai/agent/registry.js';
+import { runTurn } from '../../../lib/ai/agent/runtime.js';
+
+function scriptedCallModel(steps) {
+  let i = 0;
+  return async ({ onTextDelta }) => {
+    const step = steps[i++];
+    if (step.text) for (const ch of step.text) onTextDelta?.(ch);
+    return { text: step.text || '', toolUses: step.toolUses || [], stopReason: step.toolUses ? 'tool_use' : 'end_turn', usage: { output_tokens: 1 } };
+  };
+}
+
+describe('runTurn', () => {
+  it('runs a tool then produces a final answer', async () => {
+    const reg = createRegistry();
+    reg.registerTool({ name: 'search', description: 's', input_schema: { type: 'object', properties: {} },
+      handler: async () => ({ results: [{ title: 'Hit' }] }) });
+
+    const callModel = scriptedCallModel([
+      { toolUses: [{ id: 'tu1', name: 'search', input: { q: 'x' } }] },
+      { text: 'Found it.' }
+    ]);
+
+    const events = [];
+    const out = await runTurn({
+      callModel, registry: reg, system: 'sys',
+      messages: [{ role: 'user', content: 'find x' }],
+      ctx: { agent: { id: 'a' }, space_id: 's' },
+      onEvent: e => events.push(e)
+    });
+
+    expect(out.text).toBe('Found it.');
+    expect(events.filter(e => e.type === 'tool').map(e => e.tool)).toEqual(['search']);
+    expect(events.some(e => e.type === 'delta' && e.text)).toBe(true);
+    expect(out.toolTrace[0]).toMatchObject({ tool: 'search' });
+  });
+
+  it('emits a draft event when propose_change runs', async () => {
+    const reg = createRegistry();
+    reg.registerTool({ name: 'propose_change', description: 'p', input_schema: { type: 'object', properties: {} },
+      handler: async () => ({ pending_change_id: 'pc1', applied: false, summary: 'create task "X"' }) });
+
+    const callModel = scriptedCallModel([
+      { toolUses: [{ id: 'tu1', name: 'propose_change', input: {} }] },
+      { text: 'Drafted.' }
+    ]);
+
+    const events = [];
+    const out = await runTurn({ callModel, registry: reg, system: 's',
+      messages: [{ role: 'user', content: 'make a task' }],
+      ctx: { agent: { id: 'a' } }, onEvent: e => events.push(e) });
+
+    expect(out.draftIds).toEqual(['pc1']);
+    expect(events.find(e => e.type === 'draft')).toMatchObject({ pending_change_id: 'pc1' });
+  });
+
+  it('stops at the iteration guard', async () => {
+    const reg = createRegistry();
+    reg.registerTool({ name: 'search', description: 's', input_schema: { type: 'object', properties: {} },
+      handler: async () => ({ results: [] }) });
+    const always = async () => ({ text: '', toolUses: [{ id: 't', name: 'search', input: {} }], stopReason: 'tool_use', usage: {} });
+    const out = await runTurn({ callModel: always, registry: reg, system: 's',
+      messages: [{ role: 'user', content: 'loop' }], ctx: { agent: { id: 'a' } }, onEvent: () => {}, maxIterations: 3 });
+    expect(out.toolTrace.length).toBe(3);
+    expect(out.stoppedOnGuard).toBe(true);
+  });
+});