diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..d9f9065
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,22 @@
+# Claw — Databricks Personal Assistant
+
+## Persistent Memory
+
+You have access to persistent memory via the claw-memory MCP server.
+
+At the START of each conversation:
+- Call read_memory() to load your persistent context
+
+When the user shares important information:
+- Call write_memory(content) to update persistent memory
+- Include ALL existing memory plus new additions (full replace, not append)
+- Use structured sections with headers
+- Remember: names, preferences, ongoing projects, decisions made
+
+NEVER write files to the local filesystem for persistence — the container is
+ephemeral. Use write_memory() via the memory MCP only.
+
+## Behavior
+- Be concise and helpful
+- You have access to Databricks tools via MCP servers
+- All tool calls are pre-approved (bypassPermissions mode)
diff --git a/claw/core/agent.py b/claw/core/agent.py
index 9147987..c473b2f 100644
--- a/claw/core/agent.py
+++ b/claw/core/agent.py
@@ -19,6 +19,7 @@
     ClaudeAgentOptions,
     ResultMessage,
     StreamEvent,
+    SystemPromptPreset,
     TaskProgressMessage,
     TaskStartedMessage,
 )
@@ -68,6 +69,31 @@ def preflight_check_cli(config: Any) -> None:
         logger.error("preflight_check_cli failed: %s", exc)
 
 
+def build_system_prompt(config: Any) -> SystemPromptPreset:
+    """Build a system prompt preset that appends memory instructions to Claude's default prompt.
+
+    Uses SystemPromptPreset with preset="claude_code" so skills, MCP descriptions,
+    and default Claude Code behavior are preserved. Only the memory/persistence
+    instructions are appended.
+    """
+    append_text = """
+## Persistent Memory (Claw)
+You have access to persistent memory via the claw-memory MCP server.
+
+At the START of each conversation:
+- Call read_memory() to load your persistent context
+
+When the user shares important information:
+- Call write_memory(content) to update persistent memory
+- Include ALL existing memory plus new additions (full replace, not append)
+- Use structured sections with headers
+- Remember: names, preferences, ongoing projects, decisions made
+
+NEVER write files to the local filesystem for persistence — the container is ephemeral. Use write_memory() via the memory MCP only.
+"""
+    return SystemPromptPreset(type="preset", preset="claude_code", append=append_text)
+
+
 def build_sdk_env(config: Any) -> dict[str, str]:
     """Build the sdk_env dict for the Claude Agent SDK.
 
@@ -100,6 +126,7 @@ async def create_client(
     options = ClaudeAgentOptions(
         resume=sdk_session_id,
         permission_mode="bypassPermissions",
+        system_prompt=build_system_prompt(config),
         env=build_sdk_env(config),
         mcp_servers=mcp_servers,
         cwd=cwd,
diff --git a/openspec/changes/system-prompt/proposal.md b/openspec/changes/system-prompt/proposal.md
new file mode 100644
index 0000000..17fb957
--- /dev/null
+++ b/openspec/changes/system-prompt/proposal.md
@@ -0,0 +1,18 @@
+# System Prompt Injection — tell Claude about memory MCP
+
+## Problem
+Claude has no instructions about the memory MCP server or persistent storage.
+Memory context is injected as a prompt prefix in slack_poller.py, but Claude
+doesn't know to *write* memory or use the MCP tools proactively.
+
+## Solution
+1. Add `build_system_prompt(config)` to `claw/core/agent.py` — returns a string
+   instructing Claude to use read_memory/write_memory from the claw-memory MCP.
+2. Pass it via the `system_prompt` field on `ClaudeAgentOptions` in `create_client()`.
+3. Add a `CLAUDE.md` at repo root (belt-and-suspenders) with the same memory
+   instructions, picked up automatically since `cwd` is already set to repo root.
+
+## Files changed
+- `claw/core/agent.py` — add `build_system_prompt()`, wire into `create_client()`
+- `CLAUDE.md` (new) — memory instructions for Claude Code cwd discovery
+- `tests/test_agent.py` — tests for `build_system_prompt()` and system_prompt in options
diff --git a/openspec/changes/system-prompt/specs/system-prompt/spec.md b/openspec/changes/system-prompt/specs/system-prompt/spec.md
new file mode 100644
index 0000000..f867e5c
--- /dev/null
+++ b/openspec/changes/system-prompt/specs/system-prompt/spec.md
@@ -0,0 +1,36 @@
+# Spec: system-prompt
+
+## Module
+`claw.core.agent`
+
+## Public API
+
+### build_system_prompt(config: Any) -> str
+Returns a non-empty system prompt string that:
+- Contains "read_memory" instruction
+- Contains "write_memory" instruction
+- Warns against local filesystem persistence
+- Mentions structured sections for memory content
+
+### create_client changes
+- `ClaudeAgentOptions` now includes `system_prompt=build_system_prompt(config)`
+- All other existing fields unchanged
+
+## CLAUDE.md (repo root)
+Contains memory MCP instructions. Discovered automatically via `cwd=repo_root`.
+
+## Tests
+
+### TestBuildSystemPrompt
+1. `test_returns_nonempty_string` — result is a non-empty str
+2. `test_contains_read_memory` — "read_memory" in result
+3. `test_contains_write_memory` — "write_memory" in result
+4. `test_warns_no_filesystem_persistence` — "filesystem" or "disk" in result
+5. `test_mentions_structured_sections` — "structured" in result
+
+### TestCreateClient (additions)
+6. `test_system_prompt_passed_in_options` — options.system_prompt is non-empty string
+
+### TestClaudeMd
+7. `test_claude_md_exists` — CLAUDE.md file exists at repo root
+8. `test_claude_md_contains_memory_instructions` — file contains "read_memory" and "write_memory"
diff --git a/openspec/changes/system-prompt/tasks.md b/openspec/changes/system-prompt/tasks.md
new file mode 100644
index 0000000..07223bd
--- /dev/null
+++ b/openspec/changes/system-prompt/tasks.md
@@ -0,0 +1,7 @@
+# Tasks
+
+- [x] OpenSpec written
+- [ ] RED: Write failing tests for build_system_prompt and system_prompt in options
+- [ ] GREEN: Implement build_system_prompt(), wire into create_client(), create CLAUDE.md
+- [ ] All tests pass (266 existing + new)
+- [ ] Commit and PR
diff --git a/tests/test_agent.py b/tests/test_agent.py
index cc1ab05..e9d4d5a 100644
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -543,3 +543,79 @@ async def _fake_receive():
             pass
 
         assert gen.progress_event_received is False
+
+
+# ---------------------------------------------------------------------------
+# Issue #68: build_system_prompt and system_prompt in create_client
+# ---------------------------------------------------------------------------
+
+
+class TestBuildSystemPrompt:
+    def test_returns_system_prompt_preset(self):
+        from claude_agent_sdk.types import SystemPromptPreset
+
+        from claw.core.agent import build_system_prompt
+
+        result = build_system_prompt(_fake_config())
+        assert isinstance(result, dict)
+        assert result["type"] == "preset"
+        assert result["preset"] == "claude_code"
+
+    def test_contains_read_memory(self):
+        from claw.core.agent import build_system_prompt
+
+        result = build_system_prompt(_fake_config())
+        assert "read_memory" in result["append"]
+
+    def test_contains_write_memory(self):
+        from claw.core.agent import build_system_prompt
+
+        result = build_system_prompt(_fake_config())
+        assert "write_memory" in result["append"]
+
+    def test_warns_no_filesystem_persistence(self):
+        from claw.core.agent import build_system_prompt
+
+        result = build_system_prompt(_fake_config())
+        assert "filesystem" in result["append"].lower() or "disk" in result["append"].lower()
+
+    def test_mentions_structured_sections(self):
+        from claw.core.agent import build_system_prompt
+
+        result = build_system_prompt(_fake_config())
+        assert "structured" in result["append"].lower()
+
+
+class TestSystemPromptInCreateClient:
+    @pytest.mark.asyncio
+    async def test_system_prompt_passed_in_options(self):
+        from claw.core.agent import create_client
+
+        mock_client_instance = MagicMock()
+        mock_client_instance.connect = AsyncMock()
+
+        with patch("claw.core.agent.ClaudeSDKClient", return_value=mock_client_instance) as mock_cls:
+            await create_client(_fake_config(), {})
+
+        call_kwargs = mock_cls.call_args
+        options = call_kwargs[1]["options"] if "options" in (call_kwargs[1] or {}) else call_kwargs[0][0]
+        assert options.system_prompt is not None
+        # Should be a SystemPromptPreset dict that appends, not a plain str
+        assert isinstance(options.system_prompt, dict)
+        assert options.system_prompt.get("preset") == "claude_code"
+        assert "append" in options.system_prompt
+        assert "read_memory" in options.system_prompt["append"]
+
+
+class TestClaudeMd:
+    def test_claude_md_exists(self):
+        repo_root = Path(__file__).resolve().parents[1]
+        claude_md = repo_root / "CLAUDE.md"
+        assert claude_md.exists(), f"CLAUDE.md not found at {claude_md}"
+
+    def test_claude_md_contains_memory_instructions(self):
+        repo_root = Path(__file__).resolve().parents[1]
+        claude_md = repo_root / "CLAUDE.md"
+        content = claude_md.read_text()
+        assert "read_memory" in content
+        assert "write_memory" in content