diff --git a/src/algolia_agent/cli.py b/src/algolia_agent/cli.py
index e898773..61f6460 100644
--- a/src/algolia_agent/cli.py
+++ b/src/algolia_agent/cli.py
@@ -290,6 +290,29 @@ def cmd_create(client: AlgoliaAgentClient, args: argparse.Namespace):
 # only deviations are reported. Fields absent from this map are reported conservatively.
 _TOOL_FIELD_DEFAULTS = {"mode": "static", "allowUnlistedIndices": False}
 
+# Fields that identify a tool or are regenerated by the API, so never a real change:
+# name/type identify it, indices and description are diffed separately (the API composes
+# the tool description from the index descriptions).
+_TOOL_IDENTITY = {"name", "type", "indices", "description"}
+
+
+def _omitted_field_note(field: str, curr_val) -> str:
+    """Describe what happens to a field the payload leaves out.
+
+    The API replaces the tool object, so an omitted field takes its default. Name that
+    default when it is known rather than just saying "default", which reads as though
+    the current value were the default.
+    """
+    default = _TOOL_FIELD_DEFAULTS.get(field, _MISSING)
+    if default is _MISSING:
+        return f"{_fmt(curr_val)} would be dropped from the payload"
+    return f"{_fmt(curr_val)} would revert to its default of {_fmt(default)}"
+
+
+def _tool_key(tool: dict) -> str:
+    """Identity of a tool within the tools array."""
+    return tool.get("type") or tool.get("name") or ""
+
 
 def _fmt(value) -> str:
     """Render a diff value: JSON for objects/arrays, repr for scalars."""
@@ -397,10 +420,6 @@ def _diff(current: dict, new_payload: dict) -> list[str]:
     # that silently drops an existing tool is invisible. Only fields present in the new
     # payload are compared (same noise-avoidance as searchControls); nested index data
     # and descriptions are diffed above, so they're excluded here.
-    def _tool_key(t):
-        return t.get("type") or t.get("name") or ""
-
-    _tool_identity = {"name", "type", "indices", "description"}
     curr_tools = {_tool_key(t): t for t in current.get("tools", [])}
     new_tools = {_tool_key(t): t for t in new_payload.get("tools", [])}
 
@@ -411,7 +430,7 @@ def _tool_key(t):
         elif key not in new_tools:
             tool_lines.append(f"    - {key}")
         else:
-            new_fields = {k: v for k, v in new_tools[key].items() if k not in _tool_identity}
+            new_fields = {k: v for k, v in new_tools[key].items() if k not in _TOOL_IDENTITY}
             for k in sorted(new_fields):
                 changed, curr_shown = _field_changed(curr_tools[key].get(k), new_fields[k])
                 if changed:
@@ -422,13 +441,15 @@ def _tool_key(t):
             # reverts to its API default — verified: mode "dynamic" became "static" and
             # allowUnlistedIndices true became false. Report those, or the dry-run stays
             # silent while the update turns settings off. "description" is excluded via
-            # _tool_identity because the API regenerates it from the index descriptions.
-            for k in sorted(set(curr_tools[key]) - set(new_fields) - _tool_identity):
+            # _TOOL_IDENTITY because the API regenerates it from the index descriptions.
+            for k in sorted(set(curr_tools[key]) - set(new_fields) - _TOOL_IDENTITY):
                 curr_val = curr_tools[key][k]
                 if curr_val is None or curr_val == _TOOL_FIELD_DEFAULTS.get(k, _MISSING):
                     continue  # already at its default; omitting it changes nothing
+                default = _TOOL_FIELD_DEFAULTS.get(k, _MISSING)
+                becomes = "dropped" if default is _MISSING else _fmt(default)
                 tool_lines.append(
-                    f"    ~ {key}.{k}: {_fmt(curr_val)} → default (not sent)"
+                    f"    ~ {key}.{k}: {_fmt(curr_val)} → {becomes} (not sent)"
                 )
     if tool_lines:
         lines.append("  tools:")
@@ -454,6 +475,55 @@ def _tool_key(t):
     return lines
 
 
+def _removals(current: dict, new_payload: dict) -> list[str]:
+    """Describe losses the config model has no way to ask for.
+
+    PATCH replaces objects rather than merging them, so anything absent from the
+    payload is destroyed. agent-config.json cannot express a second tool, a tool-level
+    field like mode, or per-index searchControls, so losing those is never something the
+    user asked for — it is the config model being narrower than the API.
+
+    Index membership is deliberately excluded: the config names its indices, so
+    dropping one is an explicit choice rather than an accident.
+    """
+    out = []
+
+    curr_tools = {_tool_key(t): t for t in current.get("tools", [])}
+    new_tools = {_tool_key(t): t for t in new_payload.get("tools", [])}
+
+    for key in sorted(set(curr_tools) - set(new_tools)):
+        out.append(f"  - the {key!r} tool would be deleted")
+
+    for key in sorted(set(curr_tools) & set(new_tools)):
+        dropped = set(curr_tools[key]) - set(new_tools[key]) - _TOOL_IDENTITY
+        for field in sorted(dropped):
+            val = curr_tools[key][field]
+            if val is None or val == _TOOL_FIELD_DEFAULTS.get(field, _MISSING):
+                continue  # already at its default; omitting it changes nothing
+            out.append(f"  - {key}.{field}: {_omitted_field_note(field, val)}")
+
+    curr_sc = {
+        i["index"]: i.get("searchControls")
+        for t in current.get("tools", [])
+        for i in t.get("indices", [])
+    }
+    new_sc = {
+        i["index"]: i.get("searchControls")
+        for t in new_payload.get("tools", [])
+        for i in t.get("indices", [])
+    }
+    for idx in sorted(curr_sc):
+        if idx in new_sc and curr_sc[idx] and not new_sc[idx]:
+            out.append(f"  - searchControls on {idx!r} would be wiped")
+
+    new_cfg = new_payload.get("config")
+    if new_cfg is not None:
+        for k in sorted(set(current.get("config") or {}) - set(new_cfg)):
+            out.append(f"  - config key {k!r} would be removed")
+
+    return out
+
+
 def cmd_update(client: AlgoliaAgentClient, args: argparse.Namespace):
     current = client.get_agent(args.agent_id)
 
@@ -544,6 +614,21 @@ def cmd_update(client: AlgoliaAgentClient, args: argparse.Namespace):
             print("\n  No changes detected.")
         return
 
+    # The API replaces objects rather than merging, so a config narrower than the
+    # agent silently deletes the difference. Refuse rather than destroy.
+    removals = _removals(current, new_payload)
+    if removals and not getattr(args, "force", False):
+        source = f"{config_path}" if config_path else "the fields you supplied"
+        raise SystemExit(
+            "ERROR: this update would remove configuration the agent currently has:\n"
+            + "\n".join(removals)
+            + "\n\nThe Agent Studio API replaces these fields instead of merging them, so\n"
+            f"anything missing from the payload is lost. {source} cannot express them,\n"
+            "which is why they are absent.\n\n"
+            "Either add the values to your config, or pass --force to accept the removals.\n"
+            "Run the same command with --dry-run to see the full diff first."
+        )
+
     agent = client.update_agent(args.agent_id, new_payload)
 
     if args.json:
@@ -889,6 +974,8 @@ def build_parser() -> argparse.ArgumentParser:
                           help="Template variable substitution (repeatable)")
     update_p.add_argument("--publish", action="store_true",
                           help="Publish the agent after updating")
+    update_p.add_argument("--force", action="store_true",
+                          help="Proceed even if the update would remove existing configuration")
     update_p.add_argument("--dry-run", action="store_true",
                           help="Show what would change without making API calls")
     update_p.add_argument("--json", action="store_true", help="Output JSON")
diff --git a/tests/test_cli.py b/tests/test_cli.py
index c359395..cfb841e 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1311,14 +1311,30 @@ def test_diff_reports_tool_fields_that_revert_to_default():
     }])
     changes = _diff(current, new_payload)
     assert "  tools:" in changes
-    reverts = [line for line in changes if "default (not sent)" in line]
+    reverts = [line for line in changes if "(not sent)" in line]
     assert len(reverts) == 2, reverts
-    assert any("mode" in line and "dynamic" in line for line in reverts)
-    assert any("allowUnlistedIndices" in line for line in reverts)
+    # The actual default is named, not just the word "default".
+    assert any("mode" in line and "'dynamic' → 'static'" in line for line in reverts)
+    assert any("allowUnlistedIndices" in line and "True → False" in line for line in reverts)
     # The API regenerates tool description, so it must not be reported.
     assert not any("description" in line for line in reverts)
 
 
+def test_diff_says_dropped_when_the_default_is_unknown():
+    """Only mode and allowUnlistedIndices have known defaults; be honest about the rest."""
+    current = _agent_with_tools([{
+        "type": "algolia_search_index", "name": "algolia_search_index",
+        "someFutureField": 42,
+        "indices": [{"index": "products", "description": "d"}],
+    }])
+    new_payload = _agent_with_tools([{
+        "type": "algolia_search_index", "name": "algolia_search_index",
+        "indices": [{"index": "products", "description": "d"}],
+    }])
+    changes = _diff(current, new_payload)
+    assert any("someFutureField: 42 → dropped (not sent)" in line for line in changes)
+
+
 def test_diff_ignores_null_tool_fields_not_sent():
     """A field already null loses nothing by being omitted."""
     current = _agent_with_tools([{
@@ -1331,3 +1347,180 @@ def test_diff_ignores_null_tool_fields_not_sent():
         "indices": [{"index": "products", "description": "d"}],
     }])
     assert not _diff(current, new_payload)
+
+
+# ── destructive-update guard (#14 phase 1) ────────────────────────────────────
+
+def _rich_current_agent():
+    """An agent carrying everything agent-config.json cannot express."""
+    return {
+        "id": "agent-uuid", "name": "Rich Agent", "model": "gemini-2.5-flash",
+        "instructions": "Old instructions.", "status": "published",
+        "providerId": "provider-uuid",
+        "tools": [
+            {"name": "algolia_search_index", "type": "algolia_search_index",
+             "mode": "dynamic", "allowUnlistedIndices": True,
+             "indices": [{"index": "products", "description": "Product catalog.",
+                          "searchControls": {"hitsPerPage": {"exposed": True, "default": 7}}}]},
+            {"name": "algolia_display_results", "type": "algolia_display_results",
+             "isTerminal": False, "maxGroups": 3},
+        ],
+        "config": {"suggestions": {"enabled": True}, "memory": {"enabled": True},
+                   "max_iterations": 15},
+    }
+
+
+def _narrow_payload():
+    """What build_tool() + cmd_update produce from a config naming only the index."""
+    return {
+        "name": "Rich Agent", "providerId": "provider-uuid", "model": "gemini-2.5-flash",
+        "instructions": "Old instructions.", "status": "published",
+        "tools": [{"name": "algolia_search_index", "type": "algolia_search_index",
+                   "indices": [{"index": "products", "description": "Product catalog."}]}],
+        "config": {"suggestions": {"enabled": True}},
+    }
+
+
+def test_removals_lists_every_loss_the_config_cannot_express():
+    from algolia_agent.cli import _removals
+
+    out = _removals(_rich_current_agent(), _narrow_payload())
+    joined = "\n".join(out)
+    assert "'algolia_display_results' tool would be deleted" in joined
+    assert "algolia_search_index.mode: 'dynamic' would revert to its default of 'static'" in joined
+    assert "algolia_search_index.allowUnlistedIndices" in joined
+    assert "searchControls on 'products' would be wiped" in joined
+    assert "config key 'memory'" in joined
+    assert "config key 'max_iterations'" in joined
+    # 'suggestions' is being sent, so it is not a loss.
+    assert "'suggestions'" not in joined
+
+
+def test_removals_empty_for_a_faithful_payload():
+    from algolia_agent.cli import _removals
+
+    current = _rich_current_agent()
+    assert _removals(current, dict(current)) == []
+
+
+def test_removals_ignores_fields_already_at_their_default():
+    from algolia_agent.cli import _removals
+
+    current = _rich_current_agent()
+    current["tools"][0]["mode"] = "static"
+    current["tools"][0]["allowUnlistedIndices"] = False
+    out = "\n".join(_removals(current, _narrow_payload()))
+    assert "mode" not in out
+    assert "allowUnlistedIndices" not in out
+
+
+def test_removals_does_not_flag_index_membership():
+    """Dropping an index is expressible in config, so it is a choice, not an accident."""
+    from algolia_agent.cli import _removals
+
+    current = _rich_current_agent()
+    current["tools"][0]["indices"].append({"index": "products_asc", "description": "Asc."})
+    out = "\n".join(_removals(current, _narrow_payload()))
+    assert "products_asc" not in out
+
+
+def _guard_args(tmp_path, extra=None):
+    prompt = tmp_path / "PROMPT.md"
+    prompt.write_text("Old instructions.")
+    config = tmp_path / "config.json"
+    config.write_text(json.dumps({
+        "name": "Rich Agent", "model": "gemini-2.5-flash",
+        "instructions": str(prompt), "index": "products",
+        "index_description": "Product catalog.",
+        "config": {"suggestions": {"enabled": True}},
+    }))
+    return build_parser().parse_args(
+        ["update", "agent-uuid", "--config", str(config)] + (extra or [])
+    )
+
+
+def test_update_refuses_a_destructive_payload(tmp_path, capsys):
+    from algolia_agent.cli import cmd_update
+
+    mock_client = MagicMock()
+    mock_client.get_agent.return_value = _rich_current_agent()
+    with pytest.raises(SystemExit) as exc:
+        cmd_update(mock_client, _guard_args(tmp_path))
+    msg = str(exc.value)
+    assert "would remove configuration" in msg
+    assert "algolia_display_results" in msg
+    assert "--force" in msg
+    mock_client.update_agent.assert_not_called()
+
+
+def test_update_proceeds_with_force(tmp_path):
+    from algolia_agent.cli import cmd_update
+
+    mock_client = MagicMock()
+    mock_client.get_agent.return_value = _rich_current_agent()
+    mock_client.update_agent.return_value = {"id": "agent-uuid", "name": "Rich Agent", "status": "published"}
+    cmd_update(mock_client, _guard_args(tmp_path, ["--force"]))
+    mock_client.update_agent.assert_called_once()
+
+
+def test_dry_run_is_not_blocked_by_the_guard(tmp_path, capsys):
+    """--dry-run writes nothing, so it must still report rather than refuse."""
+    from algolia_agent.cli import cmd_update
+
+    mock_client = MagicMock()
+    mock_client.get_agent.return_value = _rich_current_agent()
+    cmd_update(mock_client, _guard_args(tmp_path, ["--dry-run"]))
+    out = capsys.readouterr().out
+    assert "DRY RUN" in out
+    assert "algolia_display_results" in out
+    mock_client.update_agent.assert_not_called()
+
+
+def test_update_allowed_when_nothing_is_removed(tmp_path):
+    """A plain agent with nothing extra must still be updatable without --force."""
+    from algolia_agent.cli import cmd_update
+
+    mock_client = MagicMock()
+    mock_client.get_agent.return_value = _make_current_agent()
+    mock_client.update_agent.return_value = {"id": "agent-uuid", "name": "Old Agent", "status": "draft"}
+    prompt = tmp_path / "PROMPT.md"
+    prompt.write_text("New instructions.")
+    config = tmp_path / "config.json"
+    config.write_text(json.dumps({
+        "name": "Old Agent", "model": "gemini-2.5-flash", "instructions": str(prompt),
+        "index": "products", "index_description": "Product catalog.",
+        "replicas": [{"index": "products_price_asc", "description": "Sorted by price ascending."}],
+    }))
+    args = build_parser().parse_args(["update", "agent-uuid", "--config", str(config)])
+    cmd_update(mock_client, args)
+    mock_client.update_agent.assert_called_once()
+
+
+def test_removals_names_the_supplied_config_path(tmp_path):
+    """update accepts --config with any path, so the message must not hard-code one."""
+    from algolia_agent.cli import cmd_update
+
+    mock_client = MagicMock()
+    mock_client.get_agent.return_value = _rich_current_agent()
+    prompt = tmp_path / "PROMPT.md"
+    prompt.write_text("Old instructions.")
+    config = tmp_path / "my-custom-name.json"
+    config.write_text(json.dumps({
+        "name": "Rich Agent", "model": "gemini-2.5-flash", "instructions": str(prompt),
+        "index": "products", "index_description": "Product catalog.",
+        "config": {"suggestions": {"enabled": True}},
+    }))
+    args = build_parser().parse_args(["update", "agent-uuid", "--config", str(config)])
+    with pytest.raises(SystemExit) as exc:
+        cmd_update(mock_client, args)
+    assert "my-custom-name.json" in str(exc.value)
+    assert "agent-config.json" not in str(exc.value)
+
+
+def test_removals_says_dropped_for_unknown_default():
+    from algolia_agent.cli import _removals
+
+    current = _rich_current_agent()
+    current["tools"][0]["someFutureField"] = 42
+    out = "\n".join(_removals(current, _narrow_payload()))
+    assert "someFutureField: 42 would be dropped from the payload" in out