diff --git a/decnet/fleet/__init__.py b/decnet/fleet/__init__.py
index f41dbeee..f0252e2e 100644
--- a/decnet/fleet/__init__.py
+++ b/decnet/fleet/__init__.py
@@ -104,8 +104,14 @@ def build_deckies_from_ini(
     host_ip: str,
     randomize: bool,
     cli_mutate_interval: int | None = None,
+    reserved_ips: set[str] | None = None,
 ) -> list[DeckyConfig]:
-    """Build DeckyConfig list from an IniConfig, auto-allocating missing IPs."""
+    """Build DeckyConfig list from an IniConfig, auto-allocating missing IPs.
+
+    *reserved_ips* lets the additive deploy path pass the IPs of the
+    already-deployed fleet so auto-allocation skips them instead of
+    handing out a colliding address.
+    """
     from ipaddress import IPv4Address, IPv4Network
     import time
     now = time.time()
@@ -121,6 +127,8 @@ def build_deckies_from_ini(
         IPv4Address(gateway),
         IPv4Address(host_ip),
     } | explicit_ips
+    if reserved_ips:
+        reserved |= {IPv4Address(ip) for ip in reserved_ips}
 
     auto_pool = (str(addr) for addr in net.hosts() if addr not in reserved)
 
diff --git a/decnet/web/db/models/deploy.py b/decnet/web/db/models/deploy.py
index 423baec9..a194c148 100644
--- a/decnet/web/db/models/deploy.py
+++ b/decnet/web/db/models/deploy.py
@@ -17,6 +17,15 @@ class DeployIniRequest(BaseModel):
     # This field now enforces strict INI structure during Pydantic initialization.
     # The OpenAPI schema correctly shows it as a required string.
     ini_content: IniContent = PydanticField(..., description="A valid INI formatted string")
+    replace_fleet: bool = PydanticField(
+        default=False,
+        description=(
+            "If true, the INI is treated as the complete desired fleet — any "
+            "existing decky absent from the INI is torn down. Default false: "
+            "the INI is appended to the existing fleet; name or IP collisions "
+            "with already-deployed deckies yield 409."
+        ),
+    )
 
 
 class DeployResponse(BaseModel):
diff --git a/decnet/web/router/fleet/api_deploy_deckies.py b/decnet/web/router/fleet/api_deploy_deckies.py
index 24a14ce3..05c80619 100644
--- a/decnet/web/router/fleet/api_deploy_deckies.py
+++ b/decnet/web/router/fleet/api_deploy_deckies.py
@@ -76,9 +76,21 @@ async def api_deploy_deckies(req: DeployIniRequest, admin: dict = Depends(requir
                        "Add a [general] section with interface=, net=, and gw= to the INI."
             )
 
+    # Snapshot the existing fleet (from prior state, NOT the freshly-built
+    # config below) so additive collision checks compare new against prior
+    # rather than against themselves. Existing IPs are passed into
+    # build_deckies_from_ini as reserved so auto-allocation skips them.
+    existing_deckies = list(config.deckies) if config is not None else []
+    reserved_ips: set[str] | None = (
+        {d.ip for d in existing_deckies if d.ip}
+        if not req.replace_fleet and existing_deckies
+        else None
+    )
+
     try:
         new_decky_configs = build_deckies_from_ini(
-            ini, subnet_cidr, gateway, host_ip, False, cli_mutate_interval=None
+            ini, subnet_cidr, gateway, host_ip, False,
+            cli_mutate_interval=None, reserved_ips=reserved_ips,
         )
     except ValueError as e:
         log.debug("deploy: build_deckies_from_ini rejected input: %s", e)
@@ -96,12 +108,40 @@ async def api_deploy_deckies(req: DeployIniRequest, admin: dict = Depends(requir
             mutate_interval=ini.mutate_interval or DEFAULT_MUTATE_INTERVAL,
         )
 
-    # The INI is the source of truth for *which* deckies exist this deploy.
-    # The old "merge with prior state" behaviour meant submitting `[decky1]`
-    # after a 3-decky run silently redeployed decky2/decky3 too — and then
-    # collided on their stale IPs ("Address already in use"). Full replace
-    # matches what the operator sees in the submitted config.
-    config.deckies = list(new_decky_configs)
+    # Two intents collapse onto one endpoint:
+    #
+    # * replace_fleet=True (explicit): INI is the complete desired fleet.
+    #   Anything absent is torn down by the reconciler. This is the path
+    #   for set-desired-state callers (CLI, declarative tooling).
+    # * replace_fleet=False (default, wizard path): INI is appended to the
+    #   existing fleet. The wizard only POSTs the new decky and would
+    #   otherwise see prior deckies silently deleted by the reconciler.
+    #
+    # The historical "always full replace" behaviour was added to dodge
+    # stale-IP collisions on redeploy ("Address already in use"); that's
+    # now scoped to replace_fleet=True.
+    if req.replace_fleet:
+        config.deckies = list(new_decky_configs)
+    else:
+        existing_names = {d.name for d in existing_deckies}
+        existing_ips = {d.ip for d in existing_deckies if d.ip}
+        for d in new_decky_configs:
+            if d.name in existing_names:
+                raise HTTPException(
+                    status_code=409,
+                    detail=(
+                        f"decky '{d.name}' already exists; "
+                        "submit with replace_fleet=true to overwrite the fleet"
+                    ),
+                )
+            if d.ip and d.ip in existing_ips:
+                raise HTTPException(
+                    status_code=409,
+                    detail=(
+                        f"IP {d.ip} is already in use by an existing decky"
+                    ),
+                )
+        config.deckies = existing_deckies + list(new_decky_configs)
 
     limits_state = await repo.get_state("config_limits")
     deployment_limit = limits_state.get("deployment_limit", 10) if limits_state else 10
@@ -152,8 +192,16 @@ async def api_deploy_deckies(req: DeployIniRequest, admin: dict = Depends(requir
     }
     await repo.set_state("deployment", new_state_payload)
 
+    # Lifecycle rows track THIS call's deployments only. In additive mode
+    # the existing deckies are already running and don't get a new
+    # lifecycle row — the caller polls /deckies/lifecycle for just the
+    # deckies they submitted. In replace mode every decky in the new
+    # config is being (re)deployed and gets a row.
+    new_names = {d.name for d in new_decky_configs}
     lifecycle_ids: dict[str, str] = {}
     for d in config.deckies:
+        if not req.replace_fleet and d.name not in new_names:
+            continue
         lid = await repo.create_lifecycle({
             "decky_name": d.name,
             "host_uuid": d.host_uuid,
@@ -174,7 +222,7 @@ async def api_deploy_deckies(req: DeployIniRequest, admin: dict = Depends(requir
 
     return {
         "message": (
-            f"Deploy accepted ({len(config.deckies)} decky/ies, mode={mode}). "
+            f"Deploy accepted ({len(lifecycle_ids)} decky/ies, mode={mode}). "
             f"Poll /deckies/lifecycle?ids=... for completion."
         ),
         "mode": mode,
diff --git a/decnet_web/src/components/DeckyFleet/DeployWizard.tsx b/decnet_web/src/components/DeckyFleet/DeployWizard.tsx
index 6ec8ded6..03ccb9e2 100644
--- a/decnet_web/src/components/DeckyFleet/DeployWizard.tsx
+++ b/decnet_web/src/components/DeckyFleet/DeployWizard.tsx
@@ -236,7 +236,7 @@ export const DeployWizard: React.FC<Props> = ({
     try {
       const res = await api.post<{ lifecycle_ids?: string[]; message?: string; mode?: string }>(
         '/deckies/deploy',
-        { ini_content: ini },
+        { ini_content: ini, replace_fleet: false },
       );
       const ids = res.data?.lifecycle_ids ?? [];
       setLifecycleIds(ids);
diff --git a/tests/api/fleet/test_deploy_additive.py b/tests/api/fleet/test_deploy_additive.py
new file mode 100644
index 00000000..17e7292d
--- /dev/null
+++ b/tests/api/fleet/test_deploy_additive.py
@@ -0,0 +1,162 @@
+"""POST /deckies/deploy additive vs replace_fleet semantics.
+
+Default behaviour (replace_fleet=False) appends the INI to the existing
+fleet so the wizard's "deploy one more decky" submit no longer wipes
+prior deckies. replace_fleet=True preserves the historical
+set-desired-state semantics for CLI / declarative callers.
+"""
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import pytest
+
+from decnet.web.dependencies import repo
+
+
+@pytest.fixture(autouse=True)
+def contract_test_mode(monkeypatch):
+    monkeypatch.setenv("DECNET_CONTRACT_TEST", "true")
+
+
+@pytest.fixture(autouse=True)
+def mock_network():
+    with patch("decnet.web.router.fleet.api_deploy_deckies.get_host_ip", return_value="192.168.1.100"):
+        with patch("decnet.web.router.fleet.api_deploy_deckies.detect_interface", return_value="eth0"):
+            with patch("decnet.web.router.fleet.api_deploy_deckies.detect_subnet", return_value=("192.168.1.0/24", "192.168.1.1")):
+                yield
+
+
+@pytest.fixture(autouse=True)
+async def _isolate_state():
+    for row in await repo.list_swarm_hosts():
+        await repo.delete_swarm_host(row["uuid"])
+    await repo.set_state("deployment", None)
+    yield
+    await repo.set_state("deployment", None)
+
+
+@pytest.mark.anyio
+async def test_additive_default_appends_to_existing_fleet(client, auth_token, monkeypatch):
+    """Two sequential deploys with replace_fleet unset → both deckies in state."""
+    monkeypatch.setenv("DECNET_MODE", "master")
+
+    r1 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-01]\nservices = ssh\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r1.status_code == 202, r1.text
+
+    r2 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-02]\nservices = http\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r2.status_code == 202, r2.text
+
+    committed = await repo.get_state("deployment")
+    assert committed is not None
+    names = {d["name"] for d in committed["config"]["deckies"]}
+    assert names == {"decky-01", "decky-02"}
+
+
+@pytest.mark.anyio
+async def test_additive_name_collision_returns_409(client, auth_token, monkeypatch):
+    """Re-submitting an existing decky name without replace_fleet → 409."""
+    monkeypatch.setenv("DECNET_MODE", "master")
+
+    r1 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-01]\nservices = ssh\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r1.status_code == 202, r1.text
+
+    r2 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-01]\nservices = http\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r2.status_code == 409, r2.text
+    assert "decky-01" in r2.json()["detail"]
+    assert "replace_fleet" in r2.json()["detail"]
+
+
+@pytest.mark.anyio
+async def test_additive_ip_collision_returns_409(client, auth_token, monkeypatch):
+    """A new decky pinned to an IP already in use → 409 with the IP."""
+    monkeypatch.setenv("DECNET_MODE", "master")
+
+    r1 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-01]\nservices = ssh\nip = 192.168.1.50\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r1.status_code == 202, r1.text
+
+    r2 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-02]\nservices = http\nip = 192.168.1.50\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r2.status_code == 409, r2.text
+    assert "192.168.1.50" in r2.json()["detail"]
+
+
+@pytest.mark.anyio
+async def test_replace_fleet_true_overwrites_existing(client, auth_token, monkeypatch):
+    """replace_fleet=True preserves the historical full-replace semantics."""
+    monkeypatch.setenv("DECNET_MODE", "master")
+
+    r1 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-01]\nservices = ssh\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r1.status_code == 202, r1.text
+
+    r2 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={
+            "ini_content": "[decky-02]\nservices = http\n",
+            "replace_fleet": True,
+        },
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r2.status_code == 202, r2.text
+
+    committed = await repo.get_state("deployment")
+    assert committed is not None
+    names = {d["name"] for d in committed["config"]["deckies"]}
+    assert names == {"decky-02"}
+
+
+@pytest.mark.anyio
+async def test_additive_lifecycle_ids_scoped_to_new_deckies(client, auth_token, monkeypatch):
+    """In additive mode the response's lifecycle_ids cover only the deckies
+    the caller submitted, not carryover. Operators polling
+    /deckies/lifecycle?ids=... see exactly what this call deployed."""
+    monkeypatch.setenv("DECNET_MODE", "master")
+
+    r1 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-01]\nservices = ssh\n[decky-02]\nservices = http\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r1.status_code == 202, r1.text
+    assert len(r1.json()["lifecycle_ids"]) == 2
+
+    r2 = await client.post(
+        "/api/v1/deckies/deploy",
+        json={"ini_content": "[decky-03]\nservices = ssh\n"},
+        headers={"Authorization": f"Bearer {auth_token}"},
+    )
+    assert r2.status_code == 202, r2.text
+    body2 = r2.json()
+    assert len(body2["lifecycle_ids"]) == 1
+
+    committed = await repo.get_state("deployment")
+    assert committed is not None
+    names = {d["name"] for d in committed["config"]["deckies"]}
+    assert names == {"decky-01", "decky-02", "decky-03"}