diff --git a/console/src/deploy.ts b/console/src/deploy.ts
index 4c0f6cf..8e2b56e 100644
--- a/console/src/deploy.ts
+++ b/console/src/deploy.ts
@@ -13,7 +13,7 @@
 // after a confirmed successful provision, never before or speculatively.
 
 import type { Source } from "./source";
-import { appendMember, appendFleetBlock } from "./fleetToml";
+import { appendMember, appendFleetBlock, fleetBlockExists } from "./fleetToml";
 import { appendK8sFleetBlock } from "./fleetsK8sToml";
 
 type Invoke = <T>(cmd: string, args?: Record<string, unknown>) => Promise<T>;
@@ -488,12 +488,35 @@ export function initDeployPanel(deps: DeployPanelDeps): DeployPanelHandle | null
 
   // Step 1 (new-fleet only): collect the fleet identity, then reveal the
   // shared Compose step — 7.5.1's "Next: first instance →".
-  identityForm.addEventListener("submit", (ev) => {
+  identityForm.addEventListener("submit", async (ev) => {
     ev.preventDefault();
-    if (!nameInput.value.trim()) {
+    const fleetName = nameInput.value.trim();
+    if (!fleetName) {
       setStatus(identityStatusEl, "fleet name is required", "err");
       return;
     }
+    // Reject a colliding name here, before Step 2 provisions anything —
+    // appendFleetBlock/appendK8sFleetBlock always append a brand-new
+    // `[fleet.<name>]` block, so reusing an existing name would otherwise
+    // only surface as a duplicate-key TOML parse error *after* the instance
+    // was already deployed (it has no partial/merge fallback; "add instance
+    // to an existing k8s fleet" isn't wired through this wizard yet either —
+    // see the isK8s check in the deploy submit handler below).
+    const isK8s = providerSel.value === "k8s";
+    try {
+      const current = await (isK8s ? deps.source.k8sFleetConfig() : deps.source.fleetConfig());
+      if (fleetBlockExists(current.text, fleetName)) {
+        setStatus(
+          identityStatusEl,
+          `a fleet named "${fleetName}" already exists — use "Add instance" on that fleet instead`,
+          "err",
+        );
+        return;
+      }
+    } catch (e) {
+      setStatus(identityStatusEl, `fleet name check unavailable: ${errText(e)}`, "err");
+      return;
+    }
     identityForm.hidden = true;
     composeSection.hidden = false;
     if (composeHeading) composeHeading.textContent = "Step 2 — first instance";
diff --git a/console/src/fleetToml.test.ts b/console/src/fleetToml.test.ts
index f3d1c19..50b6527 100644
--- a/console/src/fleetToml.test.ts
+++ b/console/src/fleetToml.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { appendMember, appendFleetBlock } from "./fleetToml";
+import { appendMember, appendFleetBlock, fleetBlockExists } from "./fleetToml";
 
 describe("appendMember", () => {
   const text = `default_cluster = "oab"
@@ -51,6 +51,26 @@ members = ["oab-default-mira-1"]
   });
 });
 
+describe("fleetBlockExists", () => {
+  const text = `default_cluster = "oab"
+
+[fleet.oab-prod-orca]
+members = ["oab-default-agent-1"]
+`;
+
+  it("is true when a [fleet.<name>] block is present", () => {
+    expect(fleetBlockExists(text, "oab-prod-orca")).toBe(true);
+  });
+
+  it("is false when the name isn't present", () => {
+    expect(fleetBlockExists(text, "no-such-fleet")).toBe(false);
+  });
+
+  it("is false against an empty file", () => {
+    expect(fleetBlockExists("", "oab-prod-orca")).toBe(false);
+  });
+});
+
 describe("appendFleetBlock", () => {
   it("appends a new [fleet.<name>] block with the given fields", () => {
     const out = appendFleetBlock("default_cluster = \"oab\"\n", {
diff --git a/console/src/fleetToml.ts b/console/src/fleetToml.ts
index dc4a348..89695be 100644
--- a/console/src/fleetToml.ts
+++ b/console/src/fleetToml.ts
@@ -32,6 +32,16 @@ function findFleetBlock(
   return { start, end, headerEnd };
 }
 
+// Whether a `[fleet.<name>]` block already exists — used by the "New fleet"
+// wizard (deploy.ts) to reject a colliding name *before* provisioning an
+// instance, rather than discovering the collision only when appendFleetBlock/
+// appendK8sFleetBlock's blind append produces a second `[fleet.<name>]`
+// header and the resulting TOML fails to parse (studio: duplicate-key crash
+// after the instance was already deployed).
+export function fleetBlockExists(text: string, name: string): boolean {
+  return findFleetBlock(text, name) !== null;
+}
+
 // Append `member` to an existing fleet's `members = [...]` array (single-line
 // TOML array — the only form fleets.toml is written in today, per the ADR's
 // mockups). A no-op if the member is already listed or the fleet isn't found.