diff --git a/src/apply.ts b/src/apply.ts
index 5302970..e083f2d 100644
--- a/src/apply.ts
+++ b/src/apply.ts
@@ -113,14 +113,46 @@ export function applyHostnameOverlay(
 // on every diff: rotating ONLY the password in the store produces no field diff,
 // therefore no PATCH, and `cast diff` says the password was not compared rather
 // than implying it matches.
+const BASIC_AUTH_KEYS = [
+  "is_http_basic_auth_enabled",
+  "http_basic_auth_username",
+  "http_basic_auth_password",
+] as const;
+
 export function completeBasicAuth(
   fields: Record<string, unknown>,
   spec: Desired | undefined,
 ): Record<string, unknown> {
-  if (fields.is_http_basic_auth_enabled !== true) return fields;
   const declared = spec?.fields ?? {};
+
+  // Only complete a payload that is ALREADY touching basic auth. This is what
+  // keeps the function from manufacturing a write, and it is the reason the
+  // honest limit above still holds.
+  if (!BASIC_AUTH_KEYS.some((k) => fields[k] !== undefined)) return fields;
+
+  // Read the INTENT from the declared spec, not from the payload. Keying on
+  // `fields.is_http_basic_auth_enabled === true` was the bug (cast#76 review):
+  // the toggle is absent from an update body exactly when it already MATCHES,
+  // so on username-only drift — auth on at both ends, username edited in the
+  // UI — computeDiff emits `http_basic_auth_username` alone, the guard returned
+  // early, and the PATCH went out as a lone username. Coolify requires the
+  // whole triple on any write that enables basic auth, so that is a 422
+  // mid-run: the failure this function exists to prevent, on the one path it
+  // was not looking at.
+  //
+  // A payload that explicitly DISABLES (toggle === false) is left alone —
+  // completing it with credentials would be manufacturing the opposite write.
+  const enabled =
+    fields.is_http_basic_auth_enabled === true ||
+    (fields.is_http_basic_auth_enabled === undefined &&
+      declared.is_http_basic_auth_enabled === true);
+  if (!enabled) return fields;
+
   const completed = { ...fields };
-  for (const k of ["http_basic_auth_username", "http_basic_auth_password"]) {
+  // The toggle is completed too, not just the credentials: Coolify's presence
+  // rule is about the write as a whole, and a username+password PATCH with no
+  // toggle asks it to infer what cast can simply state.
+  for (const k of BASIC_AUTH_KEYS) {
     if (completed[k] === undefined && declared[k] !== undefined)
       completed[k] = declared[k];
   }
diff --git a/src/cli.ts b/src/cli.ts
index dfa9069..cd915b2 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -2833,22 +2833,42 @@ export function applicationApiFields(
   // any future caller assembling a payload by hand. Failing here costs one
   // exception; failing at Coolify costs a 422 in the middle of a run that has
   // already created a project, an environment and possibly a database.
-  if (rest.is_http_basic_auth_enabled === true) {
+  // Any write that TOUCHES basic auth without disabling it must carry the whole
+  // triple. Keying this on `=== true` alone shared apply's blind spot: a PATCH
+  // body of `{http_basic_auth_username}` — the username-only drift — has no
+  // toggle to be true, so the belt never tightened either. An explicit
+  // `false` is a legitimate disable and needs no credentials.
+  const touchesBasicAuth = (
+    [
+      "is_http_basic_auth_enabled",
+      "http_basic_auth_username",
+      "http_basic_auth_password",
+    ] as const
+  ).some((k) => rest[k] !== undefined);
+  if (touchesBasicAuth && rest.is_http_basic_auth_enabled !== false) {
     const missing = (
-      ["http_basic_auth_username", "http_basic_auth_password"] as const
+      [
+        "is_http_basic_auth_enabled",
+        "http_basic_auth_username",
+        "http_basic_auth_password",
+      ] as const
     ).filter((k) => {
       const v = rest[k];
+      if (k === "is_http_basic_auth_enabled") return v !== true;
       return typeof v !== "string" || v === "";
     });
     if (missing.length > 0) {
       throw new Error(
         [
-          `refusing to enable HTTP basic auth without ${missing.join(" and ")}`,
+          `refusing a partial HTTP basic auth write — missing ${missing.join(" and ")}`,
           "",
-          "Coolify requires a username AND a password whenever basic auth is enabled, and",
-          "would answer 422 mid-run. Half-configured basic auth protects nothing anyway:",
-          "declare both under the application's `basic_auth:` (the password as a ${REF}",
-          "held by the environment's age store), or set `basic_auth.enabled: false`.",
+          "Coolify requires the toggle, a username AND a password on any write that",
+          "enables basic auth, and would answer 422 mid-run. A write carrying only some",
+          "of the three is that 422 waiting to happen — including a lone username, which",
+          "is what a username-only drift produces if nothing completes it. Half-configured",
+          "basic auth protects nothing anyway: declare the pair under the application's",
+          "`basic_auth:` (the password as a ${REF} held by the environment's age store),",
+          "or set `basic_auth.enabled: false`.",
         ].join("\n"),
       );
     }
diff --git a/test/apply.test.ts b/test/apply.test.ts
index 41a64cc..ff4d926 100644
--- a/test/apply.test.ts
+++ b/test/apply.test.ts
@@ -577,6 +577,49 @@ describe("completeBasicAuth", () => {
     ).toBe("s3cret");
   });
 
+  // The case the #76 review found, and the one the test above only LOOKED like
+  // it covered: that payload carries the toggle, so it never exercised the
+  // guard. When basic auth is already on at both ends and only the username is
+  // edited in the UI, the toggle MATCHES — so computeDiff emits no fieldDiff
+  // for it and the payload arrives as a lone username. The old guard keyed on
+  // the toggle being present and returned early, and the PATCH went out
+  // incomplete: a 422 mid-run, which is the exact failure this function exists
+  // to prevent.
+  it("completes the whole triple from a lone username — no toggle in the payload", () => {
+    expect(completeBasicAuth({ http_basic_auth_username: "ops" }, spec)).toEqual(
+      {
+        is_http_basic_auth_enabled: true,
+        http_basic_auth_username: "ops",
+        http_basic_auth_password: "s3cret",
+      },
+    );
+  });
+
+  // Same shape, other credential: a stored-password rotation riding along.
+  it("completes from a lone password too", () => {
+    expect(
+      completeBasicAuth({ http_basic_auth_password: "rotated" }, spec),
+    ).toEqual({
+      is_http_basic_auth_enabled: true,
+      http_basic_auth_username: "ops",
+      http_basic_auth_password: "rotated",
+    });
+  });
+
+  // Intent comes from the SPEC, so a spec that does not enable basic auth must
+  // not have credentials completed into its payload — otherwise reading intent
+  // from the declaration would trade one silent wrong write for another.
+  it("does not complete when the spec does not enable basic auth", () => {
+    const off: Desired = {
+      kind: "application",
+      name: "admin",
+      fields: { is_http_basic_auth_enabled: false },
+    };
+    expect(completeBasicAuth({ http_basic_auth_username: "ops" }, off)).toEqual({
+      http_basic_auth_username: "ops",
+    });
+  });
+
   it("leaves a payload that is not enabling basic auth completely alone", () => {
     // The load-bearing half: this must not MANUFACTURE a write. A run where
     // nothing about basic auth drifted sends nothing about basic auth.
diff --git a/test/wire.test.ts b/test/wire.test.ts
index 02deeeb..9512a59 100644
--- a/test/wire.test.ts
+++ b/test/wire.test.ts
@@ -86,9 +86,40 @@ describe("applicationApiFields — basic auth (#76)", () => {
     it(`refuses to enable basic auth with ${what}`, () => {
       expect(() =>
         applicationApiFields({ is_http_basic_auth_enabled: true, ...fields }),
-      ).toThrow(/refusing to enable HTTP basic auth without/);
+      ).toThrow(/refusing a partial HTTP basic auth write/);
     });
   }
+
+  // The hole the #76 review found, at the wire: a username-only drift produces
+  // a PATCH with no toggle at all, so a guard keyed on `=== true` never looked
+  // at it. These are the shapes apply must never hand over uncompleted.
+  for (const [what, fields] of [
+    ["a lone username", { http_basic_auth_username: "ops" }],
+    ["a lone password", { http_basic_auth_password: "s3cret" }],
+    [
+      "credentials with no toggle",
+      { http_basic_auth_username: "ops", http_basic_auth_password: "s3cret" },
+    ],
+  ] as const) {
+    it(`refuses ${what} — no toggle is not an exemption`, () => {
+      expect(() => applicationApiFields({ ...fields })).toThrow(
+        /refusing a partial HTTP basic auth write/,
+      );
+    });
+  }
+
+  // A disable is a legitimate one-key write: it needs no credentials, and
+  // demanding them would make turning basic auth off impossible.
+  it("allows an explicit disable to travel alone", () => {
+    expect(() =>
+      applicationApiFields({ is_http_basic_auth_enabled: false }),
+    ).not.toThrow();
+  });
+
+  // And a payload that says nothing about basic auth is not a basic-auth write.
+  it("ignores a payload that does not mention basic auth at all", () => {
+    expect(() => applicationApiFields({ is_static: true })).not.toThrow();
+  });
 });
 
 describe("databaseApiFields", () => {