diff --git a/packages/fints/src/__tests__/test-dialog.ts b/packages/fints/src/__tests__/test-dialog.ts
index ab6d83e..67235c7 100644
--- a/packages/fints/src/__tests__/test-dialog.ts
+++ b/packages/fints/src/__tests__/test-dialog.ts
@@ -2,6 +2,7 @@ import { Dialog, DialogConfig } from "../dialog";
 import { Request } from "../request";
 import { TanRequiredError } from "../errors/tan-required-error";
 import { ResponseError } from "../errors/response-error";
+import { DecoupledTanState } from "../decoupled-tan";
 
 describe("Dialog", () => {
     const baseConfig: DialogConfig = { blz: "1", name: "user", pin: "123", systemId: "0" } as any;
@@ -53,6 +54,24 @@ describe("Dialog", () => {
         }
     });
 
+    test("send throws decoupled TanRequiredError for 3955 + HITAN without 0030", async () => {
+        const hitan = { transactionReference: "ref", challengeText: "text", challengeMedia: Buffer.alloc(0) };
+        const connection = {
+            send: jest.fn().mockResolvedValue({
+                success: true,
+                returnValues: () => new Map([["3955", { message: "Sicherheitsfreigabe erfolgt über anderen Kanal." }]]),
+                findSegment: () => hitan,
+                dialogId: "4711",
+            }),
+        };
+        const dialog = new Dialog(baseConfig, connection as any);
+        const error = (await dialog.send(new Request(baseConfig)).catch((err) => err)) as TanRequiredError;
+        expect(error).toBeInstanceOf(TanRequiredError);
+        expect(error.decoupledTanState).toBe(DecoupledTanState.INITIATED);
+        expect(error.context?.returnCode).toBe("3955");
+        expect(dialog.dialogId).toBe("4711");
+    });
+
     test("capabilities getter reflects fields set during sync", () => {
         const dialog = new Dialog(baseConfig, {} as any);
 
diff --git a/packages/fints/src/dialog.ts b/packages/fints/src/dialog.ts
index b05a34d..f3f6b46 100644
--- a/packages/fints/src/dialog.ts
+++ b/packages/fints/src/dialog.ts
@@ -260,9 +260,19 @@ export class Dialog extends DialogConfig {
         if (!response.success) {
             throw new ResponseError(response);
         }
-        if (response.returnValues().has("0030")) {
-            const hitan = response.findSegment(HITAN);
-            const returnValue = response.returnValues().get("0030");
+        const returnValues = response.returnValues();
+        const hasTanRequiredCode = returnValues.has("0030");
+        let hitan: HITAN | undefined;
+        let tanRequiredCode: "0030" | "3955" | undefined;
+        if (hasTanRequiredCode) {
+            tanRequiredCode = "0030";
+        } else if (returnValues.has("3955")) {
+            hitan = response.findSegment(HITAN);
+            tanRequiredCode = hitan ? "3955" : undefined;
+        }
+        if (tanRequiredCode) {
+            hitan = hitan ?? response.findSegment(HITAN);
+            const returnValue = returnValues.get(tanRequiredCode);
 
             // Determine which segment triggered the TAN requirement
             const triggeringSegment = request.segments.length > 0 ? request.segments[0].type : undefined;
@@ -270,13 +280,18 @@ export class Dialog extends DialogConfig {
             // Check for decoupled TAN indicators per FinTS 3.0 PINTAN specification:
             // - "3956": Indicates strong customer authentication (SCA) is pending on trusted device
             // - "3076": PSD2-mandated strong customer authentication required
-            // When either code is present alongside "0030", it signals decoupled TAN flow
-            // where the user must approve the transaction in a separate app (e.g., mobile banking)
-            const returnValues = response.returnValues();
-            const isDecoupled = returnValues.has("3956") || returnValues.has("3076");
+            // - "3955": Security approval takes place in another channel
+            // These codes indicate a decoupled TAN flow where the user must approve the
+            // transaction in a separate app or device. "3955" can either accompany "0030"
+            // or be the primary TAN-required return code itself when returned with HITAN.
+            const isDecoupled = returnValues.has("3956") || returnValues.has("3076") || returnValues.has("3955");
+            const fallbackMessage =
+                tanRequiredCode === "3955"
+                    ? "TAN required: Security approval via alternate channel (3955)"
+                    : "TAN required";
 
             const error = new TanRequiredError(
-                returnValue.message,
+                returnValue?.message ?? fallbackMessage,
                 hitan.transactionReference,
                 hitan.challengeText,
                 hitan.challengeMedia,
@@ -284,7 +299,7 @@ export class Dialog extends DialogConfig {
                 TanProcessStep.CHALLENGE_RESPONSE_NEEDED,
                 triggeringSegment,
                 {
-                    returnCode: "0030",
+                    returnCode: tanRequiredCode,
                     requestSegments: request.segments.map((s) => s.type),
                 },
             );