diff --git a/electron/host-service/scm-runtime.ts b/electron/host-service/scm-runtime.ts
index a09c5c83..7411f3d0 100644
--- a/electron/host-service/scm-runtime.ts
+++ b/electron/host-service/scm-runtime.ts
@@ -17,7 +17,10 @@ import type {
   GraphResult,
 } from "../../src/lib/git-graph/types";
 import { parseWorktreePathByBranch } from "../../src/lib/source-control-worktrees";
-import type { PrMergeMethod } from "../../src/lib/pr-status";
+import type {
+  ConcretePrMergeMethod,
+  PrMergeMethod,
+} from "../../src/lib/pr-status";
 import type { DetachedCheckoutResult } from "../main/types";
 import {
   buildSourceControlDiffPreview,
@@ -1628,15 +1631,13 @@ export async function mergeScmPr(args: {
   if (!authResult.ok) {
     return { ok: false, stderr: "GitHub CLI is not authenticated." };
   }
-  const method = args.method ?? "default";
+  const method = await resolveScmMergeMethod({
+    method: args.method,
+    cwd: args.cwd,
+  });
   const result = await runCommandArgs({
     command: "gh",
-    commandArgs: [
-      "pr",
-      "merge",
-      ...(method === "default" ? [] : [`--${method}`]),
-      "--delete-branch",
-    ],
+    commandArgs: buildMergePullRequestArgs(method),
     cwd: args.cwd,
   });
   invalidateCachedGhAuthOnFailure(result, args.cwd);
@@ -1707,15 +1708,15 @@ export function buildCreatePullRequestArgs(args: {
 }
 
 export function buildAutoMergePullRequestArgs(
-  method: PrMergeMethod = "default",
+  method: ConcretePrMergeMethod = "squash",
 ) {
-  return [
-    "pr",
-    "merge",
-    "--auto",
-    ...(method === "default" ? [] : [`--${method}`]),
-    "--delete-branch",
-  ];
+  return ["pr", "merge", "--auto", `--${method}`, "--delete-branch"];
+}
+
+export function buildMergePullRequestArgs(
+  method: ConcretePrMergeMethod = "squash",
+) {
+  return ["pr", "merge", `--${method}`, "--delete-branch"];
 }
 
 export function classifyAutoMergeFailure(stderr: string) {
@@ -1798,6 +1799,49 @@ export async function fetchRepoMergeSettings(args: { cwd?: string }) {
   }
 }
 
+const MERGE_METHOD_PREFERENCE = ["squash", "merge", "rebase"] as const;
+
+/**
+ * `gh pr merge` refuses to run without an explicit strategy flag when it is not
+ * attached to a TTY, so "default" has to be resolved to a concrete strategy
+ * before the command is spawned. The repository's allowed merge methods decide
+ * which one wins; when they cannot be read we fall back to squash.
+ */
+export function pickAllowedMergeMethod(args: {
+  method?: PrMergeMethod;
+  settings?: {
+    squashMergeAllowed?: boolean;
+    mergeCommitAllowed?: boolean;
+    rebaseMergeAllowed?: boolean;
+  };
+}): ConcretePrMergeMethod {
+  if (args.method && args.method !== "default") {
+    return args.method;
+  }
+  const allowed: Record<ConcretePrMergeMethod, boolean> = {
+    squash: args.settings?.squashMergeAllowed ?? true,
+    merge: args.settings?.mergeCommitAllowed ?? true,
+    rebase: args.settings?.rebaseMergeAllowed ?? true,
+  };
+  return MERGE_METHOD_PREFERENCE.find((method) => allowed[method]) ?? "squash";
+}
+
+async function resolveScmMergeMethod(args: {
+  method?: PrMergeMethod;
+  cwd?: string;
+}): Promise<ConcretePrMergeMethod> {
+  if (args.method && args.method !== "default") {
+    return args.method;
+  }
+  const settings = await fetchRepoMergeSettings({ cwd: args.cwd }).catch(
+    () => undefined,
+  );
+  return pickAllowedMergeMethod({
+    method: args.method,
+    settings: settings?.ok ? settings : undefined,
+  });
+}
+
 export async function createScmPullRequest(args: {
   title: string;
   body?: string;
@@ -1857,9 +1901,13 @@ export async function createScmPullRequest(args: {
     return { ok: true, prUrl, autoMergeEnabled: false, stderr: "" };
   }
 
+  const resolvedMergeMethod = await resolveScmMergeMethod({
+    method: mergeMethod,
+    cwd,
+  });
   const autoMergeResult = await runCommandArgs({
     command: "gh",
-    commandArgs: buildAutoMergePullRequestArgs(mergeMethod),
+    commandArgs: buildAutoMergePullRequestArgs(resolvedMergeMethod),
     cwd,
   });
   invalidateCachedGhAuthOnFailure(autoMergeResult, cwd);
@@ -1870,12 +1918,7 @@ export async function createScmPullRequest(args: {
     if (failure === "clean-status") {
       const mergeResult = await runCommandArgs({
         command: "gh",
-        commandArgs: [
-          "pr",
-          "merge",
-          ...(mergeMethod === "default" ? [] : [`--${mergeMethod}`]),
-          "--delete-branch",
-        ],
+        commandArgs: buildMergePullRequestArgs(resolvedMergeMethod),
         cwd,
       });
       invalidateCachedGhAuthOnFailure(mergeResult, cwd);
diff --git a/src/components/layout/TopBarOpenPR.tsx b/src/components/layout/TopBarOpenPR.tsx
index 670a943f..ba99f212 100644
--- a/src/components/layout/TopBarOpenPR.tsx
+++ b/src/components/layout/TopBarOpenPR.tsx
@@ -1604,9 +1604,11 @@ export function TopBarOpenPR(props: { noDragStyle: CSSProperties }) {
     }
 
     setStep("action");
+    // "default" is resolved to a repository-allowed strategy in the host
+    // service: `gh pr merge` requires an explicit --merge/--rebase/--squash
+    // flag because it runs without a TTY here.
     const result = await mergePr({
-      method:
-        createPrMergeMethod === "default" ? undefined : createPrMergeMethod,
+      method: createPrMergeMethod,
       cwd: workspaceCwd,
     });
     setStep("idle");
diff --git a/src/components/layout/TopBarOpenPR.utils.ts b/src/components/layout/TopBarOpenPR.utils.ts
index de859045..71e098cc 100644
--- a/src/components/layout/TopBarOpenPR.utils.ts
+++ b/src/components/layout/TopBarOpenPR.utils.ts
@@ -1,12 +1,12 @@
 import { isReasonablePullRequestTitle } from "@/lib/source-control-pr";
-import type { PrMergeMethod } from "@/lib/pr-status";
+import type { ConcretePrMergeMethod, PrMergeMethod } from "@/lib/pr-status";
 
 export type CreatePrDialogStep =
   "idle" | "loading" | "ready" | "committing" | "reviewing" | "pushing" | "creating-pr" | "action";
 
 export type CreatePrSubmitAction = "pr";
 
-export type ConcretePrMergeMethod = Exclude<PrMergeMethod, "default">;
+export type { ConcretePrMergeMethod };
 
 export interface RepoMergeSettings {
   squashMergeAllowed: boolean;
diff --git a/src/lib/pr-status.ts b/src/lib/pr-status.ts
index b379e07b..48696798 100644
--- a/src/lib/pr-status.ts
+++ b/src/lib/pr-status.ts
@@ -19,6 +19,12 @@ export type WorkspacePrStatus =
 /** Merge strategy used when Create PR queues GitHub auto-merge. */
 export type PrMergeMethod = "default" | "merge" | "squash" | "rebase";
 
+/**
+ * Strategy actually handed to `gh pr merge`. "default" is not usable there:
+ * without a TTY the CLI requires one of --merge/--rebase/--squash.
+ */
+export type ConcretePrMergeMethod = Exclude<PrMergeMethod, "default">;
+
 /** Raw payload returned by the `scm:get-pr-status` IPC handler. */
 export interface GitHubPrPayload {
   number: number;
diff --git a/tests/git-graph-tag-args.test.ts b/tests/git-graph-tag-args.test.ts
index 7955a50c..ff2268a3 100644
--- a/tests/git-graph-tag-args.test.ts
+++ b/tests/git-graph-tag-args.test.ts
@@ -26,11 +26,12 @@ describe("buildCreatePullRequestArgs", () => {
 });
 
 describe("buildAutoMergePullRequestArgs", () => {
-  test("lets GitHub select the repository default merge strategy", () => {
+  test("always passes an explicit strategy because gh needs one without a TTY", () => {
     expect(buildAutoMergePullRequestArgs()).toEqual([
       "pr",
       "merge",
       "--auto",
+      "--squash",
       "--delete-branch",
     ]);
   });
diff --git a/tests/scm-runtime-pr.test.ts b/tests/scm-runtime-pr.test.ts
index 0a22b0fe..eafede01 100644
--- a/tests/scm-runtime-pr.test.ts
+++ b/tests/scm-runtime-pr.test.ts
@@ -1,12 +1,50 @@
 import { describe, expect, test } from "bun:test";
 import { ensureGhAuth, invalidateGhAuthCache } from "../electron/host-service/gh-auth";
-import { buildAutoMergePullRequestArgs, classifyAutoMergeFailure } from "../electron/host-service/scm-runtime";
+import {
+  buildAutoMergePullRequestArgs,
+  buildMergePullRequestArgs,
+  classifyAutoMergeFailure,
+  pickAllowedMergeMethod,
+} from "../electron/host-service/scm-runtime";
 
 describe("Create PR SCM runtime", () => {
   test("builds a concrete auto-merge command", () => {
     expect(buildAutoMergePullRequestArgs("squash")).toEqual(["pr", "merge", "--auto", "--squash", "--delete-branch"]);
   });
 
+  test("merge command always carries an explicit strategy flag", () => {
+    // Regression: `gh pr merge --delete-branch` exits with
+    // "--merge, --rebase, or --squash required when not running interactively",
+    // so the Merge PR action must never omit the strategy.
+    expect(buildMergePullRequestArgs("merge")).toEqual(["pr", "merge", "--merge", "--delete-branch"]);
+    expect(buildMergePullRequestArgs()).toEqual(["pr", "merge", "--squash", "--delete-branch"]);
+  });
+
+  test("resolves the default merge method against repository settings", () => {
+    expect(pickAllowedMergeMethod({ method: "rebase" })).toBe("rebase");
+    expect(pickAllowedMergeMethod({ method: "default" })).toBe("squash");
+    expect(pickAllowedMergeMethod({})).toBe("squash");
+    expect(
+      pickAllowedMergeMethod({
+        method: "default",
+        settings: { squashMergeAllowed: false, mergeCommitAllowed: true, rebaseMergeAllowed: true },
+      }),
+    ).toBe("merge");
+    expect(
+      pickAllowedMergeMethod({
+        method: "default",
+        settings: { squashMergeAllowed: false, mergeCommitAllowed: false, rebaseMergeAllowed: true },
+      }),
+    ).toBe("rebase");
+    // An explicit user choice is honored even if the repo reports it as disallowed.
+    expect(
+      pickAllowedMergeMethod({
+        method: "squash",
+        settings: { squashMergeAllowed: false, mergeCommitAllowed: true, rebaseMergeAllowed: true },
+      }),
+    ).toBe("squash");
+  });
+
   test("classifies graceful auto-merge fallback cases", () => {
     expect(classifyAutoMergeFailure("GraphQL: Pull request is in clean status")).toBe("clean-status");
     expect(classifyAutoMergeFailure("Auto-merge is disabled for this repository")).toBe("unsupported");