diff --git a/.changeset/gentle-jaguars-munch.md b/.changeset/gentle-jaguars-munch.md
new file mode 100644
index 000000000..8419e74b2
--- /dev/null
+++ b/.changeset/gentle-jaguars-munch.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1661
+---
+**`gsd install`/upgrade now recovers a malformed `~/.gsd/defaults.json` instead of leaving it broken** — a `defaults.json` containing a valid-JSON-but-non-object value (`null`, `[]`, a number, or a string) bypassed the parse `catch` and flowed through unrecovered: `null` threw a TypeError (swallowed by the outer guard, logging a confusing "Could not write" warning and leaving the file as `null`), while `[]`/`42`/`"str"` silently kept their broken shape on every install. The non-Claude finishInstall step now resets any non-object parse result to a fresh `{}` before reading/writing it, so the file is repaired and `resolve_model_ids` defaults normally.
diff --git a/bin/install.js b/bin/install.js
index 37b893f8c..d9cbfeacf 100755
--- a/bin/install.js
+++ b/bin/install.js
@@ -11322,6 +11322,14 @@ function finishInstall(settingsPath, settings, statuslineCommand, shouldInstallS
       fs.mkdirSync(gsdDir, { recursive: true });
       let defaults = {};
       try { defaults = JSON.parse(fs.readFileSync(defaultsPath, 'utf8')); } catch { /* new file */ }
+      // Recover a malformed (valid-JSON-but-non-object) defaults.json to a fresh object so
+      // the write below succeeds and the file is no longer broken. Without this, `null` /
+      // `[]` / a number / a string bypass the parse catch and either throw a TypeError on
+      // property access (swallowed by the outer try/catch, leaving the file broken) or get
+      // a property set that won't round-trip through JSON.stringify. (#1657)
+      if (defaults === null || typeof defaults !== 'object' || Array.isArray(defaults)) {
+        defaults = {};
+      }
       // Three-valued domain: false/absent → aliases; true → full IDs; "omit" → ''.
       // Honor ONLY an explicit canonical `true` opt-in (full model IDs) and an existing
       // "omit"; default everything else — absent, falsy, OR any non-canonical value — to
diff --git a/tests/bug-410-install-defaults-test-mode-guard.test.cjs b/tests/bug-410-install-defaults-test-mode-guard.test.cjs
index d0e73da92..0d5061ed7 100644
--- a/tests/bug-410-install-defaults-test-mode-guard.test.cjs
+++ b/tests/bug-410-install-defaults-test-mode-guard.test.cjs
@@ -254,3 +254,40 @@ describe('Bug #1569: non-Claude finishInstall preserves explicit resolve_model_i
     });
   });
 });
+
+// Bug #1657 — finishInstall reads ~/.gsd/defaults.json with JSON.parse but did not
+// validate the result is a plain object. A valid-JSON-but-non-object value (null, [],
+// 42, "str") bypassed the catch and flowed through, leaving the malformed file on disk
+// unrecovered (and, for null, throwing a TypeError swallowed by the outer try/catch).
+// Folded into the owning install-defaults test (no new top-level bug-NNNN file).
+describe('Bug #1657: finishInstall recovers a malformed (non-object) defaults.json', () => {
+  function seedDefaultsRaw(raw) {
+    fs.mkdirSync(GSD_DIR, { recursive: true });
+    fs.writeFileSync(DEFAULTS_PATH, raw, 'utf8');
+  }
+  function runAndRead(runtime) {
+    const saved = process.env.GSD_TEST_MODE;
+    delete process.env.GSD_TEST_MODE;
+    const log = console.log; console.log = () => {};
+    let threw = null;
+    try {
+      installModule.finishInstall(SETTINGS_PATH, {}, null, false, runtime, true, null);
+    } catch (e) { threw = e.message; } finally { console.log = log; process.env.GSD_TEST_MODE = saved; }
+    let after = null;
+    try { after = JSON.parse(fs.readFileSync(DEFAULTS_PATH, 'utf8')); } catch (e) { after = 'UNPARSEABLE: ' + e.message; }
+    return { threw, after };
+  }
+
+  for (const [label, raw] of [['null', 'null'], ['array', '[]'], ['number', '42'], ['string', '"oops"']]) {
+    test(`seed ${label} (${raw}) recovers to a valid object with resolve_model_ids:omit`, () => {
+      seedDefaultsRaw(raw);
+      const { threw, after } = runAndRead('codex');
+      assert.equal(threw, null, `must not throw for seed ${label} (got: ${threw})`);
+      assert.equal(
+        after !== null && typeof after === 'object' && !Array.isArray(after) && after.resolve_model_ids === 'omit',
+        true,
+        `seed ${label} must recover to { resolve_model_ids: 'omit' }, got: ${JSON.stringify(after)}`,
+      );
+    });
+  }
+});