diff --git a/.changeset/965-json-errors-unexpected-throw.md b/.changeset/965-json-errors-unexpected-throw.md
new file mode 100644
index 000000000..35db58e9a
--- /dev/null
+++ b/.changeset/965-json-errors-unexpected-throw.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 987
+---
+**`--json-errors` now emits a structured error even when a handler throws unexpectedly** — an unexpected (non-`ExitError`) throw fell through to a raw stack trace on stderr, breaking SDK structured-error parsing. (#965)
diff --git a/src/cli-exit.cts b/src/cli-exit.cts
index 7e4a0afda..ff7f93aa3 100644
--- a/src/cli-exit.cts
+++ b/src/cli-exit.cts
@@ -1,3 +1,8 @@
+import fs from 'node:fs';
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import ioModule = require('./io.cjs');
+const { getJsonErrorMode, ERROR_REASON } = ioModule;
+
 /**
  * Error carrying a process exit code. CLI logic throws this instead of calling
  * process.exit() (banned by n/no-process-exit); runMain() translates it into
@@ -20,7 +25,8 @@ class ExitError extends Error {
  * process.on('exit') cleanup still fires). main may be sync or async:
  *   number return -> process.exitCode = it
  *   thrown ExitError -> process.exitCode = err.code (+ stderr err.message if hasUserMessage && code!=0)
- *   other throw -> stderr stack + process.exitCode = 1
+ *   other throw -> when json-error mode is active, emits structured { ok:false, reason, message }
+ *                  to stderr; otherwise writes raw stack trace. exit code = 1 in either case.
  */
 function runMain(main: () => number | void | Promise<number | void>): void {
   Promise.resolve()
@@ -32,8 +38,18 @@ function runMain(main: () => number | void | Promise<number | void>): void {
         process.exitCode = err.code;
         return;
       }
-      const e = err as Error;
-      process.stderr.write(`${e && e.stack ? e.stack : String(err)}\n`);
+      if (getJsonErrorMode()) {
+        const e = err as Error;
+        const payload = JSON.stringify({
+          ok: false,
+          reason: ERROR_REASON.SDK_FAIL_FAST,
+          message: (e && e.message) ? e.message : String(err),
+        }) + '\n';
+        fs.writeSync(2, payload);
+      } else {
+        const e = err as Error;
+        process.stderr.write(`${e && e.stack ? e.stack : String(err)}\n`);
+      }
       process.exitCode = 1;
     });
 }
diff --git a/tests/cli-exit.test.cjs b/tests/cli-exit.test.cjs
index 75a52c6a5..03a1de939 100644
--- a/tests/cli-exit.test.cjs
+++ b/tests/cli-exit.test.cjs
@@ -2,9 +2,16 @@
 
 const { describe, test } = require('node:test');
 const assert = require('node:assert/strict');
+const { spawnSync } = require('node:child_process');
+const path = require('node:path');
 
 const { ExitError, runMain } = require('../scripts/lib/cli-exit.cjs');
 
+// Paths to the compiled product seam (src/cli-exit.cts → gsd-core/bin/lib/cli-exit.cjs)
+// used for json-error mode regression tests which require io.cjs integration.
+const BUILT_CLI_EXIT_PATH = path.resolve(__dirname, '../gsd-core/bin/lib/cli-exit.cjs');
+const IO_PATH = path.resolve(__dirname, '../gsd-core/bin/lib/io.cjs');
+
 /** Settle the runMain promise chain before asserting. */
 async function settle() {
   await new Promise((r) => setImmediate(r));
@@ -159,3 +166,84 @@ describe('runMain', () => {
     }
   });
 });
+
+// ─── Regressions ─────────────────────────────────────────────────────────────
+
+/**
+ * bug #965 — runMain unexpected throw with --json-errors active emitted a raw
+ * stack trace instead of a structured { ok:false, reason, message } envelope.
+ * SDK consumers parsing structured errors would receive an unparseable string.
+ *
+ * Fix: src/cli-exit.cts non-ExitError catch branch now checks getJsonErrorMode()
+ * and emits the same structured envelope as error() when active.
+ *
+ * Tests run against the compiled product seam (gsd-core/bin/lib/cli-exit.cjs)
+ * via subprocess so that io.cjs module-level state is isolated per spawn.
+ */
+describe('regressions', () => {
+  /** Spawn a one-shot script that sets json-error mode and calls runMain with a throwing handler. */
+  function spawnJsonErrorRun({ jsonMode, errorType = 'TypeError', message = 'unexpected boom' } = {}) {
+    const script = `
+      const io = require(${JSON.stringify(IO_PATH)});
+      const { runMain } = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});
+      io.setJsonErrorMode(${jsonMode ? 'true' : 'false'});
+      runMain(() => { throw new ${errorType}(${JSON.stringify(message)}); });
+      setImmediate(() => {});
+    `;
+    return spawnSync(process.execPath, ['-e', script], { encoding: 'utf-8' });
+  }
+
+  describe('bug-965: unexpected throw in json-error mode emits structured envelope', () => {
+    test('stderr is a single parseable JSON object (not a raw stack trace)', () => {
+      const result = spawnJsonErrorRun({ jsonMode: true });
+      assert.strictEqual(result.status, 1,
+        `expected exit code 1, got ${result.status}; stderr: ${result.stderr}`);
+      const stderrTrimmed = result.stderr.trim();
+      assert.ok(stderrTrimmed.length > 0, 'expected non-empty stderr');
+      let parsed;
+      try {
+        parsed = JSON.parse(stderrTrimmed);
+      } catch (e) {
+        assert.fail(
+          `stderr is NOT valid JSON (raw stack trace leaked through):\n${stderrTrimmed}\nparse error: ${e.message}`
+        );
+      }
+      assert.strictEqual(parsed.ok, false, `expected ok:false, got: ${JSON.stringify(parsed)}`);
+      assert.strictEqual(parsed.reason, 'sdk_fail_fast',
+        `expected reason "sdk_fail_fast", got: ${parsed.reason}`);
+      assert.ok(
+        parsed.message && parsed.message.includes('unexpected boom'),
+        `expected message to include "unexpected boom", got: ${JSON.stringify(parsed.message)}`
+      );
+    });
+
+    test('stderr JSON works for RangeError as well as TypeError', () => {
+      const result = spawnJsonErrorRun({ jsonMode: true, errorType: 'RangeError', message: 'out of bounds' });
+      assert.strictEqual(result.status, 1);
+      const parsed = JSON.parse(result.stderr.trim());
+      assert.strictEqual(parsed.ok, false);
+      assert.strictEqual(parsed.reason, 'sdk_fail_fast');
+      assert.ok(parsed.message.includes('out of bounds'));
+    });
+
+    test('stdout is empty when unexpected throw emits structured error', () => {
+      const result = spawnJsonErrorRun({ jsonMode: true });
+      assert.strictEqual(result.stdout, '',
+        `expected empty stdout, got: ${result.stdout}`);
+    });
+
+    test('plain mode (json-error off) preserves raw stack trace on stderr', () => {
+      const result = spawnJsonErrorRun({ jsonMode: false });
+      assert.strictEqual(result.status, 1);
+      const stderrTrimmed = result.stderr.trim();
+      let parsed = null;
+      try { parsed = JSON.parse(stderrTrimmed); } catch { /* expected — not JSON */ }
+      assert.strictEqual(parsed, null,
+        `expected raw stack (non-JSON) on stderr in plain mode, but got valid JSON: ${stderrTrimmed.slice(0, 200)}`);
+      assert.ok(
+        stderrTrimmed.includes('unexpected boom'),
+        `expected "unexpected boom" in stderr, got: ${stderrTrimmed.slice(0, 200)}`
+      );
+    });
+  });
+});