diff --git a/tests/behavior/tests/sdt/sdt-backspace-inside-trailing.spec.ts b/tests/behavior/tests/sdt/sdt-backspace-inside-trailing.spec.ts
new file mode 100644
index 0000000000..19f0bd45c9
--- /dev/null
+++ b/tests/behavior/tests/sdt/sdt-backspace-inside-trailing.spec.ts
@@ -0,0 +1,76 @@
+import path from 'path';
+import { fileURLToPath } from 'url';
+import { test, expect, type SuperDocFixture } from '../../fixtures/superdoc.js';
+import { getInlineSdtRange, getInlineSdtSnapshot, type InlineSdtRange } from '../../helpers/sdt.js';
+
+/**
+ * Backspace with the caret at the inline SDT trailing edge, INSIDE the content
+ * (the other half of the boundary: inside-edge editing vs the outside-edge
+ * select-then-delete in sdt-backspace-outside-trailing).
+ *
+ * Expected behavior from the word-api parity contract
+ *   sdt/inline-backspace-inside-trailing, Word 16.0
+ * Editable modes delete a character per press (content shrinks, wrapper kept);
+ * content-locked modes are blocked (content unchanged). Loads the exact .docx
+ * fixtures the contract pinned by sha256.
+ */
+
+test.use({ config: { toolbar: 'full', showSelection: true } });
+
+const DIR = path.dirname(fileURLToPath(import.meta.url));
+const FIXTURE = {
+  unlocked: path.resolve(DIR, 'fixtures/sd3237-inline-unlocked.docx'),
+  sdtLocked: path.resolve(DIR, 'fixtures/sd3218-inline-sdtLocked.docx'),
+  contentLocked: path.resolve(DIR, 'fixtures/sd3237-inline-contentlocked.docx'),
+  sdtContentLocked: path.resolve(DIR, 'fixtures/sd3218-inline-sdtContentLocked.docx'),
+} as const;
+
+async function setupInsideTrailing(superdoc: SuperDocFixture, fixture: string): Promise<InlineSdtRange> {
+  await superdoc.loadDocument(fixture);
+  await superdoc.waitForStable();
+  const sdt = await getInlineSdtRange(superdoc.page);
+  expect(sdt).not.toBeNull();
+  await superdoc.setTextSelection(sdt!.end); // last position inside the content
+  await superdoc.page.evaluate(() => (window as any).editor.view.focus());
+  await superdoc.waitForStable();
+  return sdt!;
+}
+
+async function pressN(superdoc: SuperDocFixture, key: string, n: number) {
+  for (let i = 0; i < n; i++) {
+    await superdoc.press(key);
+    await superdoc.waitForStable();
+  }
+}
+
+test.describe('SDT Backspace inside the trailing edge - Word parity', () => {
+  // Contract: sdt/inline-backspace-inside-trailing, Word 16.0
+
+  for (const mode of ['unlocked', 'sdtLocked'] as const) {
+    test(`${mode}: each Backspace deletes one character; wrapper preserved`, async ({ superdoc }) => {
+      const sdt = await setupInsideTrailing(superdoc, FIXTURE[mode]);
+      const original = sdt.content;
+      // Step-based, matching the contract: one character gone per press.
+      for (let i = 1; i <= 3; i++) {
+        await superdoc.press('Backspace');
+        await superdoc.waitForStable();
+        const s = await getInlineSdtSnapshot(superdoc.page, sdt.id);
+        expect(s.sdtExists).toBe(true); // wrapper preserved throughout
+        expect(s.empty).toBe(true); // collapsed caret
+        expect(s.sdtContent!.length).toBe(original.length - i); // one char gone per press
+        expect(original.startsWith(s.sdtContent!)).toBe(true); // removed from the trailing edge
+      }
+    });
+  }
+
+  for (const mode of ['contentLocked', 'sdtContentLocked'] as const) {
+    test(`${mode}: Backspace is blocked; content unchanged`, async ({ superdoc }) => {
+      const sdt = await setupInsideTrailing(superdoc, FIXTURE[mode]);
+      const original = sdt.content;
+      await pressN(superdoc, 'Backspace', 3);
+      const s = await getInlineSdtSnapshot(superdoc.page, sdt.id);
+      expect(s.sdtExists).toBe(true);
+      expect(s.sdtContent).toBe(original); // blocked: nothing deleted
+    });
+  }
+});
diff --git a/tests/behavior/tests/sdt/sdt-delete-inside-leading.spec.ts b/tests/behavior/tests/sdt/sdt-delete-inside-leading.spec.ts
new file mode 100644
index 0000000000..b85361b5b2
--- /dev/null
+++ b/tests/behavior/tests/sdt/sdt-delete-inside-leading.spec.ts
@@ -0,0 +1,74 @@
+import path from 'path';
+import { fileURLToPath } from 'url';
+import { test, expect, type SuperDocFixture } from '../../fixtures/superdoc.js';
+import { getInlineSdtRange, getInlineSdtSnapshot, type InlineSdtRange } from '../../helpers/sdt.js';
+
+/**
+ * Delete with the caret at the inline SDT leading edge, INSIDE the content -
+ * the front-of-content mirror of sdt-backspace-inside-trailing.
+ *
+ * Expected behavior from the word-api parity contract
+ *   sdt/inline-delete-inside-leading, Word 16.0
+ * Editable modes delete a character per press (content shrinks from the front,
+ * wrapper kept); content-locked modes are blocked (content unchanged).
+ */
+
+test.use({ config: { toolbar: 'full', showSelection: true } });
+
+const DIR = path.dirname(fileURLToPath(import.meta.url));
+const FIXTURE = {
+  unlocked: path.resolve(DIR, 'fixtures/sd3237-inline-unlocked.docx'),
+  sdtLocked: path.resolve(DIR, 'fixtures/sd3218-inline-sdtLocked.docx'),
+  contentLocked: path.resolve(DIR, 'fixtures/sd3237-inline-contentlocked.docx'),
+  sdtContentLocked: path.resolve(DIR, 'fixtures/sd3218-inline-sdtContentLocked.docx'),
+} as const;
+
+async function setupInsideLeading(superdoc: SuperDocFixture, fixture: string): Promise<InlineSdtRange> {
+  await superdoc.loadDocument(fixture);
+  await superdoc.waitForStable();
+  const sdt = await getInlineSdtRange(superdoc.page);
+  expect(sdt).not.toBeNull();
+  await superdoc.setTextSelection(sdt!.start); // first position inside the content
+  await superdoc.page.evaluate(() => (window as any).editor.view.focus());
+  await superdoc.waitForStable();
+  return sdt!;
+}
+
+async function pressN(superdoc: SuperDocFixture, key: string, n: number) {
+  for (let i = 0; i < n; i++) {
+    await superdoc.press(key);
+    await superdoc.waitForStable();
+  }
+}
+
+test.describe('SDT Delete inside the leading edge - Word parity', () => {
+  // Contract: sdt/inline-delete-inside-leading, Word 16.0
+
+  for (const mode of ['unlocked', 'sdtLocked'] as const) {
+    test(`${mode}: each Delete removes one character from the front; wrapper preserved`, async ({ superdoc }) => {
+      const sdt = await setupInsideLeading(superdoc, FIXTURE[mode]);
+      const original = sdt.content;
+      // Step-based, matching the contract: one character gone per press.
+      for (let i = 1; i <= 3; i++) {
+        await superdoc.press('Delete');
+        await superdoc.waitForStable();
+        const s = await getInlineSdtSnapshot(superdoc.page, sdt.id);
+        expect(s.sdtExists).toBe(true); // wrapper preserved throughout
+        expect(s.empty).toBe(true); // collapsed caret
+        expect(s.sdtContent!.length).toBe(original.length - i); // one char gone per press
+        expect(original.endsWith(s.sdtContent!)).toBe(true); // removed from the leading edge
+      }
+    });
+  }
+
+  for (const mode of ['contentLocked', 'sdtContentLocked'] as const) {
+    test(`${mode}: Delete is blocked; content unchanged`, async ({ superdoc }) => {
+      const sdt = await setupInsideLeading(superdoc, FIXTURE[mode]);
+      const original = sdt.content;
+      await pressN(superdoc, 'Delete', 3);
+      const s = await getInlineSdtSnapshot(superdoc.page, sdt.id);
+      expect(s.sdtExists).toBe(true);
+      expect(s.sdtContent).toBe(original); // blocked: nothing deleted
+    });
+  }
+});