diff --git a/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.md b/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.md
index 3886e838d..996552706 100644
--- a/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.md
+++ b/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.md
@@ -105,6 +105,52 @@ Generated by [AVA](https://avajs.dev).
       },
     ]
 
+## should handle session updates and validations
+
+> should unpin existing when pinning new session
+
+    [
+      {
+        docId: null,
+        id: 'session-update-id',
+        pinned: true,
+      },
+      {
+        docId: null,
+        id: 'existing-pinned-session-id',
+        pinned: false,
+      },
+    ]
+
+> session type conversion steps
+
+    [
+      {
+        session: {
+          docId: 'doc-update-id',
+          pinned: false,
+        },
+        step: 'pinned_to_doc',
+        type: 'doc',
+      },
+      {
+        session: {
+          docId: null,
+          pinned: false,
+        },
+        step: 'doc_to_workspace',
+        type: 'workspace',
+      },
+      {
+        session: {
+          docId: null,
+          pinned: true,
+        },
+        step: 'workspace_to_pinned',
+        type: 'pinned',
+      },
+    ]
+
 ## session updates and type conversions
 
 > session states after pinning - should unpin existing
diff --git a/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.snap b/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.snap
index 89544e837..441c7f561 100644
Binary files a/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.snap and b/packages/backend/server/src/__tests__/models/__snapshots__/copilot-session.spec.ts.snap differ
diff --git a/packages/backend/server/src/__tests__/models/copilot-session.spec.ts b/packages/backend/server/src/__tests__/models/copilot-session.spec.ts
index fe47c9142..0e90c2d5c 100644
--- a/packages/backend/server/src/__tests__/models/copilot-session.spec.ts
+++ b/packages/backend/server/src/__tests__/models/copilot-session.spec.ts
@@ -3,7 +3,7 @@ import { randomUUID } from 'node:crypto';
 import { PrismaClient, User, Workspace } from '@prisma/client';
 import ava, { ExecutionContext, TestFn } from 'ava';
 
-import { CopilotPromptInvalid } from '../../base';
+import { CopilotPromptInvalid, CopilotSessionInvalidInput } from '../../base';
 import {
   CopilotSessionModel,
   UpdateChatSessionData,
@@ -289,56 +289,153 @@ test('should pin and unpin sessions', async t => {
   }
 });
 
-test('session updates and type conversions', async t => {
+test('should handle session updates and validations', async t => {
   const { copilotSession, db } = t.context;
-
   await createTestPrompts(copilotSession, db);
 
   const sessionId = 'session-update-id';
+  const actionSessionId = 'action-session-id';
+  const parentSessionId = 'parent-session-id';
+  const forkedSessionId = 'forked-session-id';
   const docId = 'doc-update-id';
 
   await createTestSession(t, { sessionId });
+  await createTestSession(t, {
+    sessionId: actionSessionId,
+    promptName: 'action-prompt',
+    promptAction: 'edit',
+    docId: 'some-doc',
+  });
+  await createTestSession(t, {
+    sessionId: parentSessionId,
+    docId: 'parent-doc',
+  });
+  await db.aiSession.create({
+    data: {
+      id: forkedSessionId,
+      workspaceId: workspace.id,
+      userId: user.id,
+      docId: 'forked-doc',
+      pinned: false,
+      promptName: 'test-prompt',
+      promptAction: null,
+      parentSessionId: parentSessionId,
+    },
+  });
 
-  // should unpin existing pinned session
+  const assertUpdateThrows = async (
+    t: ExecutionContext<Context>,
+    sessionId: string,
+    updateData: UpdateChatSessionData,
+    message: string
+  ) => {
+    await t.throwsAsync(
+      t.context.copilotSession.update(user.id, sessionId, updateData),
+      { instanceOf: CopilotSessionInvalidInput },
+      message
+    );
+  };
+
+  const assertUpdate = async (
+    t: ExecutionContext<Context>,
+    sessionId: string,
+    updateData: UpdateChatSessionData,
+    message: string
+  ) => {
+    await t.notThrowsAsync(
+      t.context.copilotSession.update(user.id, sessionId, updateData),
+      message
+    );
+  };
+
+  // case 1: action sessions should reject all updates
+  {
+    const actionUpdates = [
+      { docId: 'new-doc' },
+      { pinned: true },
+      { promptName: 'test-prompt' },
+    ];
+    for (const data of actionUpdates) {
+      await assertUpdateThrows(
+        t,
+        actionSessionId,
+        data,
+        `action session should reject update: ${JSON.stringify(data)}`
+      );
+    }
+  }
+
+  // case 2: forked sessions should reject docId updates but allow others
+  {
+    await assertUpdate(
+      t,
+      forkedSessionId,
+      { pinned: true },
+      'forked session should allow pinned update'
+    );
+    await assertUpdate(
+      t,
+      forkedSessionId,
+      { promptName: 'test-prompt' },
+      'forked session should allow promptName update'
+    );
+    await assertUpdateThrows(
+      t,
+      forkedSessionId,
+      { docId: 'new-doc' },
+      'forked session should reject docId update'
+    );
+  }
+
+  {
+    // case 3: prompt update validation
+    await assertUpdate(
+      t,
+      sessionId,
+      { promptName: 'test-prompt' },
+      'should allow valid non-action prompt'
+    );
+    await assertUpdateThrows(
+      t,
+      sessionId,
+      { promptName: 'action-prompt' },
+      'should reject action prompt'
+    );
+    await assertUpdateThrows(
+      t,
+      sessionId,
+      { promptName: 'non-existent-prompt' },
+      'should reject non-existent prompt'
+    );
+  }
+
+  // cest 4: session type conversions and pinning behavior
   {
     const existingPinnedId = 'existing-pinned-session-id';
     await createTestSession(t, { sessionId: existingPinnedId, pinned: true });
 
+    // should unpin existing when pinning new session
     await copilotSession.update(user.id, sessionId, { pinned: true });
 
     const sessionStatesAfterPin = await Promise.all([
       getSessionState(db, sessionId),
       getSessionState(db, existingPinnedId),
     ]);
-
     t.snapshot(
       sessionStatesAfterPin,
-      'session states after pinning - should unpin existing'
+      'should unpin existing when pinning new session'
     );
   }
 
-  // should unpin the session
-  {
-    await copilotSession.update(user.id, sessionId, { pinned: false });
-    const sessionStateAfterUnpin = await getSessionState(db, sessionId);
-    t.snapshot(sessionStateAfterUnpin, 'session state after unpinning');
-  }
-
-  // should convert session types
+  // test type conversions
   {
     const conversionSteps: any[] = [];
-
-    let session = await db.aiSession.findUnique({
-      where: { id: sessionId },
-      select: { docId: true, pinned: true },
-    });
-
     const convertSession = async (
       step: string,
       data: UpdateChatSessionData
     ) => {
       await copilotSession.update(user.id, sessionId, data);
-      session = await db.aiSession.findUnique({
+      const session = await db.aiSession.findUnique({
         where: { id: sessionId },
         select: { docId: true, pinned: true },
       });
@@ -349,23 +446,14 @@ test('session updates and type conversions', async t => {
       });
     };
 
-    {
-      await convertSession('workspace_to_doc', { docId }); // Workspace → Doc session
-      await convertSession('doc_to_pinned', { pinned: true }); // Doc → Pinned session
-      await convertSession('pinned_to_workspace', {
-        pinned: false,
-        docId: null,
-      }); // Pinned → Workspace session
-      await convertSession('workspace_to_pinned', { pinned: true }); // Workspace → Pinned session
-    }
+    const conversions = [
+      ['pinned_to_doc', { docId, pinned: false }],
+      ['doc_to_workspace', { docId: null }],
+      ['workspace_to_pinned', { pinned: true }],
+    ] as const;
 
-    // not allow convert to action prompt
-    {
-      await t.throwsAsync(
-        copilotSession.update(user.id, sessionId, {
-          promptName: 'action-prompt',
-        })
-      );
+    for (const [step, data] of conversions) {
+      await convertSession(step, data);
     }
 
     t.snapshot(conversionSteps, 'session type conversion steps');
diff --git a/packages/backend/server/src/models/copilot-session.ts b/packages/backend/server/src/models/copilot-session.ts
index de516c036..aecc1a898 100644
--- a/packages/backend/server/src/models/copilot-session.ts
+++ b/packages/backend/server/src/models/copilot-session.ts
@@ -293,18 +293,32 @@ export class CopilotSessionModel extends BaseModel {
   ): Promise<string> {
     const session = await this.getExists(
       sessionId,
-      { id: true, workspaceId: true, docId: true, pinned: true, prompt: true },
+      {
+        id: true,
+        workspaceId: true,
+        docId: true,
+        parentSessionId: true,
+        pinned: true,
+        prompt: true,
+      },
       { userId }
     );
     if (!session) {
       throw new CopilotSessionNotFound();
     }
+
+    // not allow to update action session
+    if (session.prompt.action) {
+      throw new CopilotSessionInvalidInput(
+        `Cannot update action: ${session.id}`
+      );
+    } else if (data.docId && session.parentSessionId) {
+      throw new CopilotSessionInvalidInput(
+        `Cannot update docId for forked session: ${session.id}`
+      );
+    }
+
     if (data.promptName) {
-      if (session.prompt.action) {
-        throw new CopilotSessionInvalidInput(
-          `Cannot update prompt for action: ${session.id}`
-        );
-      }
       const prompt = await this.db.aiPrompt.findFirst({
         where: { name: data.promptName },
       });