diff --git a/.changeset/case-link-validation-message.md b/.changeset/case-link-validation-message.md
new file mode 100644
index 00000000..267ab30c
--- /dev/null
+++ b/.changeset/case-link-validation-message.md
@@ -0,0 +1,5 @@
+---
+'aicodeman': patch
+---
+
+Link Existing now says what is wrong when it rejects a case. A bad name reports "Invalid case name format" and a bad folder path reports why (not absolute, a character a session folder cannot have, ...), instead of the same "Invalid request body" for both.
diff --git a/src/web/routes/case-routes.ts b/src/web/routes/case-routes.ts
index 7515e1da..66a073f0 100644
--- a/src/web/routes/case-routes.ts
+++ b/src/web/routes/case-routes.ts
@@ -1536,7 +1536,7 @@ export function registerCaseRoutes(app: FastifyInstance, ctx: EventPort & Config
     // admin-only in multi-user mode (mirrors host CRUD + the admin-only GET listing).
     const denied = adminOnly(req, reply);
     if (denied) return denied;
-    const { name, path: folderPath } = parseBody(LinkCaseSchema, req.body, 'Invalid request body');
+    const { name, path: folderPath } = parseBody(LinkCaseSchema, req.body);
 
     // Expand ~ to home directory
     const expandedPath = folderPath.startsWith('~') ? join(homedir(), folderPath.slice(1)) : folderPath;
diff --git a/test/routes/case-routes.test.ts b/test/routes/case-routes.test.ts
index c0fdcce8..c78bfa0f 100644
--- a/test/routes/case-routes.test.ts
+++ b/test/routes/case-routes.test.ts
@@ -647,6 +647,25 @@ describe('case-routes', () => {
       expect(body.success).toBe(false);
     });
 
+    it('says which field is wrong instead of a generic message', async () => {
+      const badName = await harness.app.inject({
+        method: 'POST',
+        url: '/api/cases/link',
+        payload: { name: 'bad name!', path: '/home/user/project' },
+      });
+      expect(badName.statusCode).toBe(400);
+      expect(JSON.parse(badName.body).error).toBe('Invalid case name format');
+
+      // A leading `~` is expanded later in the route but fails the absolute-path rule first.
+      const badPath = await harness.app.inject({
+        method: 'POST',
+        url: '/api/cases/link',
+        payload: { name: 'my-project', path: '~/project' },
+      });
+      expect(badPath.statusCode).toBe(400);
+      expect(JSON.parse(badPath.body).error).toMatch(/^Invalid path:/);
+    });
+
     it('returns not found when folder does not exist', async () => {
       mockedExistsSync.mockReturnValue(false);