diff --git a/specification.md b/specification.md
index 78913f5..f794a2b 100644
--- a/specification.md
+++ b/specification.md
@@ -549,8 +549,8 @@ For all project commands the \<`path`\> option is optional:
   - If not valid json, return error.
 - Validate local package json file structure, fields and values.
   - If not validate structure, fields or values, return error.
-- Check whether the dependency has already been added.
-  - If already added, then return error.
+- Check whether the dependency has already been added at the requested version.
+  - If already added, the requested end state already holds - return the local package file as-is without error (idempotent success), rather than re-installing or re-adding it.
 - Install dependency using same logic as package install
 - Add dependency to the local package file and save.
 
@@ -586,7 +586,7 @@ Install existing dependencies listed inside a package json file.
 - Validate local package json file structure, fields and values.
   - If not validate structure, fields or values, return error.
 - Check whether the dependency exists in the local package json file.
-  - If not a dependency, then return error.
+  - If not a dependency, the requested end state already holds - return the local package file as-is without error (idempotent success, mirroring [Install and add dependency logic](#install-and-add-dependency-logic)), rather than treating it as a failure.
 - Uninstall dependency using same logic as package uninstall
 - Remove dependency from the local package file and save.
 
diff --git a/src/classes/ManagerLocal.ts b/src/classes/ManagerLocal.ts
index 95d14d4..aa10a73 100644
--- a/src/classes/ManagerLocal.ts
+++ b/src/classes/ManagerLocal.ts
@@ -717,8 +717,14 @@ export class ManagerLocal extends Manager {
   async uninstallDependency(slug: string, version?: string, filePath?: string, type = RegistryType.Plugins) {
     // Get local package file.
     const pkgFile = packageLoadFile(filePath) as any;
-    if (!pkgFile[type]) throw new Error(`Package ${type} is missing`);
-    if (!pkgFile[type][slug]) throw new Error(`Package ${type} ${slug} is not a dependency`);
+    if (!pkgFile[type] || !pkgFile[type][slug]) {
+      // Mirrors installDependency()'s "already a dependency" no-op: the requested end state
+      // (this dependency is gone) is already true, so this is idempotent success rather than an
+      // error - matches how most package managers treat "already removed"/"already installed".
+      this.log(`Package ${type} ${slug} is not a dependency`);
+      pkgFile.installed = true;
+      return pkgFile;
+    }
 
     // Uninstall dependency.
     const manager = new ManagerLocal(type, this.config.config);
diff --git a/tests/classes/ManagerLocal.test.ts b/tests/classes/ManagerLocal.test.ts
index 3764809..38f90f6 100644
--- a/tests/classes/ManagerLocal.test.ts
+++ b/tests/classes/ManagerLocal.test.ts
@@ -262,8 +262,18 @@ test('Project sync, install project, add new dependency, remove new dependency',
   const pkgDeps = await manager.installDependency(PLUGIN_PACKAGE.slug, '1.3.4', PROJECT_PATH);
   expect(omitDownloads(pkgDeps)).toEqual(omitDownloads(PROJECT_DEPS));
 
+  // Adding the same dependency again is idempotent success, not an error - the requested end
+  // state (dependency present at this version) already holds.
+  const pkgDepsAgain = await manager.installDependency(PLUGIN_PACKAGE.slug, '1.3.4', PROJECT_PATH);
+  expect(omitDownloads(pkgDepsAgain)).toEqual(omitDownloads(PROJECT_DEPS));
+
   const pkgNoDeps = await manager.uninstallDependency(PLUGIN_PACKAGE.slug, '1.3.4', PROJECT_PATH);
   expect(omitDownloads(pkgNoDeps)).toEqual(omitDownloads(PROJECT_NO_DEPS));
+
+  // Removing a dependency that's already gone is likewise idempotent success, mirroring
+  // installDependency() above, rather than throwing "not a dependency".
+  const pkgNoDepsAgain = await manager.uninstallDependency(PLUGIN_PACKAGE.slug, '1.3.4', PROJECT_PATH);
+  expect(omitDownloads(pkgNoDepsAgain)).toEqual(omitDownloads(PROJECT_NO_DEPS));
 });
 
 test('Create save persists an incomplete package without throwing', () => {