diff --git a/src/commands/project-env.spec.ts b/src/commands/project-env.spec.ts
index f56b5dff..f5f775ce 100644
--- a/src/commands/project-env.spec.ts
+++ b/src/commands/project-env.spec.ts
@@ -2274,3 +2274,115 @@ describe('environment rename validation compatibility', () => {
     });
   });
 });
+
+describe('project env input validation (empty and whitespace projectId)', () => {
+  it('rejects empty and whitespace project-id across all commands in live and dry-run modes', async () => {
+    for (const invalidProjectId of ['', '   ', '\t\n']) {
+      // list
+      await expect(
+        runEnvList(
+          { ...COMMON, projectId: invalidProjectId },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+      await expect(
+        runEnvList(
+          { ...COMMON, projectId: invalidProjectId, dryRun: true },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+
+      // get
+      await expect(
+        runEnvGet(
+          { ...COMMON, projectId: invalidProjectId, name: 'staging' },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+      await expect(
+        runEnvGet(
+          { ...COMMON, projectId: invalidProjectId, name: 'staging', dryRun: true },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+
+      // create
+      await expect(
+        runEnvCreate(
+          {
+            ...COMMON,
+            projectId: invalidProjectId,
+            name: 'staging',
+            url: 'https://staging.example.com',
+          },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+      await expect(
+        runEnvCreate(
+          {
+            ...COMMON,
+            projectId: invalidProjectId,
+            name: 'staging',
+            url: 'https://staging.example.com',
+            dryRun: true,
+          },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+
+      // update
+      await expect(
+        runEnvUpdate(
+          {
+            ...COMMON,
+            projectId: invalidProjectId,
+            name: 'staging',
+            url: 'https://updated.example.com',
+          },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+      await expect(
+        runEnvUpdate(
+          {
+            ...COMMON,
+            projectId: invalidProjectId,
+            name: 'staging',
+            url: 'https://updated.example.com',
+            dryRun: true,
+          },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+
+      // delete
+      await expect(
+        runEnvDelete(
+          { ...COMMON, projectId: invalidProjectId, name: 'staging', confirm: true },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+      await expect(
+        runEnvDelete(
+          { ...COMMON, projectId: invalidProjectId, name: 'staging', confirm: true, dryRun: true },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+
+      // set-default
+      await expect(
+        runEnvSetDefault(
+          { ...COMMON, projectId: invalidProjectId, name: 'staging' },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+      await expect(
+        runEnvSetDefault(
+          { ...COMMON, projectId: invalidProjectId, name: 'staging', dryRun: true },
+          { stdout: () => {}, stderr: () => {} },
+        ),
+      ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+    }
+  });
+});
diff --git a/src/commands/project-env.ts b/src/commands/project-env.ts
index e5576d03..0e8d32dc 100644
--- a/src/commands/project-env.ts
+++ b/src/commands/project-env.ts
@@ -148,6 +148,17 @@ function envPath(projectId: string, name?: string): string {
   return name === undefined ? base : `${base}/${encodeURIComponent(name)}`;
 }
 
+/** Trim + reject an empty project id before dry-run or network dispatch. */
+function requireProjectId(raw: string | undefined): void {
+  const id = raw?.trim() ?? '';
+  if (id.length === 0) {
+    throw localValidationError(
+      '<project-id> is required and must not be empty or whitespace-only',
+      '<project-id>',
+    );
+  }
+}
+
 /** Trim + reject an empty environment name; the server owns every other rule. */
 function requireEnvName(raw: string | undefined, flag: string): string {
   const name = raw?.trim() ?? '';
@@ -499,6 +510,7 @@ export async function runEnvList(
   opts: EnvListOptions,
   deps: ProjectDeps = {},
 ): Promise<CliProjectEnvListResponse> {
+  requireProjectId(opts.projectId);
   const out = makeOutput(opts.output, deps);
   if (opts.dryRun) {
     emitDryRunBanner(stderrOf(deps));
@@ -523,6 +535,7 @@ export async function runEnvGet(
   opts: EnvListOptions & { name: string },
   deps: ProjectDeps = {},
 ): Promise<CliProjectEnvUpdateResponse> {
+  requireProjectId(opts.projectId);
   const out = makeOutput(opts.output, deps);
   const name = requireEnvName(opts.name, '<name>');
   if (opts.dryRun) {
@@ -575,6 +588,7 @@ export async function runEnvCreate(
   opts: EnvCreateOptions,
   deps: ProjectDeps = {},
 ): Promise<CliProjectEnvCreateResponse> {
+  requireProjectId(opts.projectId);
   const out = makeOutput(opts.output, deps);
   const stderr = stderrOf(deps);
   assertIdempotencyKey(opts.idempotencyKey);
@@ -725,6 +739,7 @@ export async function runEnvUpdate(
   opts: EnvUpdateOptions,
   deps: ProjectDeps = {},
 ): Promise<CliProjectEnvUpdateResponse> {
+  requireProjectId(opts.projectId);
   const out = makeOutput(opts.output, deps);
   const stderr = stderrOf(deps);
   assertIdempotencyKey(opts.idempotencyKey);
@@ -877,6 +892,7 @@ export async function runEnvDelete(
   opts: EnvDeleteOptions,
   deps: ProjectDeps = {},
 ): Promise<CliProjectEnvDeleteResponse> {
+  requireProjectId(opts.projectId);
   const out = makeOutput(opts.output, deps);
   const stderr = stderrOf(deps);
   assertIdempotencyKey(opts.idempotencyKey);
@@ -919,6 +935,7 @@ export async function runEnvSetDefault(
   opts: EnvSetDefaultOptions,
   deps: ProjectDeps = {},
 ): Promise<CliProjectEnvUpdateResponse> {
+  requireProjectId(opts.projectId);
   const out = makeOutput(opts.output, deps);
   const stderr = stderrOf(deps);
   assertIdempotencyKey(opts.idempotencyKey);