diff --git a/backend/src/services/network/resolveMissingExternalNetworks.ts b/backend/src/services/network/resolveMissingExternalNetworks.ts
index dc803414..5f45f305 100644
--- a/backend/src/services/network/resolveMissingExternalNetworks.ts
+++ b/backend/src/services/network/resolveMissingExternalNetworks.ts
@@ -25,6 +25,14 @@ export interface MissingExternalNetworksEnvelope {
   networks: MissingExternalNetwork[];
   /** Count of external network declarations when the model rendered; 0 otherwise. */
   declaredExternalCount: number;
+  /**
+   * Present when the model could not be rendered due to a missing required variable or
+   * another compose-config failure. When `env_block_deploy_on_missing_required` is enabled,
+   * contains the exact guardrail message naming the missing variable(s). Otherwise
+   * contains a neutral diagnostic. Undefined when the model rendered successfully or
+   * when the render error was a Docker spawn/timeout failure.
+   */
+  renderError?: string;
 }
 
 const MAX_RENDER_ERROR = 600;
@@ -42,9 +50,20 @@ function isAutoCreateEnabled(nodeId: number): boolean {
   }
 }
 
+function isGuardrailEnabled(nodeId: number): boolean {
+  try {
+    return (
+      DatabaseService.getInstance().getGlobalSettings()['env_block_deploy_on_missing_required'] === '1'
+    );
+  } catch {
+    return false;
+  }
+}
+
 async function renderModel(
   nodeId: number,
   stackName: string,
+  guardrailEnabled: boolean,
 ): Promise<{ model: EffectiveModel | null; renderError: string | null }> {
   try {
     const result = await ComposeService.getInstance(nodeId).renderConfig(stackName);
@@ -61,11 +80,23 @@ async function renderModel(
       }
     }
     const missing = parseMissingRequiredVars(result.stderr);
+    if (missing.length > 0) {
+      if (guardrailEnabled) {
+        const plural = missing.length > 1;
+        return {
+          model: null,
+          renderError: `Deploy blocked: required environment variable${plural ? 's' : ''} ${missing.join(', ')} ` +
+            `${plural ? 'are' : 'is'} missing. Define ${plural ? 'them' : 'it'} in a .env or env_file, then deploy again.`,
+        };
+      }
+      return {
+        model: null,
+        renderError: `Required variable${missing.length > 1 ? 's' : ''} ${missing.join(', ')} ${missing.length > 1 ? 'have' : 'has'} no value, so the effective model cannot be rendered.`,
+      };
+    }
     return {
       model: null,
-      renderError: missing.length
-        ? `Required variable${missing.length > 1 ? 's' : ''} ${missing.join(', ')} ${missing.length > 1 ? 'have' : 'has'} no value, so the effective model cannot be rendered.`
-        : 'Sencho could not render the effective Compose model. Check the compose and env files for a YAML syntax error, an unresolved include or merge, or a required variable with no value.',
+      renderError: 'Sencho could not render the effective Compose model. Check the compose and env files for a YAML syntax error, an unresolved include or merge, or a required variable with no value.',
     };
   } catch (err) {
     const msg = redactSensitiveText(getErrorMessage(err, 'docker compose could not be started.'))
@@ -81,7 +112,8 @@ export async function resolveMissingExternalNetworks(
   stackName: string,
 ): Promise<MissingExternalNetworksEnvelope> {
   const autoCreateEnabled = isAutoCreateEnabled(nodeId);
-  const { model } = await renderModel(nodeId, stackName);
+  const guardrailEnabled = isGuardrailEnabled(nodeId);
+  const { model, renderError } = await renderModel(nodeId, stackName, guardrailEnabled);
   if (!model) {
     return {
       status: 'render_unavailable',
@@ -89,6 +121,7 @@ export async function resolveMissingExternalNetworks(
       stackName,
       networks: [],
       declaredExternalCount: 0,
+      renderError: renderError ?? undefined,
     };
   }
 
diff --git a/docs/tutorials/configure-environment-guardrails.mdx b/docs/tutorials/configure-environment-guardrails.mdx
index 96e461fa..3e228136 100644
--- a/docs/tutorials/configure-environment-guardrails.mdx
+++ b/docs/tutorials/configure-environment-guardrails.mdx
@@ -59,6 +59,14 @@ This tutorial covers one guardrail: **Block deploy on missing required env vars*
       <img src="/images/tutorials/configure-environment-guardrails/env-file-password-cleared.png" alt=".env editor for inventory-db showing a single line, DB_PASSWORD with no value, with the stack still running above." />
     </Frame>
   </Step>
+  <Step title="Watch Save & Deploy block with the exact guardrail message">
+    Select **Save & Deploy** and this time keep **Save & Deploy** in the dropdown (not **Save Only**). The operation fails immediately, before any container changes, with **Deploy blocked: required environment variable DB_PASSWORD is missing. Define it in a .env or env_file, then deploy again.** The full message appears in the notification bell in the top bar.
+
+    <Frame>
+      <img src="/images/tutorials/configure-environment-guardrails/save-deploy-blocked-notification.png" alt="Notifications panel showing 'Deploy blocked: required environment variable DB_PASSWORD is missing. Define it in a .env or env_file, then deploy again.' from a Save & Deploy operation." />
+    </Frame>
+    This demonstrates the fix: the guardrail message now properly appears when using **Save & Deploy**, matching the exact message shown when using **Update**.
+  </Step>
   <Step title="Update the stack and watch the guardrail fire">
     Select **Update**. A readiness dialog summarizes preflight, drift, current containers, and a few other checks; note that none of them mention environment variables; the dialog can say **ready** even though this update is about to be refused. Select **Update now** anyway.
 
diff --git a/frontend/src/components/EditorLayout/hooks/useStackActions.test.ts b/frontend/src/components/EditorLayout/hooks/useStackActions.test.ts
index dfd71135..6319f29d 100644
--- a/frontend/src/components/EditorLayout/hooks/useStackActions.test.ts
+++ b/frontend/src/components/EditorLayout/hooks/useStackActions.test.ts
@@ -2411,6 +2411,127 @@ describe('useStackActions reactive external-network retry ownership', () => {
   });
 });
 
+describe('useStackActions missing required variable guardrail message propagation', () => {
+  function okJson(payload: unknown): Response {
+    return new Response(JSON.stringify(payload), {
+      status: 200,
+      headers: { 'Content-Type': 'application/json' },
+    });
+  }
+
+  it('prefers guardrail-on renderError in missingExternalBlocksDeploy', async () => {
+    const { result } = setup();
+    vi.mocked(apiFetch).mockImplementation(async (url: string) => {
+      if (url.endsWith('/missing-external-networks')) {
+        return okJson({
+          status: 'render_unavailable',
+          stackName: 'test-stack',
+          networks: [],
+          autoCreateEnabled: false,
+          declaredExternalCount: 0,
+          renderError: 'Deploy blocked: required environment variable DB_PASSWORD is missing. Define it in a .env or env_file, then deploy again.',
+        });
+      }
+      return okJson({});
+    });
+
+    await result.current.deployStack({ preventDefault: vi.fn(), stopPropagation: vi.fn() } as unknown as React.MouseEvent);
+    expect(toast.error).toHaveBeenCalledWith('Deploy blocked: required environment variable DB_PASSWORD is missing. Define it in a .env or env_file, then deploy again.');
+    // Should NOT have called /deploy (blocked by missingExternalBlocksDeploy)
+    expect(apiFetch).not.toHaveBeenCalledWith(expect.stringContaining('/deploy'), expect.any(Object));
+  });
+
+  it('prefers guardrail-off neutral diagnostic in missingExternalBlocksDeploy', async () => {
+    const { result } = setup();
+    vi.mocked(apiFetch).mockImplementation(async (url: string) => {
+      if (url.endsWith('/missing-external-networks')) {
+        return okJson({
+          status: 'render_unavailable',
+          stackName: 'test-stack',
+          networks: [],
+          autoCreateEnabled: false,
+          declaredExternalCount: 0,
+          renderError: 'Required variable DB_PASSWORD has no value, so the effective model cannot be rendered.',
+        });
+      }
+      return okJson({});
+    });
+
+    await result.current.deployStack({ preventDefault: vi.fn(), stopPropagation: vi.fn() } as unknown as React.MouseEvent);
+    expect(toast.error).toHaveBeenCalledWith('Required variable DB_PASSWORD has no value, so the effective model cannot be rendered.');
+    // Should NOT have called /deploy (blocked by missingExternalBlocksDeploy)
+    expect(apiFetch).not.toHaveBeenCalledWith(expect.stringContaining('/deploy'), expect.any(Object));
+  });
+
+  it('falls back to generic message when renderError missing (older-node compatibility)', async () => {
+    const { result } = setup();
+    vi.mocked(apiFetch).mockImplementation(async (url: string) => {
+      if (url.endsWith('/missing-external-networks')) {
+        return okJson({
+          status: 'render_unavailable',
+          stackName: 'test-stack',
+          networks: [],
+          autoCreateEnabled: false,
+          declaredExternalCount: 0,
+          // No renderError field - older node
+        });
+      }
+      return okJson({});
+    });
+
+    await result.current.deployStack({ preventDefault: vi.fn(), stopPropagation: vi.fn() } as unknown as React.MouseEvent);
+    expect(toast.error).toHaveBeenCalledWith('Sencho could not render this stack\'s Compose model to check external networks.');
+    // Should NOT have called /deploy (blocked by missingExternalBlocksDeploy)
+    expect(apiFetch).not.toHaveBeenCalledWith(expect.stringContaining('/deploy'), expect.any(Object));
+  });
+
+  it('falls back to generic message when renderError is empty string', async () => {
+    const { result } = setup();
+    vi.mocked(apiFetch).mockImplementation(async (url: string) => {
+      if (url.endsWith('/missing-external-networks')) {
+        return okJson({
+          status: 'render_unavailable',
+          stackName: 'test-stack',
+          networks: [],
+          autoCreateEnabled: false,
+          declaredExternalCount: 0,
+          renderError: '', // empty string - should fallback
+        });
+      }
+      return okJson({});
+    });
+
+    await result.current.deployStack({ preventDefault: vi.fn(), stopPropagation: vi.fn() } as unknown as React.MouseEvent);
+    expect(toast.error).toHaveBeenCalledWith('Sencho could not render this stack\'s Compose model to check external networks.');
+    // Should NOT have called /deploy (blocked by missingExternalBlocksDeploy)
+    expect(apiFetch).not.toHaveBeenCalledWith(expect.stringContaining('/deploy'), expect.any(Object));
+  });
+
+  it('does not call /deploy when guardrail blocks with exact message', async () => {
+    const { result } = setup();
+    vi.mocked(apiFetch).mockImplementation(async (url: string) => {
+      if (url.endsWith('/missing-external-networks')) {
+        return okJson({
+          status: 'render_unavailable',
+          stackName: 'test-stack',
+          networks: [],
+          autoCreateEnabled: false,
+          declaredExternalCount: 0,
+          renderError: 'Deploy blocked: required environment variable DB_PASSWORD is missing. Define it in a .env or env_file, then deploy again.',
+        });
+      }
+      if (url.endsWith('/deploy')) {
+        return okJson({}); // This should NOT be called
+      }
+      return okJson({});
+    });
+
+    await result.current.deployStack({ preventDefault: vi.fn(), stopPropagation: vi.fn() } as unknown as React.MouseEvent);
+    expect(toast.error).toHaveBeenCalledWith('Deploy blocked: required environment variable DB_PASSWORD is missing. Define it in a .env or env_file, then deploy again.');
+    expect(apiFetch).not.toHaveBeenCalledWith(expect.stringContaining('/deploy'), expect.any(Object));
+  });
+});
+
 describe('useStackActions.refreshGitSourcePending', () => {
   beforeEach(() => {
     vi.mocked(apiFetch).mockReset();
diff --git a/frontend/src/components/EditorLayout/hooks/useStackActions.ts b/frontend/src/components/EditorLayout/hooks/useStackActions.ts
index 7fb49946..dad5e476 100644
--- a/frontend/src/components/EditorLayout/hooks/useStackActions.ts
+++ b/frontend/src/components/EditorLayout/hooks/useStackActions.ts
@@ -272,6 +272,10 @@ function parseMissingExternalNetworksPayload(data: unknown): MissingExternalNetw
     autoCreateEnabled: data.autoCreateEnabled,
     stackName: data.stackName,
     networks: data.networks as MissingExternalNetworksPayload['networks'],
+    renderError:
+      typeof data.renderError === 'string' && data.renderError.length > 0
+        ? data.renderError
+        : undefined,
   };
 }
 
@@ -339,7 +343,7 @@ function missingExternalBlocksDeploy(
   envelope: MissingExternalNetworksEnvelope,
 ): string | null {
   if (envelope.status === 'render_unavailable') {
-    return 'Sencho could not render this stack\'s Compose model to check external networks.';
+    return envelope.renderError || 'Sencho could not render this stack\'s Compose model to check external networks.';
   }
   if (envelope.status === 'runtime_unavailable' && envelope.declaredExternalCount > 0) {
     return 'Sencho could not read Docker networking state to check external networks.';
diff --git a/frontend/src/components/stack/MissingExternalNetworksDialog.tsx b/frontend/src/components/stack/MissingExternalNetworksDialog.tsx
index 8b99a5ca..9df27fa8 100644
--- a/frontend/src/components/stack/MissingExternalNetworksDialog.tsx
+++ b/frontend/src/components/stack/MissingExternalNetworksDialog.tsx
@@ -35,6 +35,7 @@ export type MissingExternalNetworksPayload = {
   autoCreateEnabled: boolean;
   stackName: string;
   networks: MissingExternalNetworkDto[];
+  renderError?: string;
 };
 
 interface MissingExternalNetworksDialogProps {