diff --git a/actions/setup/js/start_mcp_gateway.cjs b/actions/setup/js/start_mcp_gateway.cjs
index 17a79ad45e8..697c4d1c247 100644
--- a/actions/setup/js/start_mcp_gateway.cjs
+++ b/actions/setup/js/start_mcp_gateway.cjs
@@ -269,6 +269,100 @@ function extractOptionalServerNames(configObj) {
   return optional;
 }
 
+/**
+ * @param {unknown} serverConfig
+ * @param {NodeJS.ProcessEnv} env
+ * @returns {string[]}
+ */
+function getMissingEnvVarNamesForServer(serverConfig, env = process.env) {
+  if (!serverConfig || typeof serverConfig !== "object" || Array.isArray(serverConfig)) {
+    return [];
+  }
+  const server = /** @type {Record<string, unknown>} */ serverConfig;
+  const serverEnv = server["env"];
+  if (!serverEnv || typeof serverEnv !== "object" || Array.isArray(serverEnv)) {
+    return [];
+  }
+
+  const missing = new Set();
+  const varPattern = /\$\{([A-Z_][A-Z0-9_]*)\}/g;
+  for (const [envName, rawValue] of Object.entries(/** @type {Record<string, unknown>} */ serverEnv)) {
+    if (typeof rawValue !== "string") {
+      continue;
+    }
+    const matches = [...rawValue.matchAll(varPattern)];
+    if (matches.length === 0) {
+      if (rawValue.trim() === "") {
+        missing.add(envName);
+      }
+      continue;
+    }
+    for (const match of matches) {
+      const varName = match[1];
+      const envValue = env[varName];
+      if (envValue == null || envValue.trim() === "") {
+        missing.add(varName);
+      }
+    }
+  }
+  return Array.from(missing).sort();
+}
+
+/**
+ * Find required MCP servers that were present in the input configuration but are
+ * absent from the gateway output. Missing required servers usually indicate that
+ * startup-time configuration such as secrets or environment variables was empty.
+ *
+ * @param {Record<string, unknown>} inputConfig
+ * @param {Record<string, unknown>} gatewayOutput
+ * @param {string[]} [optionalServerNames]
+ * @param {NodeJS.ProcessEnv} [env]
+ * @returns {Array<{name: string, missingEnvVars: string[]}>}
+ */
+function findOmittedRequiredMCPServers(inputConfig, gatewayOutput, optionalServerNames = [], env = process.env) {
+  const inputServers = inputConfig && inputConfig.mcpServers;
+  if (!inputServers || typeof inputServers !== "object" || Array.isArray(inputServers)) {
+    return [];
+  }
+  const outputServers = gatewayOutput && gatewayOutput.mcpServers;
+  const outputServerMap = outputServers && typeof outputServers === "object" && !Array.isArray(outputServers) ? /** @type {Record<string, unknown>} */ outputServers : {};
+  const optional = new Set(optionalServerNames);
+
+  return Object.entries(/** @type {Record<string, unknown>} */ inputServers)
+    .filter(([name, serverConfig]) => {
+      if (optional.has(name)) {
+        return false;
+      }
+      if (serverConfig && typeof serverConfig === "object" && !Array.isArray(serverConfig)) {
+        const server = /** @type {Record<string, unknown>} */ serverConfig;
+        if (server.required === false) {
+          return false;
+        }
+      }
+      return !Object.prototype.hasOwnProperty.call(outputServerMap, name);
+    })
+    .map(([name, serverConfig]) => ({
+      name,
+      missingEnvVars: getMissingEnvVarNamesForServer(serverConfig, env),
+    }));
+}
+
+/**
+ * @param {Array<{name: string, missingEnvVars: string[]}>} omittedServers
+ * @returns {string}
+ */
+function formatOmittedRequiredMCPServersMessage(omittedServers) {
+  const details = omittedServers
+    .map(server => {
+      if (server.missingEnvVars.length === 0) {
+        return server.name;
+      }
+      return `${server.name} (missing/empty env: ${server.missingEnvVars.join(", ")})`;
+    })
+    .join("; ");
+  return `Required MCP server(s) were omitted from the gateway output: ${details}. ` + `Configure the missing secrets/environment variables, or mark intentionally best-effort servers with required: false.`;
+}
+
 /**
  * Check whether a process is alive.
  * @param {number} pid
@@ -878,6 +972,17 @@ async function main() {
     core.setFailed("ERROR: Gateway returned an error payload instead of configuration");
     return;
   }
+  const omittedRequiredServers = findOmittedRequiredMCPServers(configObj, gatewayOutput, optionalServerNames);
+  if (omittedRequiredServers.length > 0) {
+    const message = formatOmittedRequiredMCPServersMessage(omittedRequiredServers);
+    try {
+      process.kill(gatewayPid);
+    } catch {
+      // ignore
+    }
+    core.setFailed(`ERROR: ${message}`);
+    return;
+  }
 
   // -----------------------------------------------------------------------
   // Convert gateway output to agent-specific format
@@ -1100,9 +1205,12 @@ module.exports = {
   applyOTLPIgnoreIfMissing,
   detectEngineType,
   extractOptionalServerNames,
+  findOmittedRequiredMCPServers,
+  formatOmittedRequiredMCPServersMessage,
   getOTLPIfMissingMode,
   hasNonEmptyOTLPHeaders,
   isOTLPIfMissingIgnore,
+  getMissingEnvVarNamesForServer,
   getJSONParseErrorContext,
   injectCustomGatewayEnvArgs,
   normalizeSinkVisibilityEncoding,
diff --git a/actions/setup/js/start_mcp_gateway.test.cjs b/actions/setup/js/start_mcp_gateway.test.cjs
index a863a243960..dcec2e0432f 100644
--- a/actions/setup/js/start_mcp_gateway.test.cjs
+++ b/actions/setup/js/start_mcp_gateway.test.cjs
@@ -4,7 +4,10 @@ import {
   applyOTLPIgnoreIfMissing,
   detectEngineType,
   extractOptionalServerNames,
+  findOmittedRequiredMCPServers,
+  formatOmittedRequiredMCPServersMessage,
   getJSONParseErrorContext,
+  getMissingEnvVarNamesForServer,
   getOTLPIfMissingMode,
   hasNonEmptyOTLPHeaders,
   injectCustomGatewayEnvArgs,
@@ -352,3 +355,97 @@ describe("start_mcp_gateway extractOptionalServerNames", () => {
     expect(extractOptionalServerNames({ mcpServers: null })).toEqual([]);
   });
 });
+
+describe("start_mcp_gateway omitted required MCP server detection", () => {
+  it("reports configured required servers missing from gateway output with empty env names", () => {
+    const inputConfig = {
+      mcpServers: {
+        sentry: {
+          type: "stdio",
+          command: "npx",
+          env: {
+            SENTRY_ACCESS_TOKEN: "${SENTRY_ACCESS_TOKEN}",
+            SENTRY_HOST: "sentry.io",
+            OPENAI_API_KEY: "${SENTRY_OPENAI_API_KEY}",
+          },
+        },
+        grafana: {
+          type: "stdio",
+          container: "grafana/mcp-grafana:1.0.0-alpine",
+          env: {
+            GRAFANA_URL: "${GRAFANA_URL}",
+            GRAFANA_SERVICE_ACCOUNT_TOKEN: "${GRAFANA_SERVICE_ACCOUNT_TOKEN}",
+          },
+        },
+        github: {
+          type: "http",
+          url: "http://localhost:8080/mcp/github",
+        },
+      },
+    };
+    const gatewayOutput = {
+      mcpServers: {
+        github: {
+          type: "http",
+          url: "http://localhost:8080/mcp/github",
+        },
+      },
+    };
+
+    expect(
+      findOmittedRequiredMCPServers(inputConfig, gatewayOutput, [], {
+        SENTRY_ACCESS_TOKEN: "",
+        SENTRY_OPENAI_API_KEY: "",
+        GRAFANA_URL: "",
+        GRAFANA_SERVICE_ACCOUNT_TOKEN: "",
+      })
+    ).toEqual([
+      { name: "sentry", missingEnvVars: ["SENTRY_ACCESS_TOKEN", "SENTRY_OPENAI_API_KEY"] },
+      { name: "grafana", missingEnvVars: ["GRAFANA_SERVICE_ACCOUNT_TOKEN", "GRAFANA_URL"] },
+    ]);
+  });
+
+  it("does not report servers marked optional in their config", () => {
+    const inputConfig = {
+      mcpServers: {
+        datadog: { type: "http", url: "https://example.com/mcp", required: false },
+      },
+    };
+    const gatewayOutput = { mcpServers: {} };
+
+    expect(findOmittedRequiredMCPServers(inputConfig, gatewayOutput)).toEqual([]);
+  });
+
+  it("does not report servers forwarded as optional after required flags are stripped", () => {
+    const inputConfig = {
+      mcpServers: {
+        slack: { type: "http", url: "https://example.com/slack" },
+      },
+    };
+    const gatewayOutput = { mcpServers: {} };
+
+    expect(findOmittedRequiredMCPServers(inputConfig, gatewayOutput, ["slack"])).toEqual([]);
+  });
+
+  it("formats an actionable startup failure message", () => {
+    expect(
+      formatOmittedRequiredMCPServersMessage([
+        { name: "sentry", missingEnvVars: ["SENTRY_ACCESS_TOKEN"] },
+        { name: "grafana", missingEnvVars: ["GRAFANA_SERVICE_ACCOUNT_TOKEN", "GRAFANA_URL"] },
+      ])
+    ).toBe(
+      "Required MCP server(s) were omitted from the gateway output: sentry (missing/empty env: SENTRY_ACCESS_TOKEN); grafana (missing/empty env: GRAFANA_SERVICE_ACCOUNT_TOKEN, GRAFANA_URL). Configure the missing secrets/environment variables, or mark intentionally best-effort servers with required: false."
+    );
+  });
+
+  it("detects direct empty env values when no variable placeholder is present", () => {
+    expect(
+      getMissingEnvVarNamesForServer({
+        env: {
+          API_TOKEN: "",
+          HOST: "https://example.com",
+        },
+      })
+    ).toEqual(["API_TOKEN"]);
+  });
+});