diff --git a/deploy/deploy.json b/deploy/deploy.json
index 1e92f656..7e20bdb3 100644
--- a/deploy/deploy.json
+++ b/deploy/deploy.json
@@ -205,6 +205,7 @@
         "HWLAB_CODE_AGENT_PROVIDER": "openai",
         "HWLAB_CODE_AGENT_MODEL": "gpt-5.5",
         "HWLAB_CODE_AGENT_OPENAI_BASE_URL": "http://172.26.26.227:17680/v1/responses",
+        "HWLAB_CODE_AGENT_HWLAB_API_BASE_URL": "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
         "HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED": "1",
         "HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR": "repo-owned",
         "HWLAB_CODE_AGENT_CODEX_WORKSPACE": "/workspace/hwlab",
diff --git a/deploy/deploy.schema.json b/deploy/deploy.schema.json
index 92afa260..ef09f74f 100644
--- a/deploy/deploy.schema.json
+++ b/deploy/deploy.schema.json
@@ -418,6 +418,10 @@
               "type": "string",
               "const": "http://172.26.26.227:17680/v1/responses"
             },
+            "HWLAB_CODE_AGENT_HWLAB_API_BASE_URL": {
+              "type": "string",
+              "const": "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667"
+            },
             "HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED": {
               "type": "string",
               "const": "1"
diff --git a/deploy/k8s/base/workloads.yaml b/deploy/k8s/base/workloads.yaml
index 8cd96602..25439e2f 100644
--- a/deploy/k8s/base/workloads.yaml
+++ b/deploy/k8s/base/workloads.yaml
@@ -117,6 +117,10 @@
                     "name": "HWLAB_CODE_AGENT_OPENAI_BASE_URL",
                     "value": "http://172.26.26.227:17680/v1/responses"
                   },
+                  {
+                    "name": "HWLAB_CODE_AGENT_HWLAB_API_BASE_URL",
+                    "value": "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667"
+                  },
                   {
                     "name": "HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED",
                     "value": "1"
diff --git a/internal/cloud/code-agent-chat.mjs b/internal/cloud/code-agent-chat.mjs
index d0d18dea..e661d51c 100644
--- a/internal/cloud/code-agent-chat.mjs
+++ b/internal/cloud/code-agent-chat.mjs
@@ -27,8 +27,12 @@ import {
 import {
   HWLAB_AGENT_RUNTIME_SKILL_CLI_VERSION,
   HWLAB_M3_IO_CAPABILITY_LEVELS,
+  HWLAB_M3_IO_API_BASE_URL_ENV,
+  HWLAB_M3_IO_API_BASE_URL_ENVS,
   HWLAB_M3_IO_API_ROUTE,
+  HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
   HWLAB_M3_IO_SKILL_NAME,
+  configuredCloudApiBaseUrl,
   runM3IoSkillCommand
 } from "../../skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs";
 
@@ -608,6 +612,7 @@ function inspectReadOnlyRunnerAvailability(env, options = {}) {
   const skillsDirsPresent = skillsDirs.filter((dir) => existsSync(dir));
   const codexStdioFeasibility = inspectCodexStdioFeasibility(env, options);
   const sessionRegistry = resolveCodeAgentSessionRegistry(options).describe();
+  const m3IoApiBaseUrl = configuredCloudApiBaseUrl(env);
   return {
     kind: READONLY_RUNNER_KIND,
     backend: READONLY_RUNNER_BACKEND,
@@ -638,12 +643,23 @@ function inspectReadOnlyRunnerAvailability(env, options = {}) {
     skillsDirs,
     skillsDirsPresent,
     m3IoSkill: {
-      status: "available",
+      status: m3IoApiBaseUrl ? "available" : "blocked",
       service: HWLAB_M3_IO_SKILL_NAME,
       contractVersion: HWLAB_AGENT_RUNTIME_SKILL_CLI_VERSION,
       route: HWLAB_M3_IO_API_ROUTE,
-      capabilityLevel: HWLAB_M3_IO_CAPABILITY_LEVELS.ready,
+      capabilityLevel: m3IoApiBaseUrl
+        ? HWLAB_M3_IO_CAPABILITY_LEVELS.ready
+        : HWLAB_M3_IO_CAPABILITY_LEVELS.blocked,
       blockedCapabilityLevel: HWLAB_M3_IO_CAPABILITY_LEVELS.blocked,
+      hwlabApi: {
+        source: m3IoApiBaseUrl ? m3IoSkillApiBaseUrlSource(env) : "missing-config",
+        baseUrlConfigured: Boolean(m3IoApiBaseUrl),
+        redactedBaseUrl: m3IoApiBaseUrl ? redactUrl(m3IoApiBaseUrl) : null,
+        recommendedEnv: HWLAB_M3_IO_API_BASE_URL_ENV,
+        recommendedDevValue: HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+        requiredEnv: [...HWLAB_M3_IO_API_BASE_URL_ENVS]
+      },
+      blocker: m3IoApiBaseUrl ? null : m3IoApiBaseUrlMissingBlocker(),
       directGatewayCallsAllowed: false,
       directBoxCallsAllowed: false,
       directPatchPanelCallsAllowed: false,
@@ -1056,13 +1072,17 @@ async function callReadOnlyRunner({ intent, conversationId, sessionId, traceId,
 async function callM3IoSkillRunner({ intent, conversationId, sessionId, traceId, env, now, workspace, sessionRegistry, requestJson }) {
   const resolvedWorkspace = resolveRunnerWorkspace(env, { workspace });
   const registry = resolveCodeAgentSessionRegistry({ sessionRegistry });
+  const m3ApiBaseUrl = configuredCloudApiBaseUrl(env);
+  const sessionCapabilityLevel = m3ApiBaseUrl
+    ? HWLAB_M3_IO_CAPABILITY_LEVELS.ready
+    : HWLAB_M3_IO_CAPABILITY_LEVELS.blocked;
   const sessionAcquire = registry.acquire({
     conversationId,
     sessionId,
     workspace: resolvedWorkspace,
     sandbox: M3_IO_SKILL_SANDBOX,
     runnerKind: M3_IO_SKILL_RUNNER_KIND,
-    capabilityLevel: HWLAB_M3_IO_CAPABILITY_LEVELS.ready,
+    capabilityLevel: sessionCapabilityLevel,
     implementationType: M3_IO_SKILL_IMPLEMENTATION_TYPE,
     traceId,
     now
@@ -1108,12 +1128,62 @@ async function callM3IoSkillRunner({ intent, conversationId, sessionId, traceId,
   }
 
   let session = sessionAcquire.session;
+  if (!m3ApiBaseUrl) {
+    const blocker = m3IoApiBaseUrlMissingBlocker();
+    const skillResult = m3IoSkillMissingApiBaseUrlResult({
+      traceId,
+      requestId: `req_${randomUUID()}`,
+      actorId: "usr_code_agent",
+      blocker,
+      startedAt,
+      finishedAt: nowIso(now)
+    });
+    return m3IoSkillRunnerResult({
+      skillResult,
+      commandArgs: m3IoSkillArgsForIntent(intent, { env, traceId }),
+      resolvedWorkspace,
+      registry,
+      session,
+      now,
+      traceId,
+      conversationId,
+      codexStdioFeasibility,
+      startedAt
+    });
+  }
+
   const commandArgs = m3IoSkillArgsForIntent(intent, { env, traceId });
   const skillResult = await runM3IoSkillCommand(commandArgs, {
     env,
     now,
     requestJson
   });
+  return m3IoSkillRunnerResult({
+    skillResult,
+    commandArgs,
+    resolvedWorkspace,
+    registry,
+    session,
+    now,
+    traceId,
+    conversationId,
+    codexStdioFeasibility,
+    startedAt
+  });
+}
+
+function m3IoSkillRunnerResult({
+  skillResult,
+  commandArgs,
+  resolvedWorkspace,
+  registry,
+  session,
+  now,
+  traceId,
+  conversationId,
+  codexStdioFeasibility,
+  startedAt
+}) {
   const finishedAt = nowIso(now);
   const capabilityLevel = skillResult.capabilityLevel ?? (
     skillResult.ok ? HWLAB_M3_IO_CAPABILITY_LEVELS.ready : HWLAB_M3_IO_CAPABILITY_LEVELS.blocked
@@ -1231,6 +1301,101 @@ async function callM3IoSkillRunner({ intent, conversationId, sessionId, traceId,
   };
 }
 
+function m3IoSkillMissingApiBaseUrlResult({ traceId, requestId, actorId, blocker, startedAt, finishedAt }) {
+  return {
+    ok: false,
+    service: HWLAB_M3_IO_SKILL_NAME,
+    contractVersion: HWLAB_AGENT_RUNTIME_SKILL_CLI_VERSION,
+    route: HWLAB_M3_IO_API_ROUTE,
+    hwlabApi: {
+      route: HWLAB_M3_IO_API_ROUTE,
+      url: null,
+      redactedUrl: null,
+      source: "missing-config",
+      baseUrlConfigured: false,
+      requiredEnv: [...HWLAB_M3_IO_API_BASE_URL_ENVS],
+      recommendedEnv: HWLAB_M3_IO_API_BASE_URL_ENV,
+      recommendedDevValue: HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+      cloudApiOnly: true,
+      directGatewayCalls: false,
+      directBoxCalls: false,
+      directPatchPanelCalls: false
+    },
+    capabilityLevel: HWLAB_M3_IO_CAPABILITY_LEVELS.blocked,
+    controlReady: false,
+    action: "m3.io",
+    accepted: false,
+    status: "blocked",
+    traceId,
+    requestId,
+    actorId,
+    operationId: null,
+    audit: {
+      auditId: null,
+      status: "not_written",
+      durableStatus: null,
+      summary: "blocked before HWLAB API request"
+    },
+    evidence: {
+      evidenceId: null,
+      status: "blocked",
+      sourceKind: "BLOCKED",
+      blocker: blocker.code,
+      writeStatus: "not_written",
+      summary: "blocked before HWLAB API request"
+    },
+    durable: {
+      status: "blocked",
+      durable: false,
+      blocker: blocker.code,
+      category: blocker.category,
+      summary: blocker.message
+    },
+    blocker,
+    capabilityBlocker: blocker,
+    trustBlocker: null,
+    blockers: [blocker],
+    readiness: {
+      status: "blocked",
+      controlReady: false,
+      capabilityLevel: HWLAB_M3_IO_CAPABILITY_LEVELS.blocked,
+      route: HWLAB_M3_IO_API_ROUTE,
+      blocker,
+      trustBlocker: null
+    },
+    command: null,
+    result: {
+      value: null,
+      targetReadback: null
+    },
+    controlPath: {
+      cloudApi: false,
+      gatewaySimu: false,
+      boxSimu: false,
+      patchPanel: false,
+      frontendBypass: false
+    },
+    safety: {
+      cloudApiRouteOnly: true,
+      allowedRoute: HWLAB_M3_IO_API_ROUTE,
+      directGatewayCalls: false,
+      directBoxCalls: false,
+      directPatchPanelCalls: false,
+      fallbackUsed: false,
+      openAiFallbackUsed: false
+    },
+    httpStatus: 0,
+    error: {
+      code: blocker.code,
+      message: blocker.message
+    },
+    rawStatus: null,
+    startedAt,
+    finishedAt,
+    response: null
+  };
+}
+
 function detectReadOnlyRunnerIntent(message) {
   const text = String(message ?? "").trim();
   const lower = text.toLowerCase();
@@ -1900,12 +2065,7 @@ function m3IoSkillArgsForIntent(intent, { env, traceId }) {
     "--actor-id",
     "usr_code_agent"
   ];
-  const apiBaseUrl = firstNonEmpty(
-    env.HWLAB_CODE_AGENT_HWLAB_API_BASE_URL,
-    env.HWLAB_API_BASE_URL,
-    env.HWLAB_CLOUD_API_BASE_URL,
-    null
-  );
+  const apiBaseUrl = configuredCloudApiBaseUrl(env);
   if (apiBaseUrl) {
     args.push("--api-base-url", apiBaseUrl);
   }
@@ -1915,6 +2075,24 @@ function m3IoSkillArgsForIntent(intent, { env, traceId }) {
   return args;
 }
 
+function m3IoSkillApiBaseUrlSource(env = process.env) {
+  for (const name of HWLAB_M3_IO_API_BASE_URL_ENVS) {
+    if (firstNonEmpty(env[name])) return `env:${name}`;
+  }
+  return "missing-config";
+}
+
+function m3IoApiBaseUrlMissingBlocker() {
+  return {
+    code: "hwlab_api_base_url_missing",
+    category: "hwlab_api_configuration",
+    source: "code-agent-m3-skill-cli",
+    summary: `${HWLAB_M3_IO_API_BASE_URL_ENV} is required so the Skill CLI can reach cloud-api from inside the cloud-api runtime container.`,
+    message: `Set ${HWLAB_M3_IO_API_BASE_URL_ENV}=${HWLAB_M3_IO_DEV_SERVICE_BASE_URL} in the cloud-api runtime; the runner will not fall back to a loopback URL or direct hardware services.`,
+    zh: `cloud-api 运行时缺少 ${HWLAB_M3_IO_API_BASE_URL_ENV}，Skill CLI 无法从容器内访问 HWLAB API；不会回退到 loopback URL 或直连硬件服务。`
+  };
+}
+
 function m3IoSkillReply(skillResult) {
   const lines = [
     `M3 IO Skill CLI result: status=${skillResult.status}; accepted=${skillResult.accepted}; route=${skillResult.route}; traceId=${skillResult.traceId}; operationId=${skillResult.operationId ?? "null"}.`
@@ -2794,6 +2972,17 @@ function redactText(value) {
     .replace(/([A-Za-z0-9_]*KEY[A-Za-z0-9_]*=)[^\s]+/giu, "$1***");
 }
 
+function redactUrl(value) {
+  try {
+    const url = new URL(value);
+    url.username = "";
+    url.password = "";
+    return url.toString();
+  } catch {
+    return redactText(String(value ?? "")).replace(/\/\/[^/@]+@/u, "//***@");
+  }
+}
+
 function shellQuote(value) {
   return `'${String(value).replaceAll("'", "'\\''")}'`;
 }
diff --git a/internal/cloud/code-agent-session-registry.test.mjs b/internal/cloud/code-agent-session-registry.test.mjs
index c74a75b6..a8d788b5 100644
--- a/internal/cloud/code-agent-session-registry.test.mjs
+++ b/internal/cloud/code-agent-session-registry.test.mjs
@@ -9,6 +9,7 @@ import {
   classifyCodeAgentChatReadiness
 } from "../../scripts/src/code-agent-response-contract.mjs";
 import {
+  HWLAB_M3_IO_API_BASE_URL_ENV,
   HWLAB_M3_IO_CAPABILITY_LEVELS,
   HWLAB_M3_IO_API_ROUTE
 } from "../../skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs";
@@ -298,7 +299,7 @@ test("Code Agent M3 DO write uses Skill CLI to call only HWLAB API /v1/m3/io", a
       env: {
         PATH: process.env.PATH,
         HWLAB_CODE_AGENT_WORKSPACE: process.cwd(),
-        HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
+        HWLAB_CODE_AGENT_HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
         OPENAI_API_KEY: "must-not-be-used"
       },
       callProvider: async () => {
@@ -415,7 +416,7 @@ test("Code Agent M3 DI read returns structured blocker from HWLAB API without fa
       env: {
         PATH: process.env.PATH,
         HWLAB_CODE_AGENT_WORKSPACE: process.cwd(),
-        HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
+        HWLAB_CODE_AGENT_HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
         OPENAI_API_KEY: "must-not-be-used"
       },
       callProvider: async () => {
@@ -505,7 +506,7 @@ test("Code Agent blocks direct gateway or patch-panel requests instead of using
       env: {
         PATH: process.env.PATH,
         HWLAB_CODE_AGENT_WORKSPACE: process.cwd(),
-        HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667"
+        HWLAB_CODE_AGENT_HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667"
       },
       m3IoSkillRequestJson: async () => {
         throw new Error("direct gateway request must not reach the M3 skill CLI");
@@ -520,6 +521,49 @@ test("Code Agent blocks direct gateway or patch-panel requests instead of using
   assert.match(direct.error.message, /Skill CLI -> HWLAB API \/v1\/m3\/io/u);
 });
 
+test("Code Agent M3 Skill CLI blocks missing service-local HWLAB API base URL before loopback fallback", async () => {
+  const payload = await handleCodeAgentChat(
+    {
+      conversationId: "cnv_m3_missing_api_base",
+      traceId: "trc_m3_missing_api_base",
+      message: "通过 HWLAB API 把 res_boxsimu_1 的 DO1 写成 true，然后读取 res_boxsimu_2 的 DI1。"
+    },
+    {
+      now: () => "2026-05-23T00:07:30.000Z",
+      env: {
+        PATH: process.env.PATH,
+        HWLAB_CODE_AGENT_WORKSPACE: process.cwd(),
+        OPENAI_API_KEY: "must-not-be-used"
+      },
+      callProvider: async () => {
+        throw new Error("OpenAI fallback must not be used for M3 IO");
+      },
+      m3IoSkillRequestJson: async () => {
+        throw new Error("missing HWLAB API base URL must block before any HTTP request");
+      }
+    }
+  );
+
+  validateCodeAgentChatSchema(payload);
+  assert.equal(payload.status, "completed");
+  assert.equal(payload.provider, "hwlab-skill-cli");
+  assert.equal(payload.capabilityLevel, HWLAB_M3_IO_CAPABILITY_LEVELS.blocked);
+  assert.equal(payload.toolCalls.length, 1);
+  assert.equal(payload.toolCalls[0].status, "blocked");
+  assert.equal(payload.toolCalls[0].blocker.code, "hwlab_api_base_url_missing");
+  assert.equal(payload.toolCalls[0].hwlabApi.url, null);
+  assert.equal(payload.toolCalls[0].hwlabApi.source, "missing-config");
+  assert.equal(payload.toolCalls[0].hwlabApi.recommendedEnv, HWLAB_M3_IO_API_BASE_URL_ENV);
+  assert.equal(payload.toolCalls[0].command.includes("127.0.0.1:6667"), false);
+  assert.equal(payload.toolCalls[0].accepted, false);
+  assert.equal(payload.toolCalls[0].operationId, null);
+  assert.equal(payload.session.capabilityLevel, HWLAB_M3_IO_CAPABILITY_LEVELS.blocked);
+  assert.equal(payload.runnerTrace.blocker.code, "hwlab_api_base_url_missing");
+  assert.equal(payload.runnerTrace.capabilityLevel, HWLAB_M3_IO_CAPABILITY_LEVELS.blocked);
+  assert.equal(payload.providerTrace.fallbackUsed, false);
+  assert.equal(payload.skills.blockers[0].code, "hwlab_api_base_url_missing");
+});
+
 test("Codex stdio manager reports concrete blockers without falling back to readonly", async () => {
   const manager = createCodexStdioSessionManager({
     idFactory: () => "ses_stdio_blocked"
diff --git a/internal/cloud/server.test.mjs b/internal/cloud/server.test.mjs
index 70b8c495..01ce7f20 100644
--- a/internal/cloud/server.test.mjs
+++ b/internal/cloud/server.test.mjs
@@ -1032,7 +1032,7 @@ test("cloud api /v1/agent/chat routes M3 IO through Skill CLI to /v1/m3/io only"
     env: {
       PATH: process.env.PATH,
       HWLAB_CODE_AGENT_WORKSPACE: workspace,
-      HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
+      HWLAB_CODE_AGENT_HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
       OPENAI_API_KEY: "must-not-be-used"
     },
     callCodeAgentProvider: async () => {
diff --git a/scripts/code-agent-chat-smoke.mjs b/scripts/code-agent-chat-smoke.mjs
index 7681128b..c1c736df 100644
--- a/scripts/code-agent-chat-smoke.mjs
+++ b/scripts/code-agent-chat-smoke.mjs
@@ -362,7 +362,7 @@ async function runLocalContractSmoke() {
       env: {
         PATH: process.env.PATH,
         HWLAB_CODE_AGENT_WORKSPACE: process.cwd(),
-        HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
+        HWLAB_CODE_AGENT_HWLAB_API_BASE_URL: "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667",
         OPENAI_API_KEY: "must-not-be-used"
       },
       callProvider: async () => {
diff --git a/scripts/src/deploy-contract-plan.mjs b/scripts/src/deploy-contract-plan.mjs
index 557a331f..0f970aea 100644
--- a/scripts/src/deploy-contract-plan.mjs
+++ b/scripts/src/deploy-contract-plan.mjs
@@ -6,6 +6,10 @@ import {
   DEV_CODE_AGENT_PROVIDER_CONTRACT,
   codeAgentSecretRefPlaceholder
 } from "../../internal/cloud/code-agent-contract.mjs";
+import {
+  HWLAB_M3_IO_API_BASE_URL_ENV,
+  HWLAB_M3_IO_DEV_SERVICE_BASE_URL
+} from "../../skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs";
 
 const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "../..");
 const expectedPorts = Object.freeze({ frontend: 16666, api: 16667 });
@@ -377,6 +381,13 @@ function validateCloudApiCodeAgentSource(ctx, env) {
     "$.services.hwlab-cloud-api.env.HWLAB_CODE_AGENT_OPENAI_BASE_URL",
     "cloud API Code Agent DEV egress/proxy base URL"
   );
+  expectEqual(
+    ctx,
+    env[HWLAB_M3_IO_API_BASE_URL_ENV],
+    HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+    `$.services.hwlab-cloud-api.env.${HWLAB_M3_IO_API_BASE_URL_ENV}`,
+    "cloud API Code Agent Skill CLI HWLAB API service-local base URL"
+  );
   expectEqual(ctx, env.HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED, "1", "$.services.hwlab-cloud-api.env.HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED", "cloud API Codex stdio adapter enabled flag");
   expectEqual(ctx, env.HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR, "repo-owned", "$.services.hwlab-cloud-api.env.HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR", "cloud API Codex stdio supervisor mode");
   expectEqual(ctx, env.HWLAB_CODE_AGENT_CODEX_WORKSPACE, "/workspace/hwlab", "$.services.hwlab-cloud-api.env.HWLAB_CODE_AGENT_CODEX_WORKSPACE", "cloud API Codex stdio workspace mount contract");
@@ -536,6 +547,13 @@ function validateCloudApiCodeAgentArtifacts(ctx, workloads) {
     "deploy/k8s/base/workloads.yaml.hwlab-cloud-api.env.HWLAB_CODE_AGENT_OPENAI_BASE_URL",
     "cloud API workload Code Agent DEV egress/proxy base URL"
   );
+  expectEqual(
+    ctx,
+    env.get(HWLAB_M3_IO_API_BASE_URL_ENV)?.value,
+    HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+    `deploy/k8s/base/workloads.yaml.hwlab-cloud-api.env.${HWLAB_M3_IO_API_BASE_URL_ENV}`,
+    "cloud API workload Code Agent Skill CLI HWLAB API service-local base URL"
+  );
   expectEqual(ctx, env.get("HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED")?.value, "1", "deploy/k8s/base/workloads.yaml.hwlab-cloud-api.env.HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED", "cloud API workload Codex stdio adapter enabled flag");
   expectEqual(ctx, env.get("HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR")?.value, "repo-owned", "deploy/k8s/base/workloads.yaml.hwlab-cloud-api.env.HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR", "cloud API workload Codex stdio supervisor mode");
   expectEqual(ctx, env.get("HWLAB_CODE_AGENT_CODEX_WORKSPACE")?.value, "/workspace/hwlab", "deploy/k8s/base/workloads.yaml.hwlab-cloud-api.env.HWLAB_CODE_AGENT_CODEX_WORKSPACE", "cloud API workload Codex stdio workspace mount contract");
diff --git a/scripts/validate-contract.mjs b/scripts/validate-contract.mjs
index e141c3b8..26b839d2 100644
--- a/scripts/validate-contract.mjs
+++ b/scripts/validate-contract.mjs
@@ -11,6 +11,10 @@ import {
   DEV_CODE_AGENT_PROVIDER_CONTRACT,
   codeAgentSecretRefPlaceholder
 } from "../internal/cloud/code-agent-contract.mjs";
+import {
+  HWLAB_M3_IO_API_BASE_URL_ENV,
+  HWLAB_M3_IO_DEV_SERVICE_BASE_URL
+} from "../skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs";
 import {
   ENVIRONMENT_DEV,
   SERVICE_IDS,
@@ -203,6 +207,11 @@ assert.equal(
   DEV_CODE_AGENT_PROVIDER_CONTRACT.egress.defaultBaseUrl,
   "cloud-api Code Agent OpenAI base URL must use DEV egress/proxy"
 );
+assert.equal(
+  cloudApi.env[HWLAB_M3_IO_API_BASE_URL_ENV],
+  HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+  "cloud-api Code Agent Skill CLI HWLAB API base URL must use service-local cloud-api DNS"
+);
 assert.equal(cloudApi.env.HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED, "1", "cloud-api Codex stdio adapter enabled flag");
 assert.equal(cloudApi.env.HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR, "repo-owned", "cloud-api Codex stdio supervisor mode");
 assert.equal(cloudApi.env.HWLAB_CODE_AGENT_CODEX_WORKSPACE, "/workspace/hwlab", "cloud-api Codex stdio workspace mount contract");
@@ -260,6 +269,11 @@ function assertCodeAgentProviderWorkloadContract(env) {
     contract.egress.defaultBaseUrl,
     "cloud-api workload Code Agent OpenAI base URL must use DEV egress/proxy"
   );
+  assert.equal(
+    env[HWLAB_M3_IO_API_BASE_URL_ENV]?.value,
+    HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+    "cloud-api workload Code Agent Skill CLI HWLAB API base URL must use service-local cloud-api DNS"
+  );
   assert.equal(env.HWLAB_CODE_AGENT_CODEX_STDIO_ENABLED?.value, "1", "cloud-api workload Codex stdio adapter enabled flag");
   assert.equal(env.HWLAB_CODE_AGENT_CODEX_STDIO_SUPERVISOR?.value, "repo-owned", "cloud-api workload Codex stdio supervisor mode");
   assert.equal(env.HWLAB_CODE_AGENT_CODEX_WORKSPACE?.value, "/workspace/hwlab", "cloud-api workload Codex stdio workspace mount contract");
diff --git a/skills/hwlab-agent-runtime/SKILL.md b/skills/hwlab-agent-runtime/SKILL.md
index c3f84a7c..12b04eea 100644
--- a/skills/hwlab-agent-runtime/SKILL.md
+++ b/skills/hwlab-agent-runtime/SKILL.md
@@ -45,10 +45,15 @@ evidence metadata; there is no silent fallback.
 ## M3 IO Control Contract
 
 The M3 IO CLI is the controlled Code Agent -> Skill CLI -> HWLAB API adapter.
-`--api-base-url` must be a cloud-api base such as `http://127.0.0.1:6667`,
-`http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667`, or the public DEV
-API endpoint. The CLI appends `/v1/m3/io` itself and rejects direct
-gateway/box/patch-panel targets.
+`--api-base-url` must be a cloud-api base such as
+`http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667` or the public DEV API
+endpoint. In the cloud-api runtime, configure
+`HWLAB_CODE_AGENT_HWLAB_API_BASE_URL=http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667`
+so the Skill CLI can reach the service-local HWLAB API from inside the
+container. The CLI appends `/v1/m3/io` itself and rejects direct
+gateway/box/patch-panel targets. If no API base URL is configured, the CLI
+returns `hwlab_api_base_url_missing` and does not fall back to `127.0.0.1` or
+any direct hardware service.
 
 The JSON response includes `route`, `traceId`, `operationId`, `audit`,
 `evidence`, `accepted`, `status`, `capabilityLevel`, `readiness`, `blocker`,
diff --git a/skills/hwlab-agent-runtime/scripts/m3-io-skill-client.test.mjs b/skills/hwlab-agent-runtime/scripts/m3-io-skill-client.test.mjs
index e9a58918..cdfe4496 100644
--- a/skills/hwlab-agent-runtime/scripts/m3-io-skill-client.test.mjs
+++ b/skills/hwlab-agent-runtime/scripts/m3-io-skill-client.test.mjs
@@ -2,7 +2,9 @@ import assert from "node:assert/strict";
 import test from "node:test";
 
 import {
+  HWLAB_M3_IO_API_BASE_URL_ENV,
   HWLAB_M3_IO_CAPABILITY_LEVELS,
+  HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
   HWLAB_M3_IO_API_ROUTE,
   runM3IoSkillCommand,
   validateCloudApiTarget
@@ -145,3 +147,40 @@ test("M3 Skill CLI validates exact API route contract", () => {
   });
   assert.equal(invalid.code, "invalid_hwlab_api_route");
 });
+
+test("M3 Skill CLI blocks missing API base URL instead of falling back to loopback", async () => {
+  const result = await runM3IoSkillCommand(
+    [
+      "m3",
+      "io",
+      "--action",
+      "di.read",
+      "--trace-id",
+      "trc_skill_cli_missing_api_base"
+    ],
+    {
+      env: {
+        PATH: process.env.PATH
+      },
+      requestJson: async () => {
+        throw new Error("missing HWLAB API base URL must block before network call");
+      }
+    }
+  );
+
+  assert.equal(result.ok, false);
+  assert.equal(result.status, "blocked");
+  assert.equal(result.accepted, false);
+  assert.equal(result.blocker.code, "hwlab_api_base_url_missing");
+  assert.equal(result.blocker.category, "hwlab_api_configuration");
+  assert.equal(result.capabilityLevel, HWLAB_M3_IO_CAPABILITY_LEVELS.blocked);
+  assert.equal(result.hwlabApi.url, null);
+  assert.equal(result.hwlabApi.source, "missing-config");
+  assert.equal(result.hwlabApi.recommendedEnv, HWLAB_M3_IO_API_BASE_URL_ENV);
+  assert.equal(result.hwlabApi.recommendedDevValue, HWLAB_M3_IO_DEV_SERVICE_BASE_URL);
+  assert.equal(result.operationId, null);
+  assert.equal(result.audit.status, "not_written");
+  assert.equal(result.safety.directGatewayCalls, false);
+  assert.equal(result.safety.directBoxCalls, false);
+  assert.equal(result.safety.directPatchPanelCalls, false);
+});
diff --git a/skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs b/skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs
index c8b717d5..2e21b954 100644
--- a/skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs
+++ b/skills/hwlab-agent-runtime/scripts/src/m3-io-skill-client.mjs
@@ -3,6 +3,13 @@ import { randomUUID } from "node:crypto";
 export const HWLAB_AGENT_RUNTIME_SKILL_CLI_VERSION = "hwlab-agent-runtime-skill-cli-v1";
 export const HWLAB_M3_IO_API_ROUTE = "/v1/m3/io";
 export const HWLAB_M3_IO_SKILL_NAME = "hwlab-agent-runtime.m3-io";
+export const HWLAB_M3_IO_API_BASE_URL_ENV = "HWLAB_CODE_AGENT_HWLAB_API_BASE_URL";
+export const HWLAB_M3_IO_DEV_SERVICE_BASE_URL = "http://hwlab-cloud-api.hwlab-dev.svc.cluster.local:6667";
+export const HWLAB_M3_IO_API_BASE_URL_ENVS = Object.freeze([
+  HWLAB_M3_IO_API_BASE_URL_ENV,
+  "HWLAB_API_BASE_URL",
+  "HWLAB_CLOUD_API_BASE_URL"
+]);
 export const HWLAB_M3_IO_CAPABILITY_LEVELS = Object.freeze({
   blocked: "hwlab-api-control-blocked",
   ready: "hwlab-api-control-ready"
@@ -132,32 +139,54 @@ export function parseM3IoArgs(argv = [], env = process.env) {
     }
   }
 
-  parsed.apiBaseUrl = parsed.apiBaseUrl || firstNonEmpty(
-    env.HWLAB_CODE_AGENT_HWLAB_API_BASE_URL,
-    env.HWLAB_API_BASE_URL,
-    env.HWLAB_CLOUD_API_BASE_URL,
-    loopbackCloudApiBaseUrl(env)
-  );
+  parsed.apiBaseUrl = parsed.apiBaseUrl || configuredCloudApiBaseUrl(env);
   return parsed;
 }
 
 export function resolveCloudApiTarget(apiBaseUrl, env = process.env) {
-  const source = apiBaseUrl
-    ? "explicit"
-    : firstNonEmpty(env.HWLAB_CODE_AGENT_HWLAB_API_BASE_URL, null)
-      ? "env:HWLAB_CODE_AGENT_HWLAB_API_BASE_URL"
-      : firstNonEmpty(env.HWLAB_API_BASE_URL, null)
-        ? "env:HWLAB_API_BASE_URL"
-        : firstNonEmpty(env.HWLAB_CLOUD_API_BASE_URL, null)
-          ? "env:HWLAB_CLOUD_API_BASE_URL"
-          : "loopback-cloud-api-self";
-  const base = firstNonEmpty(apiBaseUrl, env.HWLAB_CODE_AGENT_HWLAB_API_BASE_URL, env.HWLAB_API_BASE_URL, env.HWLAB_CLOUD_API_BASE_URL, loopbackCloudApiBaseUrl(env));
-  const url = new URL(HWLAB_M3_IO_API_ROUTE, ensureTrailingSlash(base));
-  return {
-    route: HWLAB_M3_IO_API_ROUTE,
+  const source = cloudApiBaseUrlSource(apiBaseUrl, env);
+  const base = firstNonEmpty(apiBaseUrl, configuredCloudApiBaseUrl(env));
+  if (!base) {
+    return cloudApiTargetEnvelope({
+      source,
+      url: null,
+      redactedUrl: null,
+      baseUrlConfigured: false
+    });
+  }
+
+  let url;
+  try {
+    url = new URL(HWLAB_M3_IO_API_ROUTE, ensureTrailingSlash(base));
+  } catch {
+    return cloudApiTargetEnvelope({
+      source,
+      url: String(base),
+      redactedUrl: redactUrl(base),
+      baseUrlConfigured: true,
+      invalidBaseUrl: true
+    });
+  }
+
+  return cloudApiTargetEnvelope({
+    source,
     url: url.toString(),
     redactedUrl: redactUrl(url.toString()),
+    baseUrlConfigured: true
+  });
+}
+
+function cloudApiTargetEnvelope({ source, url, redactedUrl, baseUrlConfigured, invalidBaseUrl = false }) {
+  return {
+    route: HWLAB_M3_IO_API_ROUTE,
+    url,
+    redactedUrl,
     source,
+    baseUrlConfigured,
+    requiredEnv: [...HWLAB_M3_IO_API_BASE_URL_ENVS],
+    recommendedEnv: HWLAB_M3_IO_API_BASE_URL_ENV,
+    recommendedDevValue: HWLAB_M3_IO_DEV_SERVICE_BASE_URL,
+    invalidBaseUrl,
     cloudApiOnly: true,
     directGatewayCalls: false,
     directBoxCalls: false,
@@ -166,6 +195,20 @@ export function resolveCloudApiTarget(apiBaseUrl, env = process.env) {
 }
 
 export function validateCloudApiTarget(apiTarget) {
+  if (!apiTarget?.url) {
+    return {
+      code: "hwlab_api_base_url_missing",
+      message: `HWLAB API base URL is missing; set ${HWLAB_M3_IO_API_BASE_URL_ENV}=${HWLAB_M3_IO_DEV_SERVICE_BASE_URL} in the cloud-api runtime or pass --api-base-url explicitly.`
+    };
+  }
+
+  if (apiTarget.invalidBaseUrl === true) {
+    return {
+      code: "invalid_hwlab_api_url",
+      message: "HWLAB API base URL is not a valid URL."
+    };
+  }
+
   let url;
   try {
     url = new URL(apiTarget.url);
@@ -518,6 +561,7 @@ function dedupeBlockers(blockers) {
 
 function blockerCategory(code) {
   const value = String(code ?? "");
+  if (/api_base_url|invalid_hwlab_api_url/u.test(value)) return "hwlab_api_configuration";
   if (/readiness|control_disabled|not_ready/u.test(value)) return "readiness_blocked";
   if (/gateway/u.test(value)) return "gateway_unavailable";
   if (/box|resource/u.test(value)) return "box_unavailable";
@@ -563,10 +607,15 @@ function helpPayload() {
     service: HWLAB_M3_IO_SKILL_NAME,
     contractVersion: HWLAB_AGENT_RUNTIME_SKILL_CLI_VERSION,
     usage: [
-      "node skills/hwlab-agent-runtime/scripts/hwlab-agent-runtime-cli.mjs m3 io --action do.write --value true --api-base-url http://127.0.0.1:6667",
-      "node skills/hwlab-agent-runtime/scripts/hwlab-agent-runtime-cli.mjs m3 io --action do.write --value false --api-base-url http://127.0.0.1:6667",
-      "node skills/hwlab-agent-runtime/scripts/hwlab-agent-runtime-cli.mjs m3 io --action di.read --api-base-url http://127.0.0.1:6667"
+      `node skills/hwlab-agent-runtime/scripts/hwlab-agent-runtime-cli.mjs m3 io --action do.write --value true --api-base-url ${HWLAB_M3_IO_DEV_SERVICE_BASE_URL}`,
+      `node skills/hwlab-agent-runtime/scripts/hwlab-agent-runtime-cli.mjs m3 io --action do.write --value false --api-base-url ${HWLAB_M3_IO_DEV_SERVICE_BASE_URL}`,
+      `node skills/hwlab-agent-runtime/scripts/hwlab-agent-runtime-cli.mjs m3 io --action di.read --api-base-url ${HWLAB_M3_IO_DEV_SERVICE_BASE_URL}`
     ],
+    env: {
+      recommended: HWLAB_M3_IO_API_BASE_URL_ENV,
+      supported: [...HWLAB_M3_IO_API_BASE_URL_ENVS],
+      devValue: HWLAB_M3_IO_DEV_SERVICE_BASE_URL
+    },
     route: HWLAB_M3_IO_API_ROUTE,
     safety: {
       cloudApiRouteOnly: true,
@@ -601,9 +650,16 @@ function parseTimeout(value) {
   return parsed;
 }
 
-function loopbackCloudApiBaseUrl(env = process.env) {
-  const port = env.HWLAB_CLOUD_API_PORT || env.PORT || "6667";
-  return `http://127.0.0.1:${port}`;
+export function configuredCloudApiBaseUrl(env = process.env) {
+  return firstNonEmpty(...HWLAB_M3_IO_API_BASE_URL_ENVS.map((name) => env[name]));
+}
+
+function cloudApiBaseUrlSource(apiBaseUrl, env = process.env) {
+  if (apiBaseUrl) return "explicit";
+  for (const name of HWLAB_M3_IO_API_BASE_URL_ENVS) {
+    if (firstNonEmpty(env[name])) return `env:${name}`;
+  }
+  return "missing-config";
 }
 
 function ensureTrailingSlash(value) {