@@ -11,6 +11,6 @@ Metadata-protocol: none
 A declarative connector entry's four maps — `providerConfig.headers`, `providerConfig.defaultHeaders`, `providerConfig.transport.headers` and `providerConfig.transport.env` — are withheld whole on the metadata read path, but a literal credential in one of them is still stored in metadata. The same `credential-literal` predicate the `flow-credential-literal` advisory asks now judges those four maps and names each credential-shaped entry by its position (never its value), with the `auth.credentialRef` route:
 
 - a header entry: move it to the connector's `auth: { type: 'bearer' | 'api-key' | 'basic', credentialRef }`, which the `rest` provider and an `mcp` http transport send themselves;
-- a stdio `transport.env` entry: a stdio MCP server takes no `auth`, so the warning says `auth.credentialRef` reaches the server only over an http transport.
+- a stdio `transport.env` entry: move it to the connector's `auth: { type: 'bearer' | 'api-key' | 'basic', credentialRef }` and name the variable in `providerConfig.transport.credentialEnv`; the `mcp` provider hands the resolved credential to the stdio server in that variable itself.
 
 Always a `warning`; nothing is refused and every save lands. The rule runs on all three CLI commands and on the runtime publish gate for `connector` (`PUT /meta/connector/:name`), where it rides the save response's `advisories`. `os explain connector-credential-literal` carries the reasoning.
@@ -0,0 +1,18 @@
+---
+'@objectstack/connector-mcp': minor
+'@objectstack/lint': patch
+---
+
+Clause-②: yes (widening)
+Metadata-protocol: added
+
+`provider: 'mcp'`: a stdio MCP server can now take its credential from the secrets layer instead of a literal in `transport.env`. Declare the instance's `auth` with a `credentialRef` as for an http server, and name the environment variable the server reads in the new `providerConfig.transport.credentialEnv` key: the host resolves the reference (environment, Setup → Connector credentials, the host's resolver) and the provider hands the resolved secret (the `bearer` token, the `api-key` key, or the `basic` password) to the child in that variable. The value is never stored, logged, served on a metadata read or put in a message; a reference that resolves to nothing degrades the instance naming the reference and the server is not started.
+
+```ts
+providerConfig: { transport: { kind: 'stdio', command: 'repo-mcp-server', credentialEnv: 'REPO_MCP_TOKEN' } },
+auth: { type: 'bearer', credentialRef: 'REPO_MCP_TOKEN' },
+```
+
+Refused at materialization as configuration faults (a failed boot for the host's own entry): a `credentialEnv` that is not an environment-variable name, one on an `http` transport, one with no `bearer` / `api-key` / `basic` `auth`, and one naming a variable `transport.env` also sets. A stdio transport that names no `credentialEnv` is unchanged: it does not apply `auth`.
+
+`@objectstack/lint`: the `connector-credential-literal` advisory's finding for a credential literal in `transport.env` now steers to this route (`auth.credentialRef` with `transport.credentialEnv`) instead of telling the author to move to an http transport.
@@ -228,9 +228,10 @@ allowlist — only the listed tools become actions) and **`toolsSnapshot`**
   HTTP. Resolved `auth` is folded into the request headers; a grant-bearing
   `auth` (`oauth2`, `app-credential`) is asked for on every request instead
   (see *Grant-bearing kinds* below).
-- `{ kind: 'stdio', command, args?, env? }` — a **local child process**,
-  policy-gated (below). Credentials for a stdio server ride `transport.env`,
-  not `auth`.
+- `{ kind: 'stdio', command, args?, env?, credentialEnv? }` — a **local child
+  process**, policy-gated (below). Resolved `auth` is handed to the server in
+  its environment, under the variable `credentialEnv` names (see
+  [*A stdio server's credential*](#a-stdio-servers-credential) below).
 
 {/* os:check */}
 ```typescript
@@ -346,6 +347,49 @@ characters fails the boot before connecting, and so does a pinned tool that
 `include` leaves out — the instance never exposes it, so its pin would never be
 checked.
 
+#### A stdio server's credential
+
+A stdio MCP server reads its credential from an environment variable, and the
+value stays out of metadata the way an http server's does: declare `auth` with
+a `credentialRef`, and name the variable in `transport.credentialEnv`. The
+platform resolves the reference like any instance's (the environment, then a
+credential set under Setup → Connector credentials, then the host's resolver)
+and the provider hands the resolved secret to the child in that variable — the
+token of a `bearer`, the key of an `api-key`, the password of a `basic` (whose
+username, not a secret, goes in `env` if the server reads it).
+
+{/* os:check */}
+```typescript
+import { defineConnector } from '@objectstack/spec/integration';
+
+export const RepoToolsConnector = defineConnector({
+  name: 'repo_tools',
+  label: 'Repository Tools (MCP, stdio)',
+  type: 'api',
+  provider: 'mcp',
+  providerConfig: {
+    transport: {
+      kind: 'stdio',
+      command: 'repo-mcp-server',
+      env: { LOG_LEVEL: 'info' },
+      credentialEnv: 'REPO_MCP_TOKEN',
+    },
+  },
+  auth: { type: 'bearer', credentialRef: 'REPO_MCP_TOKEN' },
+});
+```
+
+The value is never stored, logged, served on a metadata read (which shows
+`credentialRef` and `credentialEnv`, two names) or carried in a message; it
+exists only in the environment the server is started with. A reference that
+resolves to nothing never starts the server: the instance is degraded naming
+the reference, and a credential set later starts it. These fail the boot:
+a `credentialEnv` that is not a variable name, one on an `http` transport,
+one with no `bearer` / `api-key` / `basic` `auth` to place, and one naming a
+variable `env` also sets. A stdio transport that names no `credentialEnv`
+does not apply `auth`. A literal credential typed into `env` instead is
+reported by the `connector-credential-literal` advisory, which points here.
+
 <Callout type="warn">
 **Declarative stdio transports are denied by default.** Materializing one
 spawns a local process *from metadata* — which a runtime Studio publish can
@@ -581,8 +625,9 @@ platform's to fill, so it is left out of the action's input schema, as
 `Authorization` is.
 
 An **`mcp`** instance carries a grant-bearing credential only over an `http`
-transport (a stdio server takes its credentials in `transport.env`; a handle
-on a stdio transport, or beside a static credential, fails the boot). Every
+transport (a stdio server is handed a static credential once, at spawn, in
+`transport.credentialEnv`; a handle on a stdio transport, or beside a static
+credential, fails the boot). Every
 HTTP request of its MCP session asks the handle. Each organization a dispatch
 runs for opens its own session with its own token, so organizations never
 share one. The tools are listed at materialization, outside any run, so the
 
@@ -21,7 +21,10 @@ import type { Connector } from '@objectstack/spec/integration';
  * **Credentials live with the MCP server, not in `ConnectorSchema`** (ADR-0024
  * §3). The operator supplies `env` (stdio) / `headers` (http) which we pass
  * straight to the transport; they are never copied into the serialized `def`
- * (which is exposed via discovery) and must never be logged.
+ * (which is exposed via discovery) and must never be logged. A declarative
+ * instance's resolved `auth` reaches them through the `mcp` provider factory:
+ * as a header over http, and over stdio in the variable its
+ * `transport.credentialEnv` names.
  *
  * **Trust:** launching a stdio server runs a local process. Sandboxed,
  * multi-tenant execution and managed secrets are the enterprise tier (ADR-0024
 
@@ -79,8 +79,9 @@ function isStringRecord(v: unknown): v is Record<string, string> {
  * Fold the resolved instance `auth` into an MCP **http** transport's headers
  * (ADR-0024 keeps MCP credentials with the transport). `credentialRef` has
  * already been resolved upstream, so this only maps the static credential to the
- * right header. Not applied to stdio transports — a stdio server receives its
- * credentials through `transport.env`.
+ * right header. Not applied to stdio transports — a stdio server receives the
+ * instance's credential in its environment, under the variable
+ * `transport.credentialEnv` names (see {@link credentialEnvOf}).
  */
 function applyAuthToHeaders(
   auth: ResolvedConnectorAuth | undefined,
@@ -101,11 +102,85 @@ function applyAuthToHeaders(
   }
 }
 
-/** Validate + normalize `providerConfig.transport`, injecting resolved auth for http. */
+/** An environment variable name as POSIX shells spell one. */
+const ENV_NAME_RE = /^[A-Za-z_][A-Za-z0-9_]*$/;
+
+/**
+ * The secret of a resolved static credential — what a stdio server is handed
+ * in its environment: a `bearer` token, an `api-key` key, a `basic` password.
+ * An `api-key`'s `headerName` / `paramName` place the key on an http request
+ * and are not read here; a `basic` username is not a secret, and a server that
+ * needs it takes it from `transport.env`.
+ */
+function secretOf(auth: Exclude<ResolvedConnectorAuth, { type: 'none' }>): string {
+  switch (auth.type) {
+    case 'bearer':
+      return auth.token;
+    case 'api-key':
+      return auth.key;
+    case 'basic':
+      return auth.password;
+  }
+}
+
+/**
+ * `transport.credentialEnv` of a stdio transport, judged — the route a stdio
+ * server's credential takes out of metadata (ADR-0097 §3 at ADR-0024 §3's
+ * process boundary). The instance's `auth.credentialRef` is resolved by the
+ * host like any other instance's (the environment, the stored credential, the
+ * host's resolver; a reference nothing answers degrades the instance naming it
+ * before this factory runs), and the resolved secret is put into the child's
+ * environment under the variable `credentialEnv` names. The value is never
+ * stored, logged, put on the connector `def` or carried in a message: it
+ * exists only in the environment handed to the spawn.
+ *
+ * Refused plain (configuration faults), each naming the key and never a value:
+ * a `credentialEnv` that is not a variable name; one with no static `auth` to
+ * place (a grant-bearing kind is refused by the credential-handle check, with
+ * this route named); one naming a variable `transport.env` also sets, which
+ * would give that variable two values. `undefined` when the transport names
+ * none: then `auth` is not applied to the stdio server at all, as before.
+ */
+function credentialEnvOf(
+  t: Record<string, unknown>,
+  env: Record<string, string> | undefined,
+  connectorName: string,
+  auth: ResolvedConnectorAuth | undefined,
+  borrowsCredential: boolean,
+): { name: string; value: string } | undefined {
+  const name = t.credentialEnv;
+  if (name === undefined) return undefined;
+  if (typeof name !== 'string' || !ENV_NAME_RE.test(name)) {
+    throw new Error(
+      `connector-mcp provider: connector '${connectorName}' providerConfig.transport.credentialEnv must be the name of ` +
+        `an environment variable (letters, digits and '_', not starting with a digit): the variable the stdio server ` +
+        `reads the instance's credential from.`,
+    );
+  }
+  if (borrowsCredential) return undefined;
+  if (!auth || auth.type === 'none') {
+    throw new Error(
+      `connector-mcp provider: connector '${connectorName}' names providerConfig.transport.credentialEnv '${name}' but ` +
+        `declares no credential to place there. Give the entry auth: { type: 'bearer', credentialRef } (or 'api-key' / ` +
+        `'basic'): the provider passes the resolved credential to the server in '${name}'.`,
+    );
+  }
+  if (env && Object.prototype.hasOwnProperty.call(env, name)) {
+    throw new Error(
+      `connector-mcp provider: connector '${connectorName}' sets environment variable '${name}' twice: in ` +
+        `providerConfig.transport.env and as providerConfig.transport.credentialEnv. Drop the env entry: the provider ` +
+        `passes the resolved credential to the server in '${name}' itself.`,
+    );
+  }
+  return { name, value: secretOf(auth) };
+}
+
+/** Validate + normalize `providerConfig.transport`, injecting resolved auth (http headers, stdio `credentialEnv`). */
 function normalizeTransport(
   raw: unknown,
   connectorName: string,
   auth: ResolvedConnectorAuth | undefined,
+  borrowsCredential: boolean,
 ): McpTransport {
   if (!raw || typeof raw !== 'object') {
     throw new Error(
@@ -120,14 +195,23 @@ function normalizeTransport(
         `connector-mcp provider: connector '${connectorName}' stdio transport requires a 'command' string.`,
       );
     }
+    const env = isStringRecord(t.env) ? t.env : undefined;
+    const credential = credentialEnvOf(t, env, connectorName, auth, borrowsCredential);
     return {
       kind: 'stdio',
       command: t.command,
       args: Array.isArray(t.args) ? t.args.map((a) => String(a)) : undefined,
-      env: isStringRecord(t.env) ? t.env : undefined,
+      env: credential ? { ...env, [credential.name]: credential.value } : env,
     };
   }
   if (t.kind === 'http') {
+    if (t.credentialEnv !== undefined) {
+      throw new Error(
+        `connector-mcp provider: connector '${connectorName}' names providerConfig.transport.credentialEnv on an http ` +
+          `transport. credentialEnv places the instance's credential in a stdio server's environment; over http the ` +
+          `provider sends the resolved auth as a request header itself. Delete the key.`,
+      );
+    }
     if (typeof t.url !== 'string' || t.url.length === 0) {
       throw new Error(
         `connector-mcp provider: connector '${connectorName}' http transport requires a 'url' string.`,
@@ -281,6 +365,13 @@ async function credentialRejectionStatus(err: unknown): Promise<number | undefin
  * directory. The policy check runs first and is unchanged: it judges the
  * `command` string exactly as written.
  *
+ * A declarative stdio server's credential stays out of metadata the way an
+ * http one's does: the entry declares `auth: { type, credentialRef }`, the host
+ * resolves the reference, and `providerConfig.transport.credentialEnv` names
+ * the environment variable the provider hands the resolved secret to the child
+ * in (see {@link credentialEnvOf}). A stdio transport naming no
+ * `credentialEnv` does not apply `auth` at all.
+ *
  * The connection is opened at materialization. Faults are classified (#3017):
  * an invalid transport shape is a *configuration* fault and throws plain —
  * fatal at boot per the ADR-0097 fail-loud contract — while a connect /
@@ -342,7 +433,7 @@ async function credentialRejectionStatus(err: unknown): Promise<number | undefin
 export function createMcpProviderFactory(deps: McpProviderDeps = {}): ConnectorProviderFactory {
   return async (ctx) => {
     const cfg = (ctx.providerConfig ?? {}) as McpProviderConfig;
-    let transport = normalizeTransport(cfg.transport, ctx.name, ctx.auth);
+    let transport = normalizeTransport(cfg.transport, ctx.name, ctx.auth, ctx.credentialHandle !== undefined);
     if (transport.kind === 'stdio') {
       assertDeclarativeStdioAllowed(deps.declarativeStdio, transport.command, ctx.name);
       // One anchor for the app's relative paths (#22423). A resolver that
@@ -370,15 +461,17 @@ export function createMcpProviderFactory(deps: McpProviderDeps = {}): ConnectorP
     }
     // ADR-0143 D4 — a grant-bearing instance (`oauth2`, `app-credential`)
     // borrows its credential per request. Only an http transport carries one
-    // (a stdio server takes its credentials in `transport.env`), and a
-    // request is authenticated one way.
+    // (a stdio server is handed a static credential once, at spawn, in the
+    // variable `transport.credentialEnv` names), and a request is
+    // authenticated one way.
     const handle = ctx.credentialHandle;
     if (handle) {
       if (transport.kind !== 'http') {
         throw new Error(
           `connector-mcp provider: connector '${ctx.name}' borrows its credential per request (a grant-bearing auth kind), ` +
-            `which only an http transport can carry: a stdio server takes its credentials in transport.env. Declare an ` +
-            `http transport, or a static credential for the stdio one.`,
+            `which only an http transport can carry: a stdio server is handed its credential once, at spawn. Declare an ` +
+            `http transport, or a static credential for the stdio one (auth: { type: 'bearer', credentialRef }) with ` +
+            `providerConfig.transport.credentialEnv naming the variable the server reads it from.`,
         );
       }
       const ways = [
@@ -504,7 +597,9 @@ export function createMcpProviderFactory(deps: McpProviderDeps = {}): ConnectorP
     }
     // [#27237] The handshake carried the instance's credential only over
     // http, and only as a header (an api-key `paramName` is not sent): then
-    // the server's answer was given to that credential.
+    // the server's answer was given to that credential. A stdio server handed
+    // the credential in `credentialEnv` is not counted: nothing in a stdio
+    // handshake says the server used, let alone accepted, its environment.
     //
     // A grant-bearing instance's borrowed token (`ctx.credentialHandle`)
     // reaches the server too, and is deliberately not counted here. A 403 the