diff --git a/‎.changeset/standalone-stack-stamps-env-local.md‎
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diff --git a/‎packages/metadata/src/plugin.ts‎
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diff --git a/‎packages/runtime/src/standalone-stack-default-environment-id.test.ts‎
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diff --git a/‎packages/runtime/src/standalone-stack.ts‎
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@@ -0,0 +1,47 @@
+---
+"@objectstack/runtime": patch
+"@objectstack/metadata": patch
+---
+
+fix(runtime,metadata): the default local environment id is `env_local`, not `proj_local` (#13366)
+
+The v5.0 `project` to `environment` rename changed the default local environment
+id and shipped that change on the surfaces most people meet: `packages/cli`'s
+`CHANGELOG.md` records "Default local env id: `proj_local` -> `env_local`", the
+`os dev` / `os start` / `os serve` commands emit `env_local`, and
+`content/docs/deployment/cli.mdx` documents `env_local` as the default. Two
+sites never received it and kept stamping `proj_local`.
+
+FROM: `createStandaloneStack()` — with no `environmentId` in its config and no
+`OS_ENVIRONMENT_ID` in the environment — stamped `proj_local` on the kernel it
+composed, and `MetadataPlugin` used `proj_local` to fill the environment-artifact
+validation envelope for a bare definition.
+
+TO: both stamp `env_local`.
+
+WHO SEES IT. Two audiences, both on the DEFAULT path — no `environmentId` in
+the config and no `OS_ENVIRONMENT_ID` in the environment:
+
+1. a host that calls `createStandaloneStack` / `createDefaultHostConfig`
+   **directly**;
+2. a **bare `os serve`** — one not spawned by `os dev` / `os start`. Those two
+   commands export `OS_ENVIRONMENT_ID=env_local` into the child process, which
+   the fallback yields to, so a boot they start never reached the changed line.
+   `os serve` sets no such variable for its own boot: it only READS one to name
+   the runtime state file. So a bare `os serve` used to run a kernel stamped
+   `proj_local` while publishing `runtime.env_local.json` beside it; the two now
+   agree.
+
+Where the id is observable — row scoping in `ObjectQLPlugin`, the
+`X-Environment-Id` header, `sys_metadata.environment_id` — such an embedder now
+sees `env_local` where it saw `proj_local`, so an install with rows already
+written under the old id should set `environmentId: 'proj_local'` (or
+`OS_ENVIRONMENT_ID=proj_local`) explicitly to keep them addressed. That escape
+hatch is unchanged and still wins over the default.
+
+NOT CHANGED, deliberately: `@objectstack/cloud-connection` still treats BOTH
+spellings as the local sentinel, so a persisted `OS_ENVIRONMENT_ID=proj_local`
+config keeps being recognised as local rather than presented to the control
+plane as a cloud environment id; and `package-state-store`'s separate `'default'`
+fallback keeps its own spelling, because renaming it would re-key persisted
+package-disable state files.
@@ -915,7 +915,7 @@ export class MetadataPlugin implements Plugin {
             const def = ObjectStackDefinitionSchema.parse(this._convertArtifactForward(ctx, obj, label));
             const canonical = JSON.stringify(def, Object.keys(def).sort());
             const checksum = createHash('sha256').update(canonical).digest('hex');
-            const environmentId = this.options.environmentId ?? 'proj_local';
+            const environmentId = this.options.environmentId ?? 'env_local';
             EnvironmentArtifactSchema.parse({
                 schemaVersion: '0.1',
                 environmentId,
 
@@ -0,0 +1,111 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// [#13366] The default environment id a standalone boot stamps, pinned at the
+// place it is OBSERVABLE: the two plugins `createStandaloneStack` hands it to.
+//
+// Why this file exists at all. The v5.0 `project` to `environment` rename
+// shipped the CLI default `env_local` — `packages/cli/CHANGELOG.md` records
+// "Default local env id: `proj_local` -> `env_local`" and
+// `content/docs/deployment/cli.mdx` documents `env_local` — but the runtime's
+// own fallback kept stamping `proj_local`. Nothing pinned it, in either
+// spelling, so `declared != enforced` held on a published default for a whole
+// major line without one test going red. That is the gap this closes: the
+// literal now has an assertion attached to the code path that emits it.
+//
+// It reads the id off `result.plugins` rather than off a copy of the constant,
+// because the value is only interesting where it LANDS. `MetadataPlugin` takes
+// it as `options.environmentId` and `ObjectQLPlugin` as a row-scope key; a
+// pin that re-declared the string would stay green through a change that
+// stopped passing it to either.
+//
+// ⛔ These cases must NOT be read as "the CLI default". `os dev` / `os start`
+// export `OS_ENVIRONMENT_ID` into the child boot, so a CLI-spawned kernel never
+// reaches this fallback — the CLI's own default is pinned separately (the
+// `runtime.env_local.json` publication tests in packages/cli). What this file
+// owns is the DIRECT-EMBEDDER path: `createStandaloneStack()` with no config
+// and no env var, which is the surface a `createStandaloneStack` host observes.
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mkdtempSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { createStandaloneStack } from './standalone-stack.js';
+
+const BOOT_TIMEOUT = 60_000;
+
+// The two plugin ids the stack composes. Matched by the plugin's own declared
+// `name`, not by array position: the composition order is documented as a
+// dependency-graph outcome elsewhere in this package, and an index would pin
+// that instead of this.
+const METADATA_PLUGIN = 'com.objectstack.metadata';
+const OBJECTQL_PLUGIN = 'com.objectstack.engine.objectql';
+
+/**
+ * The id as each plugin actually received it.
+ *
+ * `MetadataPlugin` keeps it under `options.environmentId`; `ObjectQLPlugin`
+ * copies it to its own `environmentId` field. Both are TypeScript-private —
+ * hence the casts — and reading them is deliberate: they are the last point at
+ * which the stamped value is still identifiable before it dissolves into row
+ * scoping and an artifact-validation envelope.
+ */
+function stampedIds(plugins: any[]): { metadata: unknown; objectql: unknown } {
+    const metadata = plugins.find((p) => p?.name === METADATA_PLUGIN);
+    const objectql = plugins.find((p) => p?.name === OBJECTQL_PLUGIN);
+    expect(metadata, `stack must carry ${METADATA_PLUGIN}`).toBeDefined();
+    expect(objectql, `stack must carry ${OBJECTQL_PLUGIN}`).toBeDefined();
+    return {
+        metadata: (metadata as any).options?.environmentId,
+        objectql: (objectql as any).environmentId,
+    };
+}
+
+describe('[#13366] createStandaloneStack — default environment id', () => {
+    let dir: string;
+    let savedEnvId: string | undefined;
+    let savedHome: string | undefined;
+
+    beforeEach(() => {
+        dir = mkdtempSync(join(tmpdir(), 'os-standalone-envid-'));
+        savedEnvId = process.env.OS_ENVIRONMENT_ID;
+        savedHome = process.env.OS_HOME;
+        delete process.env.OS_ENVIRONMENT_ID;
+        process.env.OS_HOME = dir;
+    });
+
+    afterEach(() => {
+        if (savedEnvId === undefined) delete process.env.OS_ENVIRONMENT_ID;
+        else process.env.OS_ENVIRONMENT_ID = savedEnvId;
+        if (savedHome === undefined) delete process.env.OS_HOME;
+        else process.env.OS_HOME = savedHome;
+        try { rmSync(dir, { recursive: true, force: true }); } catch { /* noop */ }
+    });
+
+    it('stamps `env_local` when neither the config nor OS_ENVIRONMENT_ID names one', async () => {
+        const stack = await createStandaloneStack({ databaseUrl: 'memory://standalone-envid-default' });
+        // The literal, at both landing sites. `proj_local` here is the pre-#13366
+        // value and is what this case exists to keep from coming back.
+        expect(stampedIds(stack.plugins)).toEqual({ metadata: 'env_local', objectql: 'env_local' });
+    }, BOOT_TIMEOUT);
+
+    it('OS_ENVIRONMENT_ID still overrides the default', async () => {
+        process.env.OS_ENVIRONMENT_ID = 'env_from_the_environment';
+        const stack = await createStandaloneStack({ databaseUrl: 'memory://standalone-envid-env' });
+        expect(stampedIds(stack.plugins)).toEqual({
+            metadata: 'env_from_the_environment',
+            objectql: 'env_from_the_environment',
+        });
+    }, BOOT_TIMEOUT);
+
+    it('an explicit `cfg.environmentId` still outranks OS_ENVIRONMENT_ID', async () => {
+        process.env.OS_ENVIRONMENT_ID = 'env_from_the_environment';
+        const stack = await createStandaloneStack({
+            environmentId: 'env_from_the_config',
+            databaseUrl: 'memory://standalone-envid-cfg',
+        });
+        expect(stampedIds(stack.plugins)).toEqual({
+            metadata: 'env_from_the_config',
+            objectql: 'env_from_the_config',
+        });
+    }, BOOT_TIMEOUT);
+});
@@ -222,8 +222,8 @@ export const StandaloneStackConfigSchema = z.object({
      * Defaults to `true`, and that default is the fix: a standalone kernel
      * OWNS its local platform tables, which is what the gate in
      * `assembleMetadataProtocol` always meant to say. It used to deduce that
-     * from `environmentId === undefined`, and line ~515 below stamps
-     * `'proj_local'` on every boot — so the block never ran and #8686's
+     * from `environmentId === undefined`, and line ~567 below stamps
+     * `'env_local'` on every boot — so the block never ran and #8686's
      * "covers every existing deployment" half covered no self-hosted install
      * at all.
      *
@@ -564,7 +564,7 @@ export async function createStandaloneStack(config?: StandaloneStackConfig): Pro
     const { DefaultDatasourcePlugin } = await import('./default-datasource-plugin.js');
     const { AppPlugin } = await import('./app-plugin.js');
 
-    const environmentId = cfg.environmentId ?? process.env.OS_ENVIRONMENT_ID ?? 'proj_local';
+    const environmentId = cfg.environmentId ?? process.env.OS_ENVIRONMENT_ID ?? 'env_local';
     const artifactPath = resolveArtifactPathInput(cfg);
 
     // `databaseAuthToken` / `OS_DATABASE_AUTH_TOKEN` / `TURSO_AUTH_TOKEN` are
@@ -747,7 +747,7 @@ export async function createStandaloneStack(config?: StandaloneStackConfig): Pro
             ...(cfg.projectRoot ? { rootDir: cfg.projectRoot } : {}),
         }),
         // [#9380] `runPlatformMigrations` is declared here, not deduced from
-        // `environmentId`: this stack stamps `'proj_local'` above, and the
+        // `environmentId`: this stack stamps `'env_local'` above, and the
         // assembly's old `environmentId === undefined` gate read that as "a
         // per-project cloud kernel" and disarmed the three boot repairs on
         // every self-hosted install. A standalone kernel owns its local