diff --git a/.changeset/25077-rest-docs-viewer-attr-escape.md b/.changeset/25077-rest-docs-viewer-attr-escape.md
new file mode 100644
index 00000000000..f048f0606fe
--- /dev/null
+++ b/.changeset/25077-rest-docs-viewer-attr-escape.md
@@ -0,0 +1,9 @@
+---
+'@objectstack/rest': patch
+---
+
+fix(rest): the API docs viewer escapes the request-derived openapi URL for the attribute it lands in
+
+The `GET {base}/docs` viewer page builds the sibling openapi URL from the request and places it into a `data-url="…"` attribute of a script element. It interpolated that value into the attribute with no escaping, so a character the attribute context interprets reached the served markup as itself rather than as the matching HTML entity. The value is now escaped for its attribute context (ruling 3C on #25040: every outlet that renders a value into HTML escapes it by the context it lands in): the five characters a quoted attribute or element text interprets become their entities, and a browser decodes each back to the original character for the page script. A URL carrying none of those characters is served byte-identically.
+
+Clause-②: no
diff --git a/packages/rest/src/discovery-routes.ts b/packages/rest/src/discovery-routes.ts
index 8bf688c0d6d..2b1c7eb3fb2 100644
--- a/packages/rest/src/discovery-routes.ts
+++ b/packages/rest/src/discovery-routes.ts
@@ -67,6 +67,8 @@
 // half is the class's `probeMcpServeable`, reached through the host.
 import { isMcpServerEnabled } from '@objectstack/types';
 import { handleRouteError, sendThrownError } from './error-response.js';
+// [#25077] The docs viewer page escapes the value it places into its attribute.
+import { escapeHtml } from './html-escape.js';
 import { logError } from './log.js';
 // [#5040 E6] The `api`-metadata endpoints the OpenAPI document folds in.
 import { enrichOpenApiWithEndpoints } from './openapi-endpoints.js';
@@ -690,6 +692,10 @@ export function registerOpenApiEndpoints(this: OpenApiRouteHost, basePath: strin
                 // Strip the trailing /docs to get the API base.
                 const apiBase = reqPath.replace(/\/docs\/?$/, '');
                 const specUrl = `${apiBase}/openapi.json`;
+                // [#25077] Escaped for the attribute it lands in (ruling 3C on
+                // #25040: every outlet that renders a value into HTML escapes it
+                // by context). A path without HTML-special characters renders
+                // byte for byte as before. `html-outlets.pin.test.ts` pins both.
                 const html = `<!doctype html>
 <html>
 <head>
@@ -698,7 +704,7 @@ export function registerOpenApiEndpoints(this: OpenApiRouteHost, basePath: strin
 <title>ObjectStack API Docs</title>
 </head>
 <body>
-<script id="api-reference" data-url="${specUrl}"></script>
+<script id="api-reference" data-url="${escapeHtml(specUrl)}"></script>
 <script src="https://cdn.jsdelivr.net/npm/@scalar/api-reference"></script>
 </body>
 </html>`;
diff --git a/packages/rest/src/html-escape.ts b/packages/rest/src/html-escape.ts
new file mode 100644
index 00000000000..6f70a3c79b1
--- /dev/null
+++ b/packages/rest/src/html-escape.ts
@@ -0,0 +1,43 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#25077] The HTML escaper this package's HTML outlets call — ruling 3C on
+ * #25040: every outlet that renders a value into HTML escapes it by the context
+ * it lands in.
+ *
+ * It covers the two contexts an outlet here may place a value into: element
+ * text and a QUOTED attribute value (single or double quotes). The five
+ * characters those contexts interpret become their entities, and every other
+ * character passes through untouched, so a value carrying none of the five
+ * renders byte for byte as it did unescaped. A browser decodes each entity back
+ * to its character, so what the page's script reads from the DOM is the
+ * original value.
+ *
+ * ⛔ Not for an unquoted attribute, a `<script>` or `<style>` body, a URL scheme
+ * decision or an event-handler attribute: each of those contexts interprets
+ * other characters and needs its own encoder. Place a value only into one of
+ * the two contexts above.
+ *
+ * Why a local module rather than a shared one: no package this one depends on
+ * exports an HTML escaper, and the repo's other escaper is private to the mail
+ * template engine, which this package does not import. It is not re-exported
+ * from the package entry, so it adds nothing to the published surface.
+ *
+ * The census in `html-outlets.pin.test.ts` lists this file as a non-outlet: it
+ * renders nothing itself.
+ */
+
+const HTML_ENTITY: Readonly<Record<string, string>> = {
+    '&': '&amp;',
+    '<': '&lt;',
+    '>': '&gt;',
+    '"': '&quot;',
+    "'": '&#39;',
+};
+
+const HTML_SPECIAL = /[&<>"']/g;
+
+/** Escape `value` for HTML element text or a quoted HTML attribute value. */
+export function escapeHtml(value: string): string {
+    return value.replace(HTML_SPECIAL, (ch) => HTML_ENTITY[ch]);
+}
diff --git a/packages/rest/src/html-outlets.pin.test.ts b/packages/rest/src/html-outlets.pin.test.ts
new file mode 100644
index 00000000000..652b7cf8a27
--- /dev/null
+++ b/packages/rest/src/html-outlets.pin.test.ts
@@ -0,0 +1,214 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#25077] Every outlet in this package that renders a value into HTML escapes
+ * it by the context it lands in — ruling 3C on #25040.
+ *
+ * ## The rule
+ *
+ * The burden sits on the OUTLET: the code that places a value into markup. A
+ * value reaching an HTML outlet may carry any character, and nothing upstream
+ * strips or refuses it, so the outlet escapes it for the context it lands in —
+ * element text or a quoted attribute value. This package has exactly one such
+ * outlet today: the `GET {base}/docs` viewer in `discovery-routes.ts`, which
+ * places the request-derived openapi URL into a `data-url="…"` attribute.
+ *
+ * ## The list, and the pin on the list itself
+ *
+ * {@link OUTLETS} names every file of this package that renders a value into
+ * HTML, each with a pin that drives its real render path. A per-entry pin
+ * cannot see an outlet nobody listed, so the LIST is pinned too: the census
+ * walks this package's source and finds every file that touches HTML at all —
+ * an identifier or a string naming `html`, or a string carrying an HTML tag —
+ * and requires each one to be on {@link OUTLETS} or on {@link NOT_OUTLETS} with
+ * the reason it renders nothing. A new file that touches HTML fails here until
+ * its author says which it is. A positive control proves the census actually
+ * finds the known outlet.
+ *
+ * ⚠️ LIMIT, stated so it is not read as more: the census reads source text of
+ * THIS package. It proves no unlisted file here touches HTML; it does not see
+ * another package (each package holding an outlet carries its own list), and it
+ * does not prove a listed outlet escapes — that is what the per-entry pin is
+ * for.
+ */
+
+import { readFileSync, readdirSync, statSync } from 'node:fs';
+import { dirname, join, relative, sep } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import ts from 'typescript';
+import { describe, it, expect, vi } from 'vitest';
+import { RestServer } from './rest-server.js';
+
+/** …/packages/rest/src — this package's whole source tree. */
+const SRC = dirname(fileURLToPath(import.meta.url));
+
+/** A file that renders a value into HTML, and the pin that holds it to the rule. */
+interface OutletEntry {
+  /** What it renders. */
+  readonly renders: string;
+  /** Registers the pin that drives its real render path — an outlet cannot be listed without one. */
+  readonly pin: () => void;
+}
+
+/** A file that touches HTML without rendering a value into it. */
+interface NotOutletEntry {
+  readonly why: string;
+}
+
+const OUTLETS: Readonly<Record<string, OutletEntry>> = {
+  'discovery-routes.ts': {
+    renders: 'the GET {base}/docs Scalar viewer page, which places the request-derived openapi URL into a data-url attribute',
+    pin: pinDocsViewer,
+  },
+};
+
+const NOT_OUTLETS: Readonly<Record<string, NotOutletEntry>> = {
+  'html-escape.ts': { why: 'the escaper the outlet calls; renders nothing itself' },
+  'import-template.ts': { why: 'names "html" as a sample field-type string in SAMPLE_TEXT_TYPES; renders nothing' },
+  'rest-route-ledger.ts': { why: 'a route-ledger note describing the docs page in prose; renders nothing' },
+};
+
+// ── The census ──────────────────────────────────────────────────────────────
+
+const HTML_WORD = /html/i;
+/** An HTML element opening or closing tag inside a string. */
+const HTML_TAG =
+  /<(?:!doctype|\/?(?:html|head|body|title|meta|style|script|link|p|a|b|i|u|em|strong|small|span|div|br|hr|img|table|thead|tbody|tfoot|tr|td|th|ul|ol|li|h[1-6]|form|input|button|textarea|select|option|label|pre|code|blockquote|section|article|header|footer|nav|main|center|font|iframe|svg|mj-[a-z-]+))(?=[\s>/])/i;
+
+/** Does this source touch HTML in CODE — comments never count. */
+function touchesHtml(fileName: string, text: string): boolean {
+  const source = ts.createSourceFile(fileName, text, ts.ScriptTarget.Latest, true);
+  let hit = false;
+  const visit = (node: ts.Node): void => {
+    if (hit) return;
+    if (ts.isIdentifier(node) || ts.isPrivateIdentifier(node)) {
+      if (HTML_WORD.test(node.text)) hit = true;
+    } else if (
+      ts.isStringLiteral(node) || ts.isNoSubstitutionTemplateLiteral(node)
+      || ts.isTemplateHead(node) || ts.isTemplateMiddle(node) || ts.isTemplateTail(node)
+    ) {
+      if (HTML_WORD.test(node.text) || HTML_TAG.test(node.text)) hit = true;
+    }
+    if (!hit) ts.forEachChild(node, visit);
+  };
+  visit(source);
+  return hit;
+}
+
+/** Every shipped source file, package-relative. Tests and test kits ship nothing. */
+function shippedSources(dir = SRC): string[] {
+  const out: string[] = [];
+  for (const entry of readdirSync(dir)) {
+    const full = join(dir, entry);
+    if (statSync(full).isDirectory()) {
+      out.push(...shippedSources(full));
+      continue;
+    }
+    if (!entry.endsWith('.ts') || entry.endsWith('.d.ts')) continue;
+    if (entry.endsWith('.test.ts') || entry.endsWith('.testkit.ts')) continue;
+    out.push(relative(SRC, full).split(sep).join('/'));
+  }
+  return out.sort();
+}
+
+describe('the outlet list — every file here that touches HTML is classified', () => {
+  it('the census reads code, not comments (control)', () => {
+    expect(touchesHtml('a.ts', '// renders html\nexport const x = 1;')).toBe(false);
+    expect(touchesHtml('a.ts', 'export const x = `<p>${1}</p>`;')).toBe(true);
+    expect(touchesHtml('a.ts', 'export const bodyHtml = 1;')).toBe(true);
+    expect(touchesHtml('a.ts', "export const t = 'text/html';")).toBe(true);
+  });
+
+  it('the census finds the known outlet (positive control)', () => {
+    const outlet = 'discovery-routes.ts';
+    expect(Object.keys(OUTLETS)).toContain(outlet);
+    expect(touchesHtml(outlet, readFileSync(join(SRC, outlet), 'utf8'))).toBe(true);
+  });
+
+  it('the files that touch HTML are exactly the listed outlets and non-outlets', () => {
+    const touching = shippedSources().filter((rel) => touchesHtml(rel, readFileSync(join(SRC, rel), 'utf8')));
+    const listed = [...Object.keys(OUTLETS), ...Object.keys(NOT_OUTLETS)].sort();
+    expect(touching).toEqual(listed);
+  });
+
+  it('no file is both an outlet and a non-outlet', () => {
+    expect(Object.keys(OUTLETS).filter((f) => f in NOT_OUTLETS)).toEqual([]);
+  });
+});
+
+// ── The per-entry pin ─────────────────────────────────────────────────────
+
+const UNSCOPED = '/api/v1';
+const SCOPED = '/api/v1/environments/:environmentId';
+
+/** Mount both bases, a signed-in context, and drive the GET {base}/docs handler. */
+function makeRest() {
+  const httpServer = {
+    get: vi.fn(), post: vi.fn(), put: vi.fn(), delete: vi.fn(), patch: vi.fn(),
+    use: vi.fn(), listen: vi.fn(), close: vi.fn(),
+  } as any;
+  const protocol: any = { getMetaItems: vi.fn(async () => ({ items: [] })) };
+  const rest = new RestServer(
+    httpServer, protocol,
+    { api: { version: 'v1', enableProjectScoping: true, projectResolution: 'auto' } } as any,
+  );
+  (rest as any).resolveExecCtx = vi.fn(async () => ({ userId: 'u_docs_reader' }));
+  rest.registerRoutes();
+  return rest;
+}
+
+/** The viewer's `data-url` attribute value, as served for a request whose path is `reqPath`. */
+async function dataUrlFor(rest: RestServer, mountPath: string, reqPath: string, environmentId?: string): Promise<string> {
+  const entry = (rest as any).routeManager.get('GET', mountPath);
+  expect(entry, `GET ${mountPath} must be mounted`).toBeDefined();
+  let html = '';
+  const res: any = {
+    status: () => res,
+    json: () => res,
+    send: (body: string) => { html = body; return res; },
+    setHeader: () => res,
+    header: () => res,
+  };
+  await entry.handler({
+    method: 'GET', path: reqPath, url: reqPath,
+    params: environmentId ? { environmentId } : {}, query: {},
+    headers: { host: 'example.test' },
+  }, res);
+  const m = html.match(/data-url="([^\n]*)"><\/script>/);
+  expect(m, 'the viewer page carries a data-url attribute').not.toBeNull();
+  return m![1];
+}
+
+function pinDocsViewer(): void {
+  /**
+   * A path segment carrying the characters a quoted HTML attribute interprets.
+   * Kept to the minimum the context reads — a quote, an angle bracket, an
+   * ampersand, an apostrophe — not an attack string.
+   */
+  const SPECIAL_ENV = 'a"<&\'b';
+  const SPECIAL_ENV_ESCAPED = 'a&quot;&lt;&amp;&#39;b';
+
+  it('a value carrying attribute-interpreted characters is escaped for the attribute context', async () => {
+    const rest = makeRest();
+    const reqPath = `/api/v1/environments/${SPECIAL_ENV}/docs`;
+    const dataUrl = await dataUrlFor(rest, `${SCOPED}/docs`, reqPath, SPECIAL_ENV);
+    expect(dataUrl).toBe(`/api/v1/environments/${SPECIAL_ENV_ESCAPED}/openapi.json`);
+    // The literal characters do not survive into the served attribute value.
+    expect(dataUrl).not.toContain('"');
+    expect(dataUrl).not.toContain('<');
+  });
+
+  it('an ordinary URL is served byte-identical', async () => {
+    const rest = makeRest();
+    const unscoped = await dataUrlFor(rest, `${UNSCOPED}/docs`, '/api/v1/docs');
+    expect(unscoped).toBe('/api/v1/openapi.json');
+    const scoped = await dataUrlFor(rest, `${SCOPED}/docs`, '/api/v1/environments/env_1/docs', 'env_1');
+    expect(scoped).toBe('/api/v1/environments/env_1/openapi.json');
+  });
+}
+
+// Registered last, so every helper above is initialised before the pin reads it.
+for (const [file, entry] of Object.entries(OUTLETS)) {
+  describe(`${file} — ${entry.renders}`, entry.pin);
+}