diff --git a/apps/daemon/src/artifact-stub-guard.ts b/apps/daemon/src/artifact-stub-guard.ts
new file mode 100644
index 000000000..2989e29ba
--- /dev/null
+++ b/apps/daemon/src/artifact-stub-guard.ts
@@ -0,0 +1,304 @@
+// Detects "stub" HTML artifact regressions: an agent emits a new artifact
+// with the same metadata.identifier as an earlier one, but the body is a
+// tiny placeholder ("see <other>.html in this project", a bare filename
+// string, an empty fallback page, etc.) instead of the full HTML.
+//
+// The guard is structural: it compares the new body's size against the
+// largest prior sibling sharing the same identifier. It does not pattern-
+// match on phrasing, so it works regardless of which agent backend produced
+// the regression. False positives are bounded by minPriorBytes (we won't
+// compare against priors that are themselves small) and minRetainedRatio.
+
+import type { Dirent } from 'node:fs';
+import { readFile, readdir, stat } from 'node:fs/promises';
+import path from 'node:path';
+
+export type ArtifactStubGuardMode = 'reject' | 'warn' | 'off';
+
+export interface ArtifactStubGuardConfig {
+  mode: ArtifactStubGuardMode;
+  minRetainedRatio: number;
+  minPriorBytes: number;
+}
+
+export interface PriorArtifactSibling {
+  name: string;
+  size: number;
+}
+
+export interface ArtifactStubGuardWarning {
+  code: 'ARTIFACT_REGRESSION';
+  message: string;
+  identifier: string;
+  newSize: number;
+  priorSize: number;
+  priorName: string;
+}
+
+export interface EvaluateArtifactStubGuardInput {
+  scanDir: string;
+  identifier: string;
+  newSize: number;
+  config: ArtifactStubGuardConfig;
+}
+
+export interface EvaluateArtifactStubGuardResult {
+  outcome: 'pass' | 'warn' | 'reject';
+  warning?: ArtifactStubGuardWarning;
+}
+
+export class ArtifactRegressionError extends Error {
+  readonly code = 'ARTIFACT_REGRESSION';
+  readonly identifier: string;
+  readonly newSize: number;
+  readonly priorSize: number;
+  readonly priorName: string;
+
+  constructor(message: string, details: { identifier: string; newSize: number; priorSize: number; priorName: string }) {
+    super(message);
+    this.name = 'ArtifactRegressionError';
+    this.identifier = details.identifier;
+    this.newSize = details.newSize;
+    this.priorSize = details.priorSize;
+    this.priorName = details.priorName;
+  }
+}
+
+export const DEFAULT_ARTIFACT_STUB_GUARD_CONFIG: ArtifactStubGuardConfig = {
+  mode: 'warn',
+  minRetainedRatio: 0.2,
+  minPriorBytes: 4096,
+};
+
+// HTML-rendered manifest kinds. Decks are HTML files on disk and have the
+// same regression failure mode as plain html artifacts (the agent emits a
+// placeholder where a multi-KB framework should be), so they're guarded
+// alongside `html`.
+export const STUB_GUARDED_MANIFEST_KINDS: ReadonlySet<string> = new Set(['html', 'deck']);
+
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+}
+
+// Mirror of the slugifier in `apps/web/src/components/ProjectView.tsx`'s
+// `persistArtifact`. The web path slugifies the identifier for the
+// filename basename but keeps the *raw* identifier in the manifest, so a
+// regex anchored on the raw identifier alone can miss its own slug-form
+// siblings on disk. We try both forms.
+export function slugifyArtifactIdentifier(value: string): string {
+  return value
+    .toLowerCase()
+    .replace(/[^a-z0-9_-]+/g, '-')
+    .replace(/^-+|-+$/g, '')
+    .slice(0, 60);
+}
+
+// Frontend fallback name used by persistArtifact when the slugified
+// identifier is empty (e.g. all-non-ASCII identifiers like "测试" strip to
+// nothing). Without matching against this, sibling discovery would miss
+// the entire "artifact*.html" family that such identifiers produce.
+export const EMPTY_SLUG_FALLBACK_NAME = 'artifact';
+
+// Two identifiers refer to the same artifact lineage when they're
+// literally equal OR one is the canonical slug form of the other (and
+// that slug is non-empty). Slugs alone matching is not enough: the
+// frontend slugifier truncates at 60 chars, so two raw identifiers that
+// only diverge after character 60 (e.g. "A...A1" and "A...A2", 70 chars
+// each) would otherwise falsely bridge. Requiring one side to *be* the
+// slug form keeps the "Landing Page" <-> "landing-page" bridge while
+// rejecting truncation-induced collisions. Empty-slug equivalence
+// (e.g. "测试" vs "首页") is also not treated as a match for the same
+// reason — distinct identifiers can both strip to empty.
+export function artifactIdentifiersMatch(a: string, b: string): boolean {
+  if (a === b) return true;
+  const slugA = slugifyArtifactIdentifier(a);
+  if (slugA.length === 0) return false;
+  const slugB = slugifyArtifactIdentifier(b);
+  if (slugA !== slugB) return false;
+  return a === slugA || b === slugB;
+}
+
+// Reads the canonical identifier from a sibling's `.artifact.json`
+// sidecar. Returns null when the sidecar is absent, malformed, or
+// missing a string identifier — callers fall back to filename-derived
+// inference for legacy artifacts that pre-date the sidecar era.
+async function readSidecarIdentifier(scanDir: string, entryName: string): Promise<string | null> {
+  try {
+    const raw = await readFile(path.join(scanDir, `${entryName}.artifact.json`), 'utf8');
+    const parsed = JSON.parse(raw) as { metadata?: { identifier?: unknown } } | null;
+    const id = parsed?.metadata?.identifier;
+    return typeof id === 'string' && id.length > 0 ? id : null;
+  } catch {
+    return null;
+  }
+}
+
+// Strips the frontend's `-N` collision suffix and the `.html` / `.htm`
+// extension to recover the per-lineage basename. Used as one of the
+// fallback identifiers for legacy HTML artifacts that don't have a
+// sidecar yet (anything written before the manifest era, or HTML
+// pasted/uploaded outside the artifact-tag flow). `inferLegacyManifest`
+// already treats these as html-kind artifacts elsewhere; we mirror
+// that so the guard doesn't silently let a stub overwrite them.
+const SYNTHETIC_IDENTIFIER_SUFFIX = /(?:-\d+)?\.html?$/;
+const HTML_EXTENSION = /\.html?$/;
+
+function syntheticIdentifierFromFilename(name: string): string {
+  return name.replace(SYNTHETIC_IDENTIFIER_SUFFIX, '');
+}
+
+// `phase-2.html` is genuinely ambiguous: it could be the identifier
+// `phase` with a `-2` collision suffix, or the standalone identifier
+// `phase-2`. Without a sidecar to disambiguate, both interpretations
+// are valid candidates — the guard accepts the file as a prior if
+// either matches the input identifier. Visible false positives
+// (rejecting a legitimate write the user can override via env) are
+// preferable to silent false negatives (stub bypasses the guard).
+function legacyCandidateIdentifiers(filename: string): string[] {
+  const fullBasename = filename.replace(HTML_EXTENSION, '');
+  const stripped = syntheticIdentifierFromFilename(filename);
+  const candidates: string[] = [];
+  if (fullBasename.length > 0) candidates.push(fullBasename);
+  if (stripped.length > 0 && stripped !== fullBasename) candidates.push(stripped);
+  return candidates;
+}
+
+// Finds prior HTML siblings on disk that share an identifier with a
+// newly-written artifact. The frontend's collision-suffixing scheme means
+// related entries match `<identifier>(-\d+)?\.html?`. The scan deliberately
+// includes any file at the same path as the new write — when an agent
+// overwrites `dashboard.html` with the same name, the file currently on
+// disk is the prior content (the overwrite happens after this scan).
+export async function findPriorArtifactSiblings(
+  scanDir: string,
+  identifier: string,
+): Promise<PriorArtifactSibling[]> {
+  if (identifier.length === 0) return [];
+  const tokens = new Set<string>();
+  tokens.add(identifier);
+  const slug = slugifyArtifactIdentifier(identifier);
+  if (slug.length > 0) tokens.add(slug);
+  // When the identifier slugifies to empty (e.g. all-non-ASCII), the web
+  // path falls back to the literal "artifact" basename. Match that family
+  // so a later artifact-2.html stub doesn't bypass the prior.
+  else tokens.add(EMPTY_SLUG_FALLBACK_NAME);
+  const alternation = Array.from(tokens, escapeRegExp).join('|');
+  const pattern = new RegExp(`^(?:${alternation})(?:-\\d+)?\\.html?$`);
+  let entries: Dirent[];
+  try {
+    entries = await readdir(scanDir, { withFileTypes: true });
+  } catch {
+    return [];
+  }
+  const results: PriorArtifactSibling[] = [];
+  for (const entry of entries) {
+    if (!entry.isFile()) continue;
+    if (!pattern.test(entry.name)) continue;
+    // Verify the sibling's canonical identifier matches before treating
+    // it as a prior. Without this check, distinct identifiers that share
+    // a sibling-name namespace (most reachably the empty-slug fallback,
+    // where 测试 and 首页 both land in artifact*.html) would falsely
+    // warn/reject across each other.
+    //
+    // For legacy HTML artifacts without a sidecar (pre-manifest era,
+    // Write-tool, paste-text, manual import), we fall back to filename-
+    // derived candidates. Because a name like `phase-2.html` is
+    // genuinely ambiguous between "phase + collision suffix -2" and "the
+    // standalone identifier phase-2", we try both interpretations and
+    // accept the file as a prior if either matches. The canonical-form
+    // anchor in artifactIdentifiersMatch still rules out truncation
+    // collisions and empty-slug conflations.
+    const sidecarIdentifier = await readSidecarIdentifier(scanDir, entry.name);
+    const candidateIdentifiers = sidecarIdentifier !== null
+      ? [sidecarIdentifier]
+      : legacyCandidateIdentifiers(entry.name);
+    if (candidateIdentifiers.length === 0) continue;
+    if (!candidateIdentifiers.some((c) => artifactIdentifiersMatch(identifier, c))) continue;
+    try {
+      const st = await stat(path.join(scanDir, entry.name));
+      results.push({ name: entry.name, size: st.size });
+    } catch {
+      // ignore unreadable entries; they don't influence the guard decision
+    }
+  }
+  return results;
+}
+
+export function readArtifactStubGuardConfigFromEnv(env: NodeJS.ProcessEnv = process.env): ArtifactStubGuardConfig {
+  const rawMode = (env.OD_ARTIFACT_STUB_GUARD ?? '').toLowerCase();
+  const mode: ArtifactStubGuardMode =
+    rawMode === 'reject' || rawMode === 'warn' || rawMode === 'off'
+      ? rawMode
+      : DEFAULT_ARTIFACT_STUB_GUARD_CONFIG.mode;
+
+  const ratioRaw = Number(env.OD_ARTIFACT_STUB_GUARD_MIN_RATIO);
+  // Accept (0, 1] so users can set 1 to reject any shrinkage. Values <=0
+  // or >1 fall back to default.
+  const minRetainedRatio =
+    Number.isFinite(ratioRaw) && ratioRaw > 0 && ratioRaw <= 1
+      ? ratioRaw
+      : DEFAULT_ARTIFACT_STUB_GUARD_CONFIG.minRetainedRatio;
+
+  const minPriorBytesRaw = Number(env.OD_ARTIFACT_STUB_GUARD_MIN_PRIOR_BYTES);
+  const minPriorBytes =
+    Number.isInteger(minPriorBytesRaw) && minPriorBytesRaw > 0
+      ? minPriorBytesRaw
+      : DEFAULT_ARTIFACT_STUB_GUARD_CONFIG.minPriorBytes;
+
+  return { mode, minRetainedRatio, minPriorBytes };
+}
+
+function buildWarning(
+  identifier: string,
+  newSize: number,
+  prior: PriorArtifactSibling,
+): ArtifactStubGuardWarning {
+  return {
+    code: 'ARTIFACT_REGRESSION',
+    message:
+      `New artifact body for identifier "${identifier}" is ${newSize} bytes, ` +
+      `but the largest prior sibling "${prior.name}" is ${prior.size} bytes. ` +
+      'This pattern usually means the agent emitted a placeholder instead of the full document. ' +
+      'Set OD_ARTIFACT_STUB_GUARD=warn to record the warning without rejecting, or =off to disable the guard entirely.',
+    identifier,
+    newSize,
+    priorSize: prior.size,
+    priorName: prior.name,
+  };
+}
+
+// Pure decision function: given the prior siblings on disk, decide whether
+// the new body is a stub regression. Splitting this from the disk scan
+// keeps the unit tests fast and lets callers pre-fetch siblings.
+export function classifyArtifactStubGuard(
+  priors: PriorArtifactSibling[],
+  identifier: string,
+  newSize: number,
+  config: ArtifactStubGuardConfig,
+): EvaluateArtifactStubGuardResult {
+  if (config.mode === 'off') return { outcome: 'pass' };
+  if (identifier.length === 0) return { outcome: 'pass' };
+  if (priors.length === 0) return { outcome: 'pass' };
+
+  let largest: PriorArtifactSibling | null = null;
+  for (const prior of priors) {
+    if (largest === null || prior.size > largest.size) largest = prior;
+  }
+  if (largest === null) return { outcome: 'pass' };
+  if (largest.size < config.minPriorBytes) return { outcome: 'pass' };
+
+  const threshold = largest.size * config.minRetainedRatio;
+  if (newSize >= threshold) return { outcome: 'pass' };
+
+  const warning = buildWarning(identifier, newSize, largest);
+  return { outcome: config.mode === 'reject' ? 'reject' : 'warn', warning };
+}
+
+export async function evaluateArtifactStubGuard(
+  input: EvaluateArtifactStubGuardInput,
+): Promise<EvaluateArtifactStubGuardResult> {
+  if (input.config.mode === 'off') return { outcome: 'pass' };
+  if (input.identifier.length === 0) return { outcome: 'pass' };
+  const priors = await findPriorArtifactSiblings(input.scanDir, input.identifier);
+  return classifyArtifactStubGuard(priors, input.identifier, input.newSize, input.config);
+}
diff --git a/apps/daemon/src/project-routes.ts b/apps/daemon/src/project-routes.ts
index 745317433..bf16ddd1c 100644
--- a/apps/daemon/src/project-routes.ts
+++ b/apps/daemon/src/project-routes.ts
@@ -1,4 +1,5 @@
 import type { Express } from 'express';
+import { ArtifactRegressionError } from './artifact-stub-guard.js';
 import type { RouteDeps } from './server-context.js';
 
 export interface RegisterProjectRoutesDeps extends RouteDeps<'db' | 'design' | 'http' | 'paths' | 'projectStore' | 'projectFiles' | 'conversations' | 'templates' | 'status' | 'events' | 'ids'> {}
@@ -900,6 +901,16 @@ export function registerProjectFileRoutes(app: Express, ctx: RegisterProjectFile
         const body = { file: meta };
         res.json(body);
       } catch (err: any) {
+        if (err instanceof ArtifactRegressionError) {
+          return sendApiError(res, 422, 'ARTIFACT_REGRESSION', err.message, {
+            details: {
+              identifier: err.identifier,
+              newSize: err.newSize,
+              priorSize: err.priorSize,
+              priorName: err.priorName,
+            },
+          });
+        }
         sendApiError(res, 500, 'INTERNAL_ERROR', 'upload failed');
       }
     },
diff --git a/apps/daemon/src/projects.ts b/apps/daemon/src/projects.ts
index 0daab4cde..8f7cc249d 100644
--- a/apps/daemon/src/projects.ts
+++ b/apps/daemon/src/projects.ts
@@ -15,6 +15,12 @@ import {
   parsePersistedManifest,
   validateArtifactManifestInput,
 } from './artifact-manifest.js';
+import {
+  ArtifactRegressionError,
+  STUB_GUARDED_MANIFEST_KINDS,
+  evaluateArtifactStubGuard,
+  readArtifactStubGuardConfigFromEnv,
+} from './artifact-stub-guard.js';
 
 const FORBIDDEN_SEGMENT = /^$|^\.\.?$/;
 const RESERVED_PROJECT_FILE_SEGMENTS = new Set(['.live-artifacts']);
@@ -395,19 +401,60 @@ export async function writeProjectFile(
     }
   }
   await mkdir(path.dirname(target), { recursive: true });
-  await writeFile(target, body);
+  let stubGuardWarning = null;
+  let validatedManifest = null;
   if (artifactManifest && typeof artifactManifest === 'object') {
-    const manifestFileName = artifactManifestNameFor(safeName);
-    const manifestTarget = await resolveSafeReal(dir, manifestFileName);
     const validated = validateArtifactManifestInput(artifactManifest, safeName);
     if (validated.ok && validated.value) {
-      const nextManifest = validated.value;
-      await writeFile(manifestTarget, JSON.stringify(nextManifest, null, 2));
+      validatedManifest = validated.value;
+      const identifier = typeof validatedManifest.metadata?.identifier === 'string'
+        ? validatedManifest.metadata.identifier
+        : '';
+      // Stub-guard applies to HTML-rendered manifest kinds (html, deck).
+      // Other kinds (markdown, svg, code-snippet) can legitimately be small
+      // and are skipped.
+      if (identifier.length > 0 && STUB_GUARDED_MANIFEST_KINDS.has(validatedManifest.kind)) {
+        // Scan the directory the new file actually lands in, not the project
+        // root — writeProjectFile accepts nested paths like reports/X.html
+        // and a root-only scan would miss prior siblings in subdirectories.
+        const guard = await evaluateArtifactStubGuard({
+          scanDir: path.dirname(target),
+          identifier,
+          newSize: Buffer.byteLength(body),
+          config: readArtifactStubGuardConfigFromEnv(),
+        });
+        if ((guard.outcome === 'reject' || guard.outcome === 'warn') && guard.warning) {
+          // Operator-visible signal regardless of mode, so on-call can see
+          // how often the guard fires without combing through 422s.
+          console.warn(
+            `[stub-guard] ${guard.outcome} identifier=${guard.warning.identifier} ` +
+              `newSize=${guard.warning.newSize} priorSize=${guard.warning.priorSize} ` +
+              `priorName=${guard.warning.priorName} project=${projectId}`,
+          );
+        }
+        if (guard.outcome === 'reject' && guard.warning) {
+          throw new ArtifactRegressionError(guard.warning.message, {
+            identifier: guard.warning.identifier,
+            newSize: guard.warning.newSize,
+            priorSize: guard.warning.priorSize,
+            priorName: guard.warning.priorName,
+          });
+        }
+        if (guard.outcome === 'warn' && guard.warning) {
+          stubGuardWarning = guard.warning;
+        }
+      }
     }
   }
+  await writeFile(target, body);
+  if (validatedManifest) {
+    const manifestFileName = artifactManifestNameFor(safeName);
+    const manifestTarget = await resolveSafeReal(dir, manifestFileName);
+    await writeFile(manifestTarget, JSON.stringify(validatedManifest, null, 2));
+  }
   const st = await stat(target);
   const persistedManifest = await readManifestForPath(dir, safeName);
-  return {
+  const result = {
     name: safeName,
     path: safeName,
     size: st.size,
@@ -417,6 +464,8 @@ export async function writeProjectFile(
     artifactKind: persistedManifest?.kind,
     artifactManifest: persistedManifest,
   };
+  if (stubGuardWarning) result.stubGuardWarning = stubGuardWarning;
+  return result;
 }
 
 function artifactManifestNameFor(name) {
diff --git a/apps/daemon/tests/artifact-stub-guard.test.ts b/apps/daemon/tests/artifact-stub-guard.test.ts
new file mode 100644
index 000000000..2f68d2f70
--- /dev/null
+++ b/apps/daemon/tests/artifact-stub-guard.test.ts
@@ -0,0 +1,432 @@
+import { mkdtemp, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import path from 'node:path';
+import { afterEach, describe, expect, it } from 'vitest';
+
+import {
+  ArtifactRegressionError,
+  DEFAULT_ARTIFACT_STUB_GUARD_CONFIG,
+  artifactIdentifiersMatch,
+  classifyArtifactStubGuard,
+  evaluateArtifactStubGuard,
+  findPriorArtifactSiblings,
+  readArtifactStubGuardConfigFromEnv,
+  slugifyArtifactIdentifier,
+  type ArtifactStubGuardConfig,
+} from '../src/artifact-stub-guard.js';
+
+// Helper: write an artifact body + the sidecar manifest the production
+// write path would produce. Sibling discovery requires the sidecar to
+// verify the canonical identifier, so unit fixtures must include both.
+async function writeArtifactPair(dir: string, name: string, body: string, identifier: string): Promise<void> {
+  await writeFile(path.join(dir, name), body);
+  const manifest = {
+    version: 1,
+    kind: 'html',
+    title: identifier,
+    entry: name,
+    renderer: 'html',
+    status: 'complete',
+    exports: ['html'],
+    metadata: { identifier, artifactType: 'text/html', inferred: false },
+  };
+  await writeFile(path.join(dir, `${name}.artifact.json`), JSON.stringify(manifest));
+}
+
+function rejectingConfig(overrides: Partial<ArtifactStubGuardConfig> = {}): ArtifactStubGuardConfig {
+  return { ...DEFAULT_ARTIFACT_STUB_GUARD_CONFIG, mode: 'reject', ...overrides };
+}
+
+function warningConfig(overrides: Partial<ArtifactStubGuardConfig> = {}): ArtifactStubGuardConfig {
+  return { ...DEFAULT_ARTIFACT_STUB_GUARD_CONFIG, mode: 'warn', ...overrides };
+}
+
+describe('classifyArtifactStubGuard', () => {
+  it('passes when no priors exist', () => {
+    const result = classifyArtifactStubGuard([], 'dashboard', 80, rejectingConfig());
+    expect(result.outcome).toBe('pass');
+    expect(result.warning).toBeUndefined();
+  });
+
+  it('passes when guard mode is off', () => {
+    const result = classifyArtifactStubGuard(
+      [{ name: 'dashboard.html', size: 80_000 }],
+      'dashboard',
+      120,
+      { ...DEFAULT_ARTIFACT_STUB_GUARD_CONFIG, mode: 'off' },
+    );
+    expect(result.outcome).toBe('pass');
+  });
+
+  it('passes when identifier is empty', () => {
+    const result = classifyArtifactStubGuard(
+      [{ name: 'dashboard.html', size: 80_000 }],
+      '',
+      120,
+      rejectingConfig(),
+    );
+    expect(result.outcome).toBe('pass');
+  });
+
+  it('passes when largest prior is below the floor', () => {
+    const result = classifyArtifactStubGuard(
+      [{ name: 'dashboard.html', size: 1_024 }],
+      'dashboard',
+      32,
+      rejectingConfig({ minPriorBytes: 4_096 }),
+    );
+    expect(result.outcome).toBe('pass');
+  });
+
+  it('passes when the new body keeps at least minRetainedRatio of the prior', () => {
+    const result = classifyArtifactStubGuard(
+      [{ name: 'dashboard.html', size: 80_000 }],
+      'dashboard',
+      40_000,
+      rejectingConfig({ minRetainedRatio: 0.2 }),
+    );
+    expect(result.outcome).toBe('pass');
+  });
+
+  it('rejects when the new body collapses below the ratio of the largest prior', () => {
+    const result = classifyArtifactStubGuard(
+      [
+        { name: 'dashboard.html', size: 80_000 },
+        { name: 'dashboard-2.html', size: 95_000 },
+      ],
+      'dashboard',
+      120,
+      rejectingConfig({ minRetainedRatio: 0.2, minPriorBytes: 4_096 }),
+    );
+    expect(result.outcome).toBe('reject');
+    expect(result.warning).toMatchObject({
+      code: 'ARTIFACT_REGRESSION',
+      identifier: 'dashboard',
+      newSize: 120,
+      priorSize: 95_000,
+      priorName: 'dashboard-2.html',
+    });
+    expect(result.warning?.message).toContain('dashboard-2.html');
+  });
+
+  it('warns instead of rejecting when mode is warn', () => {
+    const result = classifyArtifactStubGuard(
+      [{ name: 'report.html', size: 50_000 }],
+      'report',
+      300,
+      warningConfig(),
+    );
+    expect(result.outcome).toBe('warn');
+    expect(result.warning?.priorSize).toBe(50_000);
+  });
+});
+
+describe('findPriorArtifactSiblings', () => {
+  const tempDirs: string[] = [];
+
+  afterEach(async () => {
+    for (const dir of tempDirs.splice(0)) {
+      await rm(dir, { recursive: true, force: true });
+    }
+  });
+
+  async function makeDir() {
+    const dir = await mkdtemp(path.join(tmpdir(), 'od-stub-guard-'));
+    tempDirs.push(dir);
+    return dir;
+  }
+
+  it('finds bare and suffixed siblings, including the same-named target if it exists', async () => {
+    const dir = await makeDir();
+    await writeArtifactPair(dir, 'report.html', 'a'.repeat(20_000), 'report');
+    await writeArtifactPair(dir, 'report-2.html', 'b'.repeat(40_000), 'report');
+    await writeArtifactPair(dir, 'report-3.html', 'c'.repeat(60_000), 'report');
+    await writeArtifactPair(dir, 'unrelated.html', 'x'.repeat(50_000), 'unrelated');
+
+    // The target 'report-3.html' is included because it currently exists on
+    // disk and its current size is the prior content (the overwrite that
+    // would replace it has not happened yet at scan time). This is the
+    // same-name-overwrite case: see lefarcen P1.
+    const priors = await findPriorArtifactSiblings(dir, 'report');
+    const names = priors.map((p) => p.name).sort();
+    expect(names).toEqual(['report-2.html', 'report-3.html', 'report.html']);
+  });
+
+  it('returns an empty list when the directory does not exist', async () => {
+    const priors = await findPriorArtifactSiblings('/nonexistent/od/projects/missing', 'dashboard');
+    expect(priors).toEqual([]);
+  });
+
+  it('does not match identifiers that share a prefix', async () => {
+    const dir = await makeDir();
+    await writeArtifactPair(dir, 'landing.html', 'a'.repeat(1_000), 'landing');
+    await writeArtifactPair(dir, 'landing-page.html', 'b'.repeat(1_000), 'landing-page');
+
+    const priors = await findPriorArtifactSiblings(dir, 'landing');
+    const names = priors.map((p) => p.name).sort();
+    expect(names).toEqual(['landing.html']);
+  });
+
+  it('also matches .htm siblings', async () => {
+    const dir = await makeDir();
+    await writeArtifactPair(dir, 'overview-doc.htm', 'a'.repeat(20_000), 'overview-doc');
+    await writeArtifactPair(dir, 'overview-doc-2.html', 'b'.repeat(30_000), 'overview-doc');
+
+    const priors = await findPriorArtifactSiblings(dir, 'overview-doc');
+    const names = priors.map((p) => p.name).sort();
+    expect(names).toEqual(['overview-doc-2.html', 'overview-doc.htm']);
+  });
+
+  it('matches siblings using the slugified form of a non-slug identifier', async () => {
+    const dir = await makeDir();
+    // Frontend persistArtifact slugifies "Landing Page" -> "landing-page"
+    // for the filename but keeps the raw "Landing Page" in the manifest.
+    // Both forms refer to the same lineage; sidecar identity uses
+    // slug-equivalence to bridge them.
+    await writeArtifactPair(dir, 'landing-page.html', 'a'.repeat(40_000), 'Landing Page');
+
+    const priors = await findPriorArtifactSiblings(dir, 'Landing Page');
+    expect(priors.map((p) => p.name)).toEqual(['landing-page.html']);
+  });
+
+  it('falls back to the "artifact" basename when the identifier slugifies to empty', async () => {
+    const dir = await makeDir();
+    // Identifier like "测试" (or any all-non-ASCII / punctuation-only
+    // string) strips to "" through the web slugifier and persistArtifact's
+    // `|| 'artifact'` fallback writes it as artifact.html / artifact-2.html.
+    await writeArtifactPair(dir, 'artifact.html', 'a'.repeat(40_000), '测试');
+    await writeArtifactPair(dir, 'artifact-2.html', 'b'.repeat(60_000), '测试');
+
+    const priors = await findPriorArtifactSiblings(dir, '测试');
+    const names = priors.map((p) => p.name).sort();
+    expect(names).toEqual(['artifact-2.html', 'artifact.html']);
+  });
+
+  it('does NOT match a fallback sibling whose sidecar identifier differs (lefarcen/mrcfps round 4)', async () => {
+    const dir = await makeDir();
+    // Two distinct empty-slug identifiers both land in the artifact*.html
+    // namespace. A new save for "首页" must not be compared against the
+    // earlier "测试" sibling — they're unrelated artifacts that just
+    // happen to share a fallback basename.
+    await writeArtifactPair(dir, 'artifact.html', 'a'.repeat(40_000), '测试');
+
+    const priors = await findPriorArtifactSiblings(dir, '首页');
+    expect(priors).toEqual([]);
+  });
+
+  it('falls back to filename-derived identifier for legacy sidecar-less HTML (mrcfps R6)', async () => {
+    const dir = await makeDir();
+    // Pre-sidecar legacy file — `inferLegacyManifest` treats it as
+    // html-kind elsewhere, so the guard should too. Without this
+    // fallback, a stub overwrite of a legacy `dashboard.html` would
+    // bypass the guard as a "first emission".
+    await writeFile(path.join(dir, 'dashboard.html'), 'a'.repeat(40_000));
+
+    const priors = await findPriorArtifactSiblings(dir, 'dashboard');
+    expect(priors.map((p) => p.name)).toEqual(['dashboard.html']);
+  });
+
+  it('legacy fallback bridges raw <-> slug per artifactIdentifiersMatch rules', async () => {
+    const dir = await makeDir();
+    // Legacy file basename is the canonical slug form. Input identifier
+    // "Landing Page" must still bridge to it via slug-equivalence, just
+    // like the sidecar case.
+    await writeFile(path.join(dir, 'landing-page.html'), 'a'.repeat(40_000));
+
+    const priors = await findPriorArtifactSiblings(dir, 'Landing Page');
+    expect(priors.map((p) => p.name)).toEqual(['landing-page.html']);
+  });
+
+  it('legacy fallback does NOT bridge unrelated identifiers via filename inference', async () => {
+    const dir = await makeDir();
+    // Legacy file basename `dashboard` should not match identifier
+    // `legacy-dashboard` even though both are slug-form.
+    await writeFile(path.join(dir, 'dashboard.html'), 'a'.repeat(40_000));
+
+    const priors = await findPriorArtifactSiblings(dir, 'legacy-dashboard');
+    expect(priors).toEqual([]);
+  });
+
+  it('legacy fallback honors identifiers that legitimately end in -<digits> (mrcfps R7)', async () => {
+    const dir = await makeDir();
+    // `phase-2.html` is ambiguous without a sidecar: could be "phase"
+    // with -2 collision suffix, or the standalone identifier "phase-2".
+    // The guard tries both; here the input names "phase-2" so the full
+    // basename interpretation must match.
+    await writeFile(path.join(dir, 'phase-2.html'), 'a'.repeat(40_000));
+
+    const priors = await findPriorArtifactSiblings(dir, 'phase-2');
+    expect(priors.map((p) => p.name)).toEqual(['phase-2.html']);
+  });
+
+  it('legacy fallback also honors the suffix-stripped interpretation', async () => {
+    const dir = await makeDir();
+    // Same on-disk file, different input: this time the agent is
+    // emitting under the `phase` identifier (treating the -2 as a
+    // collision suffix). The suffix-stripped interpretation must match.
+    await writeFile(path.join(dir, 'phase-2.html'), 'a'.repeat(40_000));
+
+    const priors = await findPriorArtifactSiblings(dir, 'phase');
+    expect(priors.map((p) => p.name)).toEqual(['phase-2.html']);
+  });
+});
+
+describe('artifactIdentifiersMatch', () => {
+  it('matches identical raw identifiers', () => {
+    expect(artifactIdentifiersMatch('dashboard', 'dashboard')).toBe(true);
+  });
+
+  it('bridges raw form to its canonical slug form', () => {
+    expect(artifactIdentifiersMatch('Landing Page', 'landing-page')).toBe(true);
+    expect(artifactIdentifiersMatch('landing-page', 'Landing Page')).toBe(true);
+  });
+
+  it('does NOT bridge two non-canonical forms even if they slugify the same', () => {
+    // Both inputs slugify to "landing-page" but neither IS the canonical
+    // slug form, so we can't safely call them the same lineage. This is
+    // the same safety property that protects against truncation
+    // collisions for >60-char identifiers.
+    expect(artifactIdentifiersMatch('Landing Page', 'LANDING-PAGE')).toBe(false);
+  });
+
+  it('does not match distinct identifiers that both slugify to empty', () => {
+    expect(artifactIdentifiersMatch('测试', '首页')).toBe(false);
+    expect(artifactIdentifiersMatch('!!!', '???')).toBe(false);
+  });
+
+  it('matches a non-ASCII identifier with itself even when its slug is empty', () => {
+    expect(artifactIdentifiersMatch('测试', '测试')).toBe(true);
+  });
+
+  it('does not match unrelated identifiers with different slugs', () => {
+    expect(artifactIdentifiersMatch('dashboard', 'legacy-dashboard')).toBe(false);
+  });
+
+  it('does NOT bridge two long raw identifiers that share a 60-char truncated slug (mrcfps R5)', () => {
+    // Both >60 chars and identical for the first 60, differing only after.
+    // Their slugify outputs collide via truncation, but neither is the
+    // canonical slug form of itself, so they must not bridge.
+    const sixtyAs = 'a'.repeat(60);
+    const a = `${sixtyAs}-suffix-one`;
+    const b = `${sixtyAs}-suffix-two`;
+    expect(a).not.toBe(b);
+    expect(artifactIdentifiersMatch(a, b)).toBe(false);
+  });
+
+  it('still bridges raw form to truncated slug when the slug is the canonical second input', () => {
+    // The standard "Landing Page" <-> "landing-page" case must still work.
+    // Asserting via inputs that hit the truncation boundary: a 70-char raw
+    // identifier whose slug is the truncated-to-60 form, paired with that
+    // truncated form passed in directly, should still match.
+    const slug = 'a'.repeat(60);
+    const raw = 'a'.repeat(70);
+    expect(slugifyArtifactIdentifier(raw)).toBe(slug);
+    expect(artifactIdentifiersMatch(raw, slug)).toBe(true);
+  });
+});
+
+describe('evaluateArtifactStubGuard (integration with disk scan)', () => {
+  const tempDirs: string[] = [];
+
+  afterEach(async () => {
+    for (const dir of tempDirs.splice(0)) {
+      await rm(dir, { recursive: true, force: true });
+    }
+  });
+
+  async function makeDir() {
+    const dir = await mkdtemp(path.join(tmpdir(), 'od-stub-guard-eval-'));
+    tempDirs.push(dir);
+    return dir;
+  }
+
+  it('rejects a stub-sized rewrite of an existing identifier', async () => {
+    const dir = await makeDir();
+    await writeArtifactPair(dir, 'presentation.html', 'p'.repeat(60_000), 'presentation');
+
+    const result = await evaluateArtifactStubGuard({
+      scanDir: dir,
+      identifier: 'presentation',
+      newSize: 200,
+      config: rejectingConfig(),
+    });
+
+    expect(result.outcome).toBe('reject');
+    expect(result.warning?.priorName).toBe('presentation.html');
+  });
+
+  it('passes when the new body comparable in size to the prior', async () => {
+    const dir = await makeDir();
+    await writeArtifactPair(dir, 'presentation.html', 'p'.repeat(60_000), 'presentation');
+
+    const result = await evaluateArtifactStubGuard({
+      scanDir: dir,
+      identifier: 'presentation',
+      newSize: 50_000,
+      config: rejectingConfig(),
+    });
+
+    expect(result.outcome).toBe('pass');
+  });
+});
+
+describe('readArtifactStubGuardConfigFromEnv', () => {
+  it('returns defaults when env vars are absent', () => {
+    const config = readArtifactStubGuardConfigFromEnv({});
+    expect(config).toEqual(DEFAULT_ARTIFACT_STUB_GUARD_CONFIG);
+  });
+
+  it('parses recognised mode values', () => {
+    expect(readArtifactStubGuardConfigFromEnv({ OD_ARTIFACT_STUB_GUARD: 'reject' }).mode).toBe('reject');
+    expect(readArtifactStubGuardConfigFromEnv({ OD_ARTIFACT_STUB_GUARD: 'WARN' }).mode).toBe('warn');
+    expect(readArtifactStubGuardConfigFromEnv({ OD_ARTIFACT_STUB_GUARD: 'off' }).mode).toBe('off');
+  });
+
+  it('falls back to default when mode is unrecognised', () => {
+    expect(readArtifactStubGuardConfigFromEnv({ OD_ARTIFACT_STUB_GUARD: 'maybe' }).mode).toBe(
+      DEFAULT_ARTIFACT_STUB_GUARD_CONFIG.mode,
+    );
+  });
+
+  it('honours numeric overrides within range', () => {
+    const config = readArtifactStubGuardConfigFromEnv({
+      OD_ARTIFACT_STUB_GUARD_MIN_RATIO: '0.35',
+      OD_ARTIFACT_STUB_GUARD_MIN_PRIOR_BYTES: '8192',
+    });
+    expect(config.minRetainedRatio).toBeCloseTo(0.35);
+    expect(config.minPriorBytes).toBe(8_192);
+  });
+
+  it('accepts ratio = 1 to reject any shrinkage', () => {
+    const config = readArtifactStubGuardConfigFromEnv({
+      OD_ARTIFACT_STUB_GUARD_MIN_RATIO: '1',
+    });
+    expect(config.minRetainedRatio).toBe(1);
+  });
+
+  it('rejects out-of-range numeric overrides', () => {
+    const config = readArtifactStubGuardConfigFromEnv({
+      OD_ARTIFACT_STUB_GUARD_MIN_RATIO: '5',
+      OD_ARTIFACT_STUB_GUARD_MIN_PRIOR_BYTES: '-12',
+    });
+    expect(config.minRetainedRatio).toBe(DEFAULT_ARTIFACT_STUB_GUARD_CONFIG.minRetainedRatio);
+    expect(config.minPriorBytes).toBe(DEFAULT_ARTIFACT_STUB_GUARD_CONFIG.minPriorBytes);
+  });
+});
+
+describe('ArtifactRegressionError', () => {
+  it('carries identifier, sizes, and prior name in details', () => {
+    const err = new ArtifactRegressionError('regression', {
+      identifier: 'dashboard',
+      newSize: 100,
+      priorSize: 50_000,
+      priorName: 'dashboard.html',
+    });
+    expect(err.code).toBe('ARTIFACT_REGRESSION');
+    expect(err.name).toBe('ArtifactRegressionError');
+    expect(err.identifier).toBe('dashboard');
+    expect(err.newSize).toBe(100);
+    expect(err.priorSize).toBe(50_000);
+    expect(err.priorName).toBe('dashboard.html');
+  });
+});
diff --git a/apps/daemon/tests/projects-stub-guard.test.ts b/apps/daemon/tests/projects-stub-guard.test.ts
new file mode 100644
index 000000000..0d7eb1097
--- /dev/null
+++ b/apps/daemon/tests/projects-stub-guard.test.ts
@@ -0,0 +1,427 @@
+import type http from 'node:http';
+import { afterAll, afterEach, beforeAll, describe, expect, it, vi } from 'vitest';
+
+import { startServer } from '../src/server.js';
+
+interface FilePostBody {
+  name: string;
+  content: string;
+  encoding?: 'utf8' | 'base64';
+  artifactManifest?: unknown;
+}
+
+function htmlBody(byteLength: number): string {
+  const filler = 'x'.repeat(Math.max(0, byteLength - 96));
+  return `<!doctype html><html><head><title>Doc</title></head><body><main>${filler}</main></body></html>`;
+}
+
+function manifestFor(identifier: string, kind: 'html' | 'deck' = 'html') {
+  return {
+    kind,
+    renderer: kind === 'deck' ? 'deck-html' : 'html',
+    title: identifier,
+    exports: kind === 'deck' ? ['html', 'pdf'] : ['html'],
+    metadata: { identifier },
+  };
+}
+
+describe('artifact stub guard via /api/projects/:id/files', () => {
+  let server: http.Server;
+  let baseUrl: string;
+
+  beforeAll(async () => {
+    vi.stubEnv('OD_ARTIFACT_STUB_GUARD', 'reject');
+    vi.stubEnv('OD_ARTIFACT_STUB_GUARD_MIN_PRIOR_BYTES', '1024');
+    vi.stubEnv('OD_ARTIFACT_STUB_GUARD_MIN_RATIO', '0.2');
+    const started = (await startServer({ port: 0, returnServer: true })) as {
+      url: string;
+      server: http.Server;
+    };
+    baseUrl = started.url;
+    server = started.server;
+  });
+
+  afterAll(() => {
+    vi.unstubAllEnvs();
+    return new Promise<void>((resolve) => server.close(() => resolve()));
+  });
+
+  afterEach(() => {
+    // Each test resets the mode it changed; default back to reject.
+    vi.stubEnv('OD_ARTIFACT_STUB_GUARD', 'reject');
+  });
+
+  async function createProject(prefix: string) {
+    const id = `${prefix}-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const resp = await fetch(`${baseUrl}/api/projects`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ id, name: id }),
+    });
+    expect(resp.status).toBe(200);
+    return id;
+  }
+
+  async function postFile(projectId: string, body: FilePostBody) {
+    return fetch(`${baseUrl}/api/projects/${projectId}/files`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify(body),
+    });
+  }
+
+  it('rejects a stub-sized rewrite with ARTIFACT_REGRESSION', async () => {
+    const projectId = await createProject('reject');
+
+    const firstResp = await postFile(projectId, {
+      name: 'dashboard.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('dashboard'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'dashboard-2.html',
+      content: 'See dashboard.html in this project — full standalone file written to disk.',
+      artifactManifest: manifestFor('dashboard'),
+    });
+
+    expect(stubResp.status).toBe(422);
+    const stubBody = (await stubResp.json()) as {
+      error: { code: string; message: string; details?: { identifier?: string; priorName?: string } };
+    };
+    expect(stubBody.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(stubBody.error.details?.identifier).toBe('dashboard');
+    expect(stubBody.error.details?.priorName).toBe('dashboard.html');
+  });
+
+  it('does not write the new file or its manifest when rejected', async () => {
+    const projectId = await createProject('not-written');
+
+    const firstResp = await postFile(projectId, {
+      name: 'report.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('report'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'report-2.html',
+      content: '<html><body>see report.html</body></html>',
+      artifactManifest: manifestFor('report'),
+    });
+    expect(stubResp.status).toBe(422);
+
+    const listResp = await fetch(`${baseUrl}/api/projects/${projectId}/files`);
+    expect(listResp.status).toBe(200);
+    const listBody = (await listResp.json()) as { files: Array<{ name: string }> };
+    const names = listBody.files.map((f) => f.name).sort();
+    expect(names).not.toContain('report-2.html');
+    expect(names).not.toContain('report-2.html.artifact.json');
+    expect(names).toContain('report.html');
+  });
+
+  it('allows a same-size revision through', async () => {
+    const projectId = await createProject('allow');
+
+    const firstResp = await postFile(projectId, {
+      name: 'landing-page.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('landing-page'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const secondResp = await postFile(projectId, {
+      name: 'landing-page-2.html',
+      content: htmlBody(20_500),
+      artifactManifest: manifestFor('landing-page'),
+    });
+    expect(secondResp.status).toBe(200);
+  });
+
+  it('warns instead of rejecting when guard mode is warn', async () => {
+    vi.stubEnv('OD_ARTIFACT_STUB_GUARD', 'warn');
+    const projectId = await createProject('warn');
+
+    const firstResp = await postFile(projectId, {
+      name: 'overview.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('overview'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'overview-2.html',
+      content: '<html><body>placeholder</body></html>',
+      artifactManifest: manifestFor('overview'),
+    });
+    expect(stubResp.status).toBe(200);
+
+    const writeBody = (await stubResp.json()) as {
+      file: { name: string; stubGuardWarning?: { code: string; identifier: string } };
+    };
+    expect(writeBody.file.name).toBe('overview-2.html');
+    expect(writeBody.file.stubGuardWarning?.code).toBe('ARTIFACT_REGRESSION');
+    expect(writeBody.file.stubGuardWarning?.identifier).toBe('overview');
+  });
+
+  it('skips the guard entirely when mode is off', async () => {
+    vi.stubEnv('OD_ARTIFACT_STUB_GUARD', 'off');
+    const projectId = await createProject('off');
+
+    const firstResp = await postFile(projectId, {
+      name: 'briefing.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('briefing'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'briefing-2.html',
+      content: '<html><body>see briefing.html</body></html>',
+      artifactManifest: manifestFor('briefing'),
+    });
+    expect(stubResp.status).toBe(200);
+
+    const writeBody = (await stubResp.json()) as { file: { stubGuardWarning?: unknown } };
+    expect(writeBody.file.stubGuardWarning).toBeUndefined();
+  });
+
+  it('accepts a stub-sized first emission of a new identifier', async () => {
+    const projectId = await createProject('first');
+
+    const resp = await postFile(projectId, {
+      name: 'changelog.html',
+      content: '<html><body>tiny</body></html>',
+      artifactManifest: manifestFor('changelog'),
+    });
+    expect(resp.status).toBe(200);
+  });
+
+  it('rejects a stub rewrite of a deck artifact (kind: deck)', async () => {
+    const projectId = await createProject('deck');
+
+    const firstResp = await postFile(projectId, {
+      name: 'kickoff-deck.html',
+      content: htmlBody(40_000),
+      artifactManifest: manifestFor('kickoff-deck', 'deck'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'kickoff-deck-2.html',
+      content: '<html><body>see kickoff-deck.html</body></html>',
+      artifactManifest: manifestFor('kickoff-deck', 'deck'),
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as { error: { code: string } };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+  });
+
+  it('detects prior siblings written with .htm extension', async () => {
+    const projectId = await createProject('htm');
+
+    const firstResp = await postFile(projectId, {
+      name: 'overview-doc.htm',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('overview-doc'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'overview-doc-2.html',
+      content: '<html><body>see overview-doc.htm</body></html>',
+      artifactManifest: manifestFor('overview-doc'),
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as {
+      error: { code: string; details?: { priorName?: string } };
+    };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(body.error.details?.priorName).toBe('overview-doc.htm');
+  });
+
+  it('rejects a same-name overwrite that shrinks the existing file (lefarcen P1)', async () => {
+    const projectId = await createProject('overwrite');
+
+    const firstResp = await postFile(projectId, {
+      name: 'dashboard.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('dashboard'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    // Same name, same identifier, stub body: existing file is the prior.
+    const stubResp = await postFile(projectId, {
+      name: 'dashboard.html',
+      content: '<html><body>see dashboard.html</body></html>',
+      artifactManifest: manifestFor('dashboard'),
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as {
+      error: { code: string; details?: { priorName?: string } };
+    };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(body.error.details?.priorName).toBe('dashboard.html');
+
+    // Confirm the original 20 KB file is intact (not overwritten).
+    const filesResp = await fetch(`${baseUrl}/api/projects/${projectId}/files`);
+    const files = (await filesResp.json()) as { files: Array<{ name: string; size: number }> };
+    const dashboard = files.files.find((f) => f.name === 'dashboard.html');
+    expect(dashboard?.size).toBeGreaterThan(15_000);
+  });
+
+  it('rejects stub regressions in subdirectories (Codex/mrcfps P2)', async () => {
+    const projectId = await createProject('nested');
+
+    const firstResp = await postFile(projectId, {
+      name: 'reports/overview.html',
+      content: htmlBody(20_000),
+      artifactManifest: manifestFor('overview'),
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'reports/overview-2.html',
+      content: '<html><body>see reports/overview.html</body></html>',
+      artifactManifest: manifestFor('overview'),
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as {
+      error: { code: string; details?: { priorName?: string } };
+    };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(body.error.details?.priorName).toBe('overview.html');
+  });
+
+  it('finds slug-form sibling when manifest carries non-slug identifier (Codex/lefarcen/mrcfps P2)', async () => {
+    const projectId = await createProject('slug');
+
+    // Frontend wrote the previous artifact under a slugified name but the
+    // manifest carried the raw identifier "Landing Page".
+    const firstResp = await postFile(projectId, {
+      name: 'landing-page.html',
+      content: htmlBody(20_000),
+      artifactManifest: { ...manifestFor('landing-page'), metadata: { identifier: 'Landing Page' } },
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'landing-page-2.html',
+      content: '<html><body>see landing-page.html</body></html>',
+      artifactManifest: { ...manifestFor('landing-page'), metadata: { identifier: 'Landing Page' } },
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as { error: { code: string } };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+  });
+
+  it('finds artifact.html siblings when identifier slugifies to empty (lefarcen P2)', async () => {
+    const projectId = await createProject('empty-slug');
+
+    // Frontend persistArtifact slugifies "测试" -> "" -> falls back to
+    // "artifact", so the file lands as artifact.html. A subsequent stub
+    // emission with the same non-ASCII identifier must still find the
+    // prior via the empty-slug fallback.
+    const firstResp = await postFile(projectId, {
+      name: 'artifact.html',
+      content: htmlBody(20_000),
+      artifactManifest: { ...manifestFor('artifact'), metadata: { identifier: '测试' } },
+    });
+    expect(firstResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'artifact-2.html',
+      content: '<html><body>see artifact.html</body></html>',
+      artifactManifest: { ...manifestFor('artifact'), metadata: { identifier: '测试' } },
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as {
+      error: { code: string; details?: { priorName?: string } };
+    };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(body.error.details?.priorName).toBe('artifact.html');
+  });
+
+  it('does NOT cross-reject distinct empty-slug identifiers (lefarcen/mrcfps round 4)', async () => {
+    const projectId = await createProject('empty-slug-distinct');
+
+    // First save: identifier "测试", lands as artifact.html with a 20 KB
+    // body. Sidecar carries identifier="测试".
+    const firstResp = await postFile(projectId, {
+      name: 'artifact.html',
+      content: htmlBody(20_000),
+      artifactManifest: { ...manifestFor('artifact'), metadata: { identifier: '测试' } },
+    });
+    expect(firstResp.status).toBe(200);
+
+    // Second save: a *different* non-ASCII identifier "首页" that also
+    // slugifies to empty. This is a brand-new artifact lineage; the small
+    // first-emission body must not be compared against the unrelated
+    // "测试" prior just because both share the artifact*.html namespace.
+    const secondResp = await postFile(projectId, {
+      name: 'artifact-2.html',
+      content: '<html><body>tiny but legitimate first emission</body></html>',
+      artifactManifest: { ...manifestFor('artifact'), metadata: { identifier: '首页' } },
+    });
+    expect(secondResp.status).toBe(200);
+  });
+
+  it('catches stub rewrites of legacy sidecar-less HTML priors (mrcfps R6)', async () => {
+    const projectId = await createProject('legacy');
+
+    // Seed a "legacy" file by POSTing without artifactManifest — the
+    // route writes the body but no .artifact.json. Mirrors any HTML that
+    // pre-dates the sidecar era or was uploaded outside the artifact-tag
+    // flow (Write tool, paste-text, manual import).
+    const legacyResp = await postFile(projectId, {
+      name: 'dashboard.html',
+      content: htmlBody(20_000),
+      // no artifactManifest -> no sidecar on disk
+    });
+    expect(legacyResp.status).toBe(200);
+
+    // Now an agent emits a stub artifact with the matching identifier.
+    // Without the legacy fallback, the guard would skip the sidecar-less
+    // prior and let this through as a "first emission".
+    const stubResp = await postFile(projectId, {
+      name: 'dashboard-2.html',
+      content: '<html><body>see dashboard.html</body></html>',
+      artifactManifest: manifestFor('dashboard'),
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as {
+      error: { code: string; details?: { priorName?: string } };
+    };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(body.error.details?.priorName).toBe('dashboard.html');
+  });
+
+  it('catches stub rewrites of legacy priors whose identifier ends in -<digits> (mrcfps R7)', async () => {
+    const projectId = await createProject('legacy-numeric');
+
+    // Seed a sidecar-less `phase-2.html` prior (the standalone
+    // identifier "phase-2", not "phase + collision suffix"). Without
+    // the dual-candidate fallback, syntheticIdentifierFromFilename
+    // would strip the -2 and the legacy fallback would search for
+    // "phase" instead, missing the prior and bypassing the guard.
+    const legacyResp = await postFile(projectId, {
+      name: 'phase-2.html',
+      content: htmlBody(20_000),
+      // no artifactManifest -> no sidecar on disk
+    });
+    expect(legacyResp.status).toBe(200);
+
+    const stubResp = await postFile(projectId, {
+      name: 'phase-2-2.html',
+      content: '<html><body>see phase-2.html</body></html>',
+      artifactManifest: manifestFor('phase-2'),
+    });
+    expect(stubResp.status).toBe(422);
+    const body = (await stubResp.json()) as {
+      error: { code: string; details?: { priorName?: string } };
+    };
+    expect(body.error.code).toBe('ARTIFACT_REGRESSION');
+    expect(body.error.details?.priorName).toBe('phase-2.html');
+  });
+});
diff --git a/apps/web/src/components/ProjectView.tsx b/apps/web/src/components/ProjectView.tsx
index 0d798e7e6..6f074fae1 100644
--- a/apps/web/src/components/ProjectView.tsx
+++ b/apps/web/src/components/ProjectView.tsx
@@ -1618,10 +1618,34 @@ export function ProjectView({
       });
       if (file) {
         setFilesRefresh((n) => n + 1);
+        // Surface the daemon's stub-guard warning when it fires in `warn`
+        // mode (the default). Without this the warning would land in the
+        // file metadata silently and the user would never see that the
+        // model shipped a placeholder.
+        if (file.stubGuardWarning) {
+          setError(
+            `Saved "${file.name}", but the model may have shipped a placeholder: ` +
+              `${file.stubGuardWarning.message}`,
+          );
+        }
         // Auto-open the freshly-persisted artifact as a tab so the user
         // sees it without an extra click. The Write-tool path already does
         // this for tool-emitted files; this handles the artifact-tag path.
         requestOpenFile(file.name);
+      } else {
+        // writeProjectTextFile collapses non-OK responses (including
+        // 422 ARTIFACT_REGRESSION from reject-mode stub-guard) to null.
+        // Surfacing the structured error requires changing that helper's
+        // return contract for all callers — out of scope here. Until then,
+        // a generic banner makes the failure observable instead of silent.
+        // Allow the user to retry by clearing the saved-artifact ref so a
+        // retry attempt re-enters this code path.
+        savedArtifactRef.current = '';
+        setError(
+          `Couldn't save artifact "${fileName}". The daemon refused the write — ` +
+            'this is most likely OD_ARTIFACT_STUB_GUARD=reject catching a placeholder body. ' +
+            'Check the daemon logs for the structured ARTIFACT_REGRESSION details.',
+        );
       }
     },
     [project.id, projectFiles, requestOpenFile],
diff --git a/packages/contracts/src/api/files.ts b/packages/contracts/src/api/files.ts
index bb22a34b6..29d223983 100644
--- a/packages/contracts/src/api/files.ts
+++ b/packages/contracts/src/api/files.ts
@@ -15,6 +15,20 @@ export type ProjectFileKind =
   | 'spreadsheet'
   | 'binary';
 
+// Surfaced when the daemon's stub-guard runs in `warn` mode and detects a
+// likely regression (the agent emitted a placeholder body that is much
+// smaller than a prior artifact sharing the same `metadata.identifier`).
+// In `reject` mode the daemon returns `422 ARTIFACT_REGRESSION` instead and
+// no `ProjectFile` is produced.
+export interface ProjectFileStubGuardWarning {
+  code: 'ARTIFACT_REGRESSION';
+  message: string;
+  identifier: string;
+  newSize: number;
+  priorSize: number;
+  priorName: string;
+}
+
 export interface ProjectFile {
   name: string;
   path?: string;
@@ -25,6 +39,7 @@ export interface ProjectFile {
   mime: string;
   artifactKind?: ArtifactKind;
   artifactManifest?: ArtifactManifest;
+  stubGuardWarning?: ProjectFileStubGuardWarning;
 }
 
 export interface ProjectFilesResponse {
diff --git a/packages/contracts/src/errors.ts b/packages/contracts/src/errors.ts
index 08779c26f..88bd00b3f 100644
--- a/packages/contracts/src/errors.ts
+++ b/packages/contracts/src/errors.ts
@@ -16,6 +16,12 @@ export const API_ERROR_CODES = [
   'PROJECT_NOT_FOUND',
   'FILE_NOT_FOUND',
   'ARTIFACT_NOT_FOUND',
+  // The agent emitted a new artifact whose body is dramatically smaller than
+  // a prior artifact sharing the same metadata.identifier. Almost always means
+  // the agent shipped a placeholder ("see other-file.html in this project",
+  // a bare filename string, an empty fallback page) instead of the full
+  // document. Configurable via OD_ARTIFACT_STUB_GUARD (reject|warn|off).
+  'ARTIFACT_REGRESSION',
   'UPSTREAM_UNAVAILABLE',
   'RATE_LIMITED',
   // PR #974 round-4: desktop-paired daemon received an import request