diff --git a/packages/cli/src/commands/compositions.test.ts b/packages/cli/src/commands/compositions.test.ts
index 32c30caa1c..f340b94bbc 100644
--- a/packages/cli/src/commands/compositions.test.ts
+++ b/packages/cli/src/commands/compositions.test.ts
@@ -86,4 +86,17 @@ describe("parseSubComposition", () => {
       elementCount: 1,
     });
   });
+
+  it("treats a data-composition-src that points at a folder as inline instead of crashing", () => {
+    const baseDir = mkdtempSync(join(tmpdir(), "hyperframes-compositions-"));
+
+    try {
+      mkdirSync(join(baseDir, "compositions", "intro"), { recursive: true });
+      const html = `<div data-composition-id="intro" data-composition-src="compositions/intro" data-width="1920" data-height="1080"></div>`;
+
+      expect(parseCompositions(html, baseDir)).toMatchObject([{ id: "intro" }]);
+    } finally {
+      rmSync(baseDir, { recursive: true, force: true });
+    }
+  });
 });
diff --git a/packages/cli/src/commands/compositions.ts b/packages/cli/src/commands/compositions.ts
index c467e8e001..a15cbdd554 100644
--- a/packages/cli/src/commands/compositions.ts
+++ b/packages/cli/src/commands/compositions.ts
@@ -1,6 +1,7 @@
 import { defineCommand } from "citty";
 import type { Example } from "./_examples.js";
-import { existsSync, readFileSync } from "node:fs";
+import { readFileSync } from "node:fs";
+import { readProjectFile } from "@hyperframes/parsers/asset-resolution";
 import { resolve, dirname } from "node:path";
 
 export const examples: Example[] = [
@@ -64,9 +65,9 @@ export function parseCompositions(html: string, baseDir: string): CompositionInf
     // If this references an external sub-composition, parse that file
     if (compositionSrc) {
       const subPath = resolve(baseDir, compositionSrc);
-      if (existsSync(subPath)) {
-        const subHtml = readFileSync(subPath, "utf-8");
-        const subInfo = parseSubComposition(subHtml, id, width, height);
+      const sub = readProjectFile(subPath);
+      if (sub.kind === "file") {
+        const subInfo = parseSubComposition(sub.text, id, width, height);
         compositions.push({ ...subInfo, source: compositionSrc });
         return;
       }
diff --git a/packages/cli/src/commands/keyframes.test.ts b/packages/cli/src/commands/keyframes.test.ts
index 23373137c3..7f717e3694 100644
--- a/packages/cli/src/commands/keyframes.test.ts
+++ b/packages/cli/src/commands/keyframes.test.ts
@@ -13,7 +13,12 @@ import { beforeAll, describe, expect, it, onTestFinished } from "vitest";
 import type { ArgsDef } from "citty";
 import { ensureDOMParser } from "../utils/dom.js";
 import keyframesCommand from "./keyframes.js";
-import { collectShotSelectors, resolveScope, surfaceComposition } from "./keyframes.js";
+import {
+  collectCompositions,
+  collectShotSelectors,
+  resolveScope,
+  surfaceComposition,
+} from "./keyframes.js";
 import { ensureShotOutputDir } from "./motionShot.js";
 
 // citty types `args` as Resolvable<ArgsDef> (object | promise | thunk); this
@@ -366,3 +371,17 @@ describe("--layout strip help text", () => {
     expect(description.toLowerCase()).toContain("only when");
   });
 });
+
+describe("keyframes project compositions", () => {
+  it("skips a data-composition-src that points at a folder instead of crashing", () => {
+    const dir = mkdtempSync(join(tmpdir(), "hf-keyframes-folder-src-"));
+    onTestFinished(() => rmSync(dir, { recursive: true, force: true }));
+    mkdirSync(join(dir, "compositions", "intro"), { recursive: true });
+    writeFileSync(
+      join(dir, "index.html"),
+      `<!doctype html><html><body><div data-composition-id="main" data-duration="4"><div data-composition-id="intro" data-composition-src="compositions/intro"></div></div></body></html>`,
+    );
+
+    expect(collectCompositions(join(dir, "index.html"))).toHaveLength(1);
+  });
+});
diff --git a/packages/cli/src/commands/keyframes.ts b/packages/cli/src/commands/keyframes.ts
index ba85a0b7fc..4fa7f4cd84 100644
--- a/packages/cli/src/commands/keyframes.ts
+++ b/packages/cli/src/commands/keyframes.ts
@@ -1,5 +1,6 @@
 import { defineCommand } from "citty";
-import { existsSync, readFileSync, statSync } from "node:fs";
+import { existsSync, readFileSync } from "node:fs";
+import { readProjectFile } from "@hyperframes/parsers/asset-resolution";
 import { resolve, dirname, basename, join, relative, sep } from "node:path";
 import { parseGsapScript, type GsapAnimation } from "@hyperframes/core/gsap-parser";
 import type { Example } from "./_examples.js";
@@ -573,7 +574,7 @@ function groupTraces(tweens: SurfacedTween[]): SurfacedTrace[] {
   return traces;
 }
 
-function collectCompositions(indexPath: string): SurfacedComposition[] {
+export function collectCompositions(indexPath: string): SurfacedComposition[] {
   const html = readFileSync(indexPath, "utf-8");
   const baseDir = dirname(indexPath);
   const out: SurfacedComposition[] = [
@@ -589,9 +590,10 @@ function collectCompositions(indexPath: string): SurfacedComposition[] {
     const src = div.getAttribute("data-composition-src");
     if (!src) continue;
     const subPath = resolve(baseDir, src);
-    if (!existsSync(subPath)) continue;
+    const sub = readProjectFile(subPath);
+    if (sub.kind !== "file") continue;
     const id = div.getAttribute("data-composition-id") ?? src;
-    out.push(surfaceComposition(readFileSync(subPath, "utf-8"), id, src));
+    out.push(surfaceComposition(sub.text, id, src));
   }
   return out;
 }
@@ -769,9 +771,10 @@ export function resolveScope(args: { target?: string; selector?: string }): {
   let projectName: string;
   let projectDir: string | undefined;
   let entryFile: string | undefined;
-  if (raw && raw.endsWith(".html") && existsSync(raw) && statSync(raw).isFile()) {
+  const entry = raw?.endsWith(".html") ? readProjectFile(raw) : undefined;
+  if (raw && entry?.kind === "file") {
     const entryPath = resolve(raw);
-    comps = [surfaceComposition(readFileSync(entryPath, "utf-8"), basename(entryPath), entryPath)];
+    comps = [surfaceComposition(entry.text, basename(entryPath), entryPath)];
     projectName = basename(entryPath);
     projectDir = findProjectRoot(entryPath);
     entryFile = relative(projectDir, entryPath).split(sep).join("/");
diff --git a/packages/lint/src/project.test.ts b/packages/lint/src/project.test.ts
index 61b12e0add..079c076416 100644
--- a/packages/lint/src/project.test.ts
+++ b/packages/lint/src/project.test.ts
@@ -269,6 +269,20 @@ describe("missing_or_empty_sub_composition", () => {
     expect(finding?.message).toContain("does not exist");
   });
 
+  it("errors, instead of crashing, when the referenced sub-composition is a folder", async () => {
+    const project = makeProject(htmlWithSubComp("compositions/scene-title"), {});
+    mkdirSync(join(project, "compositions", "scene-title"));
+    writeFileSync(join(project, "compositions", "scene-title", "index.html"), validSubCompHtml());
+    const { results } = await lintProject(project);
+    const finding = results
+      .flatMap((r) => r.result.findings)
+      .find((f) => f.code === "missing_or_empty_sub_composition");
+
+    expect(finding?.message).toContain("compositions/scene-title");
+    expect(finding?.message).toContain("a folder, not an HTML file");
+    expect(finding?.fixHint).toContain('"compositions/scene-title/index.html"');
+  });
+
   it("errors when the referenced sub-composition file has content but no data-composition-id root", async () => {
     const { finding, totalErrors } = await lintSubComp("compositions/scene-title.html", {
       "scene-title.html": "<!doctype html><html><body><p>TODO: scene content</p></body></html>",
diff --git a/packages/lint/src/project.ts b/packages/lint/src/project.ts
index 9a116086bb..72a9981dea 100644
--- a/packages/lint/src/project.ts
+++ b/packages/lint/src/project.ts
@@ -12,6 +12,7 @@ import {
   isUnresolvedAssetPlaceholder,
   isWithinProjectRoot,
   maskNonScannableRanges,
+  readProjectFile,
   resolveExistingLocalAsset,
   resolveLocalAssetCandidates,
   resolveProjectRelativeSrc,
@@ -615,7 +616,7 @@ function lintMissingOrEmptySubComposition(
   rootHtml: string,
 ): HyperframeLintFinding[] {
   // Dedup by src path — the same reference can appear from nested sub-comps.
-  const checked = new Map<string, { srcPath: string; problem: string }>();
+  const checked = new Map<string, { srcPath: string; problem: string; folder?: true }>();
   const visited = new Set<string>();
 
   // fallow-ignore-next-line complexity
@@ -636,14 +637,25 @@ function lintMissingOrEmptySubComposition(
       if (visited.has(filePath)) continue;
       visited.add(filePath);
 
-      if (!existsSync(filePath)) {
+      const read = readProjectFile(filePath);
+      if (read.kind === "missing") {
         if (!checked.has(srcPath)) {
           checked.set(srcPath, { srcPath, problem: "the file does not exist" });
         }
         continue;
       }
+      if (read.kind === "folder") {
+        if (!checked.has(srcPath)) {
+          checked.set(srcPath, {
+            srcPath,
+            problem: "it is a folder, not an HTML file",
+            folder: true,
+          });
+        }
+        continue;
+      }
 
-      const fileHtml = readFileSync(filePath, "utf-8");
+      const fileHtml = read.text;
       const validity = checkSubCompositionUsability(fileHtml, parseSubCompHtml);
       if (!validity.ok) {
         if (!checked.has(srcPath)) {
@@ -664,17 +676,18 @@ function lintMissingOrEmptySubComposition(
   walk(rootHtml);
 
   const findings: HyperframeLintFinding[] = [];
-  for (const { srcPath, problem } of checked.values()) {
+  for (const { srcPath, problem, folder } of checked.values()) {
     findings.push({
       code: "missing_or_empty_sub_composition",
       severity: "error",
       message: `data-composition-src references "${srcPath}", but ${problem}.`,
-      fixHint:
-        `Fix this before rendering — the render pre-flight rejects unusable sub-compositions. ` +
-        `Write valid HTML into "${srcPath}" — it needs a <template> or <body> containing an element with ` +
-        `data-composition-id, data-width, and data-height. Preview/studio still tolerates and skips the ` +
-        "scene while you author it. If a scene-authoring step is still running, wait for it to finish " +
-        "before referencing the file, or re-run the step that generates it.",
+      fixHint: folder
+        ? `Point data-composition-src at the HTML file inside the folder, such as "${srcPath}/index.html".`
+        : `Fix this before rendering — the render pre-flight rejects unusable sub-compositions. ` +
+          `Write valid HTML into "${srcPath}" — it needs a <template> or <body> containing an element with ` +
+          `data-composition-id, data-width, and data-height. Preview/studio still tolerates and skips the ` +
+          "scene while you author it. If a scene-authoring step is still running, wait for it to finish " +
+          "before referencing the file, or re-run the step that generates it.",
     });
   }
 
diff --git a/packages/parsers/src/assetResolution.test.ts b/packages/parsers/src/assetResolution.test.ts
index bb9c83cf40..8db653eb21 100644
--- a/packages/parsers/src/assetResolution.test.ts
+++ b/packages/parsers/src/assetResolution.test.ts
@@ -1,5 +1,6 @@
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
-import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { execFileSync } from "node:child_process";
+import { mkdirSync, mkdtempSync, rmSync, symlinkSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join, posix, win32 } from "node:path";
 import {
@@ -7,6 +8,7 @@ import {
   isUnresolvedAssetPlaceholder,
   isWithinProjectRoot,
   maskNonScannableRanges,
+  readProjectFile,
   resolveProjectRelativeSrc,
 } from "./assetResolution.js";
 
@@ -265,3 +267,37 @@ describe("isWithinProjectRoot", () => {
     expect(isWithinProjectRoot("/project", "/project", posix)).toBe(true);
   });
 });
+
+describe("readProjectFile", () => {
+  let dir: string;
+  beforeAll(() => {
+    dir = mkdtempSync(join(tmpdir(), "hf-read-project-file-"));
+    writeFileSync(join(dir, "scene.html"), "<div></div>");
+    mkdirSync(join(dir, "intro"));
+  });
+  afterAll(() => rmSync(dir, { recursive: true, force: true }));
+
+  it("reads a file's text", () => {
+    expect(readProjectFile(join(dir, "scene.html"))).toEqual({ kind: "file", text: "<div></div>" });
+  });
+
+  it("names a folder instead of throwing EISDIR", () => {
+    expect(readProjectFile(join(dir, "intro"))).toEqual({ kind: "folder" });
+  });
+
+  it("reports a missing path, including one under a file", () => {
+    expect(readProjectFile(join(dir, "gone.html"))).toEqual({ kind: "missing" });
+    expect(readProjectFile(join(dir, "scene.html", "inner.html"))).toEqual({ kind: "missing" });
+  });
+
+  it("reports a name too long or a symlink loop as missing, as existsSync did", () => {
+    expect(readProjectFile(join(dir, `${"a".repeat(300)}.html`))).toEqual({ kind: "missing" });
+    symlinkSync(join(dir, "loop.html"), join(dir, "loop.html"));
+    expect(readProjectFile(join(dir, "loop.html"))).toEqual({ kind: "missing" });
+  });
+
+  it.skipIf(process.platform === "win32")("reports a named pipe without blocking on it", () => {
+    execFileSync("mkfifo", [join(dir, "pipe.html")]);
+    expect(readProjectFile(join(dir, "pipe.html"))).toEqual({ kind: "folder" });
+  });
+});
diff --git a/packages/parsers/src/assetResolution.ts b/packages/parsers/src/assetResolution.ts
index 0378a9d7ab..e5449ac75f 100644
--- a/packages/parsers/src/assetResolution.ts
+++ b/packages/parsers/src/assetResolution.ts
@@ -1,4 +1,4 @@
-import { existsSync } from "node:fs";
+import { closeSync, constants, existsSync, fstatSync, openSync, readFileSync } from "node:fs";
 import { isAbsolute, join, posix, relative, resolve, sep } from "node:path";
 import { decodeUrlPathVariants } from "./composition.js";
 
@@ -242,3 +242,28 @@ export function maskNonScannableRanges(html: string): string {
   out = maskRange(out, /<script\b[^>]*>[\s\S]*?<\/script\b[^>]*>/gi);
   return out;
 }
+
+export type ProjectFileRead =
+  | { kind: "file"; text: string }
+  | { kind: "folder" }
+  | { kind: "missing" };
+
+/** Reads through one descriptor, so the file-type check and the read see the same file. */
+export function readProjectFile(path: string): ProjectFileRead {
+  let fd: number;
+  try {
+    // Non-blocking so a named pipe is reported, not waited on; the mode never applies (no O_CREAT).
+    fd = openSync(path, constants.O_RDONLY | (constants.O_NONBLOCK ?? 0), 0o600);
+  } catch (error) {
+    const code = (error as NodeJS.ErrnoException).code ?? "";
+    if (["EISDIR", "ENXIO"].includes(code)) return { kind: "folder" };
+    if (["ENOENT", "ENOTDIR", "ELOOP", "ENAMETOOLONG"].includes(code)) return { kind: "missing" };
+    throw error;
+  }
+  try {
+    if (!fstatSync(fd).isFile()) return { kind: "folder" };
+    return { kind: "file", text: readFileSync(fd, "utf-8") };
+  } finally {
+    closeSync(fd);
+  }
+}
diff --git a/packages/producer/src/services/htmlCompiler.test.ts b/packages/producer/src/services/htmlCompiler.test.ts
index 030e48b81a..aec52bb5cd 100644
--- a/packages/producer/src/services/htmlCompiler.test.ts
+++ b/packages/producer/src/services/htmlCompiler.test.ts
@@ -823,6 +823,27 @@ describe("detectRenderModeHints", () => {
     ).rejects.toThrow(/compositions\/intro\.html[\s\S]*compositions\/outro\.html/);
   });
 
+  it("compileForRender aborts naming a data-composition-src that points at a folder", async () => {
+    const projectDir = makeSubCompProject(
+      "hf-folder-subcomp-",
+      [{ id: "intro", src: "compositions/intro" }],
+      {},
+    );
+    try {
+      mkdirSync(join(projectDir, "compositions", "intro"));
+      writeFileSync(
+        join(projectDir, "compositions", "intro", "index.html"),
+        validSubCompHtml("intro", "Intro"),
+      );
+
+      await expect(
+        compileForRender(projectDir, join(projectDir, "index.html"), projectDir),
+      ).rejects.toThrow(/compositions\/intro[\s\S]*a folder, not an HTML file/);
+    } finally {
+      rmSync(projectDir, { recursive: true, force: true });
+    }
+  });
+
   it("compileForRender aborts when a data-composition-src reference points at a missing file", async () => {
     const projectDir = makeSubCompProject(
       "hf-missing-subcomp-",
diff --git a/packages/producer/src/services/htmlCompiler.ts b/packages/producer/src/services/htmlCompiler.ts
index b255192eaa..f909620515 100644
--- a/packages/producer/src/services/htmlCompiler.ts
+++ b/packages/producer/src/services/htmlCompiler.ts
@@ -56,7 +56,10 @@ import {
   checkSubCompositionUsability,
   type ParsableDocumentLike,
 } from "@hyperframes/parsers/sub-composition-validity";
-import { isUnresolvedAssetPlaceholder } from "@hyperframes/parsers/asset-resolution";
+import {
+  isUnresolvedAssetPlaceholder,
+  readProjectFile,
+} from "@hyperframes/parsers/asset-resolution";
 import { extractMediaMetadata, extractAudioMetadata } from "../utils/ffprobe.js";
 import { isPathInside, toExternalAssetKey } from "../utils/paths.js";
 import { collectRenderMedia } from "./renderMediaCollector.js";
@@ -209,12 +212,17 @@ function assertSubCompositionsUsable(
     // silence here rather than pretend it surfaces an error somewhere else.
     if (visited.has(filePath)) continue;
 
-    if (!existsSync(filePath)) {
+    const read = readProjectFile(filePath);
+    if (read.kind === "missing") {
       problems.push({ srcPath, detail: "the file does not exist" });
       continue;
     }
+    if (read.kind === "folder") {
+      problems.push({ srcPath, detail: "it is a folder, not an HTML file" });
+      continue;
+    }
 
-    const fileHtml = readFileSync(filePath, "utf-8");
+    const fileHtml = read.text;
     const validity = checkSubCompositionUsability(fileHtml, parseSubCompHtmlForValidity);
     if (!validity.ok) {
       problems.push({
@@ -708,11 +716,12 @@ async function parseSubCompositions(
       continue;
     }
 
-    if (!existsSync(filePath)) {
+    const read = readProjectFile(filePath);
+    if (read.kind !== "file") {
       continue;
     }
 
-    const rawSubHtml = readFileSync(filePath, "utf-8");
+    const rawSubHtml = read.text;
     const nestedVisited = new Set(visited);
     nestedVisited.add(filePath);
 
@@ -981,10 +990,8 @@ function inlineSubCompositions(
       resolveHtml: (srcPath: string) => {
         let compHtml = subCompositions.get(srcPath) || null;
         if (!compHtml) {
-          const filePath = resolve(projectDir, srcPath);
-          if (existsSync(filePath)) {
-            compHtml = readFileSync(filePath, "utf-8");
-          }
+          const read = readProjectFile(resolve(projectDir, srcPath));
+          if (read.kind === "file") compHtml = read.text;
         }
         return compHtml;
       },
diff --git a/packages/studio-server/src/helpers/compositionInsertion.test.ts b/packages/studio-server/src/helpers/compositionInsertion.test.ts
index 954f65e468..69a1aa582f 100644
--- a/packages/studio-server/src/helpers/compositionInsertion.test.ts
+++ b/packages/studio-server/src/helpers/compositionInsertion.test.ts
@@ -1,4 +1,4 @@
-import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { afterEach, describe, expect, it } from "vitest";
@@ -175,6 +175,28 @@ describe("insertCompositionIntoSource", () => {
     expect(() => insert("../outside.html")).toThrow(CompositionInsertionError);
   });
 
+  it("rejects a source, or a source's dependency, that is a folder", () => {
+    const dir = project();
+    writeFixture(dir);
+    mkdirSync(join(dir, "intro"));
+    writeFileSync(
+      join(dir, "wraps-folder.html"),
+      `<div data-composition-id="wrap" data-width="1" data-height="1" data-duration="1"><div data-composition-src="intro"></div></div>`,
+    );
+    const insert = (sourcePath: string) =>
+      insertCompositionIntoSource({
+        projectDir: dir,
+        targetPath: "index.html",
+        sourcePath,
+        parentSource: parent,
+        start: 0,
+        desiredTrack: 0,
+      });
+
+    expect(() => insert("intro")).toThrow(/folder, not an HTML file/);
+    expect(() => insert("wraps-folder.html")).toThrow(/folder, not an HTML file/);
+  });
+
   it("says the project folder is gone rather than that the source escapes it", () => {
     const dir = project();
     rmSync(dir, { recursive: true });
diff --git a/packages/studio-server/src/helpers/compositionInsertion.ts b/packages/studio-server/src/helpers/compositionInsertion.ts
index 9b6cdd1b3f..1be4f438a3 100644
--- a/packages/studio-server/src/helpers/compositionInsertion.ts
+++ b/packages/studio-server/src/helpers/compositionInsertion.ts
@@ -1,4 +1,5 @@
-import { existsSync, readFileSync } from "node:fs";
+import { existsSync } from "node:fs";
+import { readProjectFile } from "@hyperframes/parsers/asset-resolution";
 import { randomUUID } from "node:crypto";
 import { dirname, relative, resolve, sep } from "node:path";
 import { parseHTML } from "linkedom";
@@ -52,6 +53,17 @@ function canonicalProjectPath(projectDir: string, candidate: string | null): str
   return canonical;
 }
 
+function readCompositionSource(file: string): string {
+  const read = readProjectFile(file);
+  if (read.kind === "folder") {
+    throw new CompositionInsertionError("Composition source is a folder, not an HTML file", 400);
+  }
+  if (read.kind === "missing") {
+    throw new CompositionInsertionError("Composition source was not found", 404);
+  }
+  return read.text;
+}
+
 function validateSourcePath(sourcePath: string): void {
   if (!sourcePath.trim() || sourcePath.includes("\0") || /^[a-z]+:/i.test(sourcePath)) {
     throw new CompositionInsertionError("Invalid composition source path", 400);
@@ -83,7 +95,7 @@ function validateDependencyGraph(projectDir: string, targetAbs: string, sourceAb
     }
     if (visited.has(file)) return;
     visiting.add(file);
-    const source = readFileSync(file, "utf-8");
+    const source = readCompositionSource(file);
     const { document } = compositionRoot(source);
     for (const host of descendants(document, "[data-composition-src]")) {
       const dependency = host.getAttribute("data-composition-src");
@@ -169,7 +181,7 @@ export function insertCompositionIntoSource(input: {
   const sourceAbs = canonicalProjectFile(input.projectDir, input.sourcePath);
   validateDependencyGraph(input.projectDir, targetAbs, sourceAbs);
 
-  const source = readFileSync(sourceAbs, "utf-8");
+  const source = readCompositionSource(sourceAbs);
   const sourceComposition = compositionRoot(source).root;
   const duration = positiveAttribute(
     sourceComposition,