diff --git a/.claude/commands/quarto-preview-test/SKILL.md b/.claude/commands/quarto-preview-test/SKILL.md
index 11f00b8694..b902225e21 100644
--- a/.claude/commands/quarto-preview-test/SKILL.md
+++ b/.claude/commands/quarto-preview-test/SKILL.md
@@ -100,9 +100,38 @@ See `llm-docs/preview-architecture.md` for the full architecture.
 - Testing render output only (no live preview needed) — use `quarto render`
 - CI environments without browser access
 
-## Test Fixtures and Cases
+## Test Matrix
 
-Test fixtures live in `tests/docs/manual/preview/`. The full test matrix is in `tests/docs/manual/preview/README.md`.
+The full test matrix lives in `tests/docs/manual/preview/README.md`. Test fixtures live alongside it in `tests/docs/manual/preview/`.
+
+### Running specific tests by ID
+
+When invoked with test IDs (e.g., `/quarto-preview-test T17 T18`):
+
+1. Read `tests/docs/manual/preview/README.md`
+2. Find each requested test by its ID (e.g., `#### T17:`)
+3. Parse the **Setup**, **Steps**, and **Expected** fields
+4. Execute each test following the steps, using the fixtures in `tests/docs/manual/preview/`
+5. Report PASS/FAIL for each test with the actual vs expected result
+
+### Running tests by topic
+
+When invoked with a topic description instead of IDs (e.g., `/quarto-preview-test root URL` or "run preview tests for single-file"):
+
+1. Read `tests/docs/manual/preview/README.md`
+2. Search test titles and descriptions for matches (keywords, issue numbers, feature area)
+3. Present the matched tests to the user for confirmation before running:
+   ```
+   Found these matching tests:
+   - T17: Single-file preview — root URL accessible (#14298)
+   - T18: Single-file preview — named output URL also accessible
+   Run these? [Y/n]
+   ```
+4. Only execute after user confirms
+
+### Running without arguments
+
+When invoked without test IDs or topic (e.g., `/quarto-preview-test`), use the general Edit-Verify Cycle workflow described above for ad-hoc preview testing. The test matrix is for targeted regression testing.
 
 ## Baseline Comparison
 
diff --git a/news/changelog-1.10.md b/news/changelog-1.10.md
index d43a69482c..6f85f67d76 100644
--- a/news/changelog-1.10.md
+++ b/news/changelog-1.10.md
@@ -4,6 +4,7 @@ All changes included in 1.10:
 
 - ([#14267](https://github.com/quarto-dev/quarto-cli/issues/14267)): Fix Windows paths with accented characters (e.g., `C:\Users\Sébastien\`) breaking dart-sass compilation.
 - ([#14281](https://github.com/quarto-dev/quarto-cli/issues/14281)): Fix transient `.quarto_ipynb` files accumulating during `quarto preview` with Jupyter engine.
+- ([#14298](https://github.com/quarto-dev/quarto-cli/issues/14298)): Fix `quarto preview` browse URL including output filename (e.g., `hello.html`) for single-file documents, breaking Posit Workbench proxied server access.
 
 ## Formats
 
diff --git a/src/command/preview/preview.ts b/src/command/preview/preview.ts
index f65825add2..e307948d03 100644
--- a/src/command/preview/preview.ts
+++ b/src/command/preview/preview.ts
@@ -143,6 +143,21 @@ interface PreviewOptions {
   presentation: boolean;
 }
 
+export function previewInitialPath(
+  outputFile: string,
+  project: ProjectContext | undefined,
+): string {
+  if (isPdfContent(outputFile)) {
+    return kPdfJsInitialPath;
+  }
+  if (project && !project.isSingleFile) {
+    return pathWithForwardSlashes(
+      relative(projectOutputDir(project), outputFile),
+    );
+  }
+  return "";
+}
+
 export async function preview(
   file: string,
   flags: RenderFlags,
@@ -233,7 +248,7 @@ export async function preview(
       changeHandler.render,
       project,
     )
-    : project
+    : project && !project.isSingleFile
     ? projectHtmlFileRequestHandler(
       project,
       normalizePath(file),
@@ -253,13 +268,7 @@ export async function preview(
     );
 
   // open browser if this is a browseable format
-  const initialPath = isPdfContent(result.outputFile)
-    ? kPdfJsInitialPath
-    : project
-    ? pathWithForwardSlashes(
-      relative(projectOutputDir(project), result.outputFile),
-    )
-    : "";
+  const initialPath = previewInitialPath(result.outputFile, project);
   if (
     options.browser &&
     !isServerSession() &&
diff --git a/tests/docs/manual/README.md b/tests/docs/manual/README.md
new file mode 100644
index 0000000000..df15820e31
--- /dev/null
+++ b/tests/docs/manual/README.md
@@ -0,0 +1,17 @@
+# Manual Tests
+
+Tests that require interactive sessions, external services, or browser access that cannot run in automated CI.
+
+## Test Suites
+
+| Directory / File | Area | Skill | Description |
+|-----------------|------|-------|-------------|
+| `preview/` | `quarto preview` | `/quarto-preview-test` | Live preview server behavior: URL routing, file watching, live reload, transient file cleanup |
+| `publish-connect-cloud/` | `quarto publish` | — | Posit Connect Cloud publishing with OAuth flow |
+| `mermaid-svg-pdf-tooling.qmd` | `quarto render` | — | Mermaid SVG rendering to PDF with external tooling (rsvg-convert) |
+
+## Running Tests
+
+Each suite has its own README with test matrix and execution instructions. Test fixtures live alongside the README in each directory.
+
+For preview tests, use the `/quarto-preview-test` skill which automates the start-verify-cleanup cycle.
diff --git a/tests/docs/manual/preview/README.md b/tests/docs/manual/preview/README.md
index 55b1d71d83..6575b7c547 100644
--- a/tests/docs/manual/preview/README.md
+++ b/tests/docs/manual/preview/README.md
@@ -132,6 +132,35 @@ For tests without Jupyter execution (T9, T10, T11), verify no `.quarto_ipynb` fi
 - **Steps:** Preview project, edit index, navigate to about, edit about
 - **Expected:** At most one `.quarto_ipynb` per Jupyter-using file. No accumulation.
 
+## Test Matrix: Single-file Preview Root URL (#14298)
+
+After every change to preview URL or handler logic, verify that single-file previews serve content at the root URL and print the correct Browse URL.
+
+### P1: Critical
+
+#### T17: Single-file preview — root URL accessible
+
+- **Setup:** `plain.qmd` with only markdown content (no code cells)
+- **Steps:** `quarto preview plain.qmd --port XXXX --no-browser`, then `curl -s -o /dev/null -w "%{http_code}" http://localhost:XXXX/`
+- **Expected:** HTTP 200. Browse URL prints `http://localhost:XXXX/` (no filename appended).
+- **Catches:** `projectHtmlFileRequestHandler` used for single files (defaultFile=`index.html` instead of output filename), or `previewInitialPath` returning filename instead of `""`
+
+#### T18: Single-file preview — named output URL also accessible
+
+- **Setup:** Same `plain.qmd`
+- **Steps:** `quarto preview plain.qmd --port XXXX --no-browser`, then `curl -s -o /dev/null -w "%{http_code}" http://localhost:XXXX/plain.html`
+- **Expected:** HTTP 200. The output filename path also serves the rendered content.
+- **Catches:** Handler regression where only root or only named path works
+
+### P2: Important
+
+#### T19: Project preview — non-index file URL correct
+
+- **Setup:** Website project with `_quarto.yml`, `index.qmd`, and `about.qmd`
+- **Steps:** `quarto preview --port XXXX --no-browser`, navigate to `http://localhost:XXXX/about.html`
+- **Expected:** HTTP 200. Browse URL may include path for non-index files in project context.
+- **Catches:** `isSingleFile` guard accidentally excluding real project files from path computation
+
 ## Test File Templates
 
 **Minimal Python .qmd:**
diff --git a/tests/unit/preview-initial-path.test.ts b/tests/unit/preview-initial-path.test.ts
new file mode 100644
index 0000000000..7fbd1b3ed4
--- /dev/null
+++ b/tests/unit/preview-initial-path.test.ts
@@ -0,0 +1,53 @@
+/*
+ * preview-initial-path.test.ts
+ *
+ * Tests that previewInitialPath computes the correct browse URL path
+ * for different project types. Regression test for #14298.
+ *
+ * Copyright (C) 2026 Posit Software, PBC
+ */
+
+import { unitTest } from "../test.ts";
+import { assertEquals } from "testing/asserts";
+import { join } from "../../src/deno_ral/path.ts";
+import { previewInitialPath } from "../../src/command/preview/preview.ts";
+import { createMockProjectContext } from "./project/utils.ts";
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - single file returns empty path (#14298)", async () => {
+  const project = createMockProjectContext({ isSingleFile: true });
+  const outputFile = join(project.dir, "hello.html");
+
+  const result = previewInitialPath(outputFile, project);
+  assertEquals(result, "", "Single-file preview should use root path, not filename");
+
+  project.cleanup();
+});
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - project file returns relative path", async () => {
+  const project = createMockProjectContext();
+  const outputFile = join(project.dir, "chapter.html");
+
+  const result = previewInitialPath(outputFile, project);
+  assertEquals(result, "chapter.html", "Project preview should include relative path");
+
+  project.cleanup();
+});
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - project subdir returns relative path", async () => {
+  const project = createMockProjectContext();
+  const outputFile = join(project.dir, "pages", "about.html");
+
+  const result = previewInitialPath(outputFile, project);
+  assertEquals(result, "pages/about.html", "Project preview should include subdirectory path");
+
+  project.cleanup();
+});
+
+// deno-lint-ignore require-await
+unitTest("previewInitialPath - undefined project returns empty path", async () => {
+  const result = previewInitialPath("/tmp/hello.html", undefined);
+  assertEquals(result, "", "No project should use root path");
+});
diff --git a/tests/unit/project/utils.ts b/tests/unit/project/utils.ts
index e7b6c375e2..810847678b 100644
--- a/tests/unit/project/utils.ts
+++ b/tests/unit/project/utils.ts
@@ -11,21 +11,28 @@ import { FileInformationCacheMap } from "../../../src/project/project-shared.ts"
 
 /**
  * Create a minimal mock ProjectContext for testing.
- * Only provides the essential properties needed for cache-related tests.
  *
- * @param dir - The project directory (defaults to a temp directory)
+ * @param options.dir - The project directory (defaults to a temp directory)
+ * @param options.isSingleFile - Whether this is a single-file project (defaults to false)
+ * @param options.config - Project config (defaults to { project: {} })
  * @returns A mock ProjectContext suitable for unit testing
  */
 export function createMockProjectContext(
-  dir?: string,
+  options?: {
+    dir?: string;
+    isSingleFile?: boolean;
+    config?: ProjectContext["config"];
+  },
 ): ProjectContext {
-  const projectDir = dir ?? Deno.makeTempDirSync({ prefix: "quarto-test" });
-  const ownsDir = dir === undefined;
+  const projectDir = options?.dir ??
+    Deno.makeTempDirSync({ prefix: "quarto-test" });
+  const ownsDir = options?.dir === undefined;
 
   return {
     dir: projectDir,
     engines: [],
     files: { input: [] },
+    config: options?.config ?? { project: {} },
     notebookContext: {} as ProjectContext["notebookContext"],
     fileInformationCache: new FileInformationCacheMap(),
     resolveBrand: () => Promise.resolve(undefined),
@@ -37,7 +44,7 @@ export function createMockProjectContext(
     clone: function () {
       return this;
     },
-    isSingleFile: false,
+    isSingleFile: options?.isSingleFile ?? false,
     diskCache: {} as ProjectContext["diskCache"],
     temp: {} as ProjectContext["temp"],
     cleanup: () => {