diff --git a/.claude/commands/quarto-preview-test/SKILL.md b/.claude/commands/quarto-preview-test/SKILL.md
new file mode 100644
index 00000000000..b902225e21f
--- /dev/null
+++ b/.claude/commands/quarto-preview-test/SKILL.md
@@ -0,0 +1,145 @@
+---
+name: quarto-preview-test
+description: Use when testing preview functionality, verifying live reload, or validating preview fixes. Covers starting preview with port/logging, browser verification via /agent-browser, and checking logs/filesystem for artifacts.
+---
+
+# Quarto Preview Test
+
+Interactive testing of `quarto preview` with automated browser verification.
+
+## Tools
+
+| Tool | When to use |
+|------|-------------|
+| `/agent-browser` | **Preferred.** Token-efficient browser automation. Navigate, verify content, screenshot. |
+| Chrome DevTools MCP | Deep debugging: console messages, network requests, DOM inspection. |
+| `jq` / `grep` | Parse debug log output. |
+
+## Prerequisites
+
+- Quarto dev version built (`./configure.sh` or `./configure.cmd`)
+- Test environment configured (`tests/configure-test-env.sh` or `.ps1`)
+- `/agent-browser` CLI installed (preferred), OR Chrome + Chrome DevTools MCP connected
+
+## Starting Preview
+
+Preview needs the test venv for Jupyter tests. Activate it first (`tests/.venv`), matching how `run-tests.sh` / `run-tests.ps1` do it.
+
+```bash
+# Linux/macOS
+source tests/.venv/bin/activate
+./package/dist/bin/quarto preview <file-or-dir> --no-browser --port 4444
+
+# Windows (Git Bash)
+source tests/.venv/Scripts/activate
+./package/dist/bin/quarto.cmd preview <file-or-dir> --no-browser --port 4444
+```
+
+Use `--no-browser` to control browser connection. Use `--port` for a predictable URL.
+
+### With debug logging
+
+```bash
+./package/dist/bin/quarto preview <file> --no-browser --port 4444 --log-level debug 2>&1 | tee preview.log
+```
+
+### In background
+
+```bash
+# Linux/macOS (after venv activation)
+./package/dist/bin/quarto preview <file> --no-browser --port 4444 &
+PREVIEW_PID=$!
+# ... run verification ...
+kill $PREVIEW_PID
+
+# Windows (Git Bash, after venv activation)
+./package/dist/bin/quarto.cmd preview <file> --no-browser --port 4444 &
+PREVIEW_PID=$!
+# ... run verification ...
+kill $PREVIEW_PID
+```
+
+## Edit-Verify Cycle
+
+The core test pattern:
+
+1. Start preview with `--no-browser --port 4444`
+2. Use `/agent-browser` to navigate to `http://localhost:4444/` and verify content
+3. Edit source file, wait 3-5 seconds for re-render
+4. Verify content updated in browser
+5. Check filesystem for unexpected artifacts
+6. Stop preview, verify cleanup
+
+## What to Verify
+
+**In browser** (via `/agent-browser`): Page loads, content matches source, updates reflect edits.
+
+**In terminal/logs**: No `BadResource` errors, no crashes, preview stays responsive.
+
+**On filesystem**: No orphaned temp files, cleanup happens on exit.
+
+## Windows Limitations
+
+On Windows, `kill` from Git Bash does not trigger Quarto's `onCleanup` handler (SIGINT doesn't propagate to Windows processes the same way). Cleanup-on-exit verification requires an interactive terminal with Ctrl+C. For automated testing, verify artifacts *during* preview instead.
+
+## Context Types
+
+Preview behaves differently depending on input:
+
+| Input | Code path |
+|-------|-----------|
+| Single file (no project) | `preview()` -> `renderForPreview()` |
+| File within a project | May redirect to project preview via `serveProject()` |
+| Project directory | `serveProject()` -> `watchProject()` |
+
+See `llm-docs/preview-architecture.md` for the full architecture.
+
+## When NOT to Use
+
+- Automated smoke tests — use `tests/smoke/` instead
+- Testing render output only (no live preview needed) — use `quarto render`
+- CI environments without browser access
+
+## Test Matrix
+
+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
+
+Compare dev build against installed release to distinguish regressions:
+
+```bash
+quarto --version                        # installed
+./package/dist/bin/quarto --version     # dev
+```
+
+If both show the same issue, it's pre-existing.
diff --git a/news/changelog-1.9.md b/news/changelog-1.9.md
index 46c821a13fc..5ab9145c937 100644
--- a/news/changelog-1.9.md
+++ b/news/changelog-1.9.md
@@ -4,6 +4,7 @@
 
 - ([#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.
 
 ## In previous releases
 
diff --git a/src/command/preview/preview.ts b/src/command/preview/preview.ts
index edb2ac0b75c..3141913418a 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,
@@ -230,7 +245,7 @@ export async function preview(
       changeHandler.render,
       project,
     )
-    : project
+    : project && !project.isSingleFile
     ? projectHtmlFileRequestHandler(
       project,
       normalizePath(file),
@@ -250,13 +265,7 @@ 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 00000000000..df15820e310
--- /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
new file mode 100644
index 00000000000..6575b7c5479
--- /dev/null
+++ b/tests/docs/manual/preview/README.md
@@ -0,0 +1,196 @@
+# Manual Preview Tests
+
+Tests for `quarto preview` behavior that require an interactive session with file-save events. These cannot run in automated smoke tests.
+
+## Automation
+
+Use the `/quarto-preview-test` command for the general workflow of starting preview, verifying with browser automation, and checking logs/filesystem. It documents the tools and patterns.
+
+For browser interaction, `/agent-browser` is preferred over Chrome DevTools MCP (more token-efficient). See the command for details.
+
+## Test Matrix: quarto_ipynb Accumulation (#14281)
+
+After every test involving Jupyter execution (Python/Julia cells), verify:
+1. `ls *.quarto_ipynb*` — at most one `{name}.quarto_ipynb` (no `_1`, `_2` variants)
+2. After Ctrl+C exit — no `.quarto_ipynb` files remain (unless `keep-ipynb: true`)
+
+For tests without Jupyter execution (T9, T10, T11), verify no `.quarto_ipynb` files are created at all.
+
+### P1: Critical
+
+#### T1: Single .qmd with Python — re-render accumulation
+
+- **Setup:** `test.qmd` with Python code cell
+- **Steps:** `quarto preview test.qmd`, save 5 times, check files, Ctrl+C
+- **Expected:** At most one `.quarto_ipynb` at any time. Zero after exit.
+- **Catches:** `invalidateForFile()` not deleting transient file before cache eviction
+
+#### T2: Single .qmd with Python — startup duplicate
+
+- **Setup:** Same `test.qmd`
+- **Steps:** `quarto preview test.qmd`, check files immediately after first render (before any saves), Ctrl+C
+- **Expected:** Exactly one `.quarto_ipynb` during render. Zero after exit.
+- **Catches:** `cmd.ts` not passing ProjectContext to `preview()`
+
+#### T3: .qmd in project — project-level preview
+
+- **Setup:** Website project (`_quarto.yml` with `type: website`), `index.qmd` with Python cell
+- **Steps:** `quarto preview` (project dir), save `index.qmd` 3 times, check files, Ctrl+C
+- **Expected:** At most one `index.quarto_ipynb`. Zero after exit.
+- **Catches:** Fix works when `projectContext()` finds a real project
+
+#### T4: .qmd in project — single file preview
+
+- **Setup:** Same project as T3
+- **Steps:** `quarto preview index.qmd`, save 3 times, check files, Ctrl+C
+- **Expected:** Same as T3. May redirect to project preview (expected behavior).
+- **Catches:** Context passing works for files inside serveable projects
+
+### P2: Important
+
+#### T5: .qmd with Julia code cells
+
+- **Setup:** `julia-test.qmd` with Julia cell
+- **Steps:** Same as T1
+- **Expected:** Same as T1. Julia uses the same Jupyter engine path.
+
+#### T6: Rapid successive saves
+
+- **Setup:** Same `test.qmd` as T1
+- **Steps:** Save 5 times within 2-3 seconds (faster than render completes)
+- **Expected:** At most one `.quarto_ipynb`. Debounce/queue coalesces saves.
+- **Catches:** Race condition in invalidation during in-progress render
+
+#### T7: `keep-ipynb: true`
+
+- **Setup:** `test.qmd` with `keep-ipynb: true` in YAML
+- **Steps:** Preview, save 3 times, Ctrl+C, check files
+- **Expected:** `test.quarto_ipynb` persists after exit (not cleaned up). No `_1` variants during preview.
+- **Catches:** `invalidateForFile()` respects the `transient = false` flag set by `cleanupNotebook()`
+
+#### T8: `--to pdf` format
+
+- **Setup:** Same `test.qmd` (requires TinyTeX)
+- **Steps:** `quarto preview test.qmd --to pdf`, save 3 times
+- **Expected:** Same as T1. Transient notebook logic is format-independent.
+
+#### T9: Plain .qmd — no code cells (regression)
+
+- **Setup:** `plain.qmd` with only markdown content
+- **Steps:** Preview, save 3 times, check for `.quarto_ipynb` files
+- **Expected:** No `.quarto_ipynb` files ever created.
+- **Catches:** Fix is a no-op when no Jupyter engine is involved
+
+#### T10: .qmd with R/knitr engine (regression)
+
+- **Setup:** `r-test.qmd` with R code cell and `engine: knitr`
+- **Steps:** Preview, save 3 times, check for `.quarto_ipynb` files
+- **Expected:** No `.quarto_ipynb` files. Knitr doesn't use Jupyter intermediate.
+
+#### T10b: File excluded from project inputs (regression)
+
+- **Setup:** Website project with `_quarto.yml`. Create `_excluded.qmd` with a Python cell (files starting with `_` are excluded from project inputs by default)
+- **Steps:** `quarto preview _excluded.qmd`, save 3 times, check files, Ctrl+C
+- **Expected:** Falls back to single-file preview (not project preview). At most one `.quarto_ipynb`.
+- **Catches:** Context reuse from cmd.ts incorrectly applying project semantics to excluded files
+
+### P3: Nice-to-Have
+
+#### T11: Native .ipynb file
+
+- **Setup:** `notebook.ipynb` (native Jupyter notebook)
+- **Steps:** Preview, save 3 times
+- **Expected:** No transient `.quarto_ipynb` — the `.ipynb` is the source, not transient.
+
+#### T12: File with spaces in name
+
+- **Setup:** `my document.qmd` with Python cell
+- **Steps:** `quarto preview "my document.qmd"`, save 3 times
+- **Expected:** At most one `my document.quarto_ipynb`. Path normalization handles spaces.
+
+#### T13: File in subdirectory
+
+- **Setup:** `subdir/deep/test.qmd` with Python cell
+- **Steps:** Preview from parent dir, save 3 times
+- **Expected:** At most one transient notebook in `subdir/deep/`.
+
+#### T14: Change code cell content
+
+- **Setup:** `test.qmd` with `x = 1; print(x)`
+- **Steps:** Change to `x = 2`, save; change to `x = 3`, save
+- **Expected:** At most one `.quarto_ipynb`. Code changes trigger re-execution but file is cleaned.
+
+#### T15: Change YAML metadata
+
+- **Setup:** `test.qmd` with `title: "Test"`
+- **Steps:** Change title, save; add `theme: cosmo`, save
+- **Expected:** Same as T1. Metadata changes go through the same render/invalidation path.
+
+#### T16: Multiple .qmd files in project
+
+- **Setup:** Website with `index.qmd` (Python), `about.qmd` (Python), `plain.qmd` (no code)
+- **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:**
+```yaml
+---
+title: "Preview Test"
+---
+```
+
+````
+```{python}
+print("Hello from Python")
+```
+````
+
+```
+Edit this line to trigger re-renders.
+```
+
+**Minimal website project (`_quarto.yml`):**
+```yaml
+project:
+  type: website
+```
+
+**keep-ipynb variant:**
+```yaml
+---
+title: "Keep ipynb Test"
+execute:
+  keep-ipynb: true
+---
+```
diff --git a/tests/docs/manual/preview/plain.qmd b/tests/docs/manual/preview/plain.qmd
new file mode 100644
index 00000000000..27dd844ed46
--- /dev/null
+++ b/tests/docs/manual/preview/plain.qmd
@@ -0,0 +1,9 @@
+---
+title: "Plain QMD Test"
+---
+
+## T9: No code cells
+
+No .quarto_ipynb should ever be created.
+
+Edit counter: 0
diff --git a/tests/unit/preview-initial-path.test.ts b/tests/unit/preview-initial-path.test.ts
new file mode 100644
index 00000000000..7fbd1b3ed42
--- /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 e7b6c375e29..810847678b4 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: () => {