diff --git a/extensions/vscode/src/utils/quartoProjectHelper.smoke.test.ts b/extensions/vscode/src/utils/quartoProjectHelper.smoke.test.ts
index e03b9f39f..5a94b0e4c 100644
--- a/extensions/vscode/src/utils/quartoProjectHelper.smoke.test.ts
+++ b/extensions/vscode/src/utils/quartoProjectHelper.smoke.test.ts
@@ -201,6 +201,93 @@ Hello project
   },
 );
 
+// --------------------------------------------------------------------------
+// rootDir resolution — verifies that QuartoProjectHelper resolves a relative
+// projectDir against rootDir internally, so callers don't need to.
+// --------------------------------------------------------------------------
+describe("QuartoProjectHelper - rootDir resolution", () => {
+  test("relative projectDir '.' is resolved to rootDir", () => {
+    const helper = new QuartoProjectHelper(
+      "index.qmd",
+      "index.html",
+      ".",
+      tmpDir,
+    );
+    expect(helper.projectDir).toBe(tmpDir);
+  });
+
+  test("nested relative projectDir is resolved against rootDir", () => {
+    const subDir = path.join("sub", "project");
+    const helper = new QuartoProjectHelper(
+      "index.qmd",
+      "index.html",
+      subDir,
+      tmpDir,
+    );
+    expect(helper.projectDir).toBe(path.join(tmpDir, subDir));
+  });
+
+  test("absolute projectDir is unchanged when rootDir is provided", () => {
+    const absDir = path.join(tmpDir, "already-absolute");
+    const helper = new QuartoProjectHelper(
+      "index.qmd",
+      "index.html",
+      absDir,
+      tmpDir,
+    );
+    expect(helper.projectDir).toBe(absDir);
+  });
+
+  test("rootDir enables _quarto.yml detection with relative projectDir", async () => {
+    fs.writeFileSync(
+      path.join(tmpDir, "_quarto.yml"),
+      "project:\n  type: website\n",
+    );
+    // Without rootDir, "." would resolve against cwd (not tmpDir)
+    const helperWithout = new QuartoProjectHelper(
+      "index.qmd",
+      "index.html",
+      ".",
+    );
+    // With rootDir, "." resolves to tmpDir where _quarto.yml exists
+    const helperWith = new QuartoProjectHelper(
+      "index.qmd",
+      "index.html",
+      ".",
+      tmpDir,
+    );
+    expect(await helperWithout.isQuartoYmlPresent()).toBe(false);
+    expect(await helperWith.isQuartoYmlPresent()).toBe(true);
+  });
+
+  test("rootDir triggers project render with relative projectDir", async () => {
+    fs.writeFileSync(
+      path.join(tmpDir, "_quarto.yml"),
+      "project:\n  type: website\n",
+    );
+
+    let capturedCommand: string | undefined;
+    mockRunTerminalCommand.mockImplementation((cmd: string) => {
+      if (cmd === "quarto --version") {
+        return Promise.resolve(0);
+      }
+      capturedCommand = cmd;
+      return Promise.resolve(0);
+    });
+
+    const helper = new QuartoProjectHelper(
+      "index.qmd",
+      "index.html",
+      ".",
+      tmpDir,
+    );
+    await helper.render();
+
+    // Should render the resolved absolute directory
+    expect(capturedCommand).toBe(`quarto render "${tmpDir}"`);
+  });
+});
+
 // --------------------------------------------------------------------------
 // Relative vs absolute projectDir — verifies the bug where a relative
 // projectDir (e.g. ".") caused isQuartoYmlPresent() to check the wrong
diff --git a/extensions/vscode/src/utils/quartoProjectHelper.test.ts b/extensions/vscode/src/utils/quartoProjectHelper.test.ts
index 09de62f28..5f5978710 100644
--- a/extensions/vscode/src/utils/quartoProjectHelper.test.ts
+++ b/extensions/vscode/src/utils/quartoProjectHelper.test.ts
@@ -106,6 +106,58 @@ describe("QuartoProjectHelper", () => {
     });
   });
 
+  describe("rootDir resolves relative projectDir", () => {
+    const rootDir = path.resolve(path.sep, "workspace", "my project");
+
+    test("relative projectDir is resolved against rootDir", async () => {
+      mockFileExistsAt.mockResolvedValue(false);
+
+      const helper = new QuartoProjectHelper(
+        "index.qmd",
+        "index.html",
+        ".",
+        rootDir,
+      );
+      expect(helper.projectDir).toBe(rootDir);
+      await helper.render();
+      expect(mockRenderCmd).toHaveBeenCalledWith(
+        `quarto render "${path.join(rootDir, "index.qmd")}"`,
+      );
+    });
+
+    test("nested relative projectDir is resolved against rootDir", async () => {
+      mockFileExistsAt.mockResolvedValue(true);
+
+      const relProjectDir = path.join("sub", "dir");
+      const helper = new QuartoProjectHelper(
+        "index.qmd",
+        "index.html",
+        relProjectDir,
+        rootDir,
+      );
+      const expected = path.resolve(rootDir, relProjectDir);
+      expect(helper.projectDir).toBe(expected);
+      await helper.render();
+      expect(mockRenderCmd).toHaveBeenCalledWith(`quarto render "${expected}"`);
+    });
+
+    test("absolute projectDir is unchanged when rootDir is provided", () => {
+      const absProjectDir = path.resolve(path.sep, "already", "absolute");
+      const helper = new QuartoProjectHelper(
+        "index.qmd",
+        "index.html",
+        absProjectDir,
+        rootDir,
+      );
+      expect(helper.projectDir).toBe(absProjectDir);
+    });
+
+    test("projectDir is unchanged when rootDir is omitted", () => {
+      const helper = new QuartoProjectHelper("index.qmd", "index.html", ".");
+      expect(helper.projectDir).toBe(".");
+    });
+  });
+
   describe("errors", () => {
     test("there is no quarto binary", async () => {
       mockFileExistsAt.mockResolvedValue(false);
diff --git a/extensions/vscode/src/utils/quartoProjectHelper.ts b/extensions/vscode/src/utils/quartoProjectHelper.ts
index 9a053a04f..6d6c2504c 100644
--- a/extensions/vscode/src/utils/quartoProjectHelper.ts
+++ b/extensions/vscode/src/utils/quartoProjectHelper.ts
@@ -23,10 +23,23 @@ export class QuartoProjectHelper {
   readonly renderedEntrypoint: string;
   readonly projectDir: string;
 
-  constructor(source: string, renderedEntrypoint: string, projectDir: string) {
+  /**
+   * @param source - Source entrypoint filename (e.g. "index.qmd")
+   * @param renderedEntrypoint - Rendered output filename (e.g. "index.html")
+   * @param projectDir - Project directory (may be relative)
+   * @param rootDir - If provided, projectDir is resolved against this root to
+   *   produce an absolute path. This ensures filesystem checks and quarto
+   *   render commands work correctly regardless of the extension host's cwd.
+   */
+  constructor(
+    source: string,
+    renderedEntrypoint: string,
+    projectDir: string,
+    rootDir?: string,
+  ) {
     this.source = source;
     this.renderedEntrypoint = renderedEntrypoint;
-    this.projectDir = projectDir;
+    this.projectDir = rootDir ? path.resolve(rootDir, projectDir) : projectDir;
   }
 
   async render() {
diff --git a/extensions/vscode/src/views/homeView.ts b/extensions/vscode/src/views/homeView.ts
index d579d34a3..f43fbb36d 100644
--- a/extensions/vscode/src/views/homeView.ts
+++ b/extensions/vscode/src/views/homeView.ts
@@ -305,6 +305,7 @@ export class HomeViewProvider implements WebviewViewProvider, Disposable {
       absProjectDir,
       sourceEntrypoint,
       renderedEntrypoint,
+      this.root?.uri.fsPath,
     );
   }
 
diff --git a/extensions/vscode/src/views/renders.ts b/extensions/vscode/src/views/renders.ts
index 7b6dc6305..464a83b40 100644
--- a/extensions/vscode/src/views/renders.ts
+++ b/extensions/vscode/src/views/renders.ts
@@ -14,6 +14,7 @@ export const renderQuartoContent = async (
   projectDir: string,
   sourceEntrypoint: string,
   renderedEntrypoint: string,
+  rootDir?: string,
 ) => {
   try {
     await window.withProgress(
@@ -26,6 +27,7 @@ export const renderQuartoContent = async (
           sourceEntrypoint,
           renderedEntrypoint,
           projectDir,
+          rootDir,
         );
         await quartoHelper.render();
       },