diff --git a/src/core/strategies/browserSession.ts b/src/core/strategies/browserSession.ts
index 7870900..e0bf370 100644
--- a/src/core/strategies/browserSession.ts
+++ b/src/core/strategies/browserSession.ts
@@ -124,6 +124,13 @@ async function getRepositoryId(
     );
 
     if (!response.ok) {
+      // The REST API only accepts token auth — a session cookie is rejected
+      // (404 on private repos). When authenticating by cookie, fall back to
+      // reading the id from the repository's HTML page, which the same cookie
+      // can load.
+      if (credentials.cookies && !credentials.token) {
+        return getRepositoryIdFromPage(target, credentials);
+      }
       throw new AuthenticationError(
         "Cannot access repository. Session may have expired.",
         "SESSION_EXPIRED",
@@ -134,7 +141,7 @@ async function getRepositoryId(
     const data = (await response.json()) as { id: number };
     return String(data.id);
   } catch (err) {
-    if (err instanceof AuthenticationError) {
+    if (err instanceof AuthenticationError || err instanceof UploadError) {
       throw err;
     }
     throw new UploadError(
@@ -145,6 +152,52 @@ async function getRepositoryId(
   }
 }
 
+/**
+ * Resolves the repository id from the repository's HTML page.
+ *
+ * The REST API (api.github.com) does not accept session-cookie auth, so a
+ * cookie-based browser session cannot use it to look up private repositories.
+ * The repo page embeds the numeric id in an `octolytics-dimension-repository_id`
+ * meta tag and is reachable with the same cookie used for the upload.
+ *
+ * @internal
+ */
+async function getRepositoryIdFromPage(
+  target: UploadTarget,
+  credentials: BrowserSessionCredentials,
+): Promise<string> {
+  const response = await fetch(
+    `https://github.com/${target.owner}/${target.repo}`,
+    {
+      headers: {
+        ...buildAuthHeaders(credentials),
+        Accept: "text/html",
+      },
+    },
+  );
+
+  if (!response.ok) {
+    throw new AuthenticationError(
+      "Cannot access repository. Session may have expired.",
+      "SESSION_EXPIRED",
+      { target, status: response.status },
+    );
+  }
+
+  const html = await response.text();
+  const repositoryId = html.match(
+    /octolytics-dimension-repository_id"\s+content="(\d+)"/,
+  )?.[1];
+  if (!repositoryId) {
+    throw new UploadError(
+      "Could not determine repository id from the repository page.",
+      "REPO_ID_FETCH_FAILED",
+      { target },
+    );
+  }
+  return repositoryId;
+}
+
 /**
  * Gets the upload policy and CSRF token from GitHub.
  *
diff --git a/test/unit/core/strategies/browserSession.test.ts b/test/unit/core/strategies/browserSession.test.ts
index bc11081..81b3e35 100644
--- a/test/unit/core/strategies/browserSession.test.ts
+++ b/test/unit/core/strategies/browserSession.test.ts
@@ -58,34 +58,91 @@ describe("Browser Session Strategy", () => {
   });
 
   describe("upload - getRepositoryId", () => {
-    it("throws AuthenticationError on 401 response from repo API", async () => {
+    it("throws AuthenticationError when both repo API and page reject (401)", async () => {
       const strategy = createBrowserSessionStrategy("test-cookie");
       const mockFilePath = "/tmp/test.png";
 
-      (global.fetch as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
-        ok: false,
-        status: 401,
-      });
+      (global.fetch as ReturnType<typeof vi.fn>)
+        .mockResolvedValueOnce({ ok: false, status: 401 }) // api.github.com
+        .mockResolvedValueOnce({ ok: false, status: 401 }); // HTML page fallback
 
       await expect(strategy.upload(mockFilePath, mockTarget)).rejects.toThrow(
         AuthenticationError,
       );
     });
 
-    it("throws AuthenticationError on 403 response from repo API", async () => {
+    it("throws AuthenticationError when both repo API and page reject (403)", async () => {
       const strategy = createBrowserSessionStrategy("test-cookie");
       const mockFilePath = "/tmp/test.png";
 
-      (global.fetch as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
-        ok: false,
-        status: 403,
-      });
+      (global.fetch as ReturnType<typeof vi.fn>)
+        .mockResolvedValueOnce({ ok: false, status: 403 })
+        .mockResolvedValueOnce({ ok: false, status: 403 });
 
       await expect(strategy.upload(mockFilePath, mockTarget)).rejects.toThrow(
         AuthenticationError,
       );
     });
 
+    it("falls back to the repo HTML page for the id when the API rejects a cookie (private repo)", async () => {
+      const strategy = createBrowserSessionStrategy("test-cookie");
+      const mockFilePath = "/tmp/test.png";
+
+      (global.fetch as ReturnType<typeof vi.fn>)
+        // api.github.com rejects the cookie on a private repo
+        .mockResolvedValueOnce({ ok: false, status: 404 })
+        // the HTML page loads with the cookie and embeds the repository id
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          text: async () =>
+            '<meta name="octolytics-dimension-repository_id" content="987654">',
+        })
+        // upload policy
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          json: async () => ({
+            upload_url: "https://s3.example.com/upload",
+            form: { key: "value" },
+            token: "csrf-token-123",
+          }),
+        })
+        // S3 upload
+        .mockResolvedValueOnce({ ok: true, status: 200 })
+        // confirm
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          json: async () => ({
+            url: "https://github.com/user-attachments/assets/abc",
+          }),
+        });
+
+      const result = await strategy.upload(mockFilePath, mockTarget);
+      expect(result.url).toBe("https://github.com/user-attachments/assets/abc");
+      expect(result.strategy).toBe("browser-session");
+    });
+
+    it("throws UploadError when the repo page lacks a repository id", async () => {
+      const strategy = createBrowserSessionStrategy("test-cookie");
+      const mockFilePath = "/tmp/test.png";
+
+      (global.fetch as ReturnType<typeof vi.fn>)
+        .mockResolvedValueOnce({ ok: false, status: 404 })
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          text: async () => "<html><body>no marker here</body></html>",
+        });
+
+      const error = await strategy
+        .upload(mockFilePath, mockTarget)
+        .catch((e) => e);
+      expect(error).toBeInstanceOf(UploadError);
+      expect(error.code).toBe("REPO_ID_FETCH_FAILED");
+    });
+
     it("throws UploadError when network error occurs during repo fetch", async () => {
       const strategy = createBrowserSessionStrategy("test-cookie");
       const mockFilePath = "/tmp/test.png";