fix: resolve @file virtual-file URLs to absolute in HTML renderer by VishakBaddur · Pull Request #9440 · marimo-team/marimo

VishakBaddur · 2026-05-03T02:16:58Z

Problem

mo.image() (and mo.audio(), mo.pdf()) fails to render in edit mode on molab but works in app mode. Closes #9432.

Root cause: The backend intentionally generates relative virtual file URLs like ./@file/SIZE-filename (it can't know the mount path at creation time). When the page URL has no trailing slash — e.g., molab edit mode at /notebooks/nb_xxx — the browser resolves ./@file/... to /notebooks/@file/..., dropping the notebook ID and causing a 404.

App mode works because the URL ends with /app, providing an extra path segment that makes relative resolution correct.

Verified empirically:

new URL('./@file/123.png', 'https://molab.marimo.io/notebooks/nb_xxx')
// → https://molab.marimo.io/notebooks/@file/123.png  ❌ (notebook ID dropped)

new URL('./@file/123.png', 'https://molab.marimo.io/notebooks/nb_xxx/app')
// → https://molab.marimo.io/notebooks/nb_xxx/@file/123.png  ✓

Fix

Add resolveVirtualFileUrl() helper in RenderHTML.tsx that resolves @file/ URLs against the runtime base URL with a guaranteed trailing slash, making the URL absolute and unambiguous regardless of the page URL's trailing slash.

Apply it in:

replaceVirtualFileSrc — handles <img>, <audio>, <video>, <source> (covers mo.image(), mo.audio())
replaceValidIframes — handles <iframe> (covers mo.pdf())

Data URLs and external https:// URLs are untouched.

Testing

4 new regression tests: ./@file/ rewriting, @file/ without ./, non-@file URLs (unchanged), data: URLs (unchanged)
305/305 test files pass, 4722/4722 tests pass, zero regressions

vercel · 2026-05-03T02:17:03Z

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 18, 2026 9:31pm

cubic-dev-ai

1 issue found across 3 files

Prompt for AI agents (unresolved issues)


Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="frontend/src/plugins/core/RenderHTML.tsx">

<violation number="1" location="frontend/src/plugins/core/RenderHTML.tsx:64">
P2: `replaceVirtualFileSrc` recreates `<audio>/<video>` without children, which drops nested `<source>/<track>` and fallback content.</violation>
</file>

Architecture diagram

sequenceDiagram
    participant BE as Marimo Backend
    participant RHTML as RenderHTML Component
    participant RM as RuntimeManager
    participant Browser as Browser DOM

    Note over BE, Browser: User loads notebook (e.g., /notebooks/nb_123)

    BE->>RHTML: Send HTML (mo.image, mo.pdf, etc.)
    Note right of BE: contains src="./@file/..."

    RHTML->>RHTML: parseHtml()
    
    loop For each img, audio, video, source, iframe
        RHTML->>RHTML: NEW: Check if src contains "/@file/"
        
        opt Match Found
            RHTML->>RM: getRuntimeManager().httpURL
            RM-->>RHTML: Return base URL
            
            RHTML->>RHTML: NEW: resolveVirtualFileUrl(src)
            Note right of RHTML: 1. Ensure base URL has trailing slash "/"<br/>2. Resolve relative src against base<br/>3. Convert to Absolute URL
        end
    end

    alt CHANGED: Tag is media (img/audio/video/source)
        RHTML->>RHTML: NEW: replaceVirtualFileSrc()
        RHTML-->>Browser: Render element with absolute src
    else CHANGED: Tag is iframe
        RHTML->>RHTML: CHANGED: replaceValidIframes() (absolute src injection)
        RHTML-->>Browser: Render iframe with absolute src
    else Default (External URL or Data URI)
        RHTML-->>Browser: Render element with original src
    end

    Note over Browser, BE: Network Request Resolution
    
    alt Original Behavior (Buggy in Edit Mode)
        Browser-xBE: GET /notebooks/@file/... (404: nb_123 segment lost)
    else NEW: Fixed Behavior
        Browser->>BE: GET /notebooks/nb_123/@file/...
        BE-->>Browser: 200 OK (Asset data)
    end

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review, or fix all with cubic.}

dmadisetti · 2026-05-16T01:45:57Z

Thanks! LGTM, but I think we need to make sure this is general to base URL. I'll look at it in a bit more detail but pinged @mchav who should be able to determine a bit more molab context

codecov · 2026-05-16T06:41:10Z

Bundle Report

Changes will increase total bundle size by 486 bytes (0.0%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
marimo-esm	25.17MB	486 bytes (0.0%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: marimo-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
`assets/dist-*.js`	23 bytes	183 bytes	14.37% ⚠️
`assets/dist-*.js`	299 bytes	403 bytes	287.5% ⚠️
`assets/dist-*.js`	-128 bytes	259 bytes	-33.07%
`assets/dist-*.js`	-166 bytes	169 bytes	-49.55%
`assets/dist-*.js`	152 bytes	256 bytes	146.15% ⚠️
`assets/dist-*.js`	74 bytes	176 bytes	72.55% ⚠️
`assets/dist-*.js`	40 bytes	177 bytes	29.2% ⚠️
`assets/dist-*.js`	-12 bytes	164 bytes	-6.82%
`assets/dist-*.js`	223 bytes	387 bytes	135.98% ⚠️
`assets/dist-*.js`	-155 bytes	104 bytes	-59.85%
`assets/dist-*.js`	-116 bytes	160 bytes	-42.03%
`assets/dist-*.js`	-81 bytes	102 bytes	-44.26%
`assets/dist-*.js`	20 bytes	276 bytes	7.81% ⚠️
`assets/dist-*.js`	-32 bytes	137 bytes	-18.93%
`assets/dist-*.js`	-33 bytes	104 bytes	-24.09%
`assets/dist-*.js`	-8 bytes	169 bytes	-4.52%
`assets/dist-*.js`	231 bytes	335 bytes	222.12% ⚠️
`assets/dist-*.js`	-65 bytes	104 bytes	-38.46%
`assets/dist-*.js`	-266 bytes	137 bytes	-66.0%
`assets/RenderHTML-*.js`	486 bytes	6.24kB	8.44% ⚠️
`assets/__vite-*.js`	5 bytes	98 bytes	5.38% ⚠️
`assets/__vite-*.js`	-5 bytes	93 bytes	-5.1%

Files in assets/RenderHTML-*.js:

./src/plugins/core/RenderHTML.tsx → Total Size: 7.78kB

mchav · 2026-05-18T20:14:30Z

      );

      const title = cell.getHoverTitle?.() ?? undefined;
+      const isCellSelected = cell.getIsSelected?.() || false;


How is this related to the current change?

You're right, that's unrelated, removed it.

mchav · 2026-05-18T20:17:08Z

+// Resolve a virtual file URL (./@file/... or @file/...) to an absolute URL
+// using the runtime base, ensuring a trailing slash so the notebook-ID path
+// segment is never dropped during relative resolution.
+function resolveVirtualFileUrl(src: string): string {


This seems similar to

marimo/frontend/src/core/runtime/config.ts

Line 54 in a8dd4b6

export function asRemoteURL(path: string): URL {

Can we reuse that instead of creating a new function?

I've restructured it to mirror asRemoteURL's pattern (blob: handling + same base resolution). The reason I kept it as a separate function rather than calling asRemoteURL directly is that asRemoteURL doesn't guarantee a trailing slash, and without that, new URL("./@file/...", base) drops the last path segment when the base URL has no trailing slash (the exact bug we're fixing). Added a comment in the code explaining this.

mchav · 2026-05-18T20:18:32Z

  });
 });
+
+describe("replaceVirtualFileSrc - virtual file URL rewriting", () => {


These tests don't actually repro the bug. We shoudl mock the runtime manager and check against complete strings rather than substrings.

Updated the tests to mock getRuntimeManager with a base URL that has no trailing slash (http://localhost:2718/notebooks/nb_xxx), which reproduces the actual molab edit-mode scenario. Assertions now check complete URL strings instead of substrings.

…able

When mo.image() (or mo.audio(), mo.pdf()) renders output, the backend generates a relative URL like ./@file/SIZE-filename. On molab in edit mode, the page URL is /notebooks/nb_xxx (no trailing slash), so the browser resolves ./@file/... to /notebooks/@file/... — dropping the notebook ID — causing a 404 and broken image. App mode works because the URL ends with /app, giving an extra path segment for relative resolution. Fix: add resolveVirtualFileUrl() helper that gets the runtime base URL and guarantees a trailing slash before resolving the @file path to absolute. Apply it in replaceVirtualFileSrc (img/audio/video/source) and replaceValidIframes (iframe, for mo.pdf()). Fixes marimo-team#9432

for more information, see https://pre-commit.ci

…erve children

…includes

… mo.ui.table" This reverts commit 4cb9496.

for more information, see https://pre-commit.ci

mscolnick · 2026-05-18T22:21:23Z

Going to close this so it does not get accidentally review more. the issue is on molab and it is already fixed in the latest molab that we are currently doing a slow rollout of.

vercel Bot deployed to Preview May 3, 2026 02:18 View deployment

cubic-dev-ai Bot reviewed May 3, 2026

View reviewed changes

Comment thread frontend/src/plugins/core/RenderHTML.tsx Outdated

vercel Bot deployed to Preview May 3, 2026 02:30 View deployment

dmadisetti requested a review from mchav May 16, 2026 01:44

vercel Bot deployed to Preview May 16, 2026 06:39 View deployment

vercel Bot deployed to Preview May 16, 2026 06:41 View deployment

mchav requested changes May 18, 2026

View reviewed changes

dmadisetti added the bug Something isn't working label May 18, 2026

VishakBaddur and others added 6 commits May 18, 2026 16:07

fix: restore cell selection highlight for lazy/image cells in mo.ui.t…

ef8498e

…able

[pre-commit.ci] auto fixes from pre-commit.com hooks

7ce59aa

for more information, see https://pre-commit.ci

fix: mutate domNode.attribs.src instead of recreating element to pres…

6f1f01e

…erve children

fix: tighten virtual file URL detection to use startsWith instead of …

d704986

…includes

Revert "fix: restore cell selection highlight for lazy/image cells in…

8a82c51

… mo.ui.table" This reverts commit 4cb9496.

VishakBaddur force-pushed the fix/virtual-file-url-resolution branch from 13f40f3 to 8a82c51 Compare May 18, 2026 21:30

[pre-commit.ci] auto fixes from pre-commit.com hooks

1a6a191

for more information, see https://pre-commit.ci

vercel Bot deployed to Preview May 18, 2026 21:31 View deployment

VishakBaddur requested a review from mchav May 18, 2026 21:34

mscolnick closed this May 18, 2026

Navigation Menu

Uh oh!

fix: resolve @file virtual-file URLs to absolute in HTML renderer - #9440

fix: resolve @file virtual-file URLs to absolute in HTML renderer#9440
VishakBaddur wants to merge 7 commits into
marimo-team:mainfrom
VishakBaddur:fix/virtual-file-url-resolution

VishakBaddur commented May 3, 2026

Uh oh!

vercel Bot commented May 3, 2026 •

edited

Loading

Uh oh!

cubic-dev-ai Bot left a comment

Uh oh!

Uh oh!

dmadisetti commented May 16, 2026

Uh oh!

codecov Bot commented May 16, 2026 •

edited

Loading

Assets Changed:

Uh oh!

mchav May 18, 2026

Uh oh!

VishakBaddur May 18, 2026

Uh oh!

mchav May 18, 2026

Uh oh!

VishakBaddur May 18, 2026

Uh oh!

mchav May 18, 2026

Uh oh!

VishakBaddur May 18, 2026

Uh oh!

mscolnick commented May 18, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Uh oh!

Conversation

VishakBaddur commented May 3, 2026

Problem

Fix

Testing

Uh oh!

vercel Bot commented May 3, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

cubic-dev-ai Bot left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

dmadisetti commented May 16, 2026

Uh oh!

codecov Bot commented May 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Bundle Report

Affected Assets, Files, and Routes:

Assets Changed:

Uh oh!

mchav May 18, 2026

Choose a reason for hiding this comment

Uh oh!

VishakBaddur May 18, 2026

Choose a reason for hiding this comment

Uh oh!

mchav May 18, 2026

Choose a reason for hiding this comment

Uh oh!

VishakBaddur May 18, 2026

Choose a reason for hiding this comment

Uh oh!

mchav May 18, 2026

Choose a reason for hiding this comment

Uh oh!

VishakBaddur May 18, 2026

Choose a reason for hiding this comment

Uh oh!

mscolnick commented May 18, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

vercel Bot commented May 3, 2026 •

edited

Loading

codecov Bot commented May 16, 2026 •

edited

Loading