fix: export filenames are unreadable in molab

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📝 Summary

Fixes molab export downloads failing with Export response is missing a filename.

After #10349, exports required reading Content-Disposition from the fetch response. In molab instant-launch, the editor runs on {sandbox}-session.sb.molab.run but calls the kernel on {sandbox}.sb.molab.run, so that header is not visible to JS cross-origin. The export body still succeeds; only the filename lookup failed.

This adds client-side default filenames (notebook name, or download.*) when the header cannot be read, and broadens Content-Disposition parsing.

The root cause fix is on molab-side, but regardless, downloads shouldn't fail when the header cannot be read.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview	Aug 6, 2026 6:16pm

[cubic-dev-ai]

2 issues found across 6 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/core/network/api.ts">

<violation number="1" location="frontend/src/core/network/api.ts:28">
P2: Malformed percent-encoding in a readable `filename*` causes export handling to throw instead of using `opts.defaultFilename`; catch decode failures and return `undefined` so the existing fallback runs.</violation>
</file>

<file name="frontend/src/core/network/export-filename.ts">

<violation number="1" location="frontend/src/core/network/export-filename.ts:23">
P2: Exports for notebooks whose filename uses Windows separators receive a path-containing download name instead of the notebook basename. Splitting both separators or reusing `Paths.basename` keeps the fallback filename portable.</violation>
</file>

Architecture diagram

sequenceDiagram
    participant Editor as Editor UI (Browser)
    participant Network as Network Requests
    participant Kernel as Kernel Backend (Molab)
    participant Filename as Default Filename Resolver
    participant Store as Jotai Store (filenameAtom)
    participant Api as API Client (api.ts)

    Note over Editor,Kernel: Export flow (HTML, Markdown, Script, IPYNB, PDF)

    Editor->>Network: Trigger export (e.g., Download as HTML)
    Network->>Kernel: POST export endpoint with format params
    Kernel-->>Network: Export body + Content-Disposition header

    Note over Network,Api: Molab cross-origin: editor domain differs from kernel domain, so Content-Disposition may not be exposed to JS

    alt Markdown export
        Network->>Filename: NEW: getDefaultMarkdownExportFilename(flavor)
        Filename->>Filename: Map flavor to extension (.qmd, .md, .myst.md)
    else Other export types
        Network->>Filename: NEW: getDefaultExportFilename(extension)
    end

    Filename->>Store: Read current notebook filename
    alt Notebook has a name
        Store-->>Filename: "folder/my_notebook.py"
        Filename-->>Network: "my_notebook.html"
    else Notebook is unnamed or filename missing
        Store-->>Filename: null
        Filename-->>Network: "download.html"
    end

    Network->>Api: NEW: handleExportResponse(response, { defaultFilename })
    Api->>Api: Parse export body (blob/text)

    alt Content-Disposition header readable
        Api->>Api: CHANGED: Parse RFC5987, quoted, or unquoted filename
        Api-->>Network: Use filename from header
    else Header missing or unparsable
        Api-->>Network: NEW: Use defaultFilename
    end

    Api->>Api: Read Content-Type media type
    alt Content-Type missing
        Api-->>Network: Reject export (missing media type)
    else Valid response
        Api-->>Network: ExportedFile { contents, filename, mediaType }
        Network-->>Editor: Trigger browser download with resolved filename
    end

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Coverage Report for ./frontend

Status	Category	Percentage	Covered / Total
🔵	Lines	78.83%	81643 / 103564
🔵	Statements	78.83%	81643 / 103564
🔵	Functions	71.22%	693 / 973
🔵	Branches	79.38%	4863 / 6126

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
frontend/src/core/network/api.ts	96.1%	100%	100%	96.1%	105-107, 140-142
frontend/src/core/network/export-filename.ts	100%	0%	100%	100%
frontend/src/core/network/requests-network.ts	99.8%	83.33%	100%	99.8%	423
frontend/src/core/network/types.ts	0%	0%	0%	0%

Generated in workflow #20269 for commit ffe0b16 by the Vitest Coverage Report Action