microsoft/aspire · error · InvalidOperationException

File input ' ' accepts at most .

Error message

File input '{inputName}' accepts at most {maxFileCount} {fileLabel}.

What it means

Each file input declares a maximum file count (maxFileCount). The store counts existing and in-progress upload entries for that input and, since uploads count toward the limit even while in flight, any upload beyond the cap is rejected. The message uses singular/plural 'file/files' based on the limit.

Solutions

  1. Limit client-side file selection to the input's maxFileCount before submitting
  2. Remove completed/unwanted entries for the input before uploading new files if the flow allows re-selection
  3. Catch InvalidOperationException and inform the user of the maximum ('at most N files')
  4. Serialize uploads per input and check the count immediately before each CreateEntry call

Example fix

// before
foreach (var file in selectedFiles) // may exceed limit mid-loop
    await store.CreateEntry(interactionId, inputName, file.Name, file.Stream);
// after
foreach (var file in selectedFiles.Take(maxFileCount))
    await store.CreateEntry(interactionId, inputName, file.Name, file.Stream);
Defensive patterns

Strategy: validation

Validate before calling

var current = interaction.Files.Values.Count(f => string.Equals(f.InputName, inputName, StringComparisons.InteractionInputName));
if (current >= interaction.FileInputLimits[inputName])
{
    throw new InvalidOperationException($"Input '{inputName}' already at its file limit.");
}

Type guard

bool UnderLimit(InteractionFileUploadStore.Interaction i, string inputName) =>
    i.FileInputLimits.TryGetValue(inputName, out var max) &&
    i.Files.Values.Count(f => f.InputName == inputName) < max;

Try / catch

try
{
    await store.CreateEntry(interactionId, inputName, fileName, stream);
}
catch (InvalidOperationException ex) when (ex.Message.Contains("accepts at most"))
{
    // Enforce the limit in the UI and inform the user.
}

Prevention

When it happens

Trigger: Calling CreateEntry for an input whose current fileCount already equals maxFileCount — e.g. uploading a third file to an input configured for two, or concurrent uploads racing to exceed the limit.

Common situations: Client-side UI allowing more files than the input's declared maximum; user selecting a multi-file set larger than the cap; double-submission of the same selection adding duplicate entries.

Understand the failure class

Background: "File too large" / "file size exceeds limit" errors: why libraries cap file sizes and how to fix them — this error's family across 46 libraries.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/8ca74099298d4014. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting/Dashboard/InteractionFileUploadStore.cs:69

        {
            if (interaction.State != FileInteractionState.InProgress)
            {
                throw new InvalidOperationException($"Interaction '{interactionId}' is not accepting file uploads.");
            }

            if (!interaction.FileInputLimits.TryGetValue(inputName, out var maxFileCount))
            {
                throw new InvalidOperationException($"Interaction '{interactionId}' is not accepting file uploads for input '{inputName}'.");
            }

            // Each client submits one file selection per input during an interaction. Multi-file selections upload
            // their files sequentially as part of that single selection, so every upload counts toward this limit.
            // Count uploads in progress as reserved slots so concurrent requests cannot exceed the input's limit.
            var fileCount = interaction.Files.Values.Count(entry => string.Equals(entry.InputName, inputName, StringComparisons.InteractionInputName));
            if (fileCount >= maxFileCount)
            {
                var fileLabel = maxFileCount == 1 ? "file" : "files";
                throw new InvalidOperationException($"File input '{inputName}' accepts at most {maxFileCount} {fileLabel}.");
            }

            // Keep only the leaf name as metadata. The client-supplied name is never used for the
            // on-disk path because filename rules vary by platform and some names have special semantics.
            var lastSep = originalFileName.AsSpan().LastIndexOfAny('/', '\\');
            var safeName = lastSep >= 0 ? originalFileName[(lastSep + 1)..] : originalFileName;

            var tempFile = _tempFileSystem.CreateTempFile();
            var fileId = Guid.NewGuid().ToString("N");

            interaction.Files[fileId] = new FileEntry(tempFile, inputName, safeName);
            _logger.LogDebug(
                "Created uploaded file entry {FileId} for interaction {InteractionId}, input {InputName}, and file {FileName}.",
                fileId,
                interactionId,
                inputName,
                safeName);
            return (fileId, tempFile.Path);

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