Detailed Analysis
A Reddit user has surfaced a functional regression in Claude Desktop for Windows involving the file_upload tool used by Claude's Chrome browser integration. The user's workflow—reading local files via Claude Desktop's Folder support and then using Claude in Chrome to upload those files into web forms—had worked reliably for weeks but abruptly broke. The error message returned is explicit: "file_upload no longer accepts host filesystem paths. The MCP controller must read the file and pass its contents via the files parameter." This points to a backend change on Anthropic's side that redefined how the file_upload tool expects data to be delivered, shifting from accepting a filesystem path string to requiring the actual file bytes be passed through a "files" parameter.
The technical root cause described here is a classic client-server contract mismatch. The tool's backend (likely the MCP—Model Context Protocol—server component that mediates between Claude Desktop and Chrome) was updated to expect raw file contents, but the client implementation embedded in Claude Desktop had not yet been updated to match, and continues sending a "paths" array instead. Because the user reproduced the failure on a generic HTML file input test page, the issue is clearly not tied to a specific website's upload widget but rather sits at the tool-schema level, affecting any use case that depends on Claude in Chrome to programmatically attach local files. This is a meaningful distinction for troubleshooting: it rules out permissions issues, sandboxing restrictions, or site-specific JavaScript quirks, and instead implicates a version skew introduced by a recent Anthropic-side deployment.
This kind of breakage illustrates a recurring challenge in agentic AI tooling: as Anthropic rapidly iterates on Claude's computer-use and browser-automation capabilities, backend and frontend components can drift out of sync, especially when features are rolled out incrementally or split across desktop app releases and server-side updates. MCP, which Anthropic introduced as a standardized way for Claude to connect to external tools and data sources, is central to workflows like this one, where Claude Desktop, a local file system, and a Chrome extension are all expected to interoperate seamlessly. When one link in that chain changes its expected data format without a coordinated client update, it silences a capability that users have built real workflows around, and in this case with no visible warning beforehand.
The broader significance lies in what this reveals about the current maturity level of agentic browser automation. Claude in Chrome is part of Anthropic's push to give Claude the ability to act on a user's behalf in real browsing sessions—filling forms, uploading documents, navigating sites—positioning it against similar efforts from OpenAI and Google in agentic web interaction. These capabilities are powerful precisely because they bridge local file systems and live web interfaces, but that same bridging makes them fragile to internal API changes. The user's reference to a related GitHub issue suggests this is not an isolated report, and that the community is already tracking it as a known regression. For enterprise or power users building automated pipelines atop Claude Desktop, this kind of silent breaking change underscores the risk of depending on tools still under active, fast-moving development, and highlights the importance of robust versioning, changelogs, and backward-compatibility guarantees as these agentic features move from experimental to production use.
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