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Group Type

Working Group

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Mission Statement

The Filesystems Working Group exists to make MCP Resources bidirectional, so that an agent can write a result back to the server it reads its inputs from. Agent platforms are converging on presenting services to models as filesystems, and Resources already addresses content by URI under a file:// scheme the specification defines as identifying “resources that behave like a filesystem”. Currently, Resources carries that surface in the read direction. The WG will produce a single Extensions Track SEP covering write operations, optimistic concurrency control, and the interaction with change notification and caching.

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Scope

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In Scope

  • Specification Work: One Extensions Track SEP, provisionally titled Filesystem Operations for Resources, covering:
    1. Operations. Define create, update, delete, and a metadata read (stat) that answers existence, size and last-modified for one URI without fetching the body.
    2. Optimistic concurrency control. Specify how two writers avoid a lost update, and how a client creates a resource under a create-if-absent precondition.
    3. Change notification and caching. Specify how a write interacts with notifications/resources/updated, the time-to-live and cache scope fields ttlMs and cacheScope, and the lastModified annotation.
  • Cross-Cutting Concerns: Reconcile the proposals that extend this primitive into one coherent design. SEP-2571, Resource Submission for Agent Coordination proposes resources/create and resources/delete. SEP-2532, Resource Streaming for Binary Content Delivery proposes resources/stream. SEP-1708, MCP Client-Brokered Filesystem Access proposed a files/* method family standing beside Resources, and closed on 23 January 2026.
  • Documentation: Document the new operations in the specification, and give server authors guidance on choosing between a resource write path and a tool-based one.

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Out of Scope

  • A parallel files/* primitive standing beside Resources. The WG broadens the existing Resources primitive.
  • Host-side sandbox and local-disk semantics. The WG standardizes the client and server wire format. How a host materializes resources into a filesystem for the model stays a host concern.
  • Authorization policy for writes, beyond stating where the existing MCP authorization specification applies.
  • File Uploads WG. Both groups transfer content to a server. Coordination needed on where an upload-oriented flow ends and a resource write operation begins.
  • Skills Over MCP WG. Filesystem dependency, including questions about write access, was an agenda item at the office hours of 24 March 2026 and remains open.
  • Agents WG. Multi-agent and job orchestration systems are the motivating consumers of a shared write path.

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Leadership

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Authority & Decision Rights

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Membership

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Operations

Discord: #filesystems-wg. Note The cadence above is provisional. This is subject to change once the first working session sets it.

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Deliverables & Success Metrics

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Active Work Items

The group’s first act is to take up SEP-2571 with its author, since it already covers create and delete.

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Success Criteria

  • An accepted Extensions Track SEP covering create, update, delete and stat, with a defined optimistic concurrency control mechanism and a specified interaction with notifications/resources/updated, ttlMs, cacheScope and lastModified.
  • Each proposal in this space resolved. SEP-2571 and SEP-2532 are open at time of writing, and each should end up folded in, superseded, or explicitly scoped out.
  • Reference implementations in at least two Tier 1 SDKs.
  • Two independent server implementations exercising the write path against a common client.

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Changelog