Request Idempotency — Reference Implementation

Minimal, runnable demonstration of the protocol-level mechanism proposed in SEP-3182: Request Idempotency: an idempotencyKey field on tools/call params (a sibling of arguments, not nested inside it), with deduplication performed in server-side dispatch before any tool handler runs, and explicit conflict semantics for a reused key presented with different arguments.

This is a demonstration server, not a production implementation — the dedup store is an in-memory dict with no eviction. It exists to make the before/after failure mode concrete for reviewers, per the SEP process's prototype requirement ("a standalone proof-of-concept demonstrating the key mechanics").

Setup

pip install -r requirements.txt

Note on the version pin: this demo uses FastMCP from mcp.server.fastmcp, and relies on CallToolRequestParams allowing extra fields (model_config = {"extra": "allow"}) to carry idempotencyKey as a sibling of arguments. As of the mcp SDK's 2.0.0 release, both of these changed: FastMCP was renamed and moved to MCPServer in mcp.server.mcpserver, and CallToolRequestParams no longer allows extra fields — an idempotencyKey sent under 2.0.0 is silently dropped at construction, with no error, rather than causing an import failure. Installing a plain pip install mcp today will pull 2.0.0 and this demo will appear to run while never actually deduplicating anything. The pin above (mcp>=1.9.0,<2.0.0) avoids both problems. Porting this demo to 2.0.0 would require idempotencyKey to be added as a declared field (or an equivalent extension mechanism, if MCPServer provides one) rather than relying on extra-field passthrough — that port is out of scope for this prototype.

Related MCP server: acel-toy-server

Run

python3 client_demo.py

This spawns server.py as a stdio subprocess and runs three scenarios:

1. No idempotency support — a lost-response retry double-charges.

2. With idempotencyKey — the same retry is deduplicated; the cached result is returned, no re-execution.

3. Conflict semantics — the same key reused with different arguments is rejected outright, not replayed or silently executed.

Verified working end-to-end against mcp==1.9.4 on 2026-08-01.

Note on the 2026-07-28 stateless spec: this demo's dedup logic (_dedup_store in server.py) is keyed entirely by the client-supplied idempotencyKey and never depends on protocol-level session state, so nothing here needed to change when MCP went stateless. The one thing worth calling out for anyone adapting this into a real server: in a horizontally-scaled, stateless deployment, that store needs to be shared across instances (a cache or database), not a per-process dict like this demo uses — see the SEP's "Relationship to the 2026-07-28 stateless core and MRTR" section for why.

Files

• server.py — the guarded (charge_guarded) and unguarded (charge_unguarded) tool implementations, plus a reset_ledger test helper.

• client_demo.py — drives all three scenarios against server.py.