feat!: adopt MCP revision 2026-07-28 via the mcp 2.x SDK

[CryptoJones]

What

Moves PerplexityAgent to MCP revision 2026-07-28 — the stateless revision — via the mcp 2.x SDK (mcp>=1.28.1,<2.0.0 → mcp>=2.0.0,<3.0).

FastMCP → MCPServer; Context imports from mcp.server.mcpserver; transport host/port moved off the constructor. Every tool's behaviour is unchanged and a v2 server still serves 2025-era clients from the same process.

Two things beyond the mechanical rename

1. Possession-based authorization (the stateless revision removed sessions).
The offload store and the response store both isolated one client's data from another's using id(ctx.session). 2026-07-28 removed protocol sessions, so ctx.session is a fresh object per request and that scoping no longer holds. Per the spec, cross-call state now uses server-minted handles passed as tool arguments:

• retrieve_key is now an unguessable random capability token (was a content hash, which an attacker could fabricate by guessing the content).
• Holding a retrieve_key / response_id is the authorization to fetch it.

The two isolation tests are rewritten to the possession contract. (This was a deliberate design choice — the alternative was binding state to an authenticated OAuth identity, which degrades to process-global over stdio where there's no auth.)

2. A real stdio crash, independent of the migration.
The stdin reader used a default-limit asyncio.StreamReader (64 KiB). Any JSON-RPC line over 64K raised LimitOverrunError and tore down the whole transport task group, killing the server on a single large request — legitimate or hostile. The reader now uses a 16 MiB limit and turns a still-oversized line into a clean protocol error instead of an unhandled crash. Regression-tested over a real subprocess.

This surfaced because I added a server_metrics conformance probe (read-only, no API), which activated the bad_input_does_not_crash contract that was previously skipped.

Also: the custom stdio transport now parses through the SDK's jsonrpc_message_adapter (in 2.x JSONRPCMessage is a plain union alias with no model_validate_json) and serializes with exclude_unset=True, keeping the resultType/ttlMs/cacheScope envelope fields on the wire.

Verification

Gate	Result
pytest	201 passed
ruff check .	clean
mypy src	clean (17 files)
mcp-conformance 0.2.0	17 passed, 1 skipped (injection: server tags fetched content as untrusted rather than flagging its own inputs)

uv.lock refreshed: mcp → 2.0.0, +mcp-types, +opentelemetry-api, -httpx-sse.

Note

Codeberg is the source of truth for this repo, but no Codeberg key was loaded in this session — this branch is on GitHub only and still needs a Codeberg push.

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⛔ Files ignored due to path filters (1)

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📒 Files selected for processing (9)

• CHANGELOG.md
• conformance.toml
• pyproject.toml
• src/perplexity_agent/__main__.py
• src/perplexity_agent/efficiency.py
• src/perplexity_agent/server.py
• tests/test_efficiency.py
• tests/test_main.py
• tests/test_server.py

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