chore: cap mcp <2 ahead of the v2 breaking rewrite

mcp 2.0.0a1 shipped 2026-06-11 (stable targeted ~2026-07-27). v2 removes
streamablehttp_client, changes the transport tuple arity, and renames
mcp.types fields to snake_case — all of which our client imports. The
maintainers' release note asks downstream packages to add an upper
bound now (their worked example is this exact constraint). Floor stays
at 1.27: nothing newer adds anything our surface needs, and the #2147
shutdown busy-loop we wrap remains unfixed at every released version.
Resolution is unchanged (1.27.2); lockfile re-pinned metadata only.

pyproject.toml

@@ -26,7 +26,7 @@ dependencies = [
     "openai>=2.37",
     "anthropic>=0.108",  # claude-fable-5 support; hard runtime floor is 0.105 (mid-conversation system blocks)
     "httpx>=0.28",
-    "mcp>=1.27",
+    "mcp>=1.27,<2",  # v2 is a breaking rewrite (2.0.0a1 live 2026-06-11; stable ~2026-07-27) — streamablehttp_client removed, 2-tuple transport, snake_case types; migrate deliberately
     "starlette>=1.0.1",  # PYSEC-2026-161: host-header path-injection in URL reconstruction (auth-bypass on apps comparing reconstructed URL paths)
     "uvicorn>=0.34",
     "sse-starlette>=2.0",

uv.lock

@@ -2385,7 +2385,7 @@ requires-dist = [
     { name = "httpx", specifier = ">=0.28" },
     { name = "httpx-sse", specifier = ">=0.4" },
     { name = "lacme", specifier = ">=1.0.5" },
-    { name = "mcp", specifier = ">=1.27" },
+    { name = "mcp", specifier = ">=1.27,<2" },
     { name = "mypy", marker = "extra == 'dev'", specifier = ">=1.14" },
     { name = "openai", specifier = ">=2.37" },
     { name = "psycopg", extras = ["binary"], specifier = ">=3.2" },