What I did

Before interceptors currently marshal the entire Go SDK request. On Streamable HTTP, RequestExtra contains a CloseSSEStream function field, which makes encoding/json reject the request before the interceptor or tool can run. Even an empty non-nil RequestExtra triggers this error.

Serialize only { "method": "tools/call", "params": ... }, retaining tool arguments and request _meta while excluding SDK session objects and transport state. This applies to all three interceptor runners because serialization happens before runner dispatch. After-interceptor behavior is unchanged.

The lowercase params key matches the existing jq examples. This intentionally replaces the SDK-shaped uppercase Params key; the example README now documents that migration and the payload boundary.

Regression coverage uses a real HTTP interceptor for passthrough and short-circuit results, with nil, empty and callback-bearing transport extras. A separate end-to-end test initializes a real MCP Streamable HTTP client/server pair and calls a tool through the interceptor. It verifies the tool result and the intercepted payload without Docker or a model provider.

Related issue

Fixes #358. The earlier #360 is closed and unmerged; this change includes transport-level regression coverage and does not depend on that PR.

Verification

• Linux, Go 1.25.12: full-repository golangci-lint 2.8.0 (run ./...) reported 0 issues, and the CLI build passed.
• Linux: interceptor package passed 10 consecutive runs. Modified Go files pass the repository's formatter checks.
• Linux full short suite: 34 packages passed; TestReset, TestResetEmptyCatalogs, and TestDockerCatalogProtection failed while downloading https://desktop.docker.com/mcp/catalog/v2/catalog.yaml (connection reset by peer). No tests were excluded or weakened. This is not a full-suite pass.
• Windows, Go 1.26.7: interceptor package tests passed.
• TDD: all six payload cases failed before the fix and passed after it.
• Mutation check: restoring json.Marshal(req) makes the real Streamable HTTP regression fail with the reported serialization error; restoring the fix makes the package pass again.
• Docker-backed integration tests were not run: the local Docker daemon is unavailable.
• A Windows full short-suite attempt encountered catalog/path/environment failures and was stopped; it is not reported as passing.