gateway_metrics_read still rejects a string "measures"; the v3.0.0 fix was reverted by a merge #440

The v3.0.0 release notes claim this was fixed. It was not — the fix landed and was then reverted by a merge conflict resolution, and both v3.0.0 and v3.0.1 ship the broken version.

Reproduce

Against a live project, via hookdeck gateway mcp:

{"name":"gateway_metrics_read","arguments":{"action":"events","measures":"count","granularity":"1d","start":"...","end":"..."}}

measures is required (e.g. ["count"], ["successful_count", "failed_count"])

Same for the comma-separated form, "measures": "count,failed_count". Only ["count"] works.

Cause

pkg/gateway/mcp/tool_metrics.go:109 reads the argument with in.StringSlice("measures"), which returns nil for anything that is not a JSON array (pkg/mcpcore/input.go:172). The value is dropped, len(measures) == 0, and the caller is told the argument is missing for an argument they supplied.

mcpcore.StringList (pkg/mcpcore/input.go:242) exists precisely for this and accepts both forms. tool_metrics.go is the only tool on this surface still using StringSlice — for measures and for dimensions. Every other tool uses StringList.

How it regressed

• 7dae336 (2026-08-21) fixed it, switching both measures and dimensions to mcpcore.StringList, with a comment describing this exact failure.
• b0b5710 (2026-09-22), "Merge branch main (v2.6.0) into release/v3.0.0", reintroduced in.StringSlice("measures"). The conflict was resolved in favour of the pre-fix side.

git log -S does not show the reintroduction, because it skips merge commits by default — --full-history -m is needed to see it. That is worth knowing: a fix reverted inside a merge resolution is invisible to the usual history search.

Fix

Restore 7dae336: use mcpcore.StringList(in, "measures") and mcpcore.StringList(in, "dimensions").

Add a test that passes measures as a string, not an array. The existing tests in tool_metrics_schema_test.go and tool_metrics_filters_test.go all pass []any{"count"}, so they pass against both the fixed and the reverted code — which is why the revert went unnoticed through a beta and two GA releases.

Release-note correction

The v3.0.0 note claims "measures": "count" now works. It does not, and the note should be corrected.

Impact

Models routinely send a scalar where an array is declared; that is the whole reason StringList exists. The failure is the "filter that does not filter" shape the release made a point of closing: an argument the caller supplied is silently dropped, and the error names it as missing.