fix: handle []map[string]interface{} content in FormatResponse

[MekayelAnik]

Summary

• Fix MCP envelope double-wrap in internal/delivery/mcp/response.go FormatResponse
• Tool handlers in internal/delivery/mcp/tool_types.go and pkg/dbtools/dbtools.go return responses where content is a []map[string]interface{}. The existing type assertion only matched []interface{}, so these returns fell through to return FromString(fmt.Sprintf("%v", response)), nil and the entire MCP envelope was Go-stringified into the outer content[0].text field
• Accept both []interface{} and []map[string]interface{} via a Go type switch; existing empty-slice and metadata-preservation semantics are unchanged
• Add TestFormatResponse_ContentSliceTypes that actually asserts the marshaled JSON shape — the existing TestFormatResponse loop discards return values and only verifies absence of panic, which is why this regression went unnoticed

Symptom

Calling any tool against a running server (here query_<db>) yields:

{
  "content": [{
    "type": "text",
    "text": "map[content:[map[text:Results:\n\none\tmsg\n--------------------------------------------------------------------------------\n1\thi\n\nTotal rows: 1 type:text]]]"
  }]
}

Instead of:

{
  "content": [{
    "type": "text",
    "text": "Results:\n\none\tmsg\n--------------------------------------------------------------------------------\n1\thi\n\nTotal rows: 1"
  }]
}

Reproduces against freepeak/db-mcp-server:latest for query_<db>, execute_<db>, schema_<db>, transaction_<db>, performance_<db>, list_databases, and list. The timescaledb_* tools use a separate formatter path and are not affected, which helped isolate the bug to FormatResponse.

Root cause

internal/delivery/mcp/response.go (before this change):

if respMap, ok := response.(map[string]interface{}); ok {
    ...
    if content, exists := respMap["content"]; exists {
        if contentSlice, isSlice := content.([]interface{}); isSlice {
            if len(contentSlice) == 0 {
                return NewResponse(), nil
            }
            return respMap, nil
        }
    }
    ...
}
return FromString(fmt.Sprintf("%v", response)), nil

Handler return shape is map[string]interface{}{"content": []map[string]interface{}{...}} — concrete element type, not []interface{}. The assertion fails, control flows past both inner if blocks, and the catch-all at the bottom Go-stringifies the entire envelope.

Fix

Replace the single-type assertion with a type switch covering both shapes. Inline comment in the source explains why both appear at the boundary.

Test plan

• go build ./... passes
• go test ./internal/delivery/mcp/... — 11/11 PASS, including 3 new sub-cases in TestFormatResponse_ContentSliceTypes:
  • content as []interface{} (locks in backward compat)
  • content as []map[string]interface{} (handler return type) (new behavior)
  • content with metadata preserved
• go test ./... passes for all packages (excluding pkg/db.TestRegressionAllDatabases which requires live MySQL/PostgreSQL on localhost — pre-existing, unrelated to this change)
• golangci-lint run --timeout=5m ./internal/delivery/mcp/... passes (exit 0, no warnings)
• End-to-end smoke against a locally-built image: 32/32 tool sub-checks pass; outer text no longer contains map[content:...]. Verified against a 2-DB PostgreSQL+TimescaleDB config.

Notes

The new test case includes an assert.NotContains(out, "map[") defense-in-depth check so this same class of regression cannot silently come back through any other fall-through path in FormatResponse.

[Copilot]

Pull request overview

Fixes FormatResponse so MCP handler responses that set content as []map[string]interface{} are treated as already-formatted (instead of falling through to the fmt.Sprintf("%v", response) path that Go-stringifies/double-wraps the envelope).

Changes:

• Update FormatResponse to accept both []interface{} and []map[string]interface{} for the "content" field via a type switch.
• Add targeted tests that assert the marshaled JSON shape for both content slice types and verify metadata preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/delivery/mcp/response.go	Expands "content" slice handling to cover []map[string]interface{} and avoid envelope Go-stringification.
internal/delivery/mcp/response_test.go	Adds JSON-shape assertions for both content slice types and metadata passthrough.

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[Copilot]

In TestFormatResponse, this new table entry (and the existing ones) sets expectedOutput/err/expectError, but the test body doesn’t assert the output and always calls FormatResponse(tc.input, nil). This makes the new case look covered when it’s effectively a no-op (panic-only). Consider either (a) updating TestFormatResponse to marshal/compare against expectedOutput and pass tc.err, or (b) removing this new case (and/or the unused fields) and relying on TestFormatResponse_ContentSliceTypes for shape assertions.

Suggested change

	name: "map with concrete typed content slice",
	input: map[string]interface{}{
	"content": []map[string]interface{}{
	{"type": "text", "text": "Typed content"},
	},
	},
	err: nil,
	expectError: false,
	expectedOutput: `{"content":[{"text":"Typed content","type":"text"}]}`,
	useMock: false,
	},
	{