fix(targeting): sanitize context-agent validation error responses

targeting/contextagent/handler.go

@@ -93,7 +93,8 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if err := tmproto.ValidateContextRequest(&req); err != nil {
-		writeError(w, tmproto.SafeRequestIDForEcho(req.RequestID), tmproto.ErrorCodeInvalidRequest, err.Error(), http.StatusBadRequest)
+		h.logValidationFailure(r, req.RequestID, err)
+		writeError(w, tmproto.SafeRequestIDForEcho(req.RequestID), tmproto.ErrorCodeInvalidRequest, "invalid request", http.StatusBadRequest)
 		return
 	}
 
@@ -154,6 +155,26 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// logValidationFailure logs a rejected request's validation error
+// server-side with request context, mirroring identityagent's
+// logValidationFailure. The HTTP response only ever gets the generic
+// "invalid request" message (see ServeHTTP) — validator error text
+// (which can include request-shaped detail like field names and
+// lengths) must not cross the response boundary per AGENTS.md's
+// generic-error-message invariant. request_id is only logged when it
+// passes SafeRequestIDForEcho; an id that fails that check is elided
+// and request_id_valid=false is logged instead, so a control-byte or
+// oversized request_id doesn't get written verbatim into logs either.
+func (h *handler) logValidationFailure(r *http.Request, requestID string, err error) {
+	attrs := []any{"method", r.Method, "path", r.URL.Path, "error", err}
+	if safeID := tmproto.SafeRequestIDForEcho(requestID); safeID != "" {
+		attrs = append(attrs, "request_id", safeID)
+	} else if requestID != "" {
+		attrs = append(attrs, "request_id_valid", false)
+	}
+	h.logger.Warn("invalid context-match request", attrs...)
+}
+
 // writeError writes a TMP error response. Headers must be set before
 // WriteHeader because anything after WriteHeader is silently dropped
 // by net/http.

targeting/contextagent/handler_test.go

@@ -0,0 +1,105 @@
+package contextagent
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/adcontextprotocol/adcp-go/tmproto"
+)
+
+// TestContextHandlerValidationErrorIsGenericAndLogged pins the
+// generic-error-message invariant from AGENTS.md ("Never echo err.Error()
+// in HTTP responses") for the context-match validation path. The same
+// pattern was fixed for router.HandleContextMatch and identityagent's
+// ServeHTTP in the validator-error sweep (adcontextprotocol/adcp-go#190,
+// #201; landed in PR #210) but that sweep did not touch this handler,
+// which kept echoing tmproto.ValidateContextRequest's err.Error() text —
+// e.g. "property_id contains invalid characters" — straight into the HTTP
+// response body.
+func TestContextHandlerValidationErrorIsGenericAndLogged(t *testing.T) {
+	var logs bytes.Buffer
+	h := NewHandler(HandlerConfig{
+		RequestTimeout:             time.Second,
+		RequestBodyLimit:           64 * 1024,
+		ResponseTTL:                time.Minute,
+		SupportedADCPMajorVersions: []int{3},
+		Logger:                     slog.New(slog.NewJSONHandler(&logs, nil)),
+	})
+
+	body := `{
+		"type": "context_match_request",
+		"request_id": "ctx-invalid",
+		"property_rid": "rid-1",
+		"property_id": "bad:property",
+		"property_type": "website",
+		"placement_id": "sidebar",
+		"seller_agent_url": "https://seller.example.com/agent"
+	}`
+	req := httptest.NewRequest(http.MethodPost, "/context", strings.NewReader(body))
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, req)
+
+	require.Equal(t, http.StatusBadRequest, w.Code, w.Body.String())
+	var resp tmproto.ErrorResponse
+	require.NoError(t, json.NewDecoder(w.Body).Decode(&resp))
+	assert.Equal(t, tmproto.ErrorCodeInvalidRequest, resp.Code)
+	assert.Equal(t, "ctx-invalid", resp.RequestID)
+	assert.Equal(t, "invalid request", resp.Message)
+	assert.NotContains(t, w.Body.String(), "property_id")
+
+	logText := logs.String()
+	assert.Contains(t, logText, "invalid context-match request")
+	assert.Contains(t, logText, `"method":"POST"`)
+	assert.Contains(t, logText, `"path":"/context"`)
+	assert.Contains(t, logText, "ctx-invalid")
+	assert.Contains(t, logText, "property_id contains invalid characters")
+}
+
+// TestContextHandlerInvalidRequestIDIsNotEchoed pins the companion
+// invariant: a request_id that itself fails validateSafeID (so it is
+// unsafe to echo — see tmproto.SafeRequestIDForEcho) must not appear in
+// the HTTP response body, and is elided from the structured log too.
+func TestContextHandlerInvalidRequestIDIsNotEchoed(t *testing.T) {
+	var logs bytes.Buffer
+	h := NewHandler(HandlerConfig{
+		RequestTimeout:             time.Second,
+		RequestBodyLimit:           64 * 1024,
+		ResponseTTL:                time.Minute,
+		SupportedADCPMajorVersions: []int{3},
+		Logger:                     slog.New(slog.NewJSONHandler(&logs, nil)),
+	})
+
+	body := `{
+		"type": "context_match_request",
+		"request_id": "bad/id",
+		"property_rid": "rid-1",
+		"property_id": "pub-1",
+		"property_type": "website",
+		"placement_id": "sidebar",
+		"seller_agent_url": "https://seller.example.com/agent"
+	}`
+	req := httptest.NewRequest(http.MethodPost, "/context", strings.NewReader(body))
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, req)
+
+	require.Equal(t, http.StatusBadRequest, w.Code, w.Body.String())
+	var resp tmproto.ErrorResponse
+	require.NoError(t, json.NewDecoder(w.Body).Decode(&resp))
+	assert.Empty(t, resp.RequestID)
+	assert.Equal(t, "invalid request", resp.Message)
+	assert.NotContains(t, w.Body.String(), "bad/id")
+
+	logText := logs.String()
+	assert.Contains(t, logText, "invalid context-match request")
+	assert.Contains(t, logText, `"request_id_valid":false`)
+	assert.NotContains(t, logText, "bad/id")
+}