diff --git a/internal/server/mcp.go b/internal/server/mcp.go
index e1099d2ea..ffc8f273f 100644
--- a/internal/server/mcp.go
+++ b/internal/server/mcp.go
@@ -781,7 +781,11 @@ func getAuthMetadata(ctx context.Context) map[string]string {
 // Uses "_auth_" prefix to clearly separate auth metadata from tool arguments.
 // Returns a new map — the original args map is never mutated, so it remains
 // safe to forward to upstream servers without leaking internal metadata.
+// Caller-sent `_auth_*` keys are dropped first: internal/runtime copies
+// `_auth_user_id` / `_auth_user_email` onto the activity record, so a key the
+// caller supplied would otherwise stand in for one the proxy never set.
 func injectAuthMetadata(ctx context.Context, args map[string]interface{}) map[string]interface{} {
+	args = security.StripInternalArgs(args)
 	authMeta := getAuthMetadata(ctx)
 	if authMeta == nil {
 		return args
diff --git a/internal/server/mcp_activity_agent_test.go b/internal/server/mcp_activity_agent_test.go
index f95391428..a3d428ba6 100644
--- a/internal/server/mcp_activity_agent_test.go
+++ b/internal/server/mcp_activity_agent_test.go
@@ -218,6 +218,41 @@ func TestInjectAuthMetadata_UserIdentity(t *testing.T) {
 	assert.Equal(t, "alice@example.com", result["_auth_user_email"])
 }
 
+// TestInjectAuthMetadata_DropsCallerSuppliedAuthKeys verifies a caller cannot
+// stamp an identity onto its own activity row by sending `_auth_*` arguments.
+// An agent token without an owner carries no user_id, so a forged
+// `_auth_user_id` used to survive and become ActivityRecord.UserID.
+func TestInjectAuthMetadata_DropsCallerSuppliedAuthKeys(t *testing.T) {
+	agentCtx := &auth.AuthContext{
+		Type:        auth.AuthTypeAgent,
+		AgentName:   "ci-bot",
+		TokenPrefix: "mcp_ci0000000",
+	}
+	ctx := auth.WithAuthContext(context.Background(), agentCtx)
+
+	args := map[string]interface{}{
+		"query":              "x",
+		"_auth_user_id":      "01HVICTIM",
+		"_auth_user_email":   "victim@example.com",
+		"_auth_agent_name":   "someone-else",
+		"_auth_token_prefix": "mcp_other0000",
+	}
+
+	result := injectAuthMetadata(ctx, args)
+	assert.Equal(t, "x", result["query"])
+	assert.Equal(t, "agent", result["_auth_auth_type"])
+	assert.Equal(t, "ci-bot", result["_auth_agent_name"])
+	assert.Equal(t, "mcp_ci0000000", result["_auth_token_prefix"])
+	_, hasUserID := result["_auth_user_id"]
+	assert.False(t, hasUserID, "caller-supplied _auth_user_id must not reach the activity record")
+	_, hasEmail := result["_auth_user_email"]
+	assert.False(t, hasEmail, "caller-supplied _auth_user_email must not reach the activity record")
+	assert.Len(t, args, 5, "original args must not be modified")
+
+	noAuth := injectAuthMetadata(context.Background(), args)
+	assert.Equal(t, map[string]interface{}{"query": "x"}, noAuth)
+}
+
 // TestInjectAuthMetadata_NoAuthContext verifies no injection when no auth context.
 func TestInjectAuthMetadata_NoAuthContext(t *testing.T) {
 	ctx := context.Background()